Mercurial > ecos-v3_0-branch
annotate packages/kernel/current/doc/kernel.sgml @ 2104:84bb9baeb0e1
* doc/kernel.sgml: Fix description of CYG_ISR_CALL_DSR and
CYG_ISR_HANDLED. Fix example isr_function() accordingly.
| author | asl |
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| date | Wed, 23 Nov 2005 20:58:28 +0000 |
| parents | eead27d1aa2f |
| children | d9e9cf8adc85 |
| rev | line source |
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1 <!-- {{{ Banner --> |
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3 <!-- =============================================================== --> |
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4 <!-- --> |
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5 <!-- kernel.sgml --> |
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6 <!-- --> |
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7 <!-- eCos kernel documentation. --> |
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8 <!-- --> |
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9 <!-- =============================================================== --> |
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10 <!-- ####COPYRIGHTBEGIN#### --> |
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11 <!-- --> |
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12 <!-- =============================================================== --> |
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13 <!-- Copyright (C) 1997, 1998, 1999, 2000, 2001, 2002 Red Hat, Inc. --> |
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14 <!-- This material may be distributed only subject to the terms --> |
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15 <!-- and conditions set forth in the Open Publication License, v1.0 --> |
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16 <!-- or later (the latest version is presently available at --> |
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17 <!-- http://www.opencontent.org/openpub/) --> |
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18 <!-- Distribution of the work or derivative of the work in any --> |
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19 <!-- standard (paper) book form is prohibited unless prior --> |
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20 <!-- permission obtained from the copyright holder --> |
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21 <!-- =============================================================== --> |
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22 <!-- --> |
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23 <!-- ####COPYRIGHTEND#### --> |
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24 <!-- =============================================================== --> |
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25 <!-- #####DESCRIPTIONBEGIN#### --> |
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26 <!-- --> |
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27 <!-- Author(s): nickg, bartv, markg --> |
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28 <!-- Contributors: eCos team --> |
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29 <!-- Date: 2002/02/13 --> |
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30 <!-- Version: 0.02 --> |
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31 <!-- --> |
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32 <!-- ####DESCRIPTIONEND#### --> |
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33 <!-- =============================================================== --> |
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34 |
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35 <!-- }}} --> |
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36 |
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37 <part id="kernel"> |
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38 <title>The eCos Kernel</title> |
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39 |
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40 <!-- {{{ Overview --> |
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41 |
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42 <refentry id="kernel-overview"> |
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43 |
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44 <refmeta> |
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45 <refentrytitle>Kernel Overview</refentrytitle> |
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46 </refmeta> |
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47 |
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48 <refnamediv> |
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49 <refname>Kernel</refname> |
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50 <refpurpose>Overview of the eCos Kernel</refpurpose> |
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51 </refnamediv> |
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52 |
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53 <!-- {{{ Description --> |
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54 |
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55 <refsect1 id="kernel-overview-description"> |
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56 <title>Description</title> |
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57 <para> |
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58 The kernel is one of the key packages in all of eCos. It provides the |
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59 core functionality needed for developing multi-threaded applications: |
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60 </para> |
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61 <orderedlist> |
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62 <listitem><para> |
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63 The ability to create new threads in the system, either during startup |
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64 or when the system is already running. |
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65 </para></listitem> |
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66 <listitem><para> |
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67 Control over the various threads in the system, for example |
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68 manipulating their priorities. |
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69 </para></listitem> |
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70 <listitem><para> |
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71 A choice of schedulers, determining which thread should currently be |
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72 running. |
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73 </para></listitem> |
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74 <listitem><para> |
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75 A range of synchronization primitives, allowing threads to interact |
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76 and share data safely. |
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77 </para></listitem> |
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78 <listitem><para> |
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79 Integration with the system's support for interrupts and exceptions. |
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80 </para></listitem> |
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81 </orderedlist> |
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82 <para> |
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83 In some other operating systems the kernel provides additional |
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84 functionality. For example the kernel may also provide memory |
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85 allocation functionality, and device drivers may be part of the kernel |
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86 as well. This is not the case for eCos. Memory allocation is handled |
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87 by a separate package. Similary each device driver will typically be a |
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88 separate package. Various packages are combined and configured using |
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89 the eCos configuration technology to meet the requirements of the |
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90 application. |
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91 </para> |
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92 <para> |
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93 The eCos kernel package is optional. It is possible to write |
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94 single-threaded applications which do not use any kernel |
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95 functionality, for example RedBoot. Typically such applications are |
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96 based around a central polling loop, continually checking all devices |
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97 and taking appropriate action when I/O occurs. A small amount of |
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98 calculation is possible every iteration, at the cost of an increased |
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99 delay between an I/O event occurring and the polling loop detecting |
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100 the event. When the requirements are straightforward it may well be |
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101 easier to develop the application using a polling loop, avoiding the |
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102 complexities of multiple threads and synchronization between threads. |
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103 As requirements get more complicated a multi-threaded solution becomes |
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104 more appropriate, requiring the use of the kernel. In fact some of the |
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105 more advanced packages in eCos, for example the TCP/IP stack, use |
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106 multi-threading internally. Therefore if the application uses any of |
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107 those packages then the kernel becomes a required package, not an |
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108 optional one. |
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109 </para> |
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110 <para> |
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111 The kernel functionality can be used in one of two ways. The kernel |
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112 provides its own C API, with functions like |
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113 <function>cyg_thread_create</function> and |
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114 <function>cyg_mutex_lock</function>. These can be called directly from |
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115 application code or from other packages. Alternatively there are a |
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116 number of packages which provide compatibility with existing API's, |
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117 for example POSIX threads or µITRON. These allow application |
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118 code to call standard functions such as |
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119 <function>pthread_create</function>, and those functions are |
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120 implemented using the basic functionality provided by the eCos kernel. |
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121 Using compatibility packages in an eCos application can make it much |
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122 easier to reuse code developed in other environments, and to share |
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123 code. |
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124 </para> |
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125 <para> |
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126 Although the different compatibility packages have similar |
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127 requirements on the underlying kernel, for example the ability to |
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128 create a new thread, there are differences in the exact semantics. For |
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129 example, strict µITRON compliance requires that kernel |
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130 timeslicing is disabled. This is achieved largely through the |
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131 configuration technology. The kernel provides a number of |
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132 configuration options that control the exact semantics that are |
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133 provided, and the various compatibility packages require particular |
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134 settings for those options. This has two important consequences. |
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135 First, it is not usually possible to have two different compatibility |
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136 packages in one eCos configuration because they will have conflicting |
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137 requirements on the underlying kernel. Second, the semantics of the |
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138 kernel's own API are only loosely defined because of the many |
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139 configuration options. For example <function>cyg_mutex_lock</function> |
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140 will always attempt to lock a mutex, but various configuration options |
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141 determine the behaviour when the mutex is already locked and there is |
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142 a possibility of priority inversion. |
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143 </para> |
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144 <para> |
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145 The optional nature of the kernel package presents some complications |
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146 for other code, especially device drivers. Wherever possible a device |
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147 driver should work whether or not the kernel is present. However there |
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148 are some parts of the system, especially those related to interrupt |
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149 handling, which should be implemented differently in multi-threaded |
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150 environments containing the eCos kernel and in single-threaded |
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151 environments without the kernel. To cope with both scenarios the |
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152 common HAL package provides a driver API, with functions such as |
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153 <function>cyg_drv_interrupt_attach</function>. When the kernel package |
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154 is present these driver API functions map directly on to the |
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155 equivalent kernel functions such as |
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156 <function>cyg_interrupt_attach</function>, using macros to avoid any |
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157 overheads. When the kernel is absent the common HAL package implements |
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158 the driver API directly, but this implementation is simpler than the |
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159 one in the kernel because it can assume a single-threaded environment. |
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160 </para> |
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161 </refsect1> |
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162 |
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163 <!-- }}} --> |
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164 <!-- {{{ Schedulers --> |
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165 |
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166 <refsect1 id="kernel-overview-schedulers"> |
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167 <title>Schedulers</title> |
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168 <para> |
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169 When a system involves multiple threads, a scheduler is needed to |
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170 determine which thread should currently be running. The eCos kernel |
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171 can be configured with one of two schedulers, the bitmap scheduler and |
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172 the multi-level queue (MLQ) scheduler. The bitmap scheduler is |
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173 somewhat more efficient, but has a number of limitations. Most systems |
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174 will instead use the MLQ scheduler. Other schedulers may be added in |
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175 the future, either as extensions to the kernel package or in separate |
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176 packages. |
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177 </para> |
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178 <para> |
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179 Both the bitmap and the MLQ scheduler use a simple numerical priority |
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180 to determine which thread should be running. The number of priority |
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181 levels is configurable via the option |
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182 <varname>CYGNUM_KERNEL_SCHED_PRIORITIES</varname>, but a typical |
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183 system will have up to 32 priority levels. Therefore thread priorities |
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184 will be in the range 0 to 31, with 0 being the highest priority and 31 |
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185 the lowest. Usually only the system's idle thread will run at the |
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186 lowest priority. Thread priorities are absolute, so the kernel will |
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187 only run a lower-priority thread if all higher-priority threads are |
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188 currently blocked. |
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189 </para> |
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190 <para> |
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191 The bitmap scheduler only allows one thread per priority level, so if |
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192 the system is configured with 32 priority levels then it is limited to |
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193 only 32 threads — still enough for many applications. A simple |
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194 bitmap can be used to keep track of which threads are currently |
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195 runnable. Bitmaps can also be used to keep track of threads waiting on |
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196 a mutex or other synchronization primitive. Identifying the |
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197 highest-priority runnable or waiting thread involves a simple |
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198 operation on the bitmap, and an array index operation can then be used |
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199 to get hold of the thread data structure itself. This makes the |
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200 bitmap scheduler fast and totally deterministic. |
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201 </para> |
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202 <para> |
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203 The MLQ scheduler allows multiple threads to run at the same priority. |
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204 This means that there is no limit on the number of threads in the |
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205 system, other than the amount of memory available. However operations |
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206 such as finding the highest priority runnable thread are a little bit |
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207 more expensive than for the bitmap scheduler. |
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208 </para> |
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209 <para> |
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210 Optionally the MLQ scheduler supports timeslicing, where the scheduler |
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211 automatically switches from one runnable thread to another when some |
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212 number of clock ticks have occurred. Timeslicing only comes into play |
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213 when there are two runnable threads at the same priority and no higher |
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214 priority runnable threads. If timeslicing is disabled then a thread |
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215 will not be preempted by another thread of the same priority, and will |
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216 continue running until either it explicitly yields the processor or |
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217 until it blocks by, for example, waiting on a synchronization |
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218 primitive. The configuration options |
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219 <varname>CYGSEM_KERNEL_SCHED_TIMESLICE</varname> and |
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220 <varname>CYGNUM_KERNEL_SCHED_TIMESLICE_TICKS</varname> control |
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221 timeslicing. The bitmap scheduler does not provide timeslicing |
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222 support. It only allows one thread per priority level, so it is not |
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223 possible to preempt the current thread in favour of another one with |
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224 the same priority. |
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225 </para> |
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226 <para> |
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227 Another important configuration option that affects the MLQ scheduler |
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228 is <varname>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</varname>. This |
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229 determines what happens when a thread blocks, for example by waiting |
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230 on a semaphore which has no pending events. The default behaviour of |
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231 the system is last-in-first-out queuing. For example if several |
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232 threads are waiting on a semaphore and an event is posted, the thread |
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233 that gets woken up is the last one that called |
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234 <function>cyg_semaphore_wait</function>. This allows for a simple and |
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235 fast implementation of both the queue and dequeue operations. However |
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236 if there are several queued threads with different priorities, it may |
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237 not be the highest priority one that gets woken up. In practice this |
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238 is rarely a problem: usually there will be at most one thread waiting |
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239 on a queue, or when there are several threads they will be of the same |
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240 priority. However if the application does require strict priority |
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241 queueing then the option |
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242 <varname>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</varname> should be |
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243 enabled. There are disadvantages: more work is needed whenever a |
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244 thread is queued, and the scheduler needs to be locked for this |
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245 operation so the system's dispatch latency is worse. If the bitmap |
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246 scheduler is used then priority queueing is automatic and does not |
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247 involve any penalties. |
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248 </para> |
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249 <para> |
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250 Some kernel functionality is currently only supported with the MLQ |
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251 scheduler, not the bitmap scheduler. This includes support for SMP |
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252 systems, and protection against priority inversion using either mutex |
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253 priority ceilings or priority inheritance. |
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254 </para> |
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255 </refsect1> |
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256 |
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257 <!-- }}} --> |
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258 <!-- {{{ Synch primitives --> |
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259 |
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260 <refsect1 id="kernel-overview-synch-primitives"> |
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261 <title>Synchronization Primitives</title> |
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262 <para> |
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263 The eCos kernel provides a number of different synchronization |
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264 primitives: <link linkend="kernel-mutexes">mutexes</link>, |
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265 <link linkend="kernel-condition-variables">condition variables</link>, |
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266 <link linkend="kernel-semaphores">counting semaphores</link>, |
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267 <link linkend="kernel-mail-boxes">mail boxes</link> and |
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268 <link linkend="kernel-flags">event flags</link>. |
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269 </para> |
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270 <para> |
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271 Mutexes serve a very different purpose from the other primitives. A |
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272 mutex allows multiple threads to share a resource safely: a thread |
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273 locks a mutex, manipulates the shared resource, and then unlocks the |
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274 mutex again. The other primitives are used to communicate information |
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275 between threads, or alternatively from a DSR associated with an |
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276 interrupt handler to a thread. |
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277 </para> |
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278 <para> |
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279 When a thread that has locked a mutex needs to wait for some condition |
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280 to become true, it should use a condition variable. A condition |
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281 variable is essentially just a place for a thread to wait, and which |
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282 another thread, or DSR, can use to wake it up. When a thread waits on |
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283 a condition variable it releases the mutex before waiting, and when it |
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284 wakes up it reacquires it before proceeding. These operations are |
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285 atomic so that synchronization race conditions cannot be introduced. |
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286 </para> |
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287 <para> |
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288 A counting semaphore is used to indicate that a particular event has |
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289 occurred. A consumer thread can wait for this event to occur, and a |
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290 producer thread or a DSR can post the event. There is a count |
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291 associated with the semaphore so if the event occurs multiple times in |
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292 quick succession this information is not lost, and the appropriate |
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293 number of semaphore wait operations will succeed. |
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294 </para> |
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295 <para> |
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296 Mail boxes are also used to indicate that a particular event has |
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297 occurred, and allows for one item of data to be exchanged per event. |
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298 Typically this item of data would be a pointer to some data structure. |
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299 Because of the need to store this extra data, mail boxes have a |
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300 finite capacity. If a producer thread generates mail box events |
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301 faster than they can be consumed then, to avoid overflow, it will be |
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302 blocked until space is again available in the mail box. This means |
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303 that mail boxes usually cannot be used by a DSR to wake up a |
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304 thread. Instead mail boxes are typically only used between threads. |
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305 </para> |
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306 <para> |
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307 Event flags can be used to wait on some number of different events, |
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308 and to signal that one or several of these events have occurred. This |
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309 is achieved by associating bits in a bit mask with the different |
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310 events. Unlike a counting semaphore no attempt is made to keep track |
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311 of the number of events that have occurred, only the fact that an |
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312 event has occurred at least once. Unlike a mail box it is not |
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313 possible to send additional data with the event, but this does mean |
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314 that there is no possibility of an overflow and hence event flags can |
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315 be used between a DSR and a thread as well as between threads. |
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316 </para> |
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317 <para> |
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318 The eCos common HAL package provides its own device driver API which |
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319 contains some of the above synchronization primitives. These allow |
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320 the DSR for an interrupt handler to signal events to higher-level |
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321 code. If the configuration includes the eCos kernel package then |
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322 the driver API routines map directly on to the equivalent kernel |
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323 routines, allowing interrupt handlers to interact with threads. If the |
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324 kernel package is not included and the application consists of just a |
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325 single thread running in polled mode then the driver API is |
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326 implemented entirely within the common HAL, and with no need to worry |
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327 about multiple threads the implementation can obviously be rather |
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328 simpler. |
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329 </para> |
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330 </refsect1> |
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331 |
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332 <!-- }}} --> |
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333 <!-- {{{ Threads and interrupts --> |
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334 |
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335 <refsect1 id="kernel-overview-threads-interrupts"> |
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336 <title>Threads and Interrupt Handling</title> |
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337 <para> |
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338 During normal operation the processor will be running one of the |
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339 threads in the system. This may be an application thread, a system |
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340 thread running inside say the TCP/IP stack, or the idle thread. From |
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341 time to time a hardware interrupt will occur, causing control to be |
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342 transferred briefly to an interrupt handler. When the interrupt has |
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343 been completed the system's scheduler will decide whether to return |
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344 control to the interrupted thread or to some other runnable thread. |
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345 </para> |
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346 <para> |
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347 Threads and interrupt handlers must be able to interact. If a thread |
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348 is waiting for some I/O operation to complete, the interrupt handler |
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349 associated with that I/O must be able to inform the thread that the |
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350 operation has completed. This can be achieved in a number of ways. One |
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351 very simple approach is for the interrupt handler to set a volatile |
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352 variable. A thread can then poll continuously until this flag is set, |
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353 possibly sleeping for a clock tick in between. Polling continuously |
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354 means that the cpu time is not available for other activities, which |
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355 may be acceptable for some but not all applications. Polling once |
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356 every clock tick imposes much less overhead, but means that the thread |
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357 may not detect that the I/O event has occurred until an entire clock |
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358 tick has elapsed. In typical systems this could be as long as 10 |
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359 milliseconds. Such a delay might be acceptable for some applications, |
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360 but not all. |
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361 </para> |
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362 <para> |
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363 A better solution would be to use one of the synchronization |
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364 primitives. The interrupt handler could signal a condition variable, |
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365 post to a semaphore, or use one of the other primitives. The thread |
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366 would perform a wait operation on the same primitive. It would not |
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367 consume any cpu cycles until the I/O event had occurred, and when the |
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368 event does occur the thread can start running again immediately |
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369 (subject to any higher priority threads that might also be runnable). |
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370 </para> |
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371 <para> |
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372 Synchronization primitives constitute shared data, so care must be |
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373 taken to avoid problems with concurrent access. If the thread that was |
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374 interrupted was just performing some calculations then the interrupt |
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375 handler could manipulate the synchronization primitive quite safely. |
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376 However if the interrupted thread happened to be inside some kernel |
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377 call then there is a real possibility that some kernel data structure |
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378 will be corrupted. |
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379 </para> |
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380 <para> |
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381 One way of avoiding such problems would be for the kernel functions to |
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382 disable interrupts when executing any critical region. On most |
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383 architectures this would be simple to implement and very fast, but it |
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384 would mean that interrupts would be disabled often and for quite a |
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385 long time. For some applications that might not matter, but many |
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386 embedded applications require that the interrupt handler run as soon |
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387 as possible after the hardware interrupt has occurred. If the kernel |
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388 relied on disabling interrupts then it would not be able to support |
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389 such applications. |
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390 </para> |
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391 <para> |
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392 Instead the kernel uses a two-level approach to interrupt handling. |
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393 Associated with every interrupt vector is an Interrupt Service Routine |
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394 or ISR, which will run as quickly as possible so that it can service |
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395 the hardware. However an ISR can make only a small number of kernel |
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396 calls, mostly related to the interrupt subsystem, and it cannot make |
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397 any call that would cause a thread to wake up. If an ISR detects that |
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398 an I/O operation has completed and hence that a thread should be woken |
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399 up, it can cause the associated Deferred Service Routine or DSR to |
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400 run. A DSR is allowed to make more kernel calls, for example it can |
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401 signal a condition variable or post to a semaphore. |
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402 </para> |
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403 <para> |
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404 Disabling interrupts prevents ISRs from running, but very few parts of |
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405 the system disable interrupts and then only for short periods of time. |
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406 The main reason for a thread to disable interrupts is to manipulate |
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407 some state that is shared with an ISR. For example if a thread needs |
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408 to add another buffer to a linked list of free buffers and the ISR may |
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409 remove a buffer from this list at any time, the thread would need to |
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410 disable interrupts for the few instructions needed to manipulate the |
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411 list. If the hardware raises an interrupt at this time, it remains |
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412 pending until interrupts are reenabled. |
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413 </para> |
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414 <para> |
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415 Analogous to interrupts being disabled or enabled, the kernel has a |
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416 scheduler lock. The various kernel functions such as |
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417 <function>cyg_mutex_lock</function> and |
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418 <function>cyg_semaphore_post</function> will claim the scheduler lock, |
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419 manipulate the kernel data structures, and then release the scheduler |
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420 lock. If an interrupt results in a DSR being requested and the |
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421 scheduler is currently locked, the DSR remains pending. When the |
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422 scheduler lock is released any pending DSRs will run. These may post |
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423 events to synchronization primitives, causing other higher priority |
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424 threads to be woken up. |
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425 </para> |
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426 <para> |
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427 For an example, consider the following scenario. The system has a high |
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428 priority thread A, responsible for processing some data coming from an |
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429 external device. This device will raise an interrupt when data is |
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430 available. There are two other threads B and C which spend their time |
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431 performing calculations and occasionally writing results to a display |
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432 of some sort. This display is a shared resource so a mutex is used to |
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433 control access. |
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434 </para> |
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435 <para> |
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436 At a particular moment in time thread A is likely to be blocked, |
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437 waiting on a semaphore or another synchronization primitive until data |
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438 is available. Thread B might be running performing some calculations, |
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439 and thread C is runnable waiting for its next timeslice. Interrupts |
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440 are enabled, and the scheduler is unlocked because none of the threads |
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441 are in the middle of a kernel operation. At this point the device |
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442 raises an interrupt. The hardware transfers control to a low-level |
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443 interrupt handler provided by eCos which works out exactly which |
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444 interrupt occurs, and then the corresponding ISR is run. This ISR |
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445 manipulates the hardware as appropriate, determines that there is now |
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446 data available, and wants to wake up thread A by posting to the |
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447 semaphore. However ISR's are not allowed to call |
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448 <function>cyg_semaphore_post</function> directly, so instead the ISR |
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449 requests that its associated DSR be run and returns. There are no more |
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450 interrupts to be processed, so the kernel next checks for DSR's. One |
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451 DSR is pending and the scheduler is currently unlocked, so the DSR can |
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452 run immediately and post the semaphore. This will have the effect of |
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453 making thread A runnable again, so the scheduler's data structures are |
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454 adjusted accordingly. When the DSR returns thread B is no longer the |
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455 highest priority runnable thread so it will be suspended, and instead |
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456 thread A gains control over the cpu. |
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457 </para> |
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458 <para> |
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459 In the above example no kernel data structures were being manipulated |
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460 at the exact moment that the interrupt happened. However that cannot |
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461 be assumed. Suppose that thread B had finished its current set of |
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462 calculations and wanted to write the results to the display. It would |
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463 claim the appropriate mutex and manipulate the display. Now suppose |
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464 that thread B was timesliced in favour of thread C, and that thread C |
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465 also finished its calculations and wanted to write the results to the |
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466 display. It would call <function>cyg_mutex_lock</function>. This |
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467 kernel call locks the scheduler, examines the current state of the |
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468 mutex, discovers that the mutex is already owned by another thread, |
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469 suspends the current thread, and switches control to another runnable |
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470 thread. Another interrupt happens in the middle of this |
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471 <function>cyg_mutex_lock</function> call, causing the ISR to run |
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472 immediately. The ISR decides that thread A should be woken up so it |
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473 requests that its DSR be run and returns back to the kernel. At this |
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474 point there is a pending DSR, but the scheduler is still locked by the |
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475 call to <function>cyg_mutex_lock</function> so the DSR cannot run |
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476 immediately. Instead the call to <function>cyg_mutex_lock</function> |
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477 is allowed to continue, which at some point involves unlocking the |
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478 scheduler. The pending DSR can now run, safely post the semaphore, and |
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479 thus wake up thread A. |
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480 </para> |
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481 <para> |
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482 If the ISR had called <function>cyg_semaphore_post</function> directly |
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483 rather than leaving it to a DSR, it is likely that there would have |
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484 been some sort of corruption of a kernel data structure. For example |
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485 the kernel might have completely lost track of one of the threads, and |
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486 that thread would never have run again. The two-level approach to |
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487 interrupt handling, ISR's and DSR's, prevents such problems with no |
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488 need to disable interrupts. |
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489 </para> |
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490 </refsect1> |
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491 |
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492 <!-- }}} --> |
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493 <!-- {{{ Calling contexts --> |
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494 |
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495 <refsect1 id="kernel-overview-contexts"> |
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496 <title>Calling Contexts</title> |
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497 <para> |
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498 eCos defines a number of contexts. Only certain calls are allowed from |
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499 inside each context, for example most operations on threads or |
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500 synchronization primitives are not allowed from ISR context. The |
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501 different contexts are initialization, thread, ISR and DSR. |
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502 </para> |
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503 <para> |
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504 When eCos starts up it goes through a number of phases, including |
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505 setting up the hardware and invoking C++ static constructors. During |
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506 this time interrupts are disabled and the scheduler is locked. When a |
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507 configuration includes the kernel package the final operation is a |
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508 call to <link |
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509 linkend="kernel-schedcontrol"><function>cyg_scheduler_start</function></link>. |
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510 At this point interrupts are enabled, the scheduler is unlocked, and |
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511 control is transferred to the highest priority runnable thread. If the |
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512 configuration also includes the C library package then usually the C |
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513 library startup package will have created a thread which will call the |
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514 application's <function>main</function> entry point. |
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515 </para> |
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516 <para> |
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517 Some application code can also run before the scheduler is started, |
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518 and this code runs in initialization context. If the application is |
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519 written partly or completely in C++ then the constructors for any |
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520 static objects will be run. Alternatively application code can define |
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521 a function <function>cyg_user_start</function> which gets called after |
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522 any C++ static constructors. This allows applications to be written |
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523 entirely in C. |
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524 </para> |
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525 <programlisting width=72> |
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526 void |
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527 cyg_user_start(void) |
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528 { |
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529 /* Perform application-specific initialization here */ |
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530 } |
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531 </programlisting> |
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532 <para> |
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533 It is not necessary for applications to provide a |
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534 <function>cyg_user_start</function> function since the system will |
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535 provide a default implementation which does nothing. |
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536 </para> |
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537 <para> |
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538 Typical operations that are performed from inside static constructors |
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539 or <function>cyg_user_start</function> include creating threads, |
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540 synchronization primitives, setting up alarms, and registering |
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541 application-specific interrupt handlers. In fact for many applications |
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542 all such creation operations happen at this time, using statically |
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543 allocated data, avoiding any need for dynamic memory allocation or |
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544 other overheads. |
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545 </para> |
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546 <para> |
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547 Code running in initialization context runs with interrupts disabled |
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548 and the scheduler locked. It is not permitted to reenable interrupts |
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549 or unlock the scheduler because the system is not guaranteed to be in |
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550 a totally consistent state at this point. A consequence is that |
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551 initialization code cannot use synchronization primitives such as |
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552 <function>cyg_semaphore_wait</function> to wait for an external event. |
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553 It is permitted to lock and unlock a mutex: there are no other threads |
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554 running so it is guaranteed that the mutex is not yet locked, and |
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555 therefore the lock operation will never block; this is useful when |
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556 making library calls that may use a mutex internally. |
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557 </para> |
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558 <para> |
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559 At the end of the startup sequence the system will call |
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560 <function>cyg_scheduler_start</function> and the various threads will |
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561 start running. In thread context nearly all of the kernel functions |
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562 are available. There may be some restrictions on interrupt-related |
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563 operations, depending on the target hardware. For example the hardware |
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564 may require that interrupts be acknowledged in the ISR or DSR before |
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565 control returns to thread context, in which case |
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566 <filename>cyg_interrupt_acknowledge</filename> should not be called |
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567 by a thread. |
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568 </para> |
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569 <para> |
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570 At any time the processor may receive an external interrupt, causing |
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571 control to be transferred from the current thread. Typically a VSR |
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572 provided by eCos will run and determine exactly which interrupt |
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573 occurred. Then the VSR will switch to the appropriate ISR, which can |
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574 be provided by a HAL package, a device driver, or by the application. |
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575 During this time the system is running at ISR context, and most of the |
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576 kernel function calls are disallowed. This includes the various |
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577 synchronization primitives, so for example an ISR is not allowed to |
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578 post to a semaphore to indicate that an event has happened. Usually |
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579 the only operations that should be performed from inside an ISR are |
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580 ones related to the interrupt subsystem itself, for example masking an |
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581 interrupt or acknowledging that an interrupt has been processed. On |
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582 SMP systems it is also possible to use spinlocks from ISR context. |
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583 </para> |
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584 <para> |
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585 When an ISR returns it can request that the corresponding DSR be run |
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586 as soon as it is safe to do so, and that will run in DSR context. This |
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587 context is also used for running alarm functions, and threads can |
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588 switch temporarily to DSR context by locking the scheduler. Only |
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589 certain kernel functions can be called from DSR context, although more |
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590 than in ISR context. In particular it is possible to use any |
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591 synchronization primitives which cannot block. These include |
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592 <function>cyg_semaphore_post</function>, |
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593 <filename>cyg_cond_signal</filename>, |
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594 <function>cyg_cond_broadcast</function>, |
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595 <function>cyg_flag_setbits</function>, and |
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596 <function>cyg_mbox_tryput</function>. It is not possible to use any |
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597 primitives that may block such as |
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598 <function>cyg_semaphore_wait</function>, |
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599 <function>cyg_mutex_lock</function>, or |
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600 <function>cyg_mbox_put</function>. Calling such functions from inside |
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601 a DSR may cause the system to hang. |
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602 </para> |
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603 <para> |
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604 The specific documentation for the various kernel functions gives more |
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605 details about valid contexts. |
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606 </para> |
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607 </refsect1> |
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608 |
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609 <!-- }}} --> |
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610 <!-- {{{ Error handling --> |
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611 |
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612 <refsect1 id="kernel-overview-errors"> |
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613 <title>Error Handling and Assertions</title> |
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614 <para> |
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615 In many APIs each function is expected to perform some validation of |
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616 its parameters and possibly of the current state of the system. This |
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617 is supposed to ensure that each function is used correctly, and that |
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618 application code is not attempting to perform a semaphore operation on |
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619 a mutex or anything like that. If an error is detected then a suitable |
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620 error code is returned, for example the POSIX function |
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621 <function>pthread_mutex_lock</function> can return various error codes |
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622 including <literal>EINVAL</literal> and <literal>EDEADLK</literal>. |
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623 There are a number of problems with this approach, especially in the |
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624 context of deeply embedded systems: |
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625 </para> |
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626 <orderedlist> |
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627 <listitem><para> |
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628 Performing these checks inside the mutex lock and all the other |
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629 functions requires extra cpu cycles and adds significantly to the code |
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630 size. Even if the application is written correctly and only makes |
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631 system function calls with sensible arguments and under the right |
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632 conditions, these overheads still exist. |
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633 </para></listitem> |
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634 <listitem><para> |
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635 Returning an error code is only useful if the calling code detects |
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636 these error codes and takes appropriate action. In practice the |
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637 calling code will often ignore any errors because the programmer |
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638 <emphasis>“knows”</emphasis> that the function is being |
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639 used correctly. If the programmer is mistaken then an error condition |
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640 may be detected and reported, but the application continues running |
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641 anyway and is likely to fail some time later in mysterious ways. |
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642 </para></listitem> |
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643 <listitem><para> |
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644 If the calling code does always check for error codes, that adds yet |
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645 more cpu cycles and code size overhead. |
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646 </para></listitem> |
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647 <listitem><para> |
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648 Usually there will be no way to recover from certain errors, so if the |
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649 application code detected an error such as <literal>EINVAL</literal> |
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650 then all it could do is abort the application somehow. |
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651 </para></listitem> |
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652 </orderedlist> |
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653 <para> |
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654 The approach taken within the eCos kernel is different. Functions such |
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655 as <function>cyg_mutex_lock</function> will not return an error code. |
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656 Instead they contain various assertions, which can be enabled or |
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657 disabled. During the development process assertions are normally left |
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658 enabled, and the various kernel functions will perform parameter |
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659 checks and other system consistency checks. If a problem is detected |
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660 then an assertion failure will be reported and the application will be |
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661 terminated. In a typical debug session a suitable breakpoint will have |
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662 been installed and the developer can now examine the state of the |
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663 system and work out exactly what is going on. Towards the end of the |
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664 development cycle assertions will be disabled by manipulating |
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665 configuration options within the eCos infrastructure package, and all |
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666 assertions will be eliminated at compile-time. The assumption is that |
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667 by this time the application code has been mostly debugged: the |
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668 initial version of the code might have tried to perform a semaphore |
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669 operation on a mutex, but any problems like that will have been fixed |
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670 some time ago. This approach has a number of advantages: |
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671 </para> |
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672 <orderedlist> |
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673 <listitem><para> |
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674 In the final application there will be no overheads for checking |
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675 parameters and other conditions. All that code will have been |
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676 eliminated at compile-time. |
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677 </para></listitem> |
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678 <listitem><para> |
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679 Because the final application will not suffer any overheads, it is |
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680 reasonable for the system to do more work during the development |
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681 process. In particular the various assertions can test for more error |
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682 conditions and more complicated errors. When an error is detected |
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683 it is possible to give a text message describing the error rather than |
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684 just return an error code. |
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685 </para></listitem> |
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686 <listitem><para> |
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687 There is no need for application programmers to handle error codes |
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688 returned by various kernel function calls. This simplifies the |
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689 application code. |
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690 </para></listitem> |
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691 <listitem><para> |
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692 If an error is detected then an assertion failure will be reported |
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693 immediately and the application will be halted. There is no |
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694 possibility of an error condition being ignored because application |
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695 code did not check for an error code. |
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696 </para></listitem> |
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697 </orderedlist> |
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698 <para> |
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699 Although none of the kernel functions return an error code, many of |
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700 them do return a status condition. For example the function |
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701 <function>cyg_semaphore_timed_wait</function> waits until either an |
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702 event has been posted to a semaphore, or until a certain number of |
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703 clock ticks have occurred. Usually the calling code will need to know |
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704 whether the wait operation succeeded or whether a timeout occurred. |
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705 <function>cyg_semaphore_timed_wait</function> returns a boolean: a |
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706 return value of zero or false indicates a timeout, a non-zero return |
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707 value indicates that the wait succeeded. |
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708 </para> |
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709 <para> |
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710 In conventional APIs one common error conditions is lack of memory. |
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711 For example the POSIX function <function>pthread_create</function> |
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712 usually has to allocate some memory dynamically for the thread stack |
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713 and other per-thread data. If the target hardware does not have enough |
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714 memory to meet all demands, or more commonly if the application |
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715 contains a memory leak, then there may not be enough memory available |
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716 and the function call would fail. The eCos kernel avoids such problems |
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717 by never performing any dynamic memory allocation. Instead it is the |
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718 responsibility of the application code to provide all the memory |
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719 required for kernel data structures and other needs. In the case of |
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720 <function>cyg_thread_create</function> this means a |
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721 <structname>cyg_thread</structname> data structure to hold the thread |
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722 details, and a <type>char</type> array for the thread stack. |
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723 </para> |
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724 <para> |
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725 In many applications this approach results in all data structures |
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726 being allocated statically rather than dynamically. This has several |
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727 advantages. If the application is in fact too large for the target |
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728 hardware's memory then there will be an error at link-time rather than |
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729 at run-time, making the problem much easier to diagnose. Static |
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730 allocation does not involve any of the usual overheads associated with |
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731 dynamic allocation, for example there is no need to keep track of the |
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732 various free blocks in the system, and it may be possible to eliminate |
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733 <function>malloc</function> from the system completely. Problems such |
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734 as fragmentation and memory leaks cannot occur if all data is |
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735 allocated statically. However, some applications are sufficiently |
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736 complicated that dynamic memory allocation is required, and the |
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737 various kernel functions do not distinguish between statically and |
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738 dynamically allocated memory. It still remains the responsibility of |
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739 the calling code to ensure that sufficient memory is available, and |
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740 passing null pointers to the kernel will result in assertions or |
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741 system failure. |
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742 </para> |
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743 </refsect1> |
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744 |
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745 <!-- }}} --> |
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746 |
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747 </refentry> |
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748 |
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749 <!-- }}} --> |
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750 <!-- {{{ SMP --> |
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751 |
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752 <refentry id="kernel-SMP"> |
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753 |
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754 <refmeta> |
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755 <refentrytitle>SMP Support</refentrytitle> |
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756 </refmeta> |
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757 |
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758 <refnamediv> |
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759 <refname>SMP</refname> |
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760 <refpurpose>Support Symmetric Multiprocessing Systems</refpurpose> |
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761 </refnamediv> |
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762 |
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763 <refsect1 id="kernel-smp-description"> |
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764 <title>Description</title> |
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765 <para> |
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766 eCos contains support for limited Symmetric Multi-Processing (SMP). |
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767 This is only available on selected architectures and platforms. |
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768 The implementation has a number of restrictions on the kind of |
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769 hardware supported. These are described in <xref linkend="hal-smp-support">. |
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770 </para> |
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771 |
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772 <para> |
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773 The following sections describe the changes that have been made to the |
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774 eCos kernel to support SMP operation. |
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775 </para> |
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776 </refsect1> |
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777 |
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778 <refsect1 id="kernel-smp-startup"> |
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779 <title>System Startup</title> |
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780 <para> |
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781 The system startup sequence needs to be somewhat different on an SMP |
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782 system, although this is largely transparent to application code. The |
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783 main startup takes place on only one CPU, called the primary CPU. All |
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784 other CPUs, the secondary CPUs, are either placed in suspended state |
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785 at reset, or are captured by the HAL and put into a spin as they start |
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786 up. The primary CPU is responsible for copying the DATA segment and |
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787 zeroing the BSS (if required), calling HAL variant and platform |
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788 initialization routines and invoking constructors. It then calls |
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789 <function>cyg_start</function> to enter the application. The |
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790 application may then create extra threads and other objects. |
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791 </para> |
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792 <para> |
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793 It is only when the application calls |
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794 <function>cyg_scheduler_start</function> that the secondary CPUs are |
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795 initialized. This routine scans the list of available secondary CPUs |
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796 and invokes <function>HAL_SMP_CPU_START</function> to start each |
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797 CPU. Finally it calls an internal function |
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798 <function>Cyg_Scheduler::start_cpu</function> to enter the scheduler |
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799 for the primary CPU. |
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800 </para> |
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801 <para> |
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802 Each secondary CPU starts in the HAL, where it completes any per-CPU |
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803 initialization before calling into the kernel at |
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804 <function>cyg_kernel_cpu_startup</function>. Here it claims the |
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805 scheduler lock and calls |
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806 <function>Cyg_Scheduler::start_cpu</function>. |
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807 </para> |
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808 <para> |
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809 <function>Cyg_Scheduler::start_cpu</function> is common to both the |
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810 primary and secondary CPUs. The first thing this code does is to |
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811 install an interrupt object for this CPU's inter-CPU interrupt. From |
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812 this point on the code is the same as for the single CPU case: an |
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813 initial thread is chosen and entered. |
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814 </para> |
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815 <para> |
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816 From this point on the CPUs are all equal, eCos makes no further |
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817 distinction between the primary and secondary CPUs. However, the |
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818 hardware may still distinguish between them as far as interrupt |
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819 delivery is concerned. |
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820 </para> |
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821 </refsect1> |
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822 |
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823 <refsect1 id="kernel-smp-scheduling"> |
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824 <title>Scheduling</title> |
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825 <para> |
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826 To function correctly an operating system kernel must protect its |
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827 vital data structures, such as the run queues, from concurrent |
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828 access. In a single CPU system the only concurrent activities to worry |
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829 about are asynchronous interrupts. The kernel can easily guard its |
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830 data structures against these by disabling interrupts. However, in a |
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831 multi-CPU system, this is inadequate since it does not block access by |
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832 other CPUs. |
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833 </para> |
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834 <para> |
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835 The eCos kernel protects its vital data structures using the scheduler |
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836 lock. In single CPU systems this is a simple counter that is |
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837 atomically incremented to acquire the lock and decremented to release |
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838 it. If the lock is decremented to zero then the scheduler may be |
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839 invoked to choose a different thread to run. Because interrupts may |
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840 continue to be serviced while the scheduler lock is claimed, ISRs are |
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841 not allowed to access kernel data structures, or call kernel routines |
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842 that can. Instead all such operations are deferred to an associated |
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843 DSR routine that is run during the lock release operation, when the |
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844 data structures are in a consistent state. |
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845 </para> |
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846 <para> |
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847 By choosing a kernel locking mechanism that does not rely on interrupt |
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848 manipulation to protect data structures, it is easier to convert eCos |
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849 to SMP than would otherwise be the case. The principal change needed to |
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850 make eCos SMP-safe is to convert the scheduler lock into a nestable |
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851 spin lock. This is done by adding a spinlock and a CPU id to the |
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852 original counter. |
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853 </para> |
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854 <para> |
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855 The algorithm for acquiring the scheduler lock is very simple. If the |
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856 scheduler lock's CPU id matches the current CPU then it can just increment |
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857 the counter and continue. If it does not match, the CPU must spin on |
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858 the spinlock, after which it may increment the counter and store its |
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859 own identity in the CPU id. |
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860 </para> |
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861 <para> |
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862 To release the lock, the counter is decremented. If it goes to zero |
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863 the CPU id value must be set to NONE and the spinlock cleared. |
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864 </para> |
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865 <para> |
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866 To protect these sequences against interrupts, they must be performed |
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867 with interrupts disabled. However, since these are very short code |
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868 sequences, they will not have an adverse effect on the interrupt |
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869 latency. |
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870 </para> |
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871 <para> |
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872 Beyond converting the scheduler lock, further preparing the kernel for |
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873 SMP is a relatively minor matter. The main changes are to convert |
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874 various scalar housekeeping variables into arrays indexed by CPU |
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875 id. These include the current thread pointer, the need_reschedule |
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876 flag and the timeslice counter. |
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877 </para> |
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878 <para> |
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879 At present only the Multi-Level Queue (MLQ) scheduler is capable of |
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880 supporting SMP configurations. The main change made to this scheduler |
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881 is to cope with having several threads in execution at the same |
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882 time. Running threads are marked with the CPU that they are executing on. |
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883 When scheduling a thread, the scheduler skips past any running threads |
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884 until it finds a thread that is pending. While not a constant-time |
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885 algorithm, as in the single CPU case, this is still deterministic, |
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886 since the worst case time is bounded by the number of CPUs in the |
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887 system. |
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888 </para> |
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889 <para> |
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890 A second change to the scheduler is in the code used to decide when |
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891 the scheduler should be called to choose a new thread. The scheduler |
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892 attempts to keep the <property>n</property> CPUs running the |
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893 <property>n</property> highest priority threads. Since an event or |
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894 interrupt on one CPU may require a reschedule on another CPU, there |
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895 must be a mechanism for deciding this. The algorithm currently |
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896 implemented is very simple. Given a thread that has just been awakened |
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897 (or had its priority changed), the scheduler scans the CPUs, starting |
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898 with the one it is currently running on, for a current thread that is |
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899 of lower priority than the new one. If one is found then a reschedule |
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900 interrupt is sent to that CPU and the scan continues, but now using |
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901 the current thread of the rescheduled CPU as the candidate thread. In |
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902 this way the new thread gets to run as quickly as possible, hopefully |
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903 on the current CPU, and the remaining CPUs will pick up the remaining |
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904 highest priority threads as a consequence of processing the reschedule |
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905 interrupt. |
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906 </para> |
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907 <para> |
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908 The final change to the scheduler is in the handling of |
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909 timeslicing. Only one CPU receives timer interrupts, although all CPUs |
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910 must handle timeslicing. To make this work, the CPU that receives the |
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911 timer interrupt decrements the timeslice counter for all CPUs, not |
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912 just its own. If the counter for a CPU reaches zero, then it sends a |
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913 timeslice interrupt to that CPU. On receiving the interrupt the |
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914 destination CPU enters the scheduler and looks for another thread at |
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915 the same priority to run. This is somewhat more efficient than |
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916 distributing clock ticks to all CPUs, since the interrupt is only |
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917 needed when a timeslice occurs. |
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918 </para> |
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919 <para> |
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920 All existing synchronization mechanisms work as before in an SMP |
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921 system. Additional synchronization mechanisms have been added to |
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922 provide explicit synchronization for SMP, in the form of |
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923 <link linkend="kernel-spinlocks">spinlocks</link>. |
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924 </para> |
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925 </refsect1> |
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926 |
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927 <refsect1 id="kernel-smp-interrupts"> |
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928 <title>SMP Interrupt Handling</title> |
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929 <para> |
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930 The main area where the SMP nature of a system requires special |
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931 attention is in device drivers and especially interrupt handling. It |
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932 is quite possible for the ISR, DSR and thread components of a device |
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933 driver to execute on different CPUs. For this reason it is much more |
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934 important that SMP-capable device drivers use the interrupt-related |
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935 functions correctly. Typically a device driver would use the driver |
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936 API rather than call the kernel directly, but it is unlikely that |
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937 anybody would attempt to use a multiprocessor system without the |
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938 kernel package. |
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939 </para> |
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940 <para> |
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941 Two new functions have been added to the Kernel API |
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942 to do <link linkend="kernel-interrupts-smp">interrupt |
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943 routing</link>: <function>cyg_interrupt_set_cpu</function> and |
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944 <function>cyg_interrupt_get_cpu</function>. Although not currently |
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945 supported, special values for the cpu argument may be used in future |
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946 to indicate that the interrupt is being routed dynamically or is |
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947 CPU-local. Once a vector has been routed to a new CPU, all other |
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948 interrupt masking and configuration operations are relative to that |
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949 CPU, where relevant. |
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950 </para> |
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951 |
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952 <para> |
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953 There are more details of how interrupts should be handled in SMP |
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954 systems in <xref linkend="devapi-smp-support">. |
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955 </para> |
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956 </refsect1> |
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957 |
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958 </refentry> |
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959 |
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960 <!-- }}} --> |
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961 |
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962 <!-- {{{ cyg_thread_create() --> |
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963 |
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964 <refentry id="kernel-thread-create"> |
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965 |
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966 <refmeta> |
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967 <refentrytitle>Thread creation</refentrytitle> |
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968 </refmeta> |
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969 |
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970 <refnamediv> |
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971 <refname>cyg_thread_create</refname> |
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972 <refpurpose>Create a new thread</refpurpose> |
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973 </refnamediv> |
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974 |
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975 <refsynopsisdiv> |
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976 <funcsynopsis> |
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977 <funcsynopsisinfo> |
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978 #include <cyg/kernel/kapi.h> |
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979 </funcsynopsisinfo> |
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980 <funcprototype> |
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981 <funcdef>void <function>cyg_thread_create</function></funcdef> |
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982 <paramdef>cyg_addrword_t <parameter>sched_info</parameter></paramdef> |
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983 <paramdef>cyg_thread_entry_t* <parameter>entry</parameter></paramdef> |
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984 <paramdef>cyg_addrword_t <parameter>entry_data</parameter></paramdef> |
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985 <paramdef>char* <parameter>name</parameter></paramdef> |
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986 <paramdef>void* <parameter>stack_base</parameter></paramdef> |
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987 <paramdef>cyg_ucount32 <parameter>stack_size</parameter></paramdef> |
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988 <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef> |
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989 <paramdef>cyg_thread* <parameter>thread</parameter></paramdef> |
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990 </funcprototype> |
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991 </funcsynopsis> |
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992 </refsynopsisdiv> |
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993 |
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994 <refsect1 id="kernel-thread-create-description"><title>Description</title> |
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995 <para> |
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996 The <function>cyg_thread_create</function> function allows application |
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997 code and eCos packages to create new threads. In many applications |
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998 this only happens during system initialization and all required data |
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999 is allocated statically. However additional threads can be created at |
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1000 any time, if necessary. A newly created thread is always in suspended |
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1001 state and will not start running until it has been resumed via a call |
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1002 to <function>cyg_thread_resume</function>. Also, if threads are |
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1003 created during system initialization then they will not start running |
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1004 until the eCos scheduler has been started. |
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1005 </para> |
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1006 <para> |
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1007 The <parameter class="function">name</parameter> argument is used |
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1008 primarily for debugging purposes, making it easier to keep track of |
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1009 which <structname>cyg_thread</structname> structure is associated with |
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1010 which application-level thread. The kernel configuration option |
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1011 <varname>CYGVAR_KERNEL_THREADS_NAME</varname> controls whether or not |
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1012 this name is actually used. |
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1013 </para> |
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1014 <para> |
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1015 On creation each thread is assigned a unique handle, and this will be |
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1016 stored in the location pointed at by the <parameter |
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1017 class="function">handle</parameter> argument. Subsequent operations on |
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1018 this thread including the required |
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1019 <function>cyg_thread_resume</function> should use this handle to |
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1020 identify the thread. |
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1021 </para> |
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1022 <para> |
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1023 The kernel requires a small amount of space for each thread, in the |
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1024 form of a <structname>cyg_thread</structname> data structure, to hold |
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1025 information such as the current state of that thread. To avoid any |
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1026 need for dynamic memory allocation within the kernel this space has to |
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1027 be provided by higher-level code, typically in the form of a static |
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1028 variable. The <parameter class="function">thread</parameter> argument |
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1029 provides this space. |
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1030 </para> |
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1031 </refsect1> |
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1032 |
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1033 <refsect1 id="kernel-thread-create-entry"><title>Thread Entry Point</title> |
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1034 <para> |
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1035 The entry point for a thread takes the form: |
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1036 </para> |
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1037 <programlisting width=72> |
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1038 void |
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1039 thread_entry_function(cyg_addrword_t data) |
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1040 { |
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1041 … |
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1042 } |
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1043 </programlisting> |
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1044 <para> |
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1045 The second argument to <function>cyg_thread_create</function> is a |
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1046 pointer to such a function. The third argument <parameter |
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1047 class="function">entry_data</parameter> is used to pass additional |
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1048 data to the function. Typically this takes the form of a pointer to |
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1049 some static data, or a small integer, or <literal>0</literal> if the |
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1050 thread does not require any additional data. |
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1051 </para> |
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1052 <para> |
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1053 If the thread entry function ever returns then this is equivalent to |
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1054 the thread calling <function>cyg_thread_exit</function>. Even though |
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1055 the thread will no longer run again, it remains registered with the |
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1056 scheduler. If the application needs to re-use the |
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1057 <structname>cyg_thread</structname> data structure then a call to |
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1058 <function>cyg_thread_delete</function> is required first. |
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1059 </para> |
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1060 </refsect1> |
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1061 |
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1062 <refsect1 id="kernel-thread-create-priorities"><title>Thread Priorities</title> |
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1063 <para> |
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1064 The <parameter class="function">sched_info</parameter> argument |
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1065 provides additional information to the scheduler. The exact details |
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1066 depend on the scheduler being used. For the bitmap and mlqueue |
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1067 schedulers it is a small integer, typically in the range 0 to 31, with |
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1068 0 being the highest priority. The lowest priority is normally used |
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1069 only by the system's idle thread. The exact number of priorities is |
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1070 controlled by the kernel configuration option |
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1071 <varname>CYGNUM_KERNEL_SCHED_PRIORITIES</varname>. |
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1072 </para> |
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1073 <para> |
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1074 It is the responsibility of the application developer to be aware of |
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1075 the various threads in the system, including those created by eCos |
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1076 packages, and to ensure that all threads run at suitable priorities. |
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1077 For threads created by other packages the documentation provided by |
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1078 those packages should indicate any requirements. |
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1079 </para> |
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1080 <para> |
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1081 The functions <function>cyg_thread_set_priority</function>, |
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1082 <function>cyg_thread_get_priority</function>, and |
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1083 <function>cyg_thread_get_current_priority</function> can be used to |
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1084 manipulate a thread's priority. |
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1085 </para> |
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1086 </refsect1> |
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1087 |
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1088 <refsect1 id="kernel-thread-create-stack"><title>Stacks and Stack Sizes</title> |
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1089 <para> |
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1090 Each thread needs its own stack for local variables and to keep track |
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1091 of function calls and returns. Again it is expected that this stack is |
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1092 provided by the calling code, usually in the form of static data, so |
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1093 that the kernel does not need any dynamic memory allocation |
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1094 facilities. <function>cyg_thread_create</function> takes two arguments |
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1095 related to the stack, a pointer to the base of the stack and the total |
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1096 size of this stack. On many processors stacks actually descend from the |
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1097 top down, so the kernel will add the stack size to the base address to |
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1098 determine the starting location. |
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1099 </para> |
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1100 <para> |
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1101 The exact stack size requirements for any given thread depend on a |
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1102 number of factors. The most important is of course the code that will |
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1103 be executed in the context of this code: if this involves significant |
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1104 nesting of function calls, recursion, or large local arrays, then the |
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1105 stack size needs to be set to a suitably high value. There are some |
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1106 architectural issues, for example the number of cpu registers and the |
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1107 calling conventions will have some effect on stack usage. Also, |
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1108 depending on the configuration, it is possible that some other code |
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1109 such as interrupt handlers will occasionally run on the current |
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1110 thread's stack. This depends in part on configuration options such as |
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1111 <varname>CYGIMP_HAL_COMMON_INTERRUPTS_USE_INTERRUPT_STACK</varname> |
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1112 and <varname>CYGSEM_HAL_COMMON_INTERRUPTS_ALLOW_NESTING</varname>. |
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1113 </para> |
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1114 <para> |
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1115 Determining an application's actual stack size requirements is the |
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1116 responsibility of the application developer, since the kernel cannot |
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1117 know in advance what code a given thread will run. However, the system |
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1118 does provide some hints about reasonable stack sizes in the form of |
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1119 two constants: <varname>CYGNUM_HAL_STACK_SIZE_MINIMUM</varname> and |
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1120 <varname>CYGNUM_HAL_STACK_SIZE_TYPICAL</varname>. These are defined by |
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1121 the appropriate HAL package. The <varname>MINIMUM</varname> value is |
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1122 appropriate for a thread that just runs a single function and makes |
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1123 very simple system calls. Trying to create a thread with a smaller |
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1124 stack than this is illegal. The <varname>TYPICAL</varname> value is |
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1125 appropriate for applications where application calls are nested no |
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1126 more than half a dozen or so levels, and there are no large arrays on |
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1127 the stack. |
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1128 </para> |
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1129 <para> |
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1130 If the stack sizes are not estimated correctly and a stack overflow |
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1131 occurs, the probably result is some form of memory corruption. This |
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1132 can be very hard to track down. The kernel does contain some code to |
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1133 help detect stack overflows, controlled by the configuration option |
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1134 <varname>CYGFUN_KERNEL_THREADS_STACK_CHECKING</varname>: a small |
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1135 amount of space is reserved at the stack limit and filled with a |
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1136 special signature: every time a thread context switch occurs this |
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1137 signature is checked, and if invalid that is a good indication (but |
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1138 not absolute proof) that a stack overflow has occurred. This form of |
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1139 stack checking is enabled by default when the system is built with |
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1140 debugging enabled. A related configuration option is |
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1141 <varname>CYGFUN_KERNEL_THREADS_STACK_MEASUREMENT</varname>: enabling |
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1142 this option means that a thread can call the function |
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1143 <function>cyg_thread_measure_stack_usage</function> to find out the |
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1144 maximum stack usage to date. Note that this is not necessarily the |
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1145 true maximum because, for example, it is possible that in the current |
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1146 run no interrupt occurred at the worst possible moment. |
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1147 </para> |
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1148 </refsect1> |
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1149 |
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1150 <refsect1 id="kernel-thread-create-context"><title>Valid contexts</title> |
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1151 <para> |
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1152 <function>cyg_thread_create</function> may be called during |
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1153 initialization and from within thread context. It may not be called |
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1154 from inside a DSR. |
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1155 </para> |
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1156 </refsect1> |
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1157 |
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1158 <refsect1 id="kernel-thread-create-example"><title>Example</title> |
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1159 <para> |
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1160 A simple example of thread creation is shown below. This involves |
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1161 creating five threads, one producer and four consumers or workers. The |
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1162 threads are created in the system's |
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1163 <function>cyg_user_start</function>: depending on the configuration it |
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1164 might be more appropriate to do this elsewhere, for example inside |
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1165 <function>main</function>. |
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1166 </para> |
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1167 <programlisting width=72> |
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1168 #include <cyg/hal/hal_arch.h> |
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1169 #include <cyg/kernel/kapi.h> |
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1170 |
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1171 // These numbers depend entirely on your application |
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1172 #define NUMBER_OF_WORKERS 4 |
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1173 #define PRODUCER_PRIORITY 10 |
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1174 #define WORKER_PRIORITY 11 |
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1175 #define PRODUCER_STACKSIZE CYGNUM_HAL_STACK_SIZE_TYPICAL |
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1176 #define WORKER_STACKSIZE (CYGNUM_HAL_STACK_SIZE_MINIMUM + 1024) |
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1177 |
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1178 static unsigned char producer_stack[PRODUCER_STACKSIZE]; |
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1179 static unsigned char worker_stacks[NUMBER_OF_WORKERS][WORKER_STACKSIZE]; |
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1180 static cyg_handle_t producer_handle, worker_handles[NUMBER_OF_WORKERS]; |
| 1187 | 1181 static cyg_thread producer_thread, worker_threads[NUMBER_OF_WORKERS]; |
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1182 |
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1183 static void |
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1184 producer(cyg_addrword_t data) |
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1185 { |
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1186 … |
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1187 } |
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1188 |
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1189 static void |
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1190 worker(cyg_addrword_t data) |
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1191 { |
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1192 … |
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1193 } |
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1194 |
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1195 void |
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1196 cyg_user_start(void) |
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1197 { |
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1198 int i; |
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1199 |
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1200 cyg_thread_create(PRODUCER_PRIORITY, &producer, 0, "producer", |
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1201 producer_stack, PRODUCER_STACKSIZE, |
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1202 &producer_handle, &producer_thread); |
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1203 cyg_thread_resume(producer_handle); |
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1204 for (i = 0; i < NUMBER_OF_WORKERS; i++) { |
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1205 cyg_thread_create(WORKER_PRIORITY, &worker, i, "worker", |
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1206 worker_stacks[i], WORKER_STACKSIZE, |
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1207 &(worker_handles[i]), &(worker_threads[i])); |
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1208 cyg_thread_resume(worker_handles[i]); |
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1209 } |
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1210 } |
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1211 </programlisting> |
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1212 </refsect1> |
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1213 |
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1214 |
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1215 <refsect1 id="kernel-thread-create-cxx"><title>Thread Entry Points and C++</title> |
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1216 <para> |
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1217 For code written in C++ the thread entry function must be either a |
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1218 static member function of a class or an ordinary function outside any |
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1219 class. It cannot be a normal member function of a class because such |
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1220 member functions take an implicit additional argument |
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1221 <varname>this</varname>, and the kernel has no way of knowing what |
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1222 value to use for this argument. One way around this problem is to make |
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1223 use of a special static member function, for example: |
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1224 </para> |
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1225 <programlisting width=72> |
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1226 class fred { |
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1227 public: |
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1228 void thread_function(); |
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1229 static void static_thread_aux(cyg_addrword_t); |
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1230 }; |
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1231 |
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1232 void |
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1233 fred::static_thread_aux(cyg_addrword_t objptr) |
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1234 { |
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1235 fred* object = static_cast<fred*>(objptr); |
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1236 object->thread_function(); |
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1237 } |
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1238 |
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1239 static fred instance; |
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1240 |
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1241 extern "C" void |
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1242 cyg_start( void ) |
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1243 { |
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1244 … |
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1245 cyg_thread_create( …, |
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1246 &fred::static_thread_aux, |
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1247 static_cast<cyg_addrword_t>(&instance), |
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1248 …); |
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1249 … |
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1250 } |
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1251 </programlisting> |
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1252 <para> |
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1253 Effectively this uses the <parameter |
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1254 class="function">entry_data</parameter> argument to |
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1255 <function>cyg_thread_create</function> to hold the |
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1256 <varname>this</varname> pointer. Unfortunately this approach does |
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1257 require the use of some C++ casts, so some of the type safety that can |
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1258 be achieved when programming in C++ is lost. |
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1259 </para> |
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1260 </refsect1> |
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1261 |
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1262 </refentry> |
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1263 |
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1264 <!-- }}} --> |
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1265 <!-- {{{ Thread info --> |
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1266 |
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1267 <refentry id="kernel-thread-info"> |
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1268 |
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1269 <refmeta> |
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1270 <refentrytitle>Thread information</refentrytitle> |
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1271 </refmeta> |
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1272 |
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1273 <refnamediv> |
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1274 <refname>cyg_thread_self</refname> |
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1275 <refname>cyg_thread_idle_thread</refname> |
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1276 <refname>cyg_thread_get_stack_base</refname> |
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1277 <refname>cyg_thread_get_stack_size</refname> |
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1278 <refname>cyg_thread_measure_stack_usage</refname> |
| 460 | 1279 <refname>cyg_thread_get_next</refname> |
| 1280 <refname>cyg_thread_get_info</refname> | |
|
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1281 <refname>cyg_thread_get_id</refname> |
| 460 | 1282 <refname>cyg_thread_find</refname> |
|
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1283 <refpurpose>Get basic thread information</refpurpose> |
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1284 </refnamediv> |
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1285 |
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1286 <refsynopsisdiv> |
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1287 <funcsynopsis> |
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1288 <funcsynopsisinfo> |
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1289 #include <cyg/kernel/kapi.h> |
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1290 </funcsynopsisinfo> |
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1291 <funcprototype> |
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1292 <funcdef>cyg_handle_t <function>cyg_thread_self</function></funcdef> |
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1293 <void> |
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1294 </funcprototype> |
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1295 <funcprototype> |
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1296 <funcdef>cyg_handle_t <function>cyg_thread_idle_thread</function></funcdef> |
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1297 <void> |
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1298 </funcprototype> |
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1299 <funcprototype> |
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1300 <funcdef>cyg_addrword_t <function>cyg_thread_get_stack_base</function></funcdef> |
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1301 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1302 </funcprototype> |
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1303 <funcprototype> |
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1304 <funcdef>cyg_uint32 <function>cyg_thread_get_stack_size</function></funcdef> |
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1305 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1306 </funcprototype> |
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1307 <funcprototype> |
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1308 <funcdef>cyg_uint32 <function>cyg_thread_measure_stack_usage</function></funcdef> |
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1309 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1310 </funcprototype> |
| 460 | 1311 <funcprototype> |
| 1312 <funcdef>cyg_bool <function>cyg_thread_get_next</function></funcdef> | |
| 1313 <paramdef>cyg_handle_t *<parameter>thread</parameter></paramdef> | |
| 1314 <paramdef>cyg_uint16 *<parameter>id</parameter></paramdef> | |
| 1315 </funcprototype> | |
| 1316 <funcprototype> | |
| 1317 <funcdef>cyg_bool <function>cyg_thread_get_info</function></funcdef> | |
| 1318 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> | |
| 1319 <paramdef>cyg_uint16 <parameter>id</parameter></paramdef> | |
| 1320 <paramdef>cyg_thread_info *<parameter>info</parameter></paramdef> | |
| 1321 </funcprototype> | |
| 1322 <funcprototype> | |
|
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1323 <funcdef>cyg_uint16 <function>cyg_thread_get_id</function></funcdef> |
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1324 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1325 </funcprototype> |
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1326 <funcprototype> |
| 460 | 1327 <funcdef>cyg_handle_t <function>cyg_thread_find</function></funcdef> |
| 1328 <paramdef>cyg_uint16 <parameter>id</parameter></paramdef> | |
| 1329 </funcprototype> | |
|
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1330 </funcsynopsis> |
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1331 </refsynopsisdiv> |
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1332 |
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1333 <refsect1 id="kernel-thread-info-description"><title>Description</title> |
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1334 <para> |
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1335 These functions can be used to obtain some basic information about |
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1336 various threads in the system. Typically they serve little or no |
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1337 purpose in real applications, but they can be useful during debugging. |
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1338 </para> |
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1339 <para> |
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1340 <function>cyg_thread_self</function> returns a handle corresponding |
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1341 to the current thread. It will be the same as the value filled in by |
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1342 <function>cyg_thread_create</function> when the current thread was |
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1343 created. This handle can then be passed to other functions such as |
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1344 <function>cyg_thread_get_priority</function>. |
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1345 </para> |
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1346 <para> |
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1347 <function>cyg_thread_idle_thread</function> returns the handle |
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1348 corresponding to the idle thread. This thread is created automatically |
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1349 by the kernel, so application-code has no other way of getting hold of |
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1350 this information. |
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1351 </para> |
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1352 <para> |
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1353 <function>cyg_thread_get_stack_base</function> and |
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1354 <function>cyg_thread_get_stack_size</function> return information |
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1355 about a specific thread's stack. The values returned will match the |
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1356 values passed to <function>cyg_thread_create</function> when this |
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1357 thread was created. |
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1358 </para> |
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1359 <para> |
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1360 <function>cyg_thread_measure_stack_usage</function> is only available |
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1361 if the configuration option |
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1362 <varname>CYGFUN_KERNEL_THREADS_STACK_MEASUREMENT</varname> is enabled. |
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1363 The return value is the maximum number of bytes of stack space used so |
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1364 far by the specified thread. Note that this should not be considered a |
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1365 true upper bound, for example it is possible that in the current test |
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1366 run the specified thread has not yet been interrupted at the deepest |
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1367 point in the function call graph. Never the less the value returned |
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1368 can give some useful indication of the thread's stack requirements. |
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1369 </para> |
| 460 | 1370 <para> |
| 1371 <function>cyg_thread_get_next</function> is used to enumerate all the | |
| 526 | 1372 current threads in the system. It should be called initially with the |
| 460 | 1373 locations pointed to by <parameter>thread</parameter> and |
| 1374 <parameter>id</parameter> set to zero. On return these will be set to | |
| 1375 the handle and ID of the first thread. On subsequent calls, these | |
| 1376 parameters should be left set to the values returned by the previous | |
| 1377 call. The handle and ID of the next thread in the system will be | |
| 1378 installed each time, until a <literal>false</literal> return value | |
| 1379 indicates the end of the list. | |
| 1380 </para> | |
| 1381 <para> | |
| 1382 <function>cyg_thread_get_info</function> fills in the | |
| 1383 <type>cyg_thread_info</type> structure with information about the | |
| 1384 thread described by the <parameter>thread</parameter> and | |
| 1385 <parameter>id</parameter> arguments. The information returned includes | |
| 1386 the thread's handle and id, its state and name, priorities and stack | |
| 1387 parameters. If the thread does not exist the function returns | |
| 1388 <literal>false</literal>. | |
| 1389 </para> | |
| 1390 <para> | |
| 526 | 1391 The <type>cyg_thread_info</type> structure is defined as follows by |
| 1392 <<filename class=headerfile>cyg/kernel/kapi.h</filename>>, but may | |
| 1393 be extended in future with additional members, and so its size should | |
| 1394 not be relied upon: | |
| 1395 <programlisting> | |
| 1396 typedef struct | |
| 1397 { | |
| 1398 <type>cyg_handle_t</type> <structfield>handle</structfield>; | |
| 1399 <type>cyg_uint16</type> <structfield>id</structfield>; | |
| 1400 <type>cyg_uint32</type> <structfield>state</structfield>; | |
| 1401 <type>char</type> <structfield>*name</structfield>; | |
| 1402 <type>cyg_priority_t</type> <structfield>set_pri</structfield>; | |
| 1403 <type>cyg_priority_t</type> <structfield>cur_pri</structfield>; | |
| 1404 <type>cyg_addrword_t</type> <structfield>stack_base</structfield>; | |
| 1405 <type>cyg_uint32</type> <structfield>stack_size</structfield>; | |
| 1406 <type>cyg_uint32</type> <structfield>stack_used</structfield>; | |
| 1407 } cyg_thread_info; | |
| 1408 </programlisting> | |
| 1409 </para> | |
| 1410 <para> | |
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1411 <function>cyg_thread_get_id</function> returns the unique thread ID for |
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1412 the thread identified by <parameter>thread</parameter>. |
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1413 </para> |
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1414 <para> |
| 460 | 1415 <function>cyg_thread_find</function> returns a handle for the thread |
| 1416 whose ID is <parameter>id</parameter>. If no such thread exists, a | |
| 1417 zero handle is returned. | |
| 1418 </para> | |
|
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1419 </refsect1> |
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1420 |
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1421 <refsect1 id="kernel-thread-info-context"><title>Valid contexts</title> |
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1422 <para> |
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1423 <function>cyg_thread_self</function> may only be called from thread |
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1424 context. <function>cyg_thread_idle_thread</function> may be called |
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1425 from thread or DSR context, but only after the system has been |
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1426 initialized. <function>cyg_thread_get_stack_base</function>, |
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1427 <function>cyg_thread_get_stack_size</function> and |
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1428 <function>cyg_thread_measure_stack_usage</function> may be called |
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1429 any time after the specified thread has been created, but measuring |
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1430 stack usage involves looping over at least part of the thread's stack |
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1431 so this should normally only be done from thread context. |
|
1436
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1432 <function>cyg_thread_get_id</function> may be called from any context |
|
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1433 as long as the caller can guarantee that the supplied thread handle |
|
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1434 remains valid. |
|
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1435 </para> |
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1436 </refsect1> |
| 460 | 1437 |
| 1438 <refsect1 id="kernel-thread-info-examples"><title>Examples</title> | |
| 1439 <para> | |
| 1440 A simple example of the use of the | |
| 1441 <function>cyg_thread_get_next</function> and | |
| 1442 <function>cyg_thread_get_info</function> follows: | |
| 1443 </para> | |
| 1444 <programlisting width=72> | |
| 1445 | |
| 1446 #include <cyg/kernel/kapi.h> | |
| 1447 #include <stdio.h> | |
| 1448 | |
| 1449 void show_threads(void) | |
| 1450 { | |
| 1451 cyg_handle_t thread = 0; | |
| 1452 cyg_uint16 id = 0; | |
| 1453 | |
| 1454 while( cyg_thread_get_next( &thread, &id ) ) | |
| 1455 { | |
| 1456 cyg_thread_info info; | |
| 1457 | |
| 1458 if( !cyg_thread_get_info( thread, id, &info ) ) | |
| 1459 break; | |
| 1460 | |
| 1461 printf("ID: %04x name: %10s pri: %d\n", | |
| 1462 info.id, info.name?info.name:"----", info.set_pri ); | |
| 1463 } | |
| 1464 } | |
| 1465 | |
| 1466 </programlisting> | |
| 1467 </refsect1> | |
| 1468 | |
|
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1469 </refentry> |
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1470 |
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1471 <!-- }}} --> |
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1472 <!-- {{{ Thread control --> |
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1473 |
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1474 <refentry id="kernel-thread-control"> |
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1475 |
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1476 <refmeta> |
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1477 <refentrytitle>Thread control</refentrytitle> |
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1478 </refmeta> |
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1479 |
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1480 <refnamediv> |
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1481 <refname>cyg_thread_yield</refname> |
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1482 <refname>cyg_thread_delay</refname> |
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1483 <refname>cyg_thread_suspend</refname> |
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1484 <refname>cyg_thread_resume</refname> |
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1485 <refname>cyg_thread_release</refname> |
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1486 <refpurpose>Control whether or not a thread is running</refpurpose> |
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1487 </refnamediv> |
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1488 |
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1489 <refsynopsisdiv> |
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1490 <funcsynopsis> |
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1491 <funcsynopsisinfo> |
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1492 #include <cyg/kernel/kapi.h> |
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1493 </funcsynopsisinfo> |
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1494 <funcprototype> |
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1495 <funcdef>void <function>cyg_thread_yield</function></funcdef> |
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1496 <void> |
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1497 </funcprototype> |
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1498 <funcprototype> |
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1499 <funcdef>void <function>cyg_thread_delay</function></funcdef> |
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1500 <paramdef>cyg_tick_count_t <parameter>delay</parameter></paramdef> |
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1501 </funcprototype> |
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1502 <funcprototype> |
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1503 <funcdef>void <function>cyg_thread_suspend</function></funcdef> |
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1504 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1505 </funcprototype> |
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1506 <funcprototype> |
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1507 <funcdef>void <function>cyg_thread_resume</function></funcdef> |
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1508 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1509 </funcprototype> |
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1510 <funcprototype> |
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1511 <funcdef>void <function>cyg_thread_release</function></funcdef> |
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1512 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1513 </funcprototype> |
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1514 </funcsynopsis> |
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1515 </refsynopsisdiv> |
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1516 |
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1517 <refsect1><title id="kernel-thread-control-description">Description</title> |
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1518 <para> |
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1519 These functions provide some control over whether or not a particular |
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1520 thread can run. Apart from the required use of |
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1521 <function>cyg_thread_resume</function> to start a newly-created |
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1522 thread, application code should normally use proper synchronization |
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1523 primitives such as condition variables or mail boxes. |
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1524 </para> |
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1525 </refsect1> |
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1526 |
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1527 <refsect1><title id="kernel-thread-control-yield">Yield</title> |
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1528 <para> |
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1529 <function>cyg_thread_yield</function> allows a thread to relinquish |
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1530 control of the processor to some other runnable thread which has the |
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1531 same priority. This can have no effect on any higher-priority thread |
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1532 since, if such a thread were runnable, the current thread would have |
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1533 been preempted in its favour. Similarly it can have no effect on any |
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1534 lower-priority thread because the current thread will always be run in |
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1535 preference to those. As a consequence this function is only useful |
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1536 in configurations with a scheduler that allows multiple threads to run |
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1537 at the same priority, for example the mlqueue scheduler. If instead |
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1538 the bitmap scheduler was being used then |
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1539 <function>cyg_thread_yield()</function> would serve no purpose. |
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1540 </para> |
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1541 <para> |
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1542 Even if a suitable scheduler such as the mlqueue scheduler has been |
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1543 configured, <function>cyg_thread_yield</function> will still rarely |
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1544 prove useful: instead timeslicing will be used to ensure that all |
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1545 threads of a given priority get a fair slice of the available |
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1546 processor time. However it is possible to disable timeslicing via the |
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1547 configuration option <varname>CYGSEM_KERNEL_SCHED_TIMESLICE</varname>, |
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1548 in which case <function>cyg_thread_yield</function> can be used to |
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1549 implement a form of cooperative multitasking. |
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1550 </para> |
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1551 </refsect1> |
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1552 |
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1553 <refsect1><title id="kernel-thread-control-delay">Delay</title> |
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1554 <para> |
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1555 <function>cyg_thread_delay</function> allows a thread to suspend until |
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1556 the specified number of clock ticks have occurred. For example, if a |
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1557 value of 1 is used and the system clock runs at a frequency of 100Hz |
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1558 then the thread will sleep for up to 10 milliseconds. This |
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1559 functionality depends on the presence of a real-time system clock, as |
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1560 controlled by the configuration option |
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1561 <varname>CYGVAR_KERNEL_COUNTERS_CLOCK</varname>. |
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1562 </para> |
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1563 <para> |
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1564 If the application requires delays measured in milliseconds or similar |
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1565 units rather than in clock ticks, some calculations are needed to |
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1566 convert between these units as described in <xref |
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1567 linkend="kernel-clocks">. Usually these calculations can be done by |
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1568 the application developer, or at compile-time. Performing such |
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1569 calculations prior to every call to |
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1570 <function>cyg_thread_delay</function> adds unnecessary overhead to the |
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1571 system. |
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1572 </para> |
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1573 </refsect1> |
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1574 |
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1575 <refsect1><title id="kernel-thread-control-suspend">Suspend and Resume</title> |
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1576 <para> |
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1577 Associated with each thread is a suspend counter. When a thread is |
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1578 first created this counter is initialized to 1. |
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1579 <function>cyg_thread_suspend</function> can be used to increment the |
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1580 suspend counter, and <function>cyg_thread_resume</function> decrements |
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1581 it. The scheduler will never run a thread with a non-zero suspend |
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1582 counter. Therefore a newly created thread will not run until it has |
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1583 been resumed. |
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1584 </para> |
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1585 <para> |
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1586 An occasional problem with the use of suspend and resume functionality |
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1587 is that a thread gets suspended more times than it is resumed and |
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1588 hence never becomes runnable again. This can lead to very confusing |
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1589 behaviour. To help with debugging such problems the kernel provides a |
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1590 configuration option |
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1591 <varname>CYGNUM_KERNEL_MAX_SUSPEND_COUNT_ASSERT</varname> which |
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1592 imposes an upper bound on the number of suspend calls without matching |
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1593 resumes, with a reasonable default value. This functionality depends |
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1594 on infrastructure assertions being enabled. |
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1595 </para> |
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1596 </refsect1> |
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1597 |
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1598 <refsect1><title id="kernel-thread-control-release">Releasing a Blocked Thread</title> |
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1599 <para> |
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1600 When a thread is blocked on a synchronization primitive such as a |
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1601 semaphore or a mutex, or when it is waiting for an alarm to trigger, |
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1602 it can be forcibly woken up using |
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1603 <function>cyg_thread_release</function>. Typically this will call the |
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1604 affected synchronization primitive to return false, indicating that |
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1605 the operation was not completed successfully. This function has to be |
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1606 used with great care, and in particular it should only be used on |
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1607 threads that have been designed appropriately and check all return |
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1608 codes. If instead it were to be used on, say, an arbitrary thread that |
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1609 is attempting to claim a mutex then that thread might not bother to |
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1610 check the result of the mutex lock operation - usually there would be |
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1611 no reason to do so. Therefore the thread will now continue running in |
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1612 the false belief that it has successfully claimed a mutex lock, and |
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1613 the resulting behaviour is undefined. If the system has been built |
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1614 with assertions enabled then it is possible that an assertion will |
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1615 trigger when the thread tries to release the mutex it does not |
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1616 actually own. |
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1617 </para> |
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1618 <para> |
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1619 The main use of <function>cyg_thread_release</function> is in the |
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1620 POSIX compatibility layer, where it is used in the implementation of |
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1621 per-thread signals and cancellation handlers. |
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1622 </para> |
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1623 </refsect1> |
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1624 |
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1625 <refsect1 id="kernel-thread-control-context"><title>Valid contexts</title> |
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1626 <para> |
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1627 <function>cyg_thread_yield</function> can only be called from thread |
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1628 context, A DSR must always run to completion and cannot yield the |
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1629 processor to some thread. <function>cyg_thread_suspend</function>, |
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1630 <function>cyg_thread_resume</function>, and |
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1631 <function>cyg_thread_release</function> may be called from thread or |
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1632 DSR context. |
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1633 </para> |
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1634 </refsect1> |
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1635 |
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1636 </refentry> |
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1637 |
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1638 <!-- }}} --> |
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1639 <!-- {{{ Thread termination --> |
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1640 |
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1641 <refentry id="kernel-thread-termination"> |
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1642 |
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1643 <refmeta> |
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1644 <refentrytitle>Thread termination</refentrytitle> |
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1645 </refmeta> |
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1646 |
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1647 <refnamediv> |
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1648 <refname>cyg_thread_exit</refname> |
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1649 <refname>cyg_thread_kill</refname> |
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1650 <refname>cyg_thread_delete</refname> |
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1651 <refpurpose>Allow threads to terminate</refpurpose> |
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1652 </refnamediv> |
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1653 |
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1654 <refsynopsisdiv> |
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1655 <funcsynopsis> |
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1656 <funcsynopsisinfo> |
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1657 #include <cyg/kernel/kapi.h> |
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1658 </funcsynopsisinfo> |
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1659 <funcprototype> |
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1660 <funcdef>void <function>cyg_thread_exit</function></funcdef> |
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1661 <void> |
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1662 </funcprototype> |
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1663 <funcprototype> |
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1664 <funcdef>void <function>cyg_thread_kill</function></funcdef> |
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1665 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1666 </funcprototype> |
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1667 <funcprototype> |
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1529
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1668 <funcdef>cyg_bool_t <function>cyg_thread_delete</function></funcdef> |
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1669 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1670 </funcprototype> |
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1671 </funcsynopsis> |
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1672 </refsynopsisdiv> |
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1673 |
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1674 <refsect1><title id="kernel-thread-termination-description">Description</title> |
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1675 <para> |
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1676 In many embedded systems the various threads are allocated statically, |
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1677 created during initialization, and never need to terminate. This |
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1678 avoids any need for dynamic memory allocation or other resource |
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1679 management facilities. However if a given application does have a |
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1680 requirement that some threads be created dynamically, must terminate, |
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1681 and their resources such as the stack be reclaimed, then the kernel |
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1682 provides the functions <function>cyg_thread_exit</function>, |
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1683 <function>cyg_thread_kill</function>, and |
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1684 <function>cyg_thread_delete</function>. |
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1685 </para> |
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1686 <para> |
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1687 <function>cyg_thread_exit</function> allows a thread to terminate |
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1688 itself, thus ensuring that it will not be run again by the scheduler. |
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1689 However the <structname>cyg_thread</structname> data structure passed |
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1690 to <function>cyg_thread_create</function> remains in use, and the |
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1691 handle returned by <function>cyg_thread_create</function> remains |
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1692 valid. This allows other threads to perform certain operations on the |
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1693 terminated thread, for example to determine its stack usage via |
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1694 <function>cyg_thread_measure_stack_usage</function>. When the handle |
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1695 and <structname>cyg_thread</structname> structure are no longer |
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1696 required, <function>cyg_thread_delete</function> should be called to |
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1697 release these resources. If the stack was dynamically allocated then |
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1698 this should not be freed until after the call to |
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1699 <function>cyg_thread_delete</function>. |
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1700 </para> |
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1701 <para> |
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1702 Alternatively, one thread may use <function>cyg_thread_kill</function> |
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1703 on another This has much the same effect as the affected thread |
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1704 calling <function>cyg_thread_exit</function>. However killing a thread |
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1705 is generally rather dangerous because no attempt is made to unlock any |
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1706 synchronization primitives currently owned by that thread or release |
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1707 any other resources that thread may have claimed. Therefore use of |
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1708 this function should be avoided, and |
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1709 <function>cyg_thread_exit</function> is preferred. |
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1710 <function>cyg_thread_kill</function> cannot be used by a thread to |
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1711 kill itself. |
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1712 </para> |
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1713 <para> |
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1714 <function>cyg_thread_delete</function> should be used on a thread |
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1715 after it has exited and is no longer required. After this call the |
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1716 thread handle is no longer valid, and both the |
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1717 <structname>cyg_thread</structname> structure and the thread stack can |
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1718 be re-used or freed. If <function>cyg_thread_delete</function> is |
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1719 invoked on a thread that is still running then there is an implicit |
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1720 call to <function>cyg_thread_kill</function>. This function returns |
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1721 <literal>true</literal> if the delete was successful, and |
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1722 <literal>false</literal> if the delete did not happen. The delete |
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1723 may not happen for example if the thread being destroyed is a lower |
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1724 priority thread than the running thread, and will thus not wake up |
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1725 in order to exit until it is rescheduled. |
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1726 </para> |
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1727 </refsect1> |
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1728 |
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1729 <refsect1 id="kernel-thread-termination-context"><title>Valid contexts</title> |
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1730 <para> |
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1731 <function>cyg_thread_exit</function>, |
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1732 <function>cyg_thread_kill</function> and |
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1733 <function>cyg_thread_delete</function> can only be called from thread |
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1734 context. |
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1735 </para> |
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1736 </refsect1> |
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1737 |
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1738 </refentry> |
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1739 |
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1740 <!-- }}} --> |
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1741 <!-- {{{ Thread priorities --> |
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1742 |
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1743 <refentry id="kernel-thread-priorities"> |
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1744 |
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1745 <refmeta> |
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1746 <refentrytitle>Thread priorities</refentrytitle> |
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1747 </refmeta> |
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1748 |
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1749 <refnamediv> |
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1750 <refname>cyg_thread_get_priority</refname> |
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1751 <refname>cyg_thread_get_current_priority</refname> |
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1752 <refname>cyg_thread_set_priority</refname> |
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1753 <refpurpose>Examine and manipulate thread priorities</refpurpose> |
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1754 </refnamediv> |
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1755 <refsynopsisdiv> |
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1756 <funcsynopsis> |
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1757 <funcsynopsisinfo> |
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1758 #include <cyg/kernel/kapi.h> |
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1759 </funcsynopsisinfo> |
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1760 <funcprototype> |
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1761 <funcdef>cyg_priority_t <function>cyg_thread_get_priority</function></funcdef> |
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1762 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1763 </funcprototype> |
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1764 <funcprototype> |
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1765 <funcdef>cyg_priority_t <function>cyg_thread_get_current_priority</function></funcdef> |
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1766 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1767 </funcprototype> |
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1768 <funcprototype> |
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1769 <funcdef>void <function>cyg_thread_set_priority</function></funcdef> |
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1770 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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1771 <paramdef>cyg_priority_t <parameter>priority</parameter></paramdef> |
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1772 </funcprototype> |
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1773 </funcsynopsis> |
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1774 </refsynopsisdiv> |
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1775 |
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1776 <refsect1><title id="kernel-thread-priorities-description">Description</title> |
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1777 <para> |
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1778 Typical schedulers use the concept of a thread priority to determine |
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1779 which thread should run next. Exactly what this priority consists of |
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1780 will depend on the scheduler, but a typical implementation would be a |
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1781 small integer in the range 0 to 31, with 0 being the highest priority. |
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1782 Usually only the idle thread will run at the lowest priority. The |
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1783 exact number of priority levels available depends on the |
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1784 configuration, typically the option |
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1785 <varname>CYGNUM_KERNEL_SCHED_PRIORITIES</varname>. |
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1786 </para> |
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1787 <para> |
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1788 <function>cyg_thread_get_priority</function> can be used to determine |
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1789 the priority of a thread, or more correctly the value last used in a |
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1790 <function>cyg_thread_set_priority</function> call or when the thread |
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1791 was first created. In some circumstances it is possible that the |
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1792 thread is actually running at a higher priority. For example, if it |
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1793 owns a mutex and priority ceilings or inheritance is being used to |
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1794 prevent priority inversion problems, then the thread's priority may |
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1795 have been boosted temporarily. |
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1796 <function>cyg_thread_get_current_priority</function> returns the real |
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1797 current priority. |
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1798 </para> |
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1799 <para> |
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1800 In many applications appropriate thread priorities can be determined |
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1801 and allocated statically. However, if it is necessary for a thread's |
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1802 priority to change at run-time then the |
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1803 <function>cyg_thread_set_priority</function> function provides this |
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1804 functionality. |
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1805 </para> |
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1806 </refsect1> |
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1807 |
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1808 <refsect1 id="kernel-thread-priorities-context"><title>Valid contexts</title> |
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1809 <para> |
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1810 <function>cyg_thread_get_priority</function> and |
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1811 <function>cyg_thread_get_current_priority</function> can be called |
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1812 from thread or DSR context, although the latter is rarely useful. |
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1813 <function>cyg_thread_set_priority</function> should also only be |
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1814 called from thread context. |
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1815 </para> |
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1816 </refsect1> |
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1817 </refentry> |
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1818 |
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1819 <!-- }}} --> |
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1820 <!-- {{{ Per-thread data --> |
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1821 |
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1822 <refentry id="kernel-thread-data"> |
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1823 |
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1824 <refmeta> |
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1825 <refentrytitle>Per-thread data</refentrytitle> |
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1826 </refmeta> |
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1827 |
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1828 <refnamediv> |
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1829 <refname>cyg_thread_new_data_index</refname> |
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1830 <refname>cyg_thread_free_data_index</refname> |
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1831 <refname>cyg_thread_get_data</refname> |
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1832 <refname>cyg_thread_get_data_ptr</refname> |
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1833 <refname>cyg_thread_set_data</refname> |
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1834 <refpurpose>Manipulate per-thread data</refpurpose> |
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1835 </refnamediv> |
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1836 <refsynopsisdiv> |
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1837 <funcsynopsis> |
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1838 <funcsynopsisinfo> |
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1839 #include <cyg/kernel/kapi.h> |
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1840 </funcsynopsisinfo> |
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1841 <funcprototype> |
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1842 <funcdef>cyg_ucount32 <function>cyg_thread_new_data_index</function></funcdef> |
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1843 <void> |
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1844 </funcprototype> |
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1845 <funcprototype> |
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1846 <funcdef>void <function>cyg_thread_free_data_index</function></funcdef> |
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1847 <paramdef>cyg_ucount32 <parameter>index</parameter></paramdef> |
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1848 </funcprototype> |
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1849 <funcprototype> |
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1850 <funcdef>cyg_addrword_t <function>cyg_thread_get_data</function></funcdef> |
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1851 <paramdef>cyg_ucount32 <parameter>index</parameter></paramdef> |
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1852 </funcprototype> |
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1853 <funcprototype> |
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1854 <funcdef>cyg_addrword_t* <function>cyg_thread_get_data_ptr</function></funcdef> |
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1855 <paramdef>cyg_ucount32 <parameter>index</parameter></paramdef> |
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1856 </funcprototype> |
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1857 <funcprototype> |
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1858 <funcdef>void <function>cyg_thread_set_data</function></funcdef> |
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1859 <paramdef>cyg_ucount32 <parameter>index</parameter></paramdef> |
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1860 <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef> |
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1861 </funcprototype> |
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1862 </funcsynopsis> |
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1863 </refsynopsisdiv> |
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1864 |
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1865 <refsect1><title id="kernel-thread-data-description">Description</title> |
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1866 <para> |
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1867 In some applications and libraries it is useful to have some data that |
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1868 is specific to each thread. For example, many of the functions in the |
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1869 POSIX compatibility package return -1 to indicate an error and store |
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1870 additional information in what appears to be a global variable |
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1871 <varname>errno</varname>. However, if multiple threads make concurrent |
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1872 calls into the POSIX library and if <varname>errno</varname> were |
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1873 really a global variable then a thread would have no way of knowing |
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1874 whether the current <varname>errno</varname> value really corresponded |
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1875 to the last POSIX call it made, or whether some other thread had run |
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1876 in the meantime and made a different POSIX call which updated the |
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1877 variable. To avoid such confusion <varname>errno</varname> is instead |
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1878 implemented as a per-thread variable, and each thread has its own |
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1879 instance. |
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1880 </para> |
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1881 <para> |
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1882 The support for per-thread data can be disabled via the configuration |
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1883 option <varname>CYGVAR_KERNEL_THREADS_DATA</varname>. If enabled, each |
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1884 <structname>cyg_thread</structname> data structure holds a small array |
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1885 of words. The size of this array is determined by the configuration |
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1886 option <varname>CYGNUM_KERNEL_THREADS_DATA_MAX</varname>. When a |
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1887 thread is created the array is filled with zeroes. |
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1888 </para> |
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1889 <para> |
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1890 If an application needs to use per-thread data then it needs an index |
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1891 into this array which has not yet been allocated to other code. This |
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1892 index can be obtained by calling |
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1893 <function>cyg_thread_new_data_index</function>, and then used in |
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1894 subsequent calls to <function>cyg_thread_get_data</function>. |
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1895 Typically indices are allocated during system initialization and |
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1896 stored in static variables. If for some reason a slot in the array is |
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1897 no longer required and can be re-used then it can be released by calling |
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1898 <function>cyg_thread_free_data_index</function>, |
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1899 </para> |
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1900 <para> |
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1901 The current per-thread data in a given slot can be obtained using |
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1902 <function>cyg_thread_get_data</function>. This implicitly operates on |
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1903 the current thread, and its single argument should be an index as |
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1904 returned by <function>cyg_thread_new_data_index</function>. The |
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1905 per-thread data can be updated using |
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1906 <function>cyg_thread_set_data</function>. If a particular item of |
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1907 per-thread data is needed repeatedly then |
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1908 <function>cyg_thread_get_data_ptr</function> can be used to obtain the |
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1909 address of the data, and indirecting through this pointer allows the |
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1910 data to be examined and updated efficiently. |
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1911 </para> |
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1912 <para> |
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1913 Some packages, for example the error and POSIX packages, have |
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1914 pre-allocated slots in the array of per-thread data. These slots |
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1915 should not normally be used by application code, and instead slots |
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1916 should be allocated during initialization by a call to |
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1917 <function>cyg_thread_new_data_index</function>. If it is known that, |
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1918 for example, the configuration will never include the POSIX |
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1919 compatibility package then application code may instead decide to |
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1920 re-use the slot allocated to that package, |
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1921 <varname>CYGNUM_KERNEL_THREADS_DATA_POSIX</varname>, but obviously |
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1922 this does involve a risk of strange and subtle bugs if the |
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1923 application's requirements ever change. |
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1924 </para> |
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1925 </refsect1> |
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1926 |
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1927 <refsect1 id="kernel-thread-data-context"><title>Valid contexts</title> |
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1928 <para> |
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1929 Typically <function>cyg_thread_new_data_index</function> is only |
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1930 called during initialization, but may also be called at any time in |
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1931 thread context. <function>cyg_thread_free_data_index</function>, if |
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1932 used at all, can also be called during initialization or from thread |
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1933 context. <function>cyg_thread_get_data</function>, |
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1934 <function>cyg_thread_get_data_ptr</function>, and |
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1935 <function>cyg_thread_set_data</function> may only be called from |
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1936 thread context because they implicitly operate on the current thread. |
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1937 </para> |
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1938 </refsect1> |
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1939 |
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1940 </refentry> |
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1941 |
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1942 <!-- }}} --> |
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1943 <!-- {{{ Thread destructors --> |
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1944 |
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1945 <refentry id="kernel-thread-destructors"> |
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1946 |
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1947 <refmeta> |
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1948 <refentrytitle>Thread destructors</refentrytitle> |
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1949 </refmeta> |
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1950 |
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1951 <refnamediv> |
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1952 <refname>cyg_thread_add_destructor</refname> |
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1953 <refname>cyg_thread_rem_destructor</refname> |
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1954 <refpurpose>Call functions on thread termination</refpurpose> |
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1955 </refnamediv> |
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1956 <refsynopsisdiv> |
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1957 <funcsynopsis> |
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1958 <funcsynopsisinfo> |
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1959 #include <cyg/kernel/kapi.h> |
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1960 typedef void (*<type>cyg_thread_destructor_fn</type>)(<type>cyg_addrword_t</type>); |
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1961 </funcsynopsisinfo> |
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1962 <funcprototype> |
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1963 <funcdef>cyg_bool_t <function>cyg_thread_add_destructor</function></funcdef> |
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1964 <paramdef>cyg_thread_destructor_fn <parameter>fn</parameter></paramdef> |
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1965 <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef> |
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1966 </funcprototype> |
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1967 <funcprototype> |
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1968 <funcdef>cyg_bool_t <function>cyg_thread_rem_destructor</function></funcdef> |
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1969 <paramdef>cyg_thread_destructor_fn <parameter>fn</parameter></paramdef> |
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1970 <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef> |
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1971 </funcprototype> |
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1972 </funcsynopsis> |
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1973 </refsynopsisdiv> |
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1974 |
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1975 <refsect1><title id="kernel-thread-destructors-description">Description</title> |
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1976 <para> |
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1977 These functions are provided for cases when an application requires a |
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1978 function to be automatically called when a thread exits. This is often |
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1979 useful when, for example, freeing up resources allocated by the thread. |
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1980 </para> |
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1981 <para> |
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1982 This support must be enabled with the configuration option |
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1983 <varname>CYGPKG_KERNEL_THREADS_DESTRUCTORS</varname>. When enabled, |
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1984 you may register a function of type |
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1985 <type>cyg_thread_destructor_fn</type> to be called on thread |
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1986 termination using <function>cyg_thread_add_destructor</function>. You |
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1987 may also provide it with a piece of arbitrary information in the |
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1988 <parameter>data</parameter> argument which will be passed to the |
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1989 destructor function <parameter>fn</parameter> when the thread |
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1990 terminates. If you no longer wish to call a function previous |
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1991 registered with <function>cyg_thread_add_destructor</function>, you |
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1992 may call <function>cyg_thread_rem_destructor</function> with the same |
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1993 parameters used to register the destructor function. Both these |
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1994 functions return <literal>true</literal> on success and |
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1995 <literal>false</literal> on failure. |
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1996 </para> |
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1997 <para> |
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1998 By default, thread destructors are per-thread, which means that registering |
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1999 a destructor function only registers that function for the current thread. |
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2000 In other words, each thread has its own list of destructors. |
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2001 Alternatively you may disable the configuration option |
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2002 <varname>CYGSEM_KERNEL_THREADS_DESTRUCTORS_PER_THREAD</varname> in which |
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2003 case any registered destructors will be run when <emphasis>any</emphasis> |
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2004 threads exit. In other words, the thread destructor list is global and all |
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2005 threads have the same destructors. |
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2006 </para> |
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2007 <para> |
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2008 There is a limit to the number of destructors which may be registered, |
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2009 which can be controlled with the |
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2010 <varname>CYGNUM_KERNEL_THREADS_DESTRUCTORS</varname> configuration |
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2011 option. Increasing this value will very slightly increase the amount |
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2012 of memory in use, and when |
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2013 <varname>CYGSEM_KERNEL_THREADS_DESTRUCTORS_PER_THREAD</varname> is |
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2014 enabled, the amount of memory used per thread will increase. When the |
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2015 limit has been reached, <function>cyg_thread_add_destructor</function> |
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2016 will return <literal>false</literal>. |
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2017 </para> |
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2018 </refsect1> |
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2019 |
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2020 <refsect1 id="kernel-thread-destructors-context"><title>Valid contexts</title> |
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2021 <para> |
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2022 When <varname>CYGSEM_KERNEL_THREADS_DESTRUCTORS_PER_THREAD</varname> |
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2023 is enabled, these functions must only be called from a thread context |
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2024 as they implicitly operate on the current thread. When |
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2025 <varname>CYGSEM_KERNEL_THREADS_DESTRUCTORS_PER_THREAD</varname> is |
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2026 disabled, these functions may be called from thread or DSR context, |
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2027 or at initialization time. |
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2028 </para> |
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2029 </refsect1> |
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2030 |
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2031 </refentry> |
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2032 |
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2033 <!-- }}} --> |
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2034 <!-- {{{ Exceptions --> |
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2035 |
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2036 <refentry id="kernel-exceptions"> |
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2037 |
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2038 <refmeta> |
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2039 <refentrytitle>Exception handling</refentrytitle> |
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2040 </refmeta> |
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2041 |
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2042 <refnamediv> |
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2043 <refname>cyg_exception_set_handler</refname> |
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2044 <refname>cyg_exception_clear_handler</refname> |
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2045 <refname>cyg_exception_call_handler</refname> |
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2046 <refpurpose>Handle processor exceptions</refpurpose> |
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2047 </refnamediv> |
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2048 <refsynopsisdiv> |
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2049 <funcsynopsis> |
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2050 <funcsynopsisinfo> |
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2051 #include <cyg/kernel/kapi.h> |
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2052 </funcsynopsisinfo> |
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2053 <funcprototype> |
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2054 <funcdef>void <function>cyg_exception_set_handler</function></funcdef> |
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2055 <paramdef>cyg_code_t <parameter>exception_number</parameter></paramdef> |
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2056 <paramdef>cyg_exception_handler_t* <parameter>new_handler</parameter></paramdef> |
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2057 <paramdef>cyg_addrword_t <parameter>new_data</parameter></paramdef> |
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2058 <paramdef>cyg_exception_handler_t** <parameter>old_handler</parameter></paramdef> |
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2059 <paramdef>cyg_addrword_t* <parameter>old_data</parameter></paramdef> |
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2060 </funcprototype> |
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2061 <funcprototype> |
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2062 <funcdef>void <function>cyg_exception_clear_handler</function></funcdef> |
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2063 <paramdef>cyg_code_t <parameter>exception_number</parameter></paramdef> |
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2064 </funcprototype> |
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2065 <funcprototype> |
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2066 <funcdef>void <function>cyg_exception_call_handler</function></funcdef> |
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2067 <paramdef>cyg_handle_t <parameter>thread</parameter></paramdef> |
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2068 <paramdef>cyg_code_t <parameter>exception_number</parameter></paramdef> |
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2069 <paramdef>cyg_addrword_t <parameter>exception_info</parameter></paramdef> |
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2070 </funcprototype> |
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2071 </funcsynopsis> |
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2072 </refsynopsisdiv> |
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2073 |
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2074 <refsect1><title id="kernel-exceptions-description">Description</title> |
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2075 <para> |
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2076 Sometimes code attempts operations that are not legal on the current |
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2077 hardware, for example dividing by zero, or accessing data through a |
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2078 pointer that is not properly aligned. When this happens the hardware |
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2079 will raise an exception. This is very similar to an interrupt, but |
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2080 happens synchronously with code execution rather than asynchronously |
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2081 and hence can be tied to the thread that is currently running. |
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2082 </para> |
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2083 <para> |
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2084 The exceptions that can be raised depend very much on the hardware, |
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2085 especially the processor. The corresponding documentation should be |
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2086 consulted for more details. Alternatively the architectural HAL header |
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2087 file <filename class="headerfile">hal_intr.h</filename>, or one of the |
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2088 variant or platform header files it includes, will contain appropriate |
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2089 definitions. The details of how to handle exceptions, including |
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2090 whether or not it is possible to recover from them, also depend on the |
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2091 hardware. |
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2092 </para> |
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2093 <para> |
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2094 Exception handling is optional, and can be disabled through the |
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2095 configuration option <varname>CYGPKG_KERNEL_EXCEPTIONS</varname>. If |
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2096 an application has been exhaustively tested and is trusted never to |
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2097 raise a hardware exception then this option can be disabled and code |
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2098 and data sizes will be reduced somewhat. If exceptions are left |
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2099 enabled then the system will provide default handlers for the various |
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2100 exceptions, but these do nothing. Even the specific type of exception |
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2101 is ignored, so there is no point in attempting to decode this and |
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2102 distinguish between say a divide-by-zero and an unaligned access. |
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2103 If the application installs its own handlers and wants details of the |
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2104 specific exception being raised then the configuration option |
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2105 <varname>CYGSEM_KERNEL_EXCEPTIONS_DECODE</varname> has to be enabled. |
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2106 </para> |
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2107 <para> |
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2108 An alternative handler can be installed using |
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2109 <function>cyg_exception_set_handler</function>. This requires a code |
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2110 for the exception, a function pointer for the new exception handler, |
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2111 and a parameter to be passed to this handler. Details of the |
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2112 previously installed exception handler will be returned via the |
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2113 remaining two arguments, allowing that handler to be reinstated, or |
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2114 null pointers can be used if this information is of no interest. An |
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2115 exception handling function should take the following form: |
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2116 </para> |
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2117 <programlisting width=72> |
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2118 void |
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2119 my_exception_handler(cyg_addrword_t data, cyg_code_t exception, cyg_addrword_t info) |
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2120 { |
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2121 … |
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2122 } |
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2123 </programlisting> |
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2124 <para> |
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2125 The data argument corresponds to the <parameter class="function">new_data</parameter> |
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2126 parameter supplied to <function>cyg_exception_set_handler</function>. |
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2127 The exception code is provided as well, in case a single handler is |
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2128 expected to support multiple exceptions. The <parameter class="function">info</parameter> |
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2129 argument will depend on the hardware and on the specific exception. |
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2130 </para> |
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2131 <para> |
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2132 <function>cyg_exception_clear_handler</function> can be used to |
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2133 restore the default handler, if desired. It is also possible for |
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2134 software to raise an exception and cause the current handler to be |
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2135 invoked, but generally this is useful only for testing. |
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2136 </para> |
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2137 <para> |
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2138 By default the system maintains a single set of global exception |
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2139 handlers. However, since exceptions occur synchronously it is |
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2140 sometimes useful to handle them on a per-thread basis, and have a |
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2141 different set of handlers for each thread. This behaviour can be |
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2142 obtained by disabling the configuration option |
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2143 <varname>CYGSEM_KERNEL_EXCEPTIONS_GLOBAL</varname>. If per-thread |
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2144 exception handlers are being used then |
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2145 <function>cyg_exception_set_handler</function> and |
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2146 <function>cyg_exception_clear_handler</function> apply to the current |
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2147 thread. Otherwise they apply to the global set of handlers. |
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2148 </para> |
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2149 |
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2150 <caution><para> |
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2151 In the current implementation |
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2152 <function>cyg_exception_call_handler</function> can only be used on |
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2153 the current thread. There is no support for delivering an exception to |
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2154 another thread. |
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2155 </para></caution> |
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2156 <note><para> |
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2157 Exceptions at the eCos kernel level refer specifically to |
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2158 hardware-related events such as unaligned accesses to memory or |
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2159 division by zero. There is no relation with other concepts that are |
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2160 also known as exceptions, for example the <literal>throw</literal> and |
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2161 <literal>catch</literal> facilities associated with C++. |
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2162 </para></note> |
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2163 |
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2164 </refsect1> |
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2165 |
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2166 <refsect1 id="kernel-exceptions-context"><title>Valid contexts</title> |
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2167 <para> |
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2168 If the system is configured with a single set of global exception |
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2169 handlers then |
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2170 <function>cyg_exception_set_handler</function> and |
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2171 <function>cyg_exception_clear_handler</function> may be called during |
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2172 initialization or from thread context. If instead per-thread exception |
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2173 handlers are being used then it is not possible to install new |
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2174 handlers during initialization because the functions operate |
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2175 implicitly on the current thread, so they can only be called from |
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2176 thread context. <function>cyg_exception_call_handler</function> should |
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2177 only be called from thread context. |
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2178 </para> |
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2179 </refsect1> |
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2180 |
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2181 </refentry> |
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2182 |
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2183 <!-- }}} --> |
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2184 <!-- {{{ Counters --> |
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2185 |
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2186 <refentry id="kernel-counters"> |
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2187 |
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2188 <refmeta> |
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2189 <refentrytitle>Counters</refentrytitle> |
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2190 </refmeta> |
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2191 |
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2192 <refnamediv> |
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2193 <refname>cyg_counter_create</refname> |
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2194 <refname>cyg_counter_delete</refname> |
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2195 <refname>cyg_counter_current_value</refname> |
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2196 <refname>cyg_counter_set_value</refname> |
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2197 <refname>cyg_counter_tick</refname> |
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2198 <refpurpose>Count event occurrences</refpurpose> |
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2199 </refnamediv> |
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2200 |
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2201 <refsynopsisdiv> |
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2202 <funcsynopsis> |
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2203 <funcsynopsisinfo> |
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2204 #include <cyg/kernel/kapi.h> |
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2205 </funcsynopsisinfo> |
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2206 <funcprototype> |
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2207 <funcdef>void <function>cyg_counter_create</function></funcdef> |
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2208 <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef> |
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2209 <paramdef>cyg_counter* <parameter>counter</parameter></paramdef> |
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2210 </funcprototype> |
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2211 <funcprototype> |
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2212 <funcdef>void <function>cyg_counter_delete</function></funcdef> |
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2213 <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef> |
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2214 </funcprototype> |
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2215 <funcprototype> |
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2216 <funcdef>cyg_tick_count_t <function>cyg_counter_current_value</function></funcdef> |
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2217 <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef> |
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2218 </funcprototype> |
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2219 <funcprototype> |
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2220 <funcdef>void <function>cyg_counter_set_value</function></funcdef> |
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2221 <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef> |
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2222 <paramdef>cyg_tick_count_t <parameter>new_value</parameter></paramdef> |
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2223 </funcprototype> |
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2224 <funcprototype> |
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2225 <funcdef>void <function>cyg_counter_tick</function></funcdef> |
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2226 <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef> |
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2227 </funcprototype> |
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2228 </funcsynopsis> |
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2229 </refsynopsisdiv> |
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2230 |
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2231 <refsect1 id="kernel-counters-description"><title>Description</title> |
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2232 <para> |
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2233 Kernel counters can be used to keep track of how many times a |
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2234 particular event has occurred. Usually this event is an external |
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2235 signal of some sort. The most common use of counters is in the |
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2236 implementation of clocks, but they can be useful with other event |
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2237 sources as well. Application code can attach <link |
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2238 linkend="kernel-alarms">alarms</link> to counters, causing a function |
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2239 to be called when some number of events have occurred. |
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2240 </para> |
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2241 <para> |
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2242 A new counter is initialized by a call to |
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2243 <function>cyg_counter_create</function>. The first argument is used to |
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2244 return a handle to the new counter which can be used for subsequent |
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2245 operations. The second argument allows the application to provide the |
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2246 memory needed for the object, thus eliminating any need for dynamic |
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2247 memory allocation within the kernel. If a counter is no longer |
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2248 required and does not have any alarms attached then |
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2249 <function>cyg_counter_delete</function> can be used to release the |
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2250 resources, allowing the <structname>cyg_counter</structname> data |
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2251 structure to be re-used. |
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2252 </para> |
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2253 <para> |
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2254 Initializing a counter does not automatically attach it to any source |
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2255 of events. Instead some other code needs to call |
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2256 <function>cyg_counter_tick</function> whenever a suitable event |
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2257 occurs, which will cause the counter to be incremented and may cause |
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2258 alarms to trigger. The current value associated with the counter can |
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2259 be retrieved using <function>cyg_counter_current_value</function> and |
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2260 modified with <function>cyg_counter_set_value</function>. Typically |
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2261 the latter function is only used during initialization, for example to |
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2262 set a clock to wallclock time, but it can be used to reset a counter |
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2263 if necessary. However <function>cyg_counter_set_value</function> will |
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2264 never trigger any alarms. A newly initialized counter has a starting |
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2265 value of 0. |
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2266 </para> |
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2267 <para> |
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2268 The kernel provides two different implementations of counters. The |
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2269 default is <varname>CYGIMP_KERNEL_COUNTERS_SINGLE_LIST</varname> which |
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2270 stores all alarms attached to the counter on a single list. This is |
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2271 simple and usually efficient. However when a tick occurs the kernel |
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2272 code has to traverse this list, typically at DSR level, so if there |
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2273 are a significant number of alarms attached to a single counter this |
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2274 will affect the system's dispatch latency. The alternative |
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2275 implementation, <varname>CYGIMP_KERNEL_COUNTERS_MULTI_LIST</varname>, |
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2276 stores each alarm in one of an array of lists such that at most one of |
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2277 the lists needs to be searched per clock tick. This involves extra |
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2278 code and data, but can improve real-time responsiveness in some |
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2279 circumstances. Another configuration option that is relevant here |
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2280 is <varname>CYGIMP_KERNEL_COUNTERS_SORT_LIST</varname>, which is |
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2281 disabled by default. This provides a trade off between doing work |
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2282 whenever a new alarm is added to a counter and doing work whenever a |
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2283 tick occurs. It is application-dependent which of these is more |
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2284 appropriate. |
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2285 </para> |
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2286 </refsect1> |
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2287 |
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2288 <refsect1 id="kernel-counters-context"><title>Valid contexts</title> |
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2289 <para> |
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2290 <function>cyg_counter_create</function> is typically called during |
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2291 system initialization but may also be called in thread context. |
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2292 Similarly <function>cyg_counter_delete</function> may be called during |
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2293 initialization or in thread context. |
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2294 <function>cyg_counter_current_value</function>, |
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2295 <function>cyg_counter_set_value</function> and |
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2296 <function>cyg_counter_tick</function> may be called during |
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2297 initialization or from thread or DSR context. In fact, |
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2298 <function>cyg_counter_tick</function> is usually called from inside a |
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2299 DSR in response to an external event of some sort. |
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2300 </para> |
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2301 </refsect1> |
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2302 |
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2303 </refentry> |
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2304 |
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2305 <!-- }}} --> |
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2306 <!-- {{{ Clocks --> |
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2307 |
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2308 <refentry id="kernel-clocks"> |
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2309 |
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2310 <refmeta> |
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2311 <refentrytitle>Clocks</refentrytitle> |
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2312 </refmeta> |
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2313 |
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2314 <refnamediv> |
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2315 <refname>cyg_clock_create</refname> |
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2316 <refname>cyg_clock_delete</refname> |
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2317 <refname>cyg_clock_to_counter</refname> |
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2318 <refname>cyg_clock_set_resolution</refname> |
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2319 <refname>cyg_clock_get_resolution</refname> |
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2320 <refname>cyg_real_time_clock</refname> |
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2321 <refname>cyg_current_time</refname> |
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2322 <refpurpose>Provide system clocks</refpurpose> |
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2323 </refnamediv> |
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2324 |
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2325 <refsynopsisdiv> |
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2326 <funcsynopsis> |
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2327 <funcsynopsisinfo> |
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2328 #include <cyg/kernel/kapi.h> |
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2329 </funcsynopsisinfo> |
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2330 <funcprototype> |
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2331 <funcdef>void <function>cyg_clock_create</function></funcdef> |
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2332 <paramdef>cyg_resolution_t <parameter>resolution</parameter></paramdef> |
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2333 <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef> |
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2334 <paramdef>cyg_clock* <parameter>clock</parameter></paramdef> |
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2335 </funcprototype> |
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2336 <funcprototype> |
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2337 <funcdef>void <function>cyg_clock_delete</function></funcdef> |
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2338 <paramdef>cyg_handle_t <parameter>clock</parameter></paramdef> |
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2339 </funcprototype> |
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2340 <funcprototype> |
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2341 <funcdef>void <function>cyg_clock_to_counter</function></funcdef> |
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2342 <paramdef>cyg_handle_t <parameter>clock</parameter></paramdef> |
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2343 <paramdef>cyg_handle_t* <parameter>counter</parameter></paramdef> |
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2344 </funcprototype> |
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2345 <funcprototype> |
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2346 <funcdef>void <function>cyg_clock_set_resolution</function></funcdef> |
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2347 <paramdef>cyg_handle_t <parameter>clock</parameter></paramdef> |
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2348 <paramdef>cyg_resolution_t <parameter>resolution</parameter></paramdef> |
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2349 </funcprototype> |
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2350 <funcprototype> |
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2351 <funcdef>cyg_resolution_t <function>cyg_clock_get_resolution</function></funcdef> |
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2352 <paramdef>cyg_handle_t <parameter>clock</parameter></paramdef> |
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2353 </funcprototype> |
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2354 <funcprototype> |
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2355 <funcdef>cyg_handle_t <function>cyg_real_time_clock</function></funcdef> |
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2356 <void> |
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2357 </funcprototype> |
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2358 <funcprototype> |
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2359 <funcdef>cyg_tick_count_t <function>cyg_current_time</function></funcdef> |
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2360 <void> |
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2361 </funcprototype> |
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2362 </funcsynopsis> |
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2363 </refsynopsisdiv> |
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2364 |
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2365 <refsect1 id="kernel-clocks-description"><title>Description</title> |
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2366 <para> |
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2367 In the eCos kernel clock objects are a special form of <link |
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2368 linkend="kernel-counters">counter</link> objects. They are attached to |
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2369 a specific type of hardware, clocks that generate ticks at very |
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2370 specific time intervals, whereas counters can be used with any event |
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2371 source. |
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2372 </para> |
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2373 <para> |
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2374 In a default configuration the kernel provides a single clock |
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2375 instance, the real-time clock. This gets used for timeslicing and for |
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2376 operations that involve a timeout, for example |
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2377 <function>cyg_semaphore_timed_wait</function>. If this functionality |
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2378 is not required it can be removed from the system using the |
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2379 configuration option <varname>CYGVAR_KERNEL_COUNTERS_CLOCK</varname>. |
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2380 Otherwise the real-time clock can be accessed by a call to |
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2381 <function>cyg_real_time_clock</function>, allowing applications to |
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2382 attach alarms, and the current counter value can be obtained using |
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2383 <function>cyg_current_time</function>. |
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2384 </para> |
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2385 <para> |
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2386 Applications can create and destroy additional clocks if desired, |
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2387 using <function>cyg_clock_create</function> and |
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2388 <function>cyg_clock_delete</function>. The first argument to |
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2389 <function>cyg_clock_create</function> specifies the |
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2390 <link linkend="kernel-clocks-resolution">resolution</link> this clock |
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2391 will run at. The second argument is used to return a handle for this |
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2392 clock object, and the third argument provides the kernel with the |
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2393 memory needed to hold this object. This clock will not actually tick |
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2394 by itself. Instead it is the responsibility of application code to |
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2395 initialize a suitable hardware timer to generate interrupts at the |
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2396 appropriate frequency, install an interrupt handler for this, and |
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2397 call <function>cyg_counter_tick</function> from inside the DSR. |
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2398 Associated with each clock is a kernel counter, a handle for which can |
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2399 be obtained using <function>cyg_clock_to_counter</function>. |
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2400 </para> |
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2401 </refsect1> |
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2402 |
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2403 <refsect1 id="kernel-clocks-resolution"><title>Clock Resolutions and Ticks</title> |
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2404 <para> |
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2405 At the kernel level all clock-related operations including delays, |
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2406 timeouts and alarms work in units of clock ticks, rather than in units |
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2407 of seconds or milliseconds. If the calling code, whether the |
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2408 application or some other package, needs to operate using units such |
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2409 as milliseconds then it has to convert from these units to clock |
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2410 ticks. |
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2411 </para> |
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2412 <para> |
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2413 The main reason for this is that it accurately reflects the |
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2414 hardware: calling something like <function>nanosleep</function> with a |
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2415 delay of ten nanoseconds will not work as intended on any real |
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2416 hardware because timer interrupts simply will not happen that |
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2417 frequently; instead calling <function>cyg_thread_delay</function> with |
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2418 the equivalent delay of 0 ticks gives a much clearer indication that |
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2419 the application is attempting something inappropriate for the target |
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2420 hardware. Similarly, passing a delay of five ticks to |
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2421 <function>cyg_thread_delay</function> makes it fairly obvious that |
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2422 the current thread will be suspended for somewhere between four and |
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2423 five clock periods, as opposed to passing 50000000 to |
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2424 <function>nanosleep</function> which suggests a granularity that is |
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2425 not actually provided. |
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2426 </para> |
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2427 <para> |
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2428 A secondary reason is that conversion between clock ticks and units |
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2429 such as milliseconds can be somewhat expensive, and whenever possible |
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2430 should be done at compile-time or by the application developer rather |
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2431 than at run-time. This saves code size and cpu cycles. |
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2432 </para> |
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2433 <para> |
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2434 The information needed to perform these conversions is the clock |
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2435 resolution. This is a structure with two fields, a dividend and a |
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2436 divisor, and specifies the number of nanoseconds between clock ticks. |
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2437 For example a clock that runs at 100Hz will have 10 milliseconds |
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2438 between clock ticks, or 10000000 nanoseconds. The ratio between the |
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2439 resolution's dividend and divisor will therefore be 10000000 to 1, and |
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2440 typical values for these might be 1000000000 and 100. If the clock |
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2441 runs at a different frequency, say 60Hz, the numbers could be |
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2442 1000000000 and 60 respectively. Given a delay in nanoseconds, this can |
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2443 be converted to clock ticks by multiplying with the the divisor and |
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2444 then dividing by the dividend. For example a delay of 50 milliseconds |
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2445 corresponds to 50000000 nanoseconds, and with a clock frequency of |
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2446 100Hz this can be converted to |
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2447 ((50000000 * 100) / 1000000000) = 5 |
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2448 clock ticks. Given the large numbers involved this arithmetic normally |
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2449 has to be done using 64-bit precision and the |
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2450 <type>long long</type> data type, but allows code to run on |
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2451 hardware with unusual clock frequencies. |
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2452 </para> |
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2453 <para> |
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2454 The default frequency for the real-time clock on any platform is |
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2455 usually about 100Hz, but platform-specific documentation should be |
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2456 consulted for this information. Usually it is possible to override |
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2457 this default by configuration options, but again this depends on the |
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2458 capabilities of the underlying hardware. The resolution for any clock |
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2459 can be obtained using <function>cyg_clock_get_resolution</function>. |
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2460 For clocks created by application code, there is also a function |
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2461 <function>cyg_clock_set_resolution</function>. This does not affect |
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2462 the underlying hardware timer in any way, it merely updates the |
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2463 information that will be returned in subsequent calls to |
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2464 <function>cyg_clock_get_resolution</function>: changing the actual |
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2465 underlying clock frequency will require appropriate manipulation of |
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2466 the timer hardware. |
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2467 </para> |
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2468 </refsect1> |
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2469 |
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2470 <refsect1 id="kernel-clocks-context"><title>Valid contexts</title> |
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2471 <para> |
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2472 <function>cyg_clock_create</function> is usually only called during |
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2473 system initialization (if at all), but may also be called from thread |
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2474 context. The same applies to <function>cyg_clock_delete</function>. |
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2475 The remaining functions may be called during initialization, from |
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2476 thread context, or from DSR context, although it should be noted that |
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2477 there is no locking between |
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2478 <function>cyg_clock_get_resolution</function> and |
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2479 <function>cyg_clock_set_resolution</function> so theoretically it is |
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2480 possible that the former returns an inconsistent data structure. |
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2481 </para> |
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2482 </refsect1> |
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2483 |
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2484 </refentry> |
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2485 |
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2486 <!-- }}} --> |
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2487 <!-- {{{ Alarms --> |
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2488 |
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2489 <refentry id="kernel-alarms"> |
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2490 |
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2491 <refmeta> |
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2492 <refentrytitle>Alarms</refentrytitle> |
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2493 </refmeta> |
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2494 |
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2495 <refnamediv> |
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2496 <refname>cyg_alarm_create</refname> |
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2497 <refname>cyg_alarm_delete</refname> |
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2498 <refname>cyg_alarm_initialize</refname> |
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2499 <refname>cyg_alarm_enable</refname> |
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2500 <refname>cyg_alarm_disable</refname> |
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2501 <refpurpose>Run an alarm function when a number of events have occurred</refpurpose> |
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2502 </refnamediv> |
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2503 |
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2504 <refsynopsisdiv> |
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2505 <funcsynopsis> |
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2506 <funcsynopsisinfo> |
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2507 #include <cyg/kernel/kapi.h> |
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2508 </funcsynopsisinfo> |
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2509 <funcprototype> |
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2510 <funcdef>void <function>cyg_alarm_create</function></funcdef> |
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2511 <paramdef>cyg_handle_t <parameter>counter</parameter></paramdef> |
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2512 <paramdef>cyg_alarm_t* <parameter>alarmfn</parameter></paramdef> |
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2513 <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef> |
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2514 <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef> |
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2515 <paramdef>cyg_alarm* <parameter>alarm</parameter></paramdef> |
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2516 </funcprototype> |
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2517 <funcprototype> |
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2518 <funcdef>void <function>cyg_alarm_delete</function></funcdef> |
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2519 <paramdef>cyg_handle_t <parameter>alarm</parameter></paramdef> |
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2520 </funcprototype> |
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2521 <funcprototype> |
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2522 <funcdef>void <function>cyg_alarm_initialize</function></funcdef> |
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2523 <paramdef>cyg_handle_t <parameter>alarm</parameter></paramdef> |
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2524 <paramdef>cyg_tick_count_t <parameter>trigger</parameter></paramdef> |
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2525 <paramdef>cyg_tick_count_t <parameter>interval</parameter></paramdef> |
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2526 </funcprototype> |
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2527 <funcprototype> |
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2528 <funcdef>void <function>cyg_alarm_enable</function></funcdef> |
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2529 <paramdef>cyg_handle_t <parameter>alarm</parameter></paramdef> |
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2530 </funcprototype> |
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2531 <funcprototype> |
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2532 <funcdef>void <function>cyg_alarm_disable</function></funcdef> |
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2533 <paramdef>cyg_handle_t <parameter>alarm</parameter></paramdef> |
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2534 </funcprototype> |
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2535 </funcsynopsis> |
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2536 </refsynopsisdiv> |
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2537 |
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2538 <refsect1 id="kernel-alarms-description"><title>Description</title> |
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2539 <para> |
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2540 Kernel alarms are used together with counters and allow for action to |
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2541 be taken when a certain number of events have occurred. If the counter |
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2542 is associated with a clock then the alarm action happens when the |
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2543 appropriate number of clock ticks have occurred, in other words after |
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2544 a certain period of time. |
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2545 </para> |
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2546 <para> |
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2547 Setting up an alarm involves a two-step process. First the alarm must |
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2548 be created with a call to <function>cyg_alarm_create</function>. This |
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2549 takes five arguments. The first identifies the counter to which the |
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2550 alarm should be attached. If the alarm should be attached to the |
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2551 system's real-time clock then <function>cyg_real_time_clock</function> |
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2552 and <function>cyg_clock_to_counter</function> can be used to get hold |
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2553 of the appropriate handle. The next two arguments specify the action |
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2554 to be taken when the alarm is triggered, in the form of a function |
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2555 pointer and some data. This function should take the form: |
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2556 </para> |
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2557 <programlisting width=72> |
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2558 void |
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2559 alarm_handler(cyg_handle_t alarm, cyg_addrword_t data) |
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2560 { |
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2561 … |
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2562 } |
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2563 </programlisting> |
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2564 <para> |
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2565 The data argument passed to the alarm function corresponds to the |
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2566 third argument passed to <function>cyg_alarm_create</function>. |
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2567 The fourth argument to <function>cyg_alarm_create</function> is used |
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2568 to return a handle to the newly-created alarm object, and the final |
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2569 argument provides the memory needed for the alarm object and thus |
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2570 avoids any need for dynamic memory allocation within the kernel. |
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2571 </para> |
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2572 <para> |
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2573 Once an alarm has been created a further call to |
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2574 <function>cyg_alarm_initialize</function> is needed to activate it. |
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2575 The first argument specifies the alarm. The second argument indicates |
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2576 the number of events, for example clock ticks, that need to occur |
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2577 before the alarm triggers. If the third argument is 0 then the alarm |
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2578 will only trigger once. A non-zero value specifies that the alarm |
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2579 should trigger repeatedly, with an interval of the specified number of |
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2580 events. |
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2581 </para> |
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2582 <para> |
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2583 Alarms can be temporarily disabled and reenabled using |
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2584 <function>cyg_alarm_disable</function> and |
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2585 <function>cyg_alarm_enable</function>. Alternatively another call to |
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2586 <function>cyg_alarm_initialize</function> can be used to modify the |
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2587 behaviour of an existing alarm. If an alarm is no longer required then |
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2588 the associated resources can be released using |
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2589 <function>cyg_alarm_delete</function>. |
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2590 </para> |
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2591 <para> |
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2592 The alarm function is invoked when a counter tick occurs, in other |
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2593 words when there is a call to <function>cyg_counter_tick</function>, |
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2594 and will happen in the same context. If the alarm is associated with |
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2595 the system's real-time clock then this will be DSR context, following |
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2596 a clock interrupt. If the alarm is associated with some other |
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2597 application-specific counter then the details will depend on how that |
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2598 counter is updated. |
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2599 </para> |
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2600 <para> |
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2601 If two or more alarms are registered for precisely the same counter tick, |
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2602 the order of execution of the alarm functions is unspecified. |
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2603 </para> |
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2604 </refsect1> |
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2605 |
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2606 <refsect1 id="kernel-alarms-context"><title>Valid contexts</title> |
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2607 <para> |
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2608 <function>cyg_alarm_create</function> |
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2609 <function>cyg_alarm_initialize</function> is typically called during |
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2610 system initialization but may also be called in thread context. The |
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2611 same applies to <function>cyg_alarm_delete</function>. |
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2612 <function>cyg_alarm_initialize</function>, |
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2613 <function>cyg_alarm_disable</function> and |
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2614 <function>cyg_alarm_enable</function> may be called during |
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2615 initialization or from thread or DSR context, but |
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2616 <function>cyg_alarm_enable</function> and |
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2617 <function>cyg_alarm_initialize</function> may be expensive operations |
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2618 and should only be called when necessary. |
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2619 </para> |
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2620 </refsect1> |
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2621 |
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2622 </refentry> |
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2623 |
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2624 <!-- }}} --> |
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2625 <!-- {{{ Mutexes --> |
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2626 |
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2627 <refentry id="kernel-mutexes"> |
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2628 |
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2629 <refmeta> |
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2630 <refentrytitle>Mutexes</refentrytitle> |
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2631 </refmeta> |
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2632 |
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2633 <refnamediv> |
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2634 <refname>cyg_mutex_init</refname> |
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2635 <refname>cyg_mutex_destroy</refname> |
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2636 <refname>cyg_mutex_lock</refname> |
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2637 <refname>cyg_mutex_trylock</refname> |
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2638 <refname>cyg_mutex_unlock</refname> |
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2639 <refname>cyg_mutex_release</refname> |
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2640 <refname>cyg_mutex_set_ceiling</refname> |
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2641 <refname>cyg_mutex_set_protocol</refname> |
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2642 <refpurpose>Synchronization primitive</refpurpose> |
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2643 </refnamediv> |
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2644 |
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2645 <refsynopsisdiv> |
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2646 <funcsynopsis> |
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2647 <funcsynopsisinfo> |
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2648 #include <cyg/kernel/kapi.h> |
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2649 </funcsynopsisinfo> |
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2650 <funcprototype> |
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2651 <funcdef>void <function>cyg_mutex_init</function></funcdef> |
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2652 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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2653 </funcprototype> |
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2654 <funcprototype> |
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2655 <funcdef>void <function>cyg_mutex_destroy</function></funcdef> |
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2656 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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2657 </funcprototype> |
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2658 <funcprototype> |
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2659 <funcdef>cyg_bool_t <function>cyg_mutex_lock</function></funcdef> |
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2660 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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2661 </funcprototype> |
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2662 <funcprototype> |
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2663 <funcdef>cyg_bool_t <function>cyg_mutex_trylock</function></funcdef> |
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2664 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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2665 </funcprototype> |
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2666 <funcprototype> |
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2667 <funcdef>void <function>cyg_mutex_unlock</function></funcdef> |
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2668 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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2669 </funcprototype> |
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2670 <funcprototype> |
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2671 <funcdef>void <function>cyg_mutex_release</function></funcdef> |
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2672 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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2673 </funcprototype> |
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2674 <funcprototype> |
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2675 <funcdef>void <function>cyg_mutex_set_ceiling</function></funcdef> |
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2676 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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2677 <paramdef>cyg_priority_t <parameter>priority</parameter></paramdef> |
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2678 </funcprototype> |
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2679 <funcprototype> |
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2680 <funcdef>void <function>cyg_mutex_set_protocol</function></funcdef> |
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2681 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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2682 <paramdef>enum cyg_mutex_protocol <parameter>protocol/</parameter></paramdef> |
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2683 </funcprototype> |
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2684 </funcsynopsis> |
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2685 </refsynopsisdiv> |
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2686 |
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2687 <refsect1 id="kernel-mutexes-description"><title>Description</title> |
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2688 <para> |
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2689 The purpose of mutexes is to let threads share resources safely. If |
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2690 two or more threads attempt to manipulate a data structure with no |
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2691 locking between them then the system may run for quite some time |
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2692 without apparent problems, but sooner or later the data structure will |
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2693 become inconsistent and the application will start behaving strangely |
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2694 and is quite likely to crash. The same can apply even when |
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2695 manipulating a single variable or some other resource. For example, |
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2696 consider: |
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2697 </para> |
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2698 <programlisting width=72> |
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2699 static volatile int counter = 0; |
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2700 |
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2701 void |
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2702 process_event(void) |
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2703 { |
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2704 … |
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2705 |
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2706 counter++; |
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2707 } |
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2708 </programlisting> |
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2709 <para> |
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2710 Assume that after a certain period of time <varname>counter</varname> |
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2711 has a value of 42, and two threads A and B running at the same |
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2712 priority call <function>process_event</function>. Typically thread A |
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2713 will read the value of <varname>counter</varname> into a register, |
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2714 increment this register to 43, and write this updated value back to |
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2715 memory. Thread B will do the same, so usually |
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2716 <varname>counter</varname> will end up with a value of 44. However if |
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2717 thread A is timesliced after reading the old value 42 but before |
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2718 writing back 43, thread B will still read back the old value and will |
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2719 also write back 43. The net result is that the counter only gets |
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2720 incremented once, not twice, which depending on the application may |
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2721 prove disastrous. |
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2722 </para> |
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2723 <para> |
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2724 Sections of code like the above which involve manipulating shared data |
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2725 are generally known as critical regions. Code should claim a lock |
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2726 before entering a critical region and release the lock when leaving. |
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2727 Mutexes provide an appropriate synchronization primitive for this. |
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2728 </para> |
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2729 <programlisting width=72> |
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2730 static volatile int counter = 0; |
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2731 static cyg_mutex_t lock; |
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2732 |
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2733 void |
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2734 process_event(void) |
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2735 { |
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2736 … |
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2737 |
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2738 cyg_mutex_lock(&lock); |
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2739 counter++; |
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2740 cyg_mutex_unlock(&lock); |
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2741 } |
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2742 </programlisting> |
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2743 <para> |
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2744 A mutex must be initialized before it can be used, by calling |
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2745 <function>cyg_mutex_init</function>. This takes a pointer to a |
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2746 <structname>cyg_mutex_t</structname> data structure which is typically |
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2747 statically allocated, and may be part of a larger data structure. If a |
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2748 mutex is no longer required and there are no threads waiting on it |
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2749 then <function>cyg_mutex_destroy</function> can be used. |
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2750 </para> |
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2751 <para> |
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2752 The main functions for using a mutex are |
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2753 <function>cyg_mutex_lock</function> and |
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2754 <function>cyg_mutex_unlock</function>. In normal operation |
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2755 <function>cyg_mutex_lock</function> will return success after claiming |
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2756 the mutex lock, blocking if another thread currently owns the mutex. |
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2757 However the lock operation may fail if other code calls |
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2758 <function>cyg_mutex_release</function> or |
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2759 <function>cyg_thread_release</function>, so if these functions may get |
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2760 used then it is important to check the return value. The current owner |
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2761 of a mutex should call <function>cyg_mutex_unlock</function> when a |
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2762 lock is no longer required. This operation must be performed by the |
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2763 owner, not by another thread. |
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2764 </para> |
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2765 <para> |
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2766 <function>cyg_mutex_trylock</function> is a variant of |
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2767 <function>cyg_mutex_lock</function> that will always return |
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2768 immediately, returning success or failure as appropriate. This |
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2769 function is rarely useful. Typical code locks a mutex just before |
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2770 entering a critical region, so if the lock cannot be claimed then |
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2771 there may be nothing else for the current thread to do. Use of this |
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2772 function may also cause a form of priority inversion if the owner |
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2773 owner runs at a lower priority, because the priority inheritance code |
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2774 will not be triggered. Instead the current thread continues running, |
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2775 preventing the owner from getting any cpu time, completing the |
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2776 critical region, and releasing the mutex. |
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2777 </para> |
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2778 <para> |
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2779 <function>cyg_mutex_release</function> can be used to wake up all |
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2780 threads that are currently blocked inside a call to |
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2781 <function>cyg_mutex_lock</function> for a specific mutex. These lock |
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2782 calls will return failure. The current mutex owner is not affected. |
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2783 </para> |
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2784 </refsect1> |
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2785 |
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2786 <refsect1 id="kernel-mutexes-priority-inversion"><title>Priority Inversion</title> |
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2787 <para> |
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2788 The use of mutexes gives rise to a problem known as priority |
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2789 inversion. In a typical scenario this requires three threads A, B, and |
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2790 C, running at high, medium and low priority respectively. Thread A and |
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2791 thread B are temporarily blocked waiting for some event, so thread C |
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2792 gets a chance to run, needs to enter a critical region, and locks |
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2793 a mutex. At this point threads A and B are woken up - the exact order |
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2794 does not matter. Thread A needs to claim the same mutex but has to |
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2795 wait until C has left the critical region and can release the mutex. |
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2796 Meanwhile thread B works on something completely different and can |
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2797 continue running without problems. Because thread C is running a lower |
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2798 priority than B it will not get a chance to run until B blocks for |
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2799 some reason, and hence thread A cannot run either. The overall effect |
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2800 is that a high-priority thread A cannot proceed because of a lower |
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2801 priority thread B, and priority inversion has occurred. |
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2802 </para> |
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2803 <para> |
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2804 In simple applications it may be possible to arrange the code such |
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2805 that priority inversion cannot occur, for example by ensuring that a |
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2806 given mutex is never shared by threads running at different priority |
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2807 levels. However this may not always be possible even at the |
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2808 application level. In addition mutexes may be used internally by |
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2809 underlying code, for example the memory allocation package, so careful |
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2810 analysis of the whole system would be needed to be sure that priority |
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2811 inversion cannot occur. Instead it is common practice to use one of |
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2812 two techniques: priority ceilings and priority inheritance. |
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2813 </para> |
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2814 <para> |
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2815 Priority ceilings involve associating a priority with each mutex. |
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2816 Usually this will match the highest priority thread that will ever |
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2817 lock the mutex. When a thread running at a lower priority makes a |
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2818 successful call to <function>cyg_mutex_lock</function> or |
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2819 <function>cyg_mutex_trylock</function> its priority will be boosted to |
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2820 that of the mutex. For example, given the previous example the |
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2821 priority associated with the mutex would be that of thread A, so for |
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2822 as long as it owns the mutex thread C will run in preference to thread |
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2823 B. When C releases the mutex its priority drops to the normal value |
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2824 again, allowing A to run and claim the mutex. Setting the |
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2825 priority for a mutex involves a call to |
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2826 <function>cyg_mutex_set_ceiling</function>, which is typically called |
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2827 during initialization. It is possible to change the ceiling |
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2828 dynamically but this will only affect subsequent lock operations, not |
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2829 the current owner of the mutex. |
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2830 </para> |
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2831 <para> |
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2832 Priority ceilings are very suitable for simple applications, where for |
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2833 every thread in the system it is possible to work out which mutexes |
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2834 will be accessed. For more complicated applications this may prove |
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2835 difficult, especially if thread priorities change at run-time. An |
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2836 additional problem occurs for any mutexes outside the application, for |
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2837 example used internally within eCos packages. A typical eCos package |
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2838 will be unaware of the details of the various threads in the system, |
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2839 so it will have no way of setting suitable ceilings for its internal |
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2840 mutexes. If those mutexes are not exported to application code then |
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2841 using priority ceilings may not be viable. The kernel does provide a |
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2842 configuration option |
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2843 <varname>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL_DEFAULT_PRIORITY</varname> |
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2844 that can be used to set the default priority ceiling for all mutexes, |
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2845 which may prove sufficient. |
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2846 </para> |
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2847 <para> |
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2848 The alternative approach is to use priority inheritance: if a thread |
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2849 calls <function>cyg_mutex_lock</function> for a mutex that it |
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2850 currently owned by a lower-priority thread, then the owner will have |
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2851 its priority raised to that of the current thread. Often this is more |
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2852 efficient than priority ceilings because priority boosting only |
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2853 happens when necessary, not for every lock operation, and the required |
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2854 priority is determined at run-time rather than by static analysis. |
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2855 However there are complications when multiple threads running at |
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2856 different priorities try to lock a single mutex, or when the current |
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2857 owner of a mutex then tries to lock additional mutexes, and this makes |
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2858 the implementation significantly more complicated than priority |
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2859 ceilings. |
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2860 </para> |
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2861 <para> |
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2862 There are a number of configuration options associated with priority |
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2863 inversion. First, if after careful analysis it is known that priority |
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2864 inversion cannot arise then the component |
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2865 <function>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL</function> |
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2866 can be disabled. More commonly this component will be enabled, and one |
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2867 of either |
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2868 <varname>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL_INHERIT</varname> |
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2869 or |
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2870 <varname>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL_CEILING</varname> |
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2871 will be selected, so that one of the two protocols is available for |
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2872 all mutexes. It is possible to select multiple protocols, so that some |
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2873 mutexes can have priority ceilings while others use priority |
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2874 inheritance or no priority inversion protection at all. Obviously this |
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2875 flexibility will add to the code size and to the cost of mutex |
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2876 operations. The default for all mutexes will be controlled by |
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2877 <varname>CYGSEM_KERNEL_SYNCH_MUTEX_PRIORITY_INVERSION_PROTOCOL_DEFAULT</varname>, |
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2878 and can be changed at run-time using |
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2879 <function>cyg_mutex_set_protocol</function>. |
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2880 </para> |
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2881 <para> |
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2882 Priority inversion problems can also occur with other synchronization |
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2883 primitives such as semaphores. For example there could be a situation |
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2884 where a high-priority thread A is waiting on a semaphore, a |
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2885 low-priority thread C needs to do just a little bit more work before |
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2886 posting the semaphore, but a medium priority thread B is running and |
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2887 preventing C from making progress. However a semaphore does not have |
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2888 the concept of an owner, so there is no way for the system to know |
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2889 that it is thread C which would next post to the semaphore. Hence |
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2890 there is no way for the system to boost the priority of C |
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2891 automatically and prevent the priority inversion. Instead situations |
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2892 like this have to be detected by application developers and |
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2893 appropriate precautions have to be taken, for example making sure that |
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2894 all the threads run at suitable priorities at all times. |
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2895 </para> |
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2896 <warning><para> |
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2897 The current implementation of priority inheritance within the eCos |
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2898 kernel does not handle certain exceptional circumstances completely |
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2899 correctly. Problems will only arise if a thread owns one mutex, |
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2900 then attempts to claim another mutex, and there are other threads |
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2901 attempting to lock these same mutexes. Although the system will |
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2902 continue running, the current owners of the various mutexes involved |
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2903 may not run at the priority they should. This situation never arises |
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2904 in typical code because a mutex will only be locked for a small |
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2905 critical region, and there is no need to manipulate other shared resources |
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2906 inside this region. A more complicated implementation of priority |
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2907 inheritance is possible but would add significant overhead and certain |
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2908 operations would no longer be deterministic. |
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2909 </para></warning> |
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2910 <warning><para> |
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2911 Support for priority ceilings and priority inheritance is not |
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2912 implemented for all schedulers. In particular neither priority |
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2913 ceilings nor priority inheritance are currently available for the |
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2914 bitmap scheduler. |
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2915 </para></warning> |
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2916 </refsect1> |
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2917 |
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2918 <refsect1 id="kernel-mutexes-alternatives"><title>Alternatives</title> |
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2919 <para> |
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2920 In nearly all circumstances, if two or more threads need to share some |
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2921 data then protecting this data with a mutex is the correct thing to |
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2922 do. Mutexes are the only primitive that combine a locking mechanism |
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2923 and protection against priority inversion problems. However this |
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2924 functionality is achieved at a cost, and in exceptional circumstances |
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2925 such as an application's most critical inner loop it may be desirable |
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2926 to use some other means of locking. |
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2927 </para> |
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2928 <para> |
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2929 When a critical region is very very small it is possible to lock the |
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2930 scheduler, thus ensuring that no other thread can run until the |
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2931 scheduler is unlocked again. This is achieved with calls to <link |
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2932 linkend="kernel-schedcontrol"><function>cyg_scheduler_lock</function></link> |
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2933 and <function>cyg_scheduler_unlock</function>. If the critical region |
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2934 is sufficiently small then this can actually improve both performance |
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2935 and dispatch latency because <function>cyg_mutex_lock</function> also |
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2936 locks the scheduler for a brief period of time. This approach will not |
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2937 work on SMP systems because another thread may already be running on a |
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2938 different processor and accessing the critical region. |
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2939 </para> |
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2940 <para> |
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2941 Another way of avoiding the use of mutexes is to make sure that all |
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2942 threads that access a particular critical region run at the same |
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2943 priority and configure the system with timeslicing disabled |
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2944 (<varname>CYGSEM_KERNEL_SCHED_TIMESLICE</varname>). Without |
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2945 timeslicing a thread can only be preempted by a higher-priority one, |
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2946 or if it performs some operation that can block. This approach |
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2947 requires that none of the operations in the critical region can block, |
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2948 so for example it is not legal to call |
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2949 <function>cyg_semaphore_wait</function>. It is also vulnerable to |
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2950 any changes in the configuration or to the various thread priorities: |
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2951 any such changes may now have unexpected side effects. It will not |
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2952 work on SMP systems. |
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2953 </para> |
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2954 </refsect1> |
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2955 |
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2956 <refsect1 id="kernel-mutexes-recursive"><title>Recursive Mutexes</title> |
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2957 <para> |
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2958 The implementation of mutexes within the eCos kernel does not support |
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2959 recursive locks. If a thread has locked a mutex and then attempts to |
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2960 lock the mutex again, typically as a result of some recursive call in |
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2961 a complicated call graph, then either an assertion failure will be |
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2962 reported or the thread will deadlock. This behaviour is deliberate. |
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2963 When a thread has just locked a mutex associated with some data |
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2964 structure, it can assume that that data structure is in a consistent |
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2965 state. Before unlocking the mutex again it must ensure that the data |
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2966 structure is again in a consistent state. Recursive mutexes allow a |
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2967 thread to make arbitrary changes to a data structure, then in a |
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2968 recursive call lock the mutex again while the data structure is still |
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2969 inconsistent. The net result is that code can no longer make any |
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2970 assumptions about data structure consistency, which defeats the |
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2971 purpose of using mutexes. |
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2972 </para> |
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2973 </refsect1> |
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2974 |
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2975 <refsect1 id="kernel-mutexes-context"><title>Valid contexts</title> |
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2976 <para> |
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2977 <function>cyg_mutex_init</function>, |
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2978 <function>cyg_mutex_set_ceiling</function> and |
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2979 <function>cyg_mutex_set_protocol</function> are normally called during |
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2980 initialization but may also be called from thread context. The |
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2981 remaining functions should only be called from thread context. Mutexes |
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2982 serve as a mutual exclusion mechanism between threads, and cannot be |
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2983 used to synchronize between threads and the interrupt handling |
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2984 subsystem. If a critical region is shared between a thread and a DSR |
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2985 then it must be protected using <link |
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2986 linkend="kernel-schedcontrol"><function>cyg_scheduler_lock</function></link> |
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2987 and <function>cyg_scheduler_unlock</function>. If a critical region is |
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2988 shared between a thread and an ISR, it must be protected by disabling |
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2989 or masking interrupts. Obviously these operations must be used with |
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2990 care because they can affect dispatch and interrupt latencies. |
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2991 </para> |
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2992 </refsect1> |
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2993 |
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2994 </refentry> |
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2995 |
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2996 <!-- }}} --> |
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2997 <!-- {{{ Condition variables --> |
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2998 |
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2999 <refentry id="kernel-condition-variables"> |
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3000 |
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3001 <refmeta> |
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3002 <refentrytitle>Condition Variables</refentrytitle> |
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3003 </refmeta> |
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3004 |
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3005 <refnamediv> |
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3006 <refname>cyg_cond_init</refname> |
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3007 <refname>cyg_cond_destroy</refname> |
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3008 <refname>cyg_cond_wait</refname> |
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3009 <refname>cyg_cond_timed_wait</refname> |
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3010 <refname>cyg_cond_signal</refname> |
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3011 <refname>cyg_cond_broadcast</refname> |
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3012 <refpurpose>Synchronization primitive</refpurpose> |
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3013 </refnamediv> |
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3014 |
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3015 <refsynopsisdiv> |
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3016 <funcsynopsis> |
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3017 <funcsynopsisinfo> |
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3018 #include <cyg/kernel/kapi.h> |
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3019 </funcsynopsisinfo> |
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3020 <funcprototype> |
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3021 <funcdef>void <function>cyg_cond_init</function></funcdef> |
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3022 <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef> |
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3023 <paramdef>cyg_mutex_t* <parameter>mutex</parameter></paramdef> |
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3024 </funcprototype> |
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3025 <funcprototype> |
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3026 <funcdef>void <function>cyg_cond_destroy</function></funcdef> |
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3027 <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef> |
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3028 </funcprototype> |
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3029 <funcprototype> |
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3030 <funcdef>cyg_bool_t <function>cyg_cond_wait</function></funcdef> |
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3031 <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef> |
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3032 </funcprototype> |
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3033 <funcprototype> |
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3034 <funcdef>cyg_bool_t <function>cyg_cond_timed_wait</function></funcdef> |
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3035 <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef> |
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3036 <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef> |
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3037 </funcprototype> |
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3038 <funcprototype> |
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3039 <funcdef>void <function>cyg_cond_signal</function></funcdef> |
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3040 <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef> |
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3041 </funcprototype> |
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3042 <funcprototype> |
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3043 <funcdef>void <function>cyg_cond_broadcast</function></funcdef> |
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3044 <paramdef>cyg_cond_t* <parameter>cond</parameter></paramdef> |
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3045 </funcprototype> |
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3046 </funcsynopsis> |
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3047 </refsynopsisdiv> |
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3048 |
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3049 <refsect1 id="kernel-condition-variables-description"><title>Description</title> |
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3050 |
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3051 <para> |
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3052 Condition variables are used in conjunction with mutexes to implement |
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3053 long-term waits for some condition to become true. For example |
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3054 consider a set of functions that control access to a pool of |
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3055 resources: |
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3056 </para> |
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3057 |
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3058 <programlisting width=72> |
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3059 |
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3060 cyg_mutex_t res_lock; |
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3061 res_t res_pool[RES_MAX]; |
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3062 int res_count = RES_MAX; |
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3063 |
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3064 void res_init(void) |
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3065 { |
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3066 cyg_mutex_init(&res_lock); |
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3067 <fill pool with resources> |
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3068 } |
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3069 |
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3070 res_t res_allocate(void) |
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3071 { |
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3072 res_t res; |
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3073 |
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3074 cyg_mutex_lock(&res_lock); // lock the mutex |
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3075 |
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3076 if( res_count == 0 ) // check for free resource |
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3077 res = RES_NONE; // return RES_NONE if none |
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3078 else |
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3079 { |
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3080 res_count--; // allocate a resources |
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3081 res = res_pool[res_count]; |
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3082 } |
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3083 |
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3084 cyg_mutex_unlock(&res_lock); // unlock the mutex |
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3085 |
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3086 return res; |
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3087 } |
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3088 |
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3089 void res_free(res_t res) |
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3090 { |
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3091 cyg_mutex_lock(&res_lock); // lock the mutex |
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3092 |
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3093 res_pool[res_count] = res; // free the resource |
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3094 res_count++; |
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3095 |
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3096 cyg_mutex_unlock(&res_lock); // unlock the mutex |
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3097 } |
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3098 </programlisting> |
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3099 |
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3100 <para> |
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3101 These routines use the variable <varname>res_count</varname> to keep |
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3102 track of the resources available. If there are none then |
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3103 <function>res_allocate</function> returns <literal>RES_NONE</literal>, |
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3104 which the caller must check for and take appropriate error handling |
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3105 actions. |
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3106 </para> |
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3107 |
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3108 <para> |
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3109 Now suppose that we do not want to return |
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3110 <literal>RES_NONE</literal> when there are no resources, but want to |
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3111 wait for one to become available. This is where a condition variable |
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3112 can be used: |
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3113 </para> |
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3114 |
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3115 <programlisting width=72> |
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3116 |
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3117 cyg_mutex_t res_lock; |
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3118 cyg_cond_t res_wait; |
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3119 res_t res_pool[RES_MAX]; |
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3120 int res_count = RES_MAX; |
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3121 |
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3122 void res_init(void) |
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3123 { |
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3124 cyg_mutex_init(&res_lock); |
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3125 cyg_cond_init(&res_wait, &res_lock); |
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3126 <fill pool with resources> |
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3127 } |
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3128 |
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3129 res_t res_allocate(void) |
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3130 { |
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3131 res_t res; |
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3132 |
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3133 cyg_mutex_lock(&res_lock); // lock the mutex |
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3134 |
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3135 while( res_count == 0 ) // wait for a resources |
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3136 cyg_cond_wait(&res_wait); |
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3137 |
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3138 res_count--; // allocate a resource |
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3139 res = res_pool[res_count]; |
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3140 |
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3141 cyg_mutex_unlock(&res_lock); // unlock the mutex |
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3142 |
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3143 return res; |
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3144 } |
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3145 |
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3146 void res_free(res_t res) |
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3147 { |
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3148 cyg_mutex_lock(&res_lock); // lock the mutex |
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3149 |
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3150 res_pool[res_count] = res; // free the resource |
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3151 res_count++; |
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3152 |
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3153 cyg_cond_signal(&res_wait); // wake up any waiting allocators |
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3154 |
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3155 cyg_mutex_unlock(&res_lock); // unlock the mutex |
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3156 } |
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3157 </programlisting> |
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3158 |
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3159 <para> |
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3160 In this version of the code, when <function>res_allocate</function> |
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3161 detects that there are no resources it calls |
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3162 <function>cyg_cond_wait</function>. This does two things: it unlocks |
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3163 the mutex, and puts the calling thread to sleep on the condition |
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3164 variable. When <function>res_free</function> is eventually called, it |
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3165 puts a resource back into the pool and calls |
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3166 <function>cyg_cond_signal</function> to wake up any thread waiting on |
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3167 the condition variable. When the waiting thread eventually gets to run again, |
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3168 it will re-lock the mutex before returning from |
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3169 <function>cyg_cond_wait</function>. |
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3170 </para> |
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3171 |
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3172 <para> |
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3173 There are two important things to note about the way in which this |
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3174 code works. The first is that the mutex unlock and wait in |
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3175 <function>cyg_cond_wait</function> are atomic: no other thread can run |
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3176 between the unlock and the wait. If this were not the case then a call |
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3177 to <function>res_free</function> by that thread would release the |
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3178 resource but the call to <function>cyg_cond_signal</function> would be |
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3179 lost, and the first thread would end up waiting when there were |
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3180 resources available. |
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3181 </para> |
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3182 |
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3183 <para> |
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3184 The second feature is that the call to |
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3185 <function>cyg_cond_wait</function> is in a <literal>while</literal> |
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3186 loop and not a simple <literal>if</literal> statement. This is because |
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3187 of the need to re-lock the mutex in <function>cyg_cond_wait</function> |
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3188 when the signalled thread reawakens. If there are other threads |
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3189 already queued to claim the lock then this thread must wait. Depending |
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3190 on the scheduler and the queue order, many other threads may have |
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3191 entered the critical section before this one gets to run. So the |
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3192 condition that it was waiting for may have been rendered false. Using |
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3193 a loop around all condition variable wait operations is the only way |
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3194 to guarantee that the condition being waited for is still true after |
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3195 waiting. |
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3196 </para> |
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3197 |
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3198 <para> |
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3199 Before a condition variable can be used it must be initialized with a |
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3200 call to <function>cyg_cond_init</function>. This requires two |
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3201 arguments, memory for the data structure and a pointer to an existing |
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3202 mutex. This mutex will not be initialized by |
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3203 <function>cyg_cond_init</function>, instead a separate call to |
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3204 <function>cyg_mutex_init</function> is required. If a condition |
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3205 variable is no longer required and there are no threads waiting on it |
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3206 then <function>cyg_cond_destroy</function> can be used. |
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3207 </para> |
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3208 <para> |
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3209 When a thread needs to wait for a condition to be satisfied it can |
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3210 call <function>cyg_cond_wait</function>. The thread must have already |
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3211 locked the mutex that was specified in the |
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3212 <function>cyg_cond_init</function> call. This mutex will be unlocked |
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3213 and the current thread will be suspended in an atomic operation. When |
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3214 some other thread performs a signal or broadcast operation the current |
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3215 thread will be woken up and automatically reclaim ownership of the mutex |
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3216 again, allowing it to examine global state and determine whether or |
|
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3217 not the condition is now satisfied. |
|
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3218 </para> |
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3219 <para> |
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3220 The kernel supplies a variant of this function, |
|
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3221 <function>cyg_cond_timed_wait</function>, which can be used to wait on |
|
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3222 the condition variable or until some number of clock ticks have |
|
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3223 occurred. The number of ticks is specified as an absolute, not |
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3224 relative tick count, and so in order to wait for a relative number of |
|
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3225 ticks, the return value of the <function>cyg_current_time()</function> |
|
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3226 function should be added to determine the absolute number of ticks. |
|
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3227 The mutex will always be reclaimed before |
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3228 <function>cyg_cond_timed_wait</function> returns, regardless of |
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3229 whether it was a result of a signal operation or a timeout. |
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3230 </para> |
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3231 <para> |
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3232 There is no <function>cyg_cond_trywait</function> function because |
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3233 this would not serve any purpose. If a thread has locked the mutex and |
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3234 determined that the condition is satisfied, it can just release the |
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3235 mutex and return. There is no need to perform any operation on the |
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3236 condition variable. |
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3237 </para> |
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3238 <para> |
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3239 When a thread changes shared state that may affect some other thread |
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3240 blocked on a condition variable, it should call either |
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3241 <function>cyg_cond_signal</function> or |
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3242 <function>cyg_cond_broadcast</function>. These calls do not require |
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3243 ownership of the mutex, but usually the mutex will have been claimed |
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3244 before updating the shared state. A signal operation only wakes up the |
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3245 first thread that is waiting on the condition variable, while a |
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3246 broadcast wakes up all the threads. If there are no threads waiting on |
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3247 the condition variable at the time, then the signal or broadcast will |
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3248 have no effect: past signals are not counted up or remembered in any |
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3249 way. Typically a signal should be used when all threads will check the |
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3250 same condition and at most one thread can continue running. A |
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3251 broadcast should be used if threads check slightly different |
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3252 conditions, or if the change to the global state might allow multiple |
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3253 threads to proceed. |
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3254 </para> |
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3255 </refsect1> |
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3256 |
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3257 <refsect1 id="kernel-condition-variables-context"><title>Valid contexts</title> |
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3258 <para> |
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3259 <function>cyg_cond_init</function> is typically called during system |
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3260 initialization but may also be called in thread context. The same |
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3261 applies to <function>cyg_cond_delete</function>. |
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3262 <function>cyg_cond_wait</function> and |
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3263 <function>cyg_cond_timedwait</function> may only be called from thread |
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3264 context since they may block. <function>cyg_cond_signal</function> and |
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3265 <function>cyg_cond_broadcast</function> may be called from thread or |
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3266 DSR context. |
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3267 </para> |
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3268 </refsect1> |
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3269 |
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3270 </refentry> |
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3271 |
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3272 <!-- }}} --> |
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3273 <!-- {{{ Semaphores --> |
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3274 |
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3275 <refentry id="kernel-semaphores"> |
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3276 |
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3277 <refmeta> |
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3278 <refentrytitle>Semaphores</refentrytitle> |
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3279 </refmeta> |
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3280 |
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3281 <refnamediv> |
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3282 <refname>cyg_semaphore_init</refname> |
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3283 <refname>cyg_semaphore_destroy</refname> |
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3284 <refname>cyg_semaphore_wait</refname> |
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3285 <refname>cyg_semaphore_timed_wait</refname> |
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3286 <refname>cyg_semaphore_post</refname> |
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3287 <refname>cyg_semaphore_peek</refname> |
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3288 <refpurpose>Synchronization primitive</refpurpose> |
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3289 </refnamediv> |
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3290 |
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3291 <refsynopsisdiv> |
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3292 <funcsynopsis> |
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3293 <funcsynopsisinfo> |
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3294 #include <cyg/kernel/kapi.h> |
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3295 </funcsynopsisinfo> |
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3296 <funcprototype> |
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3297 <funcdef>void <function>cyg_semaphore_init</function></funcdef> |
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3298 <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef> |
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3299 <paramdef>cyg_count32 <parameter>val</parameter></paramdef> |
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3300 </funcprototype> |
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3301 <funcprototype> |
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3302 <funcdef>void <function>cyg_semaphore_destroy</function></funcdef> |
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3303 <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef> |
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3304 </funcprototype> |
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3305 <funcprototype> |
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3306 <funcdef>cyg_bool_t <function>cyg_semaphore_wait</function></funcdef> |
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3307 <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef> |
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3308 </funcprototype> |
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3309 <funcprototype> |
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3310 <funcdef>cyg_bool_t <function>cyg_semaphore_timed_wait</function></funcdef> |
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3311 <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef> |
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3312 <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef> |
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3313 </funcprototype> |
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3314 <funcprototype> |
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3315 <funcdef>cyg_bool_t <function>cyg_semaphore_trywait</function></funcdef> |
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3316 <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef> |
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3317 </funcprototype> |
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3318 <funcprototype> |
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3319 <funcdef>void <function>cyg_semaphore_post</function></funcdef> |
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3320 <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef> |
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3321 </funcprototype> |
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3322 <funcprototype> |
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3323 <funcdef>void <function>cyg_semaphore_peek</function></funcdef> |
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3324 <paramdef>cyg_sem_t* <parameter>sem</parameter></paramdef> |
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3325 <paramdef>cyg_count32* <parameter>val</parameter></paramdef> |
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3326 </funcprototype> |
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3327 </funcsynopsis> |
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3328 </refsynopsisdiv> |
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3329 |
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3330 <refsect1 id="kernel-semaphores-description"><title>Description</title> |
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3331 <para> |
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3332 Counting semaphores are a <link |
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3333 linkend="kernel-overview-synch-primitives">synchronization |
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3334 primitive</link> that allow threads to wait until an event has |
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3335 occurred. The event may be generated by a producer thread, or by a DSR |
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3336 in response to a hardware interrupt. Associated with each semaphore is |
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3337 an integer counter that keeps track of the number of events that have |
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3338 not yet been processed. If this counter is zero, an attempt by a |
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3339 consumer thread to wait on the semaphore will block until some other |
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3340 thread or a DSR posts a new event to the semaphore. If the counter is |
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3341 greater than zero then an attempt to wait on the semaphore will |
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3342 consume one event, in other words decrement the counter, and return |
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3343 immediately. Posting to a semaphore will wake up the first thread that |
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3344 is currently waiting, which will then resume inside the semaphore wait |
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3345 operation and decrement the counter again. |
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3346 </para> |
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3347 <para> |
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3348 Another use of semaphores is for certain forms of resource management. |
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3349 The counter would correspond to how many of a certain type of resource |
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3350 are currently available, with threads waiting on the semaphore to |
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3351 claim a resource and posting to release the resource again. In |
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3352 practice <link linkend="kernel-condition-variables">condition |
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3353 variables</link> are usually much better suited for operations like |
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3354 this. |
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3355 </para> |
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3356 <para> |
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3357 <function>cyg_semaphore_init</function> is used to initialize a |
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3358 semaphore. It takes two arguments, a pointer to a |
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3359 <structname>cyg_sem_t</structname> structure and an initial value for |
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3360 the counter. Note that semaphore operations, unlike some other parts |
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3361 of the kernel API, use pointers to data structures rather than |
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3362 handles. This makes it easier to embed semaphores in a larger data |
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3363 structure. The initial counter value can be any number, zero, positive |
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3364 or negative, but typically a value of zero is used to indicate that no |
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3365 events have occurred yet. |
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3366 </para> |
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3367 <para> |
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3368 <function>cyg_semaphore_wait</function> is used by a consumer thread |
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3369 to wait for an event. If the current counter is greater than 0, in |
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3370 other words if the event has already occurred in the past, then the |
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3371 counter will be decremented and the call will return immediately. |
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3372 Otherwise the current thread will be blocked until there is a |
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3373 <function>cyg_semaphore_post</function> call. |
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3374 </para> |
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3375 <para> |
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3376 <function>cyg_semaphore_post</function> is called when an event has |
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3377 occurs. This increments the counter and wakes up the first thread |
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3378 waiting on the semaphore (if any). Usually that thread will then |
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3379 continue running inside <function>cyg_semaphore_wait</function> and |
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3380 decrement the counter again. However other scenarioes are possible. |
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3381 For example the thread calling <function>cyg_semaphore_post</function> |
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3382 may be running at high priority, some other thread running at medium |
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3383 priority may be about to call <function>cyg_semaphore_wait</function> |
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3384 when it next gets a chance to run, and a low priority thread may be |
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3385 waiting on the semaphore. What will happen is that the current high |
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3386 priority thread continues running until it is descheduled for some |
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3387 reason, then the medium priority thread runs and its call to |
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3388 <function>cyg_semaphore_wait</function> succeeds immediately, and |
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3389 later on the low priority thread runs again, discovers a counter value |
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3390 of 0, and blocks until another event is posted. If there are multiple |
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3391 threads blocked on a semaphore then the configuration option |
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3392 <varname>CYGIMP_KERNEL_SCHED_SORTED_QUEUES</varname> determines which |
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3393 one will be woken up by a post operation. |
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3394 </para> |
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3395 <para> |
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3396 <function>cyg_semaphore_wait</function> returns a boolean. Normally it |
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3397 will block until it has successfully decremented the counter, retrying |
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3398 as necessary, and return success. However the wait operation may be |
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3399 aborted by a call to <link |
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3400 linkend="kernel-thread-control"><function>cyg_thread_release</function></link>, |
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3401 and <function>cyg_semaphore_wait</function> will then return false. |
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3402 </para> |
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3403 <para> |
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3404 <function>cyg_semaphore_timed_wait</function> is a variant of |
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3405 <function>cyg_semaphore_wait</function>. It can be used to wait until |
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3406 either an event has occurred or a number of clock ticks have happened. |
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3407 The number of ticks is specified as an absolute, not relative tick |
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3408 count, and so in order to wait for a relative number of ticks, the |
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3409 return value of the <function>cyg_current_time()</function> function |
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3410 should be added to determine the absolute number of ticks. The |
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3411 function returns success if the semaphore wait operation succeeded, or |
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3412 false if the operation timed out or was aborted by |
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3413 <function>cyg_thread_release</function>. |
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3414 If support for the real-time |
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3415 clock has been removed from the current configuration then this |
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3416 function will not be available. |
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3417 <function>cyg_semaphore_trywait</function> is another variant which |
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3418 will always return immediately rather than block, again returning |
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3419 success or failure. If <function>cyg_semaphore_timedwait</function> |
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3420 is given a timeout in the past, it operates like |
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3421 <function>cyg_semaphore_trywait</function>. |
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3422 </para> |
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3423 <para> |
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3424 <function>cyg_semaphore_peek</function> can be used to get hold of the |
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3425 current counter value. This function is rarely useful except for |
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3426 debugging purposes since the counter value may change at any time if |
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3427 some other thread or a DSR performs a semaphore operation. |
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3428 </para> |
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3429 </refsect1> |
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3430 |
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3431 <refsect1 id="kernel-semaphores-context"><title>Valid contexts</title> |
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3432 <para> |
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3433 <function>cyg_semaphore_init</function> is normally called during |
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3434 initialization but may also be called from thread context. |
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3435 <function>cyg_semaphore_wait</function> and |
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3436 <function>cyg_semaphore_timed_wait</function> may only be called from |
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3437 thread context because these operations may block. |
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3438 <function>cyg_semaphore_trywait</function>, |
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3439 <function>cyg_semaphore_post</function> and |
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3440 <function>cyg_semaphore_peek</function> may be called from thread or |
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3441 DSR context. |
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3442 </para> |
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3443 </refsect1> |
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3444 |
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3445 </refentry> |
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3446 |
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3447 <!-- }}} --> |
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3448 <!-- {{{ Mail boxes --> |
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3449 |
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3450 <refentry id="kernel-mail-boxes"> |
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3451 |
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3452 <refmeta> |
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3453 <refentrytitle>Mail boxes</refentrytitle> |
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3454 </refmeta> |
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3455 |
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3456 <refnamediv> |
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3457 <refname>cyg_mbox_create</refname> |
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3458 <refname>cyg_mbox_delete</refname> |
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3459 <refname>cyg_mbox_get</refname> |
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3460 <refname>cyg_mbox_timed_get</refname> |
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3461 <refname>cyg_mbox_tryget</refname> |
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3462 <refname>cyg_mbox_peek_item</refname> |
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3463 <refname>cyg_mbox_put</refname> |
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3464 <refname>cyg_mbox_timed_put</refname> |
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3465 <refname>cyg_mbox_tryput</refname> |
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3466 <refname>cyg_mbox_peek</refname> |
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3467 <refname>cyg_mbox_waiting_to_get</refname> |
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3468 <refname>cyg_mbox_waiting_to_put</refname> |
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3469 <refpurpose>Synchronization primitive</refpurpose> |
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3470 </refnamediv> |
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3471 |
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3472 <refsynopsisdiv> |
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3473 <funcsynopsis> |
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3474 <funcsynopsisinfo> |
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3475 #include <cyg/kernel/kapi.h> |
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3476 </funcsynopsisinfo> |
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3477 <funcprototype> |
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3478 <funcdef>void <function>cyg_mbox_create</function></funcdef> |
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3479 <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef> |
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3480 <paramdef>cyg_mbox* <parameter>mbox</parameter></paramdef> |
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3481 </funcprototype> |
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3482 <funcprototype> |
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3483 <funcdef>void <function>cyg_mbox_delete</function></funcdef> |
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3484 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3485 </funcprototype> |
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3486 <funcprototype> |
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3487 <funcdef>void* <function>cyg_mbox_get</function></funcdef> |
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3488 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3489 </funcprototype> |
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3490 <funcprototype> |
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3491 <funcdef>void* <function>cyg_mbox_timed_get</function></funcdef> |
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3492 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3493 <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef> |
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3494 </funcprototype> |
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3495 <funcprototype> |
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3496 <funcdef>void* <function>cyg_mbox_tryget</function></funcdef> |
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3497 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3498 </funcprototype> |
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3499 <funcprototype> |
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3500 <funcdef>cyg_count32 <function>cyg_mbox_peek</function></funcdef> |
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3501 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3502 </funcprototype> |
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3503 <funcprototype> |
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3504 <funcdef>void* <function>cyg_mbox_peek_item</function></funcdef> |
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3505 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3506 </funcprototype> |
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3507 <funcprototype> |
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3508 <funcdef>cyg_bool_t <function>cyg_mbox_put</function></funcdef> |
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3509 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3510 <paramdef>void* <parameter>item</parameter></paramdef> |
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3511 </funcprototype> |
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3512 <funcprototype> |
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3513 <funcdef>cyg_bool_t <function>cyg_mbox_timed_put</function></funcdef> |
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3514 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3515 <paramdef>void* <parameter>item</parameter></paramdef> |
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3516 <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef> |
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3517 </funcprototype> |
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3518 <funcprototype> |
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3519 <funcdef>cyg_bool_t <function>cyg_mbox_tryput</function></funcdef> |
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3520 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3521 <paramdef>void* <parameter>item</parameter></paramdef> |
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3522 </funcprototype> |
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3523 <funcprototype> |
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3524 <funcdef>cyg_bool_t <function>cyg_mbox_waiting_to_get</function></funcdef> |
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3525 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3526 </funcprototype> |
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3527 <funcprototype> |
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3528 <funcdef>cyg_bool_t <function>cyg_mbox_waiting_to_put</function></funcdef> |
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3529 <paramdef>cyg_handle_t <parameter>mbox</parameter></paramdef> |
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3530 </funcprototype> |
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3531 </funcsynopsis> |
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3532 </refsynopsisdiv> |
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3533 |
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3534 <refsect1 id="kernel-mail-boxes-description"><title>Description</title> |
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3535 <para> |
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3536 Mail boxes are a synchronization primitive. Like semaphores they |
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3537 can be used by a consumer thread to wait until a certain event has |
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3538 occurred, but the producer also has the ability to transmit some data |
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3539 along with each event. This data, the message, is normally a pointer |
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3540 to some data structure. It is stored in the mail box itself, so the |
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3541 producer thread that generates the event and provides the data usually |
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3542 does not have to block until some consumer thread is ready to receive |
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3543 the event. However a mail box will only have a finite capacity, |
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3544 typically ten slots. Even if the system is balanced and events are |
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3545 typically consumed at least as fast as they are generated, a burst of |
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3546 events can cause the mail box to fill up and the generating thread |
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3547 will block until space is available again. This behaviour is very |
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3548 different from semaphores, where it is only necessary to maintain a |
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3549 counter and hence an overflow is unlikely. |
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3550 </para> |
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3551 <para> |
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3552 Before a mail box can be used it must be created with a call to |
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3553 <function>cyg_mbox_create</function>. Each mail box has a unique |
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3554 handle which will be returned via the first argument and which should |
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3555 be used for subsequent operations. |
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3556 <function>cyg_mbox_create</function> also requires an area of memory |
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3557 for the kernel structure, which is provided by the |
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3558 <structname>cyg_mbox</structname> second argument. If a mail box is |
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3559 no longer required then <function>cyg_mbox_delete</function> can be |
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3560 used. This will simply discard any messages that remain posted. |
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3561 </para> |
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3562 <para> |
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3563 The main function for waiting on a mail box is |
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3564 <function>cyg_mbox_get</function>. If there is a pending message |
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3565 because of a call to <function>cyg_mbox_put</function> then |
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3566 <function>cyg_mbox_get</function> will return immediately with the |
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3567 message that was put into the mail box. Otherwise this function |
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3568 will block until there is a put operation. Exceptionally the thread |
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3569 can instead be unblocked by a call to |
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3570 <function>cyg_thread_release</function>, in which case |
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3571 <function>cyg_mbox_get</function> will return a null pointer. It is |
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3572 assumed that there will never be a call to |
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3573 <function>cyg_mbox_put</function> with a null pointer, because it |
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3574 would not be possible to distinguish between that and a release |
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3575 operation. Messages are always retrieved in the order in which they |
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3576 were put into the mail box, and there is no support for messages |
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3577 with different priorities. |
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3578 </para> |
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3579 <para> |
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3580 There are two variants of <function>cyg_mbox_get</function>. The |
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3581 first, <function>cyg_mbox_timed_get</function> will wait until either |
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3582 a message is available or until a number of clock ticks have occurred. |
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3583 The number of ticks is specified as an absolute, not relative tick |
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3584 count, and so in order to wait for a relative number of ticks, the |
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3585 return value of the <function>cyg_current_time()</function> function |
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3586 should be added to determine the absolute number of ticks. If no |
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3587 message is posted within the timeout then a null pointer will be |
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3588 returned. <function>cyg_mbox_tryget</function> is a non-blocking |
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3589 operation which will either return a message if one is available or a |
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3590 null pointer. |
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3591 </para> |
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3592 <para> |
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3593 New messages are placed in the mail box by calling |
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3594 <function>cyg_mbox_put</function> or one of its variants. The main put |
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3595 function takes two arguments, a handle to the mail box and a |
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3596 pointer for the message itself. If there is a spare slot in the |
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3597 mail box then the new message can be placed there immediately, and |
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3598 if there is a waiting thread it will be woken up so that it can |
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3599 receive the message. If the mail box is currently full then |
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3600 <function>cyg_mbox_put</function> will block until there has been a |
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3601 get operation and a slot is available. The |
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3602 <function>cyg_mbox_timed_put</function> variant imposes a time limit |
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3603 on the put operation, returning false if the operation cannot be |
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3604 completed within the specified number of clock ticks and as for |
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3605 <function>cyg_mbox_timed_get</function> this is an absolute tick |
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3606 count. The <function>cyg_mbox_tryput</function> variant is |
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3607 non-blocking, returning false if there are no free slots available and |
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3608 the message cannot be posted without blocking. |
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3609 </para> |
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3610 <para> |
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3611 There are a further four functions available for examining the current |
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3612 state of a mailbox. The results of these functions must be used with |
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3613 care because usually the state can change at any time as a result of |
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3614 activity within other threads, but they may prove occasionally useful |
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3615 during debugging or in special situations. |
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3616 <function>cyg_mbox_peek</function> returns a count of the number of |
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3617 messages currently stored in the mail box. |
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3618 <function>cyg_mbox_peek_item</function> retrieves the first message, |
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3619 but it remains in the mail box until a get operation is performed. |
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3620 <function>cyg_mbox_waiting_to_get</function> and |
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3621 <function>cyg_mbox_waiting_to_put</function> indicate whether or not |
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3622 there are currently threads blocked in a get or a put operation on a |
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3623 given mail box. |
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3624 </para> |
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3625 <para> |
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3626 The number of slots in each mail box is controlled by a |
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3627 configuration option |
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3628 <varname>CYGNUM_KERNEL_SYNCH_MBOX_QUEUE_SIZE</varname>, with a default |
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3629 value of 10. All mail boxes are the same size. |
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3630 </para> |
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3631 </refsect1> |
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3632 |
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3633 <refsect1 id="kernel-mail-boxes-context"><title>Valid contexts</title> |
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3634 <para> |
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3635 <function>cyg_mbox_create</function> is typically called during |
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3636 system initialization but may also be called in thread context. |
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3637 The remaining functions are normally called only during thread |
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3638 context. Of special note is <function>cyg_mbox_put</function> which |
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3639 can be a blocking operation when the mail box is full, and which |
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3640 therefore must never be called from DSR context. It is permitted to |
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3641 call <function>cyg_mbox_tryput</function>, |
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3642 <function>cyg_mbox_tryget</function>, and the information functions |
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3643 from DSR context but this is rarely useful. |
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3644 </para> |
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3645 </refsect1> |
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3646 |
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3647 </refentry> |
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3648 |
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3649 <!-- }}} --> |
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3650 <!-- {{{ Flags --> |
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3651 |
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3652 <refentry id="kernel-flags"> |
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3653 |
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3654 <refmeta> |
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3655 <refentrytitle>Event Flags</refentrytitle> |
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3656 </refmeta> |
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3657 |
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3658 <refnamediv> |
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3659 <refname>cyg_flag_init</refname> |
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3660 <refname>cyg_flag_destroy</refname> |
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3661 <refname>cyg_flag_setbits</refname> |
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3662 <refname>cyg_flag_maskbits</refname> |
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3663 <refname>cyg_flag_wait</refname> |
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3664 <refname>cyg_flag_timed_wait</refname> |
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3665 <refname>cyg_flag_poll</refname> |
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3666 <refname>cyg_flag_peek</refname> |
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3667 <refname>cyg_flag_waiting</refname> |
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3668 <refpurpose>Synchronization primitive</refpurpose> |
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3669 </refnamediv> |
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3670 |
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3671 <refsynopsisdiv> |
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3672 <funcsynopsis> |
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3673 <funcsynopsisinfo> |
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3674 #include <cyg/kernel/kapi.h> |
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3675 </funcsynopsisinfo> |
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3676 <funcprototype> |
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3677 <funcdef>void <function>cyg_flag_init</function></funcdef> |
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3678 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3679 </funcprototype> |
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3680 <funcprototype> |
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3681 <funcdef>void <function>cyg_flag_destroy</function></funcdef> |
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3682 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3683 </funcprototype> |
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3684 <funcprototype> |
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3685 <funcdef>void <function>cyg_flag_setbits</function></funcdef> |
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3686 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3687 <paramdef>cyg_flag_value_t <parameter>value</parameter></paramdef> |
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3688 </funcprototype> |
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3689 <funcprototype> |
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3690 <funcdef>void <function>cyg_flag_maskbits</function></funcdef> |
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3691 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3692 <paramdef>cyg_flag_value_t <parameter>value</parameter></paramdef> |
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3693 </funcprototype> |
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3694 <funcprototype> |
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3695 <funcdef>cyg_flag_value_t <function>cyg_flag_wait</function></funcdef> |
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3696 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3697 <paramdef>cyg_flag_value_t <parameter>pattern</parameter></paramdef> |
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3698 <paramdef>cyg_flag_mode_t <parameter>mode</parameter></paramdef> |
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3699 </funcprototype> |
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3700 <funcprototype> |
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3701 <funcdef>cyg_flag_value_t <function>cyg_flag_timed_wait</function></funcdef> |
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3702 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3703 <paramdef>cyg_flag_value_t <parameter>pattern</parameter></paramdef> |
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3704 <paramdef>cyg_flag_mode_t <parameter>mode</parameter></paramdef> |
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3705 <paramdef>cyg_tick_count_t <parameter>abstime</parameter></paramdef> |
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3706 </funcprototype> |
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3707 <funcprototype> |
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3708 <funcdef>cyg_flag_value_t <function>cyg_flag_poll</function></funcdef> |
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3709 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3710 <paramdef>cyg_flag_value_t <parameter>pattern</parameter></paramdef> |
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3711 <paramdef>cyg_flag_mode_t <parameter>mode</parameter></paramdef> |
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3712 </funcprototype> |
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3713 <funcprototype> |
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3714 <funcdef>cyg_flag_value_t <function>cyg_flag_peek</function></funcdef> |
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3715 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3716 </funcprototype> |
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3717 <funcprototype> |
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3718 <funcdef>cyg_bool_t <function>cyg_flag_waiting</function></funcdef> |
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3719 <paramdef>cyg_flag_t* <parameter>flag</parameter></paramdef> |
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3720 </funcprototype> |
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3721 </funcsynopsis> |
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3722 </refsynopsisdiv> |
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3723 |
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3724 <refsect1 id="kernel-flags-description"><title>Description</title> |
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3725 <para> |
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3726 Event flags allow a consumer thread to wait for one of several |
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3727 different types of event to occur. Alternatively it is possible to |
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3728 wait for some combination of events. The implementation is relatively |
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3729 straightforward. Each event flag contains a 32-bit integer. |
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3730 Application code associates these bits with specific events, so for |
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3731 example bit 0 could indicate that an I/O operation has completed and |
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3732 data is available, while bit 1 could indicate that the user has |
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3733 pressed a start button. A producer thread or a DSR can cause one or |
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3734 more of the bits to be set, and a consumer thread currently waiting |
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3735 for these bits will be woken up. |
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3736 </para> |
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3737 <para> |
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3738 Unlike semaphores no attempt is made to keep track of event counts. It |
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3739 does not matter whether a given event occurs once or multiple times |
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3740 before being consumed, the corresponding bit in the event flag will |
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3741 change only once. However semaphores cannot easily be used to handle |
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3742 multiple event sources. Event flags can often be used as an |
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3743 alternative to condition variables, although they cannot be used for |
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3744 completely arbitrary conditions and they only support the equivalent |
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3745 of condition variable broadcasts, not signals. |
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3746 </para> |
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3747 <para> |
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3748 Before an event flag can be used it must be initialized by a call to |
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3749 <function>cyg_flag_init</function>. This takes a pointer to a |
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3750 <structname>cyg_flag_t</structname> data structure, which can be part of a |
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3751 larger structure. All 32 bits in the event flag will be set to 0, |
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3752 indicating that no events have yet occurred. If an event flag is no |
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3753 longer required it can be cleaned up with a call to |
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3754 <function>cyg_flag_destroy</function>, allowing the memory for the |
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3755 <structfield>cyg_flag_t</structfield> structure to be re-used. |
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3756 </para> |
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3757 <para> |
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3758 A consumer thread can wait for one or more events by calling |
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3759 <function>cyg_flag_wait</function>. This takes three arguments. The |
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3760 first identifies a particular event flag. The second is some |
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3761 combination of bits, indicating which events are of interest. The |
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3762 final argument should be one of the following: |
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3763 </para> |
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3764 <variablelist> |
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3765 <varlistentry> |
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3766 <term><literal>CYG_FLAG_WAITMODE_AND</literal></term> |
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3767 <listitem><para> |
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3768 The call to <function>cyg_flag_wait</function> will block until all |
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3769 the specified event bits are set. The event flag is not cleared when |
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3770 the wait succeeds, in other words all the bits remain set. |
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3771 </para></listitem> |
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3772 </varlistentry> |
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3773 <varlistentry> |
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3774 <term><literal>CYG_FLAG_WAITMODE_OR</literal></term> |
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3775 <listitem><para> |
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3776 The call will block until at least one of the specified event bits is |
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3777 set. The event flag is not cleared on return. |
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3778 </para></listitem> |
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3779 </varlistentry> |
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3780 <varlistentry> |
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3781 <term><literal>CYG_FLAG_WAITMODE_AND | CYG_FLAG_WAITMODE_CLR</literal></term> |
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3782 <listitem><para> |
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3783 The call will block until all the specified event bits are set, and |
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3784 the entire event flag is cleared when the call succeeds. Note that |
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3785 if this mode of operation is used then a single event flag cannot be |
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3786 used to store disjoint sets of events, even though enough bits might |
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3787 be available. Instead each disjoint set of events requires its own |
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3788 event flag. |
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3789 </para></listitem> |
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3790 </varlistentry> |
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3791 <varlistentry> |
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3792 <term><literal>CYG_FLAG_WAITMODE_OR | CYG_FLAG_WAITMODE_CLR</literal></term> |
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3793 <listitem><para> |
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3794 The call will block until at least one of the specified event bits is |
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3795 set, and the entire flag is cleared when the call succeeds. |
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3796 </para></listitem> |
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3797 </varlistentry> |
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3798 </variablelist> |
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3799 <para> |
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3800 A call to <function>cyg_flag_wait</function> normally blocks until the |
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3801 required condition is satisfied. It will return the value of the event |
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3802 flag at the point that the operation succeeded, which may be a |
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3803 superset of the requested events. If |
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3804 <function>cyg_thread_release</function> is used to unblock a thread |
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3805 that is currently in a wait operation, the |
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3806 <function>cyg_flag_wait</function> call will instead return 0. |
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3807 </para> |
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3808 <para> |
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3809 <function>cyg_flag_timed_wait</function> is a variant of |
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3810 <function>cyg_flag_wait</function> which adds a timeout: the wait |
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3811 operation must succeed within the specified number of ticks, or it |
|
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3812 will fail with a return value of 0. The number of ticks is specified |
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3813 as an absolute, not relative tick count, and so in order to wait for a |
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3814 relative number of ticks, the return value of the |
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3815 <function>cyg_current_time()</function> function should be added to |
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3816 determine the absolute number of ticks. |
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3817 <function>cyg_flag_poll</function> is a non-blocking variant: if the |
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3818 wait operation can succeed immediately it acts like |
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3819 <function>cyg_flag_wait</function>, otherwise it returns immediately |
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3820 with a value of 0. |
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3821 </para> |
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3822 <para> |
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3823 <function>cyg_flag_setbits</function> is called by a producer thread |
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3824 or from inside a DSR when an event occurs. The specified bits are or'd |
|
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3825 into the current event flag value. This may cause one or more waiting |
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3826 threads to be woken up, if their conditions are now satisfied. How many |
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3827 threads are awoken depends on the use of <literal>CYG_FLAG_WAITMODE_CLR |
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3828 </literal>. The queue of threads waiting on the flag is walked to find |
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3829 threads which now have their wake condition fulfilled. If the awoken thread |
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3830 has passed <literal>CYG_FLAG_WAITMODE_CLR</literal> the walking of the queue |
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3831 is terminated, otherwise the walk continues. Thus if no threads have passed |
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3832 <literal>CYG_FLAG_WAITMORE_CLR</literal> all threads with fulfilled |
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3833 conditions will be awoken. If <literal>CYG_FLAG_WAITMODE_CLR</literal> is |
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3834 passed by threads with fulfilled conditions, the number of awoken threads |
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3835 will depend on the order the threads are in the queue. |
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3836 </para> |
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3837 <para> |
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3838 <function>cyg_flag_maskbits</function> can be used to clear one or |
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3839 more bits in the event flag. This can be called from a producer when a |
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3840 particular condition is no longer satisfied, for example when the user |
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3841 is no longer pressing a particular button. It can also be used by a |
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3842 consumer thread if <literal>CYG_FLAG_WAITMODE_CLR</literal> was not |
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3843 used as part of the wait operation, to indicate that some but not all |
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3844 of the active events have been consumed. If there are multiple |
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3845 consumer threads performing wait operations without using |
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3846 <function>CYG_FLAG_WAITMODE_CLR</function> then typically some |
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3847 additional synchronization such as a mutex is needed to prevent |
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3848 multiple threads consuming the same event. |
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3849 </para> |
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3850 <para> |
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3851 Two additional functions are provided to query the current state of an |
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3852 event flag. <function>cyg_flag_peek</function> returns the current |
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3853 value of the event flag, and <function>cyg_flag_waiting</function> can |
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3854 be used to find out whether or not there are any threads currently |
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3855 blocked on the event flag. Both of these functions must be used with |
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3856 care because other threads may be operating on the event flag. |
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3857 </para> |
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3858 </refsect1> |
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3859 |
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3860 <refsect1 id="kernel-flags-context"><title>Valid contexts</title> |
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3861 <para> |
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3862 <function>cyg_flag_init</function> is typically called during system |
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3863 initialization but may also be called in thread context. The same |
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3864 applies to <function>cyg_flag_destroy</function>. |
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3865 <function>cyg_flag_wait</function> and |
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3866 <function>cyg_flag_timed_wait</function> may only be called from |
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3867 thread context. The remaining functions may be called from thread or |
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3868 DSR context. |
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3869 </para> |
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3870 </refsect1> |
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3871 |
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3872 </refentry> |
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3873 |
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3874 <!-- }}} --> |
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3875 <!-- {{{ Spinlocks --> |
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3876 |
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3877 <refentry id="kernel-spinlocks"> |
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3878 |
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3879 <refmeta> |
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3880 <refentrytitle>Spinlocks</refentrytitle> |
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3881 </refmeta> |
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3882 |
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3883 <refnamediv> |
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3884 <refname>cyg_spinlock_create</refname> |
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3885 <refname>cyg_spinlock_destroy</refname> |
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3886 <refname>cyg_spinlock_spin</refname> |
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3887 <refname>cyg_spinlock_clear</refname> |
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3888 <refname>cyg_spinlock_test</refname> |
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3889 <refname>cyg_spinlock_spin_intsave</refname> |
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3890 <refname>cyg_spinlock_clear_intsave</refname> |
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3891 <refpurpose>Low-level Synchronization Primitive</refpurpose> |
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3892 </refnamediv> |
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3893 |
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3894 <refsynopsisdiv> |
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3895 <funcsynopsis> |
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3896 <funcsynopsisinfo> |
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3897 #include <cyg/kernel/kapi.h> |
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3898 </funcsynopsisinfo> |
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3899 <funcprototype> |
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3900 <funcdef>void <function>cyg_spinlock_init</function></funcdef> |
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3901 <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef> |
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3902 <paramdef>cyg_bool_t <parameter>locked</parameter></paramdef> |
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3903 </funcprototype> |
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3904 <funcprototype> |
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3905 <funcdef>void <function>cyg_spinlock_destroy</function></funcdef> |
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3906 <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef> |
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3907 </funcprototype> |
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3908 <funcprototype> |
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3909 <funcdef>void <function>cyg_spinlock_spin</function></funcdef> |
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3910 <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef> |
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3911 </funcprototype> |
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3912 <funcprototype> |
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3913 <funcdef>void <function>cyg_spinlock_clear</function></funcdef> |
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3914 <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef> |
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3915 </funcprototype> |
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3916 <funcprototype> |
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3917 <funcdef>cyg_bool_t <function>cyg_spinlock_try</function></funcdef> |
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3918 <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef> |
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3919 </funcprototype> |
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3920 <funcprototype> |
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3921 <funcdef>cyg_bool_t <function>cyg_spinlock_test</function></funcdef> |
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3922 <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef> |
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3923 </funcprototype> |
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3924 <funcprototype> |
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3925 <funcdef>void <function>cyg_spinlock_spin_intsave</function></funcdef> |
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3926 <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef> |
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3927 <paramdef>cyg_addrword_t* <parameter>istate</parameter></paramdef> |
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3928 </funcprototype> |
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3929 <funcprototype> |
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3930 <funcdef>void <function>cyg_spinlock_clear_intsave</function></funcdef> |
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3931 <paramdef>cyg_spinlock_t* <parameter>lock</parameter></paramdef> |
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3932 <paramdef>cyg_addrword_t <parameter>istate</parameter></paramdef> |
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3933 </funcprototype> |
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3934 </funcsynopsis> |
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3935 </refsynopsisdiv> |
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3936 |
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3937 <refsect1 id="kernel-spinlocks-description"><title>Description</title> |
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3938 <para> |
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3939 Spinlocks provide an additional synchronization primitive for |
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3940 applications running on SMP systems. They operate at a lower level |
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3941 than the other primitives such as mutexes, and for most purposes the |
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3942 higher-level primitives should be preferred. However there are some |
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3943 circumstances where a spinlock is appropriate, especially when |
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3944 interrupt handlers and threads need to share access to hardware, and |
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3945 on SMP systems the kernel implementation itself depends on spinlocks. |
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3946 </para> |
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3947 <para> |
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3948 Essentially a spinlock is just a simple flag. When code tries to claim |
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3949 a spinlock it checks whether or not the flag is already set. If not |
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3950 then the flag is set and the operation succeeds immediately. The exact |
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3951 implementation of this is hardware-specific, for example it may use a |
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3952 test-and-set instruction to guarantee the desired behaviour even if |
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3953 several processors try to access the spinlock at the exact same time. |
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3954 If it is not possible to claim a spinlock then the current thead spins |
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3955 in a tight loop, repeatedly checking the flag until it is clear. This |
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3956 behaviour is very different from other synchronization primitives such |
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3957 as mutexes, where contention would cause a thread to be suspended. The |
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3958 assumption is that a spinlock will only be held for a very short time. |
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3959 If claiming a spinlock could cause the current thread to be suspended |
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3960 then spinlocks could not be used inside interrupt handlers, which is |
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3961 not acceptable. |
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3962 </para> |
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3963 <para> |
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3964 This does impose a constraint on any code which uses spinlocks. |
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3965 Specifically it is important that spinlocks are held only for a short |
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3966 period of time, typically just some dozens of instructions. Otherwise |
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3967 another processor could be blocked on the spinlock for a long time, |
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3968 unable to do any useful work. It is also important that a thread which |
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3969 owns a spinlock does not get preempted because that might cause |
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3970 another processor to spin for a whole timeslice period, or longer. One |
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3971 way of achieving this is to disable interrupts on the current |
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3972 processor, and the function |
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3973 <function>cyg_spinlock_spin_intsave</function> is provided to |
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3974 facilitate this. |
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3975 </para> |
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3976 <para> |
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3977 Spinlocks should not be used on single-processor systems. Consider a |
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3978 high priority thread which attempts to claim a spinlock already held |
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3979 by a lower priority thread: it will just loop forever and the lower |
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3980 priority thread will never get another chance to run and release the |
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3981 spinlock. Even if the two threads were running at the same priority, |
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3982 the one attempting to claim the spinlock would spin until it was |
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3983 timesliced and a lot of cpu time would be wasted. If an interrupt |
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3984 handler tried to claim a spinlock owned by a thread, the interrupt |
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3985 handler would loop forever. Therefore spinlocks are only appropriate |
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3986 for SMP systems where the current owner of a spinlock can continue |
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3987 running on a different processor. |
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3988 </para> |
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3989 <para> |
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3990 Before a spinlock can be used it must be initialized by a call to |
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3991 <function>cyg_spinlock_init</function>. This takes two arguments, a |
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3992 pointer to a <function>cyg_spinlock_t</function> data structure, and |
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3993 a flag to specify whether the spinlock starts off locked or unlocked. |
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3994 If a spinlock is no longer required then it can be destroyed by a call |
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3995 to <function>cyg_spinlock_destroy</function>. |
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3996 </para> |
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3997 <para> |
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3998 There are two routines for claiming a spinlock: |
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3999 <function>cyg_spinlock_spin</function> and |
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4000 <function>cyg_spinlock_spin_intsave</function>. The former can be used |
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4001 when it is known the current code will not be preempted, for example |
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4002 because it is running in an interrupt handler or because interrupts |
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4003 are disabled. The latter will disable interrupts in addition to |
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4004 claiming the spinlock, so is safe to use in all circumstances. The |
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4005 previous interrupt state is returned via the second argument, and |
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4006 should be used in a subsequent call to |
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4007 <function>cyg_spinlock_clear_intsave</function>. |
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4008 </para> |
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4009 <para> |
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4010 Similarly there are two routines for releasing a spinlock: |
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4011 <function>cyg_spinlock_clear</function> and |
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4012 <function>cyg_spinlock_clear_intsave</function>. Typically |
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4013 the former will be used if the spinlock was claimed by a call to |
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4014 <function>cyg_spinlock_spin</function>, and the latter when |
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4015 <function>cyg_spinlock_intsave</function> was used. |
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4016 </para> |
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4017 <para> |
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4018 There are two additional routines. |
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4019 <function>cyg_spinlock_try</function> is a non-blocking version of |
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4020 <function>cyg_spinlock_spin</function>: if possible the lock will be |
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4021 claimed and the function will return <literal>true</literal>; otherwise the function |
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4022 will return immediately with failure. |
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4023 <function>cyg_spinlock_test</function> can be used to find out whether |
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4024 or not the spinlock is currently locked. This function must be used |
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4025 with care because, especially on a multiprocessor system, the state of |
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4026 the spinlock can change at any time. |
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4027 </para> |
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4028 <para> |
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4029 Spinlocks should only be held for a short period of time, and |
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4030 attempting to claim a spinlock will never cause a thread to be |
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4031 suspended. This means that there is no need to worry about priority |
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4032 inversion problems, and concepts such as priority ceilings and |
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4033 inheritance do not apply. |
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4034 </para> |
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4035 </refsect1> |
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4036 |
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4037 <refsect1 id="kernel-spinlocks-context"><title>Valid contexts</title> |
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4038 <para> |
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4039 All of the spinlock functions can be called from any context, |
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4040 including ISR and DSR context. Typically |
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4041 <function>cyg_spinlock_init</function> is only called during system |
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4042 initialization. |
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4043 </para> |
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4044 </refsect1> |
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4045 |
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4046 </refentry> |
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4047 |
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4048 <!-- }}} --> |
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4049 <!-- {{{ Scheduler control --> |
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4050 |
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4051 <refentry id="kernel-schedcontrol"> |
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4052 |
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4053 <refmeta> |
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4054 <refentrytitle>Scheduler Control</refentrytitle> |
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4055 </refmeta> |
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4056 |
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4057 <refnamediv> |
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4058 <refname>cyg_scheduler_start</refname> |
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4059 <refname>cyg_scheduler_lock</refname> |
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4060 <refname>cyg_scheduler_unlock</refname> |
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4061 <refname>cyg_scheduler_safe_lock</refname> |
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4062 <refname>cyg_scheduler_read_lock</refname> |
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4063 <refpurpose>Control the state of the scheduler</refpurpose> |
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4064 </refnamediv> |
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4065 |
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4066 <refsynopsisdiv> |
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4067 <funcsynopsis> |
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4068 <funcsynopsisinfo> |
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4069 #include <cyg/kernel/kapi.h> |
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4070 </funcsynopsisinfo> |
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4071 <funcprototype> |
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4072 <funcdef>void <function>cyg_scheduler_start</function></funcdef> |
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4073 <void> |
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4074 </funcprototype> |
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4075 <funcprototype> |
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4076 <funcdef>void <function>cyg_scheduler_lock</function></funcdef> |
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4077 <void> |
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4078 </funcprototype> |
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4079 <funcprototype> |
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4080 <funcdef>void <function>cyg_scheduler_unlock</function></funcdef> |
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4081 <void> |
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4082 </funcprototype> |
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4083 <funcprototype> |
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4084 <funcdef>cyg_ucount32 <function>cyg_scheduler_read_lock</function></funcdef> |
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4085 <void> |
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4086 </funcprototype> |
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4087 </funcsynopsis> |
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4088 </refsynopsisdiv> |
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4089 |
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4090 <refsect1 id="kernel-schedcontrol-description"><title>Description</title> |
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4091 <para> |
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4092 <function>cyg_scheduler_start</function> should only be called once, |
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4093 to mark the end of system initialization. In typical configurations it |
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4094 is called automatically by the system startup, but some applications |
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4095 may bypass the standard startup in which case |
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4096 <function>cyg_scheduler_start</function> will have to be called |
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4097 explicitly. The call will enable system interrupts, allowing I/O |
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4098 operations to commence. Then the scheduler will be invoked and control |
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4099 will be transferred to the highest priority runnable thread. The call |
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4100 will never return. |
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4101 </para> |
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4102 <para> |
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4103 The various data structures inside the eCos kernel must be protected |
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4104 against concurrent updates. Consider a call to |
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4105 <function>cyg_semaphore_post</function> which causes a thread to be |
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4106 woken up: the semaphore data structure must be updated to remove the |
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4107 thread from its queue; the scheduler data structure must also be |
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4108 updated to mark the thread as runnable; it is possible that the newly |
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4109 runnable thread has a higher priority than the current one, in which |
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4110 case preemption is required. If in the middle of the semaphore post |
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4111 call an interrupt occurred and the interrupt handler tried to |
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4112 manipulate the same data structures, for example by making another |
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4113 thread runnable, then it is likely that the structures will be left in |
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4114 an inconsistent state and the system will fail. |
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4115 </para> |
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4116 <para> |
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4117 To prevent such problems the kernel contains a special lock known as |
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4118 the scheduler lock. A typical kernel function such as |
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4119 <function>cyg_semaphore_post</function> will claim the scheduler lock, |
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4120 do all its manipulation of kernel data structures, and then release |
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4121 the scheduler lock. The current thread cannot be preempted while it |
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4122 holds the scheduler lock. If an interrupt occurs and a DSR is supposed |
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4123 to run to signal that some event has occurred, that DSR is postponed |
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4124 until the scheduler unlock operation. This prevents concurrent updates |
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4125 of kernel data structures. |
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4126 </para> |
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4127 <para> |
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4128 The kernel exports three routines for manipulating the scheduler lock. |
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4129 <function>cyg_scheduler_lock</function> can be called to claim the |
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4130 lock. On return it is guaranteed that the current thread will not be |
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4131 preempted, and that no other code is manipulating any kernel data |
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4132 structures. <function>cyg_scheduler_unlock</function> can be used to |
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4133 release the lock, which may cause the current thread to be preempted. |
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4134 <function>cyg_scheduler_read_lock</function> can be used to query the |
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4135 current state of the scheduler lock. This function should never be |
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4136 needed because well-written code should always know whether or not the |
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4137 scheduler is currently locked, but may prove useful during debugging. |
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4138 </para> |
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4139 <para> |
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4140 The implementation of the scheduler lock involves a simple counter. |
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4141 Code can call <function>cyg_scheduler_lock</function> multiple times, |
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4142 causing the counter to be incremented each time, as long as |
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4143 <function>cyg_scheduler_unlock</function> is called the same number of |
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4144 times. This behaviour is different from mutexes where an attempt by a |
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4145 thread to lock a mutex multiple times will result in deadlock or an |
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4146 assertion failure. |
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4147 </para> |
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4148 <para> |
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4149 Typical application code should not use the scheduler lock. Instead |
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4150 other synchronization primitives such as mutexes and semaphores should |
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4151 be used. While the scheduler is locked the current thread cannot be |
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4152 preempted, so any higher priority threads will not be able to run. |
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4153 Also no DSRs can run, so device drivers may not be able to service |
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4154 I/O requests. However there is one situation where locking the |
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4155 scheduler is appropriate: if some data structure needs to be shared |
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4156 between an application thread and a DSR associated with some interrupt |
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4157 source, the thread can use the scheduler lock to prevent concurrent |
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4158 invocations of the DSR and then safely manipulate the structure. It is |
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4159 desirable that the scheduler lock is held for only a short period of |
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4160 time, typically some tens of instructions. In exceptional cases there |
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4161 may also be some performance-critical code where it is more |
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4162 appropriate to use the scheduler lock rather than a mutex, because the |
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4163 former is more efficient. |
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4164 </para> |
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4165 </refsect1> |
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4166 |
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4167 <refsect1 id="kernel-schedcontrol-context"><title>Valid contexts</title> |
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4168 <para> |
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4169 <function>cyg_scheduler_start</function> can only be called during |
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4170 system initialization, since it marks the end of that phase. The |
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4171 remaining functions may be called from thread or DSR context. Locking |
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4172 the scheduler from inside the DSR has no practical effect because the |
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4173 lock is claimed automatically by the interrupt subsystem before |
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4174 running DSRs, but allows functions to be shared between normal thread |
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4175 code and DSRs. |
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4176 </para> |
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4177 </refsect1> |
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4178 |
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4179 </refentry> |
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4180 |
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4181 <!-- }}} --> |
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4182 <!-- {{{ Interrupt handling --> |
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4183 |
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4184 <refentry id="kernel-interrupts"> |
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4185 |
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4186 <refmeta> |
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4187 <refentrytitle>Interrupt Handling</refentrytitle> |
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4188 </refmeta> |
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4189 |
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4190 <refnamediv> |
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4191 <refname>cyg_interrupt_create</refname> |
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4192 <refname>cyg_interrupt_delete</refname> |
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4193 <refname>cyg_interrupt_attach</refname> |
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4194 <refname>cyg_interrupt_detach</refname> |
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4195 <refname>cyg_interrupt_configure</refname> |
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4196 <refname>cyg_interrupt_acknowledge</refname> |
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4197 <refname>cyg_interrupt_enable</refname> |
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4198 <refname>cyg_interrupt_disable</refname> |
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4199 <refname>cyg_interrupt_mask</refname> |
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4200 <refname>cyg_interrupt_mask_intunsafe</refname> |
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4201 <refname>cyg_interrupt_unmask</refname> |
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4202 <refname>cyg_interrupt_unmask_intunsafe</refname> |
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4203 <refname>cyg_interrupt_set_cpu</refname> |
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4204 <refname>cyg_interrupt_get_cpu</refname> |
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4205 <refname>cyg_interrupt_get_vsr</refname> |
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4206 <refname>cyg_interrupt_set_vsr</refname> |
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4207 <refpurpose>Manage interrupt handlers</refpurpose> |
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4208 </refnamediv> |
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4209 |
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4210 <refsynopsisdiv> |
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4211 <funcsynopsis> |
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4212 <funcsynopsisinfo> |
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4213 #include <cyg/kernel/kapi.h> |
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4214 </funcsynopsisinfo> |
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4215 <funcprototype> |
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4216 <funcdef>void <function>cyg_interrupt_create</function></funcdef> |
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4217 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4218 <paramdef>cyg_priority_t <parameter>priority</parameter></paramdef> |
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4219 <paramdef>cyg_addrword_t <parameter>data</parameter></paramdef> |
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4220 <paramdef>cyg_ISR_t* <parameter>isr</parameter></paramdef> |
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4221 <paramdef>cyg_DSR_t* <parameter>dsr</parameter></paramdef> |
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4222 <paramdef>cyg_handle_t* <parameter>handle</parameter></paramdef> |
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4223 <paramdef>cyg_interrupt* <parameter>intr</parameter></paramdef> |
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4224 </funcprototype> |
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4225 <funcprototype> |
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4226 <funcdef>void <function>cyg_interrupt_delete</function></funcdef> |
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4227 <paramdef>cyg_handle_t <parameter>interrupt</parameter></paramdef> |
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4228 </funcprototype> |
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4229 <funcprototype> |
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4230 <funcdef>void <function>cyg_interrupt_attach</function></funcdef> |
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4231 <paramdef>cyg_handle_t <parameter>interrupt</parameter></paramdef> |
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4232 </funcprototype> |
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4233 <funcprototype> |
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4234 <funcdef>void <function>cyg_interrupt_detach</function></funcdef> |
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4235 <paramdef>cyg_handle_t <parameter>interrupt</parameter></paramdef> |
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4236 </funcprototype> |
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4237 <funcprototype> |
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4238 <funcdef>void <function>cyg_interrupt_configure</function></funcdef> |
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4239 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4240 <paramdef>cyg_bool_t <parameter>level</parameter></paramdef> |
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4241 <paramdef>cyg_bool_t <parameter>up</parameter></paramdef> |
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4242 </funcprototype> |
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4243 <funcprototype> |
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4244 <funcdef>void <function>cyg_interrupt_acknowledge</function></funcdef> |
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4245 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4246 </funcprototype> |
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4247 <funcprototype> |
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4248 <funcdef>void <function>cyg_interrupt_disable</function></funcdef> |
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4249 <void> |
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4250 </funcprototype> |
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4251 <funcprototype> |
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4252 <funcdef>void <function>cyg_interrupt_enable</function></funcdef> |
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4253 <void> |
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4254 </funcprototype> |
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4255 <funcprototype> |
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4256 <funcdef>void <function>cyg_interrupt_mask</function></funcdef> |
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4257 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4258 </funcprototype> |
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4259 <funcprototype> |
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4260 <funcdef>void <function>cyg_interrupt_mask_intunsafe</function></funcdef> |
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4261 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4262 </funcprototype> |
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4263 <funcprototype> |
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4264 <funcdef>void <function>cyg_interrupt_unmask</function></funcdef> |
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4265 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4266 </funcprototype> |
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4267 <funcprototype> |
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4268 <funcdef>void <function>cyg_interrupt_unmask_intunsafe</function></funcdef> |
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4269 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4270 </funcprototype> |
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4271 <funcprototype> |
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4272 <funcdef>void <function>cyg_interrupt_set_cpu</function></funcdef> |
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4273 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4274 <paramdef>cyg_cpu_t <parameter>cpu</parameter></paramdef> |
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4275 </funcprototype> |
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4276 <funcprototype> |
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4277 <funcdef>cyg_cpu_t <function>cyg_interrupt_get_cpu</function></funcdef> |
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4278 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4279 </funcprototype> |
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4280 <funcprototype> |
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4281 <funcdef>void <function>cyg_interrupt_get_vsr</function></funcdef> |
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4282 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4283 <paramdef>cyg_VSR_t** <parameter>vsr</parameter></paramdef> |
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4284 </funcprototype> |
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4285 <funcprototype> |
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4286 <funcdef>void <function>cyg_interrupt_set_vsr</function></funcdef> |
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4287 <paramdef>cyg_vector_t <parameter>vector</parameter></paramdef> |
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4288 <paramdef>cyg_VSR_t* <parameter>vsr</parameter></paramdef> |
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4289 </funcprototype> |
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4290 </funcsynopsis> |
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4291 </refsynopsisdiv> |
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4292 |
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4293 <refsect1 id="kernel-interrupts-description"><title>Description</title> |
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4294 <para> |
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4295 The kernel provides an interface for installing interrupt handlers and |
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4296 controlling when interrupts occur. This functionality is used |
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4297 primarily by eCos device drivers and by any application code that |
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4298 interacts directly with hardware. However in most cases it is better |
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4299 to avoid using this kernel functionality directly, and instead the |
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4300 device driver API provided by the common HAL package should be used. |
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4301 Use of the kernel package is optional, and some applications such as |
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4302 RedBoot work with no need for multiple threads or synchronization |
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4303 primitives. Any code which calls the kernel directly rather than the |
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4304 device driver API will not function in such a configuration. When the |
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4305 kernel package is present the device driver API is implemented as |
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4306 <literal>#define</literal>'s to the equivalent kernel calls, otherwise |
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4307 it is implemented inside the common HAL package. The latter |
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4308 implementation can be simpler than the kernel one because there is no |
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4309 need to consider thread preemption and similar issues. |
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4310 </para> |
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4311 <para> |
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4312 The exact details of interrupt handling vary widely between |
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4313 architectures. The functionality provided by the kernel abstracts away |
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4314 from many of the details of the underlying hardware, thus simplifying |
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4315 application development. However this is not always successful. For |
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4316 example, if some hardware does not provide any support at all for |
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4317 masking specific interrupts then calling |
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4318 <function>cyg_interrupt_mask</function> may not behave as intended: |
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4319 instead of masking just the one interrupt source it might disable all |
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4320 interrupts, because that is as close to the desired behaviour as is |
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4321 possible given the hardware restrictions. Another possibility is that |
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4322 masking a given interrupt source also affects all lower-priority |
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4323 interrupts, but still allows higher-priority ones. The documentation |
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4324 for the appropriate HAL packages should be consulted for more |
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4325 information about exactly how interrupts are handled on any given |
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4326 hardware. The HAL header files will also contain useful information. |
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4327 </para> |
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4328 </refsect1> |
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4329 |
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4330 <refsect1 id="kernel-interrupts-handlers"><title>Interrupt Handlers</title> |
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4331 <para> |
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4332 Interrupt handlers are created by a call to |
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4333 <function>cyg_interrupt_create</function>. This takes the following |
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4334 arguments: |
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4335 </para> |
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4336 <variablelist> |
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4337 <varlistentry> |
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4338 <term>cyg_vector_t <parameter>vector</parameter></term> |
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4339 <listitem><para> |
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4340 The interrupt vector, a small integer, identifies the specific |
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4341 interrupt source. The appropriate hardware documentation or HAL header |
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4342 files should be consulted for details of which vector corresponds to |
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4343 which device. |
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4344 </para></listitem> |
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4345 </varlistentry> |
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4346 <varlistentry> |
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4347 <term>cyg_priority_t <parameter>priority</parameter></term> |
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4348 <listitem><para> |
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4349 Some hardware may support interrupt priorities, where a low priority |
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4350 interrupt handler can in turn be interrupted by a higher priority one. |
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4351 Again hardware-specific documentation should be consulted for details |
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4352 about what the valid interrupt priority levels are. |
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4353 </para></listitem> |
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4354 </varlistentry> |
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4355 <varlistentry> |
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4356 <term>cyg_addrword_t <parameter>data</parameter></term> |
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4357 <listitem><para> |
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4358 When an interrupt occurs eCos will first call the associated |
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4359 interrupt service routine or ISR, then optionally a deferred service |
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4360 routine or DSR. The <parameter>data</parameter> argument to |
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4361 <function>cyg_interrupt_create</function> will be passed to both these |
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4362 functions. Typically it will be a pointer to some data structure. |
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4363 </para></listitem> |
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4364 </varlistentry> |
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4365 <varlistentry> |
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4366 <term>cyg_ISR_t <parameter>isr</parameter></term> |
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4367 <listitem><para> |
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4368 When an interrupt occurs the hardware will transfer control to the |
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4369 appropriate vector service routine or VSR, which is usually provided |
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4370 by eCos. This performs any appropriate processing, for example to work |
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4371 out exactly which interrupt occurred, and then as quickly as possible |
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4372 transfers control the installed ISR. An ISR is a C function which |
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4373 takes the following form: |
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4374 </para> |
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4375 <programlisting width=72> |
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4376 cyg_uint32 |
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4377 isr_function(cyg_vector_t vector, cyg_addrword_t data) |
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4378 { |
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4379 cyg_bool_t dsr_required = 0; |
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4380 |
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4381 … |
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4382 |
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4383 return dsr_required ? |
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4384 (CYG_ISR_CALL_DSR : CYG_ISR_HANDLED) : |
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4385 CYG_ISR_HANDLED; |
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4386 } |
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4387 </programlisting> |
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4388 <para> |
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4389 The first argument identifies the particular interrupt source, |
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4390 especially useful if there multiple instances of a given device and a |
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4391 single ISR can be used for several different interrupt vectors. The |
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4392 second argument is the <parameter>data</parameter> field passed to |
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4393 <function>cyg_interrupt_create</function>, usually a pointer to some |
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4394 data structure. The exact conditions under which an ISR runs will |
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4395 depend partly on the hardware and partly on configuration options. |
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4396 Interrupts may currently be disabled globally, especially if the |
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4397 hardware does not support interrupt priorities. Alternatively |
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4398 interrupts may be enabled such that higher priority interrupts are |
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4399 allowed through. The ISR may be running on a separate interrupt stack, |
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4400 or on the stack of whichever thread was running at the time the |
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4401 interrupt happened. |
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4402 </para> |
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4403 <para> |
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4404 A typical ISR will do as little work as possible, just enough to meet |
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4405 the needs of the hardware and then acknowledge the interrupt by |
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4406 calling <function>cyg_interrupt_acknowledge</function>. This ensures |
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4407 that interrupts will be quickly reenabled, so higher priority devices |
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4408 can be serviced. For some applications there may be one device which |
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4409 is especially important and whose ISR can take much longer than |
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4410 normal. However eCos device drivers usually will not assume that they |
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4411 are especially important, so their ISRs will be as short as possible. |
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4412 </para> |
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4413 <para> |
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4414 The return value of an ISR is normally a bit mask containing zero, one |
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4415 or both of the following bits: <literal>CYG_ISR_CALL_DSR</literal> or |
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4416 <literal>CYG_ISR_HANDLED</literal>. The former indicates that further |
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4417 processing is required at DSR level, and the interrupt handler's DSR |
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4418 will be run as soon as possible. The latter indicates that the |
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4419 interrupt was handled by this ISR so there is no need to call other |
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4420 interrupt handlers which might be chained on this interrupt vector. If |
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4421 this ISR did not handle the interrupt it should not set the |
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4422 CYG_ISR_HANDLED bit so that other chained interrupt handlers may |
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4423 handle the interrupt. |
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4424 </para> |
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4425 <para> |
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4426 An ISR is allowed to make very few kernel calls. It can manipulate the |
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4427 interrupt mask, and on SMP systems it can use spinlocks. However an |
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4428 ISR must not make higher-level kernel calls such as posting to a |
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4429 semaphore, instead any such calls must be made from the DSR. This |
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4430 avoids having to disable interrupts throughout the kernel and thus |
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4431 improves interrupt latency. |
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4432 </para></listitem> |
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4433 </varlistentry> |
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4434 <varlistentry> |
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4435 <term>cyg_DSR_t <parameter>dsr</parameter></term> |
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4436 <listitem><para> |
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4437 If an interrupt has occurred and the ISR has returned a value |
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4438 <literal>CYG_ISR_CALL_DSR</literal> bit being set, the system |
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4439 will call the deferred service routine or DSR associated with this interrupt |
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4440 handler. If the scheduler is not currently locked then the DSR will |
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4441 run immediately. However if the interrupted thread was in the middle |
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4442 of a kernel call and had locked the scheduler, then the DSR will be |
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4443 deferred until the scheduler is again unlocked. This allows the |
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4444 DSR to make certain kernel calls safely, for example posting to a |
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4445 semaphore or signalling a condition variable. A DSR is a C function |
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4446 which takes the following form: |
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4447 </para> |
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4448 <programlisting width=72> |
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4449 void |
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4450 dsr_function(cyg_vector_t vector, |
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4451 cyg_ucount32 count, |
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4452 cyg_addrword_t data) |
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4453 { |
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4454 } |
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4455 </programlisting> |
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4456 <para> |
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4457 The first argument identifies the specific interrupt that has caused |
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4458 the DSR to run. The second argument indicates the number of these |
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4459 interrupts that have occurred and for which the ISR requested a DSR. |
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4460 Usually this will be <literal>1</literal>, unless the system is |
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4461 suffering from a very heavy load. The third argument is the |
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4462 <parameter>data</parameter> field passed to |
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4463 <function>cyg_interrupt_create</function>. |
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4464 </para></listitem> |
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4465 </varlistentry> |
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4466 <varlistentry> |
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4467 <term>cyg_handle_t* <parameter>handle</parameter></term> |
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4468 <listitem><para> |
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4469 The kernel will return a handle to the newly created interrupt handler |
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4470 via this argument. Subsequent operations on the interrupt handler such |
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4471 as attaching it to the interrupt source will use this handle. |
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4472 </para></listitem> |
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4473 </varlistentry> |
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4474 <varlistentry> |
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4475 <term>cyg_interrupt* <parameter>intr</parameter></term> |
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4476 <listitem><para> |
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4477 This provides the kernel with an area of memory for holding this |
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4478 interrupt handler and associated data. |
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4479 </para></listitem> |
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4480 </varlistentry> |
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4481 </variablelist> |
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4482 <para> |
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4483 The call to <function>cyg_interrupt_create</function> simply fills in |
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4484 a kernel data structure. A typical next step is to call |
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4485 <function>cyg_interrupt_attach</function> using the handle returned by |
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4486 the create operation. This makes it possible to have several different |
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4487 interrupt handlers for a given vector, attaching whichever one is |
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4488 currently appropriate. Replacing an interrupt handler requires a call |
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4489 to <function>cyg_interrupt_detach</function>, followed by another call |
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4490 to <function>cyg_interrupt_attach</function> for the replacement |
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4491 handler. <function>cyg_interrupt_delete</function> can be used if an |
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4492 interrupt handler is no longer required. |
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4493 </para> |
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4494 <para> |
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4495 Some hardware may allow for further control over specific interrupts, |
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4496 for example whether an interrupt is level or edge triggered. Any such |
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4497 hardware functionality can be accessed using |
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4498 <function>cyg_interrupt_configure</function>: the |
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4499 <parameter>level</parameter> argument selects between level versus |
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4500 edge triggered; the <parameter>up</parameter> argument selects between |
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4501 high and low level, or between rising and falling edges. |
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4502 </para> |
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4503 <para> |
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4504 Usually interrupt handlers are created, attached and configured during |
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4505 system initialization, while global interrupts are still disabled. On |
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4506 most hardware it will also be necessary to call |
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4507 <function>cyg_interrupt_unmask</function>, since the sensible default |
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4508 for interrupt masking is to ignore any interrupts for which no handler |
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4509 is installed. |
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4510 </para> |
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4511 </refsect1> |
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4512 |
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4513 <refsect1 id="kernel-interrupts-enable"><title>Controlling Interrupts</title> |
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4514 <para> |
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4515 eCos provides two ways of controlling whether or not interrupts |
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4516 happen. It is possible to disable and reenable all interrupts |
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4517 globally, using <function>cyg_interrupt_disable</function> and |
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4518 <function>cyg_interrupt_enable</function>. Typically this works by |
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4519 manipulating state inside the cpu itself, for example setting a flag |
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4520 in a status register or executing special instructions. Alternatively |
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4521 it may be possible to mask a specific interrupt source by writing to |
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4522 one or to several interrupt mask registers. Hardware-specific |
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4523 documentation should be consulted for the exact details of how |
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4524 interrupt masking works, because a full implementation is not possible |
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4525 on all hardware. |
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4526 </para> |
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4527 <para> |
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4528 The primary use for these functions is to allow data to be shared |
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4529 between ISRs and other code such as DSRs or threads. If both a thread |
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4530 and an ISR need to manipulate either a data structure or the hardware |
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4531 itself, there is a possible conflict if an interrupt happens just when |
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4532 the thread is doing such manipulation. Problems can be avoided by the |
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4533 thread either disabling or masking interrupts during the critical |
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4534 region. If this critical region requires only a few instructions then |
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4535 usually it is more efficient to disable interrupts. For larger |
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4536 critical regions it may be more appropriate to use interrupt masking, |
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4537 allowing other interrupts to occur. There are other uses for interrupt |
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4538 masking. For example if a device is not currently being used by the |
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4539 application then it may be desirable to mask all interrupts generated |
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4540 by that device. |
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4541 </para> |
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4542 <para> |
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4543 There are two functions for masking a specific interrupt source, |
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4544 <function>cyg_interrupt_mask</function> and |
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4545 <function>cyg_interrupt_mask_intunsafe</function>. On typical hardware |
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4546 masking an interrupt is not an atomic operation, so if two threads |
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4547 were to perform interrupt masking operations at the same time there |
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4548 could be problems. <function>cyg_interrupt_mask</function> disables |
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4549 all interrupts while it manipulates the interrupt mask. In situations |
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4550 where interrupts are already know to be disabled, |
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4551 <function>cyg_interrupt_mask_intunsafe</function> can be used |
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4552 instead. There are matching functions |
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4553 <function>cyg_interrupt_unmask</function> and |
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4554 <function>cyg_interrupt_unmask_intsafe</function>. |
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4555 </para> |
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4556 </refsect1> |
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4557 |
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4558 <refsect1 id="kernel-interrupts-smp"><title>SMP Support</title> |
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4559 <para> |
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4560 On SMP systems the kernel provides an additional two functions related |
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4561 to interrupt handling. <function>cyg_interrupt_set_cpu</function> |
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4562 specifies that a particular hardware interrupt should always be |
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4563 handled on one specific processor in the system. In other words when |
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4564 the interrupt triggers it is only that processor which detects it, and |
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4565 it is only on that processor that the VSR and ISR will run. If a DSR |
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4566 is requested then it will also run on the same CPU. The |
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4567 function <function>cyg_interrupt_get_cpu</function> can be used to |
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4568 find out which interrupts are handled on which processor. |
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4569 </para> |
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4570 </refsect1> |
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4571 |
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4572 <refsect1 id="kernel-interrupts-vsr"><title>VSR Support</title> |
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4573 <para> |
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4574 When an interrupt occurs the hardware will transfer control to a piece |
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4575 of code known as the VSR, or Vector Service Routine. By default this |
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4576 code is provided by eCos. Usually it is written in assembler, but on |
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4577 some architectures it may be possible to implement VSRs in C by |
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4578 specifying an interrupt attribute. Compiler documentation should be |
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4579 consulted for more information on this. The default eCos VSR will work |
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4580 out which ISR function should process the interrupt, and set up a C |
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4581 environment suitable for this ISR. |
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4582 </para> |
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4583 <para> |
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4584 For some applications it may be desirable to replace the default eCos |
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4585 VSR and handle some interrupts directly. This minimizes interrupt |
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4586 latency, but it requires application developers to program at a lower |
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4587 level. Usually the best way to write a custom VSR is to copy the |
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4588 existing one supplied by eCos and then make appropriate modifications. |
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4589 The function <function>cyg_interrupt_get_vsr</function> can be used to |
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4590 get hold of the current VSR for a given interrupt vector, allowing it |
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4591 to be restored if the custom VSR is no longer required. |
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4592 <function>cyg_interrupt_set_vsr</function> can be used to install a |
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4593 replacement VSR. Usually the <parameter>vsr</parameter> argument will |
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4594 correspond to an exported label in an assembler source file. |
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4595 </para> |
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4596 </refsect1> |
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4597 |
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4598 <refsect1 id="kernel-interrupts-context"><title>Valid contexts</title> |
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4599 <para> |
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4600 In a typical configuration interrupt handlers are created and attached |
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4601 during system initialization, and never detached or deleted. However |
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4602 it is possible to perform these operations at thread level, if |
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4603 desired. Similarly <function>cyg_interrupt_configure</function>, |
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4604 <function>cyg_interrupt_set_vsr</function>, and |
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4605 <function>cyg_interrupt_set_cpu</function> are usually called only |
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4606 during system initialization, but on typical hardware may be called at |
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4607 any time. <function>cyg_interrupt_get_vsr</function> and |
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4608 <function>cyg_interrupt_get_cpu</function> may be called at any time. |
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4609 </para> |
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4610 <para> |
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4611 The functions for enabling, disabling, masking and unmasking |
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4612 interrupts can be called in any context, when appropriate. It is the |
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4613 responsibility of application developers to determine when the use of |
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4614 these functions is appropriate. |
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4615 </para> |
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4616 </refsect1> |
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4617 |
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4618 </refentry> |
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4619 |
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4620 <!-- }}} --> |
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4621 <!-- {{{ tm_basic --> |
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4622 |
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4623 <refentry id="kernel-characterization"> |
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4624 |
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4625 <refmeta> |
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4626 <refentrytitle>Kernel Real-time Characterization</refentrytitle> |
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4627 </refmeta> |
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4628 <refnamediv> |
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4629 <refname>tm_basic</refname> |
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4630 <refpurpose>Measure the performance of the eCos kernel</refpurpose> |
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4631 </refnamediv> |
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4632 |
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4633 <refsect1 id="kernel-characterization-description"> |
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4634 <title>Description</title> |
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4635 <para> |
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4636 When building a real-time system, care must be taken to ensure that |
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4637 the system will be able to perform properly within the constraints of |
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4638 that system. One of these constraints may be how fast certain |
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4639 operations can be performed. Another might be how deterministic the |
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4640 overall behavior of the system is. Lastly the memory footprint (size) |
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4641 and unit cost may be important. |
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4642 </para> |
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4643 <para> |
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4644 One of the major problems encountered while evaluating a system will |
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4645 be how to compare it with possible alternatives. Most manufacturers of |
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4646 real-time systems publish performance numbers, ostensibly so that |
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4647 users can compare the different offerings. However, what these numbers |
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4648 mean and how they were gathered is often not clear. The values are |
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4649 typically measured on a particular piece of hardware, so in order to |
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4650 truly compare, one must obtain measurements for exactly the same set |
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4651 of hardware that were gathered in a similar fashion. |
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4652 </para> |
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4653 <para> |
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4654 Two major items need to be present in any given set of measurements. |
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4655 First, the raw values for the various operations; these are typically |
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4656 quite easy to measure and will be available for most systems. Second, |
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4657 the determinacy of the numbers; in other words how much the value |
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4658 might change depending on other factors within the system. This value |
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4659 is affected by a number of factors: how long interrupts might be |
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4660 masked, whether or not the function can be interrupted, even very |
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4661 hardware-specific effects such as cache locality and pipeline usage. |
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4662 It is very difficult to measure the determinacy of any given |
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4663 operation, but that determinacy is fundamentally important to proper |
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4664 overall characterization of a system. |
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4665 </para> |
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4666 <para> |
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4667 In the discussion and numbers that follow, three key measurements are |
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4668 provided. The first measurement is an estimate of the interrupt |
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4669 latency: this is the length of time from when a hardware interrupt |
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4670 occurs until its Interrupt Service Routine (ISR) is called. The second |
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4671 measurement is an estimate of overall interrupt overhead: this is the |
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4672 length of time average interrupt processing takes, as measured by the |
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4673 real-time clock interrupt (other interrupt sources will certainly take |
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4674 a different amount of time, but this data cannot be easily gathered). |
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4675 The third measurement consists of the timings for the various kernel |
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4676 primitives. |
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4677 </para> |
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4678 </refsect1> |
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4679 <refsect1 id="kernel-characterization-methodology"> |
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4680 <title>Methodology</title> |
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4681 <para> |
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4682 Key operations in the kernel were measured by using a simple test |
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4683 program which exercises the various kernel primitive operations. A |
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4684 hardware timer, normally the one used to drive the real-time clock, |
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4685 was used for these measurements. In most cases this timer can be read |
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4686 with quite high resolution, typically in the range of a few |
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4687 microseconds. For each measurement, the operation was repeated a |
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4688 number of times. Time stamps were obtained directly before and after |
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4689 the operation was performed. The data gathered for the entire set of |
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4690 operations was then analyzed, generating average (mean), maximum and |
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4691 minimum values. The sample variance (a measure of how close most |
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4692 samples are to the mean) was also calculated. The cost of obtaining |
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4693 the real-time clock timer values was also measured, and was subtracted |
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4694 from all other times. |
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4695 </para> |
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4696 <para> |
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4697 Most kernel functions can be measured separately. In each case, a |
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4698 reasonable number of iterations are performed. Where the test case |
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4699 involves a kernel object, for example creating a task, each iteration |
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4700 is performed on a different object. There is also a set of tests which |
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4701 measures the interactions between multiple tasks and certain kernel |
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4702 primitives. Most functions are tested in such a way as to determine |
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4703 the variations introduced by varying numbers of objects in the system. |
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4704 For example, the mailbox tests measure the cost of a 'peek' operation |
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4705 when the mailbox is empty, has a single item, and has multiple items |
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4706 present. In this way, any effects of the state of the object or how |
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4707 many items it contains can be determined. |
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4708 </para> |
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4709 <para> |
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4710 There are a few things to consider about these measurements. Firstly, |
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4711 they are quite micro in scale and only measure the operation in |
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4712 question. These measurements do not adequately describe how the |
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4713 timings would be perturbed in a real system with multiple interrupting |
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4714 sources. Secondly, the possible aberration incurred by the real-time |
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4715 clock (system heartbeat tick) is explicitly avoided. Virtually all |
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4716 kernel functions have been designed to be interruptible. Thus the |
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4717 times presented are typical, but best case, since any particular |
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4718 function may be interrupted by the clock tick processing. This number |
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4719 is explicitly calculated so that the value may be included in any |
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4720 deadline calculations required by the end user. Lastly, the reported |
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4721 measurements were obtained from a system built with all options at |
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4722 their default values. Kernel instrumentation and asserts are also |
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4723 disabled for these measurements. Any number of configuration options |
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4724 can change the measured results, sometimes quite dramatically. For |
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4725 example, mutexes are using priority inheritance in these measurements. |
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4726 The numbers will change if the system is built with priority |
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4727 inheritance on mutex variables turned off. |
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4728 </para> |
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4729 <para> |
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4730 The final value that is measured is an estimate of interrupt latency. |
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4731 This particular value is not explicitly calculated in the test program |
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4732 used, but rather by instrumenting the kernel itself. The raw number of |
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4733 timer ticks that elapse between the time the timer generates an |
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4734 interrupt and the start of the timer ISR is kept in the kernel. These |
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4735 values are printed by the test program after all other operations have |
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4736 been tested. Thus this should be a reasonable estimate of the |
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4737 interrupt latency over time. |
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4738 </para> |
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4739 </refsect1> |
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4740 |
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4741 <refsect1 id="kernel-characterization-using-measurements"> |
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4742 <title>Using these Measurements</title> |
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4743 <para> |
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4744 These measurements can be used in a number of ways. The most typical |
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4745 use will be to compare different real-time kernel offerings on similar |
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4746 hardware, another will be to estimate the cost of implementing a task |
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4747 using eCos (applications can be examined to see what effect the kernel |
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4748 operations will have on the total execution time). Another use would |
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4749 be to observe how the tuning of the kernel affects overall operation. |
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4750 </para> |
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4751 </refsect1> |
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4752 |
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4753 <refsect1 id="kernel-characterization-influences"> |
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4754 <title>Influences on Performance</title> |
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4755 <para> |
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4756 A number of factors can affect real-time performance in a system. One |
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4757 of the most common factors, yet most difficult to characterize, is the |
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4758 effect of device drivers and interrupts on system timings. Different |
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4759 device drivers will have differing requirements as to how long |
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4760 interrupts are suppressed, for example. The eCos system has been |
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4761 designed with this in mind, by separating the management of interrupts |
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4762 (ISR handlers) and the processing required by the interrupt |
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4763 (DSR—Deferred Service Routine— handlers). However, since |
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4764 there is so much variability here, and indeed most device drivers will |
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4765 come from the end users themselves, these effects cannot be reliably |
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4766 measured. Attempts have been made to measure the overhead of the |
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4767 single interrupt that eCos relies on, the real-time clock timer. This |
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4768 should give you a reasonable idea of the cost of executing interrupt |
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4769 handling for devices. |
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4770 </para> |
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4771 </refsect1> |
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4772 |
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4773 <refsect1 id="kernel-characterization-measured-items"> |
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4774 <title>Measured Items</title> |
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4775 <para> |
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4776 This section describes the various tests and the numbers presented. |
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4777 All tests use the C kernel API (available by way of |
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4778 <filename>cyg/kernel/kapi.h</filename>). There is a single main thread |
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4779 in the system that performs the various tests. Additional threads may |
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4780 be created as part of the testing, but these are short lived and are |
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4781 destroyed between tests unless otherwise noted. The terminology |
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4782 “lower priority” means a priority that is less important, |
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4783 not necessarily lower in numerical value. A higher priority thread |
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4784 will run in preference to a lower priority thread even though the |
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4785 priority value of the higher priority thread may be numerically less |
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4786 than that of the lower priority thread. |
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4787 </para> |
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4788 |
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4789 <refsect2 id="kernel-characterization-measure-threads"> |
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4790 <title>Thread Primitives</title> |
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4791 <variablelist> |
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4792 <varlistentry> |
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4793 <term>Create thread</term> |
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4794 <listitem><para> |
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4795 This test measures the <function>cyg_thread_create()</function> call. |
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4796 Each call creates a totally new thread. The set of threads created by |
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4797 this test will be reused in the subsequent thread primitive tests. |
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4798 </para></listitem> |
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4799 </varlistentry> |
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4800 <varlistentry> |
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4801 <term>Yield thread</term> |
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4802 <listitem><para> |
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4803 This test measures the <function>cyg_thread_yield()</function> call. |
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4804 For this test, there are no other runnable threads, thus the test |
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4805 should just measure the overhead of trying to give up the CPU. |
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4806 </para></listitem> |
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4807 </varlistentry> |
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4808 <varlistentry> |
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4809 <term>Suspend [suspended] thread</term> |
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4810 <listitem><para> |
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4811 This test measures the <function>cyg_thread_suspend()</function> call. |
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4812 A thread may be suspended multiple times; each thread is already |
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4813 suspended from its initial creation, and is suspended again. |
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4814 </para></listitem> |
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4815 </varlistentry> |
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4816 <varlistentry> |
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4817 <term>Resume thread</term> |
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4818 <listitem><para> |
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4819 This test measures the <function>cyg_thread_resume()</function> call. |
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4820 All of the threads have a suspend count of 2, thus this call does not |
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4821 make them runnable. This test just measures the overhead of resuming a |
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4822 thread. |
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4823 </para></listitem> |
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4824 </varlistentry> |
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4825 <varlistentry> |
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4826 <term>Set priority</term> |
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4827 <listitem><para> |
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4828 This test measures the <function>cyg_thread_set_priority()</function> |
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4829 call. Each thread, currently suspended, has its priority set to a new |
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4830 value. |
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4831 </para></listitem> |
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4832 </varlistentry> |
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4833 <varlistentry> |
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4834 <term>Get priority</term> |
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4835 <listitem><para> |
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4836 This test measures the <function>cyg_thread_get_priority()</function> |
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4837 call. |
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4838 </para></listitem> |
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4839 </varlistentry> |
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4840 <varlistentry> |
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4841 <term>Kill [suspended] thread</term> |
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4842 <listitem><para> |
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4843 This test measures the <function>cyg_thread_kill()</function> call. |
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4844 Each thread in the set is killed. All threads are known to be |
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4845 suspended before being killed. |
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4846 </para></listitem> |
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4847 </varlistentry> |
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4848 <varlistentry> |
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4849 <term>Yield [no other] thread</term> |
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4850 <listitem><para> |
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4851 This test measures the <function>cyg_thread_yield()</function> call |
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4852 again. This is to demonstrate that the |
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4853 <function>cyg_thread_yield()</function> call has a fixed overhead, |
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4854 regardless of whether there are other threads in the system. |
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4855 </para></listitem> |
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4856 </varlistentry> |
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4857 <varlistentry> |
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4858 <term>Resume [suspended low priority] thread</term> |
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4859 <listitem><para> |
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4860 This test measures the <function>cyg_thread_resume()</function> call |
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4861 again. In this case, the thread being resumed is lower priority than |
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4862 the main thread, thus it will simply become ready to run but not be |
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4863 granted the CPU. This test measures the cost of making a thread ready |
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4864 to run. |
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4865 </para></listitem> |
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4866 </varlistentry> |
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4867 <varlistentry> |
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4868 <term>Resume [runnable low priority] thread</term> |
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4869 <listitem><para> |
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4870 This test measures the <function>cyg_thread_resume()</function> call |
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4871 again. In this case, the thread being resumed is lower priority than |
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4872 the main thread and has already been made runnable, so in fact the |
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4873 resume call has no effect. |
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4874 </para></listitem> |
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4875 </varlistentry> |
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4876 <varlistentry> |
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4877 <term>Suspend [runnable] thread</term> |
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4878 <listitem><para> |
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4879 This test measures the <function>cyg_thread_suspend()</function> call |
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4880 again. In this case, each thread has already been made runnable (by |
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4881 previous tests). |
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4882 </para></listitem> |
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4883 </varlistentry> |
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4884 <varlistentry> |
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4885 <term>Yield [only low priority] thread</term> |
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4886 <listitem><para> |
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4887 This test measures the <function>cyg_thread_yield()</function> call. |
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4888 In this case, there are many other runnable threads, but they are all |
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4889 lower priority than the main thread, thus no thread switches will take |
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4890 place. |
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4891 </para></listitem> |
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4892 </varlistentry> |
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4893 <varlistentry> |
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4894 <term>Suspend [runnable->not runnable] thread</term> |
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4895 <listitem><para> |
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4896 This test measures the <function>cyg_thread_suspend()</function> call |
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4897 again. The thread being suspended will become non-runnable by this |
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4898 action. |
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4899 </para></listitem> |
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4900 </varlistentry> |
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4901 <varlistentry> |
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4902 <term>Kill [runnable] thread</term> |
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4903 <listitem><para> |
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4904 This test measures the <function>cyg_thread_kill()</function> call |
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4905 again. In this case, the thread being killed is currently runnable, |
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4906 but lower priority than the main thread. |
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4907 </para></listitem> |
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4908 </varlistentry> |
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4909 <varlistentry> |
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4910 <term>Resume [high priority] thread</term> |
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4911 <listitem><para> |
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4912 This test measures the <function>cyg_thread_resume()</function> call. |
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4913 The thread being resumed is higher priority than the main thread, thus |
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4914 a thread switch will take place on each call. In fact there will be |
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4915 two thread switches; one to the new higher priority thread and a |
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4916 second back to the test thread. The test thread exits immediately. |
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4917 </para></listitem> |
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4918 </varlistentry> |
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4919 <varlistentry> |
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4920 <term>Thread switch</term> |
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4921 <listitem><para> |
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4922 This test attempts to measure the cost of switching from one thread to |
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4923 another. Two equal priority threads are started and they will each |
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4924 yield to the other for a number of iterations. A time stamp is |
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4925 gathered in one thread before the |
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4926 <function>cyg_thread_yield()</function> call and after the call in the |
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4927 other thread. |
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4928 </para></listitem> |
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4929 </varlistentry> |
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4930 </variablelist> |
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4931 </refsect2> |
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4932 |
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4933 <refsect2 id="kernel-characterization-measure-scheduler" |
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4934 <title>Scheduler Primitives</title> |
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4935 <variablelist> |
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4936 <varlistentry> |
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4937 <term>Scheduler lock</term> |
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4938 <listitem><para> |
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4939 This test measures the <function>cyg_scheduler_lock()</function> call. |
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4940 </para></listitem> |
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4941 </varlistentry> |
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4942 <varlistentry> |
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4943 <term>Scheduler unlock [0 threads]</term> |
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4944 <listitem><para> |
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4945 This test measures the <function>cyg_scheduler_unlock()</function> |
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4946 call. There are no other threads in the system and the unlock happens |
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4947 immediately after a lock so there will be no pending DSR’s to |
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4948 run. |
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4949 </para></listitem> |
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4950 </varlistentry> |
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4951 <varlistentry> |
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4952 <term>Scheduler unlock [1 suspended thread]</term> |
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4953 <listitem><para> |
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4954 This test measures the <function>cyg_scheduler_unlock()</function> |
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4955 call. There is one other thread in the system which is currently |
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4956 suspended. |
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4957 </para></listitem> |
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4958 </varlistentry> |
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4959 <varlistentry> |
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4960 <term>Scheduler unlock [many suspended threads]</term> |
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4961 <listitem><para> |
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4962 This test measures the <function>cyg_scheduler_unlock()</function> |
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4963 call. There are many other threads in the system which are currently |
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4964 suspended. The purpose of this test is to determine the cost of having |
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4965 additional threads in the system when the scheduler is activated by |
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4966 way of <function>cyg_scheduler_unlock()</function>. |
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4967 </para></listitem> |
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4968 </varlistentry> |
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4969 <varlistentry> |
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4970 <term>Scheduler unlock [many low priority threads]</term> |
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4971 <listitem><para> |
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4972 This test measures the <function>cyg_scheduler_unlock()</function> |
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4973 call. There are many other threads in the system which are runnable |
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4974 but are lower priority than the main thread. The purpose of this test |
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4975 is to determine the cost of having additional threads in the system |
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4976 when the scheduler is activated by way of |
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4977 <function>cyg_scheduler_unlock()</function>. |
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4978 </para></listitem> |
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4979 </varlistentry> |
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4980 </variablelist> |
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4981 </refsect2> |
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4982 |
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4983 <refsect2 id="kernel-characterization-measure-mutex"> |
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4984 <title>Mutex Primitives</title> |
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4985 <variablelist> |
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4986 <varlistentry> |
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4987 <term>Init mutex</term> |
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4988 <listitem><para> |
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4989 This test measures the <function>cyg_mutex_init()</function> call. A |
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4990 number of separate mutex variables are created. The purpose of this |
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4991 test is to measure the cost of creating a new mutex and introducing it |
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4992 to the system. |
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4993 </para></listitem> |
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4994 </varlistentry> |
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4995 <varlistentry> |
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4996 <term>Lock [unlocked] mutex</term> |
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4997 <listitem><para> |
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4998 This test measures the <function>cyg_mutex_lock()</function> call. The |
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4999 purpose of this test is to measure the cost of locking a mutex which |
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5000 is currently unlocked. There are no other threads executing in the |
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5001 system while this test runs. |
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5002 </para></listitem> |
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5003 </varlistentry> |
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5004 <varlistentry> |
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5005 <term>Unlock [locked] mutex</term> |
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5006 <listitem><para> |
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5007 This test measures the <function>cyg_mutex_unlock()</function> call. |
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5008 The purpose of this test is to measure the cost of unlocking a mutex |
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5009 which is currently locked. There are no other threads executing in the |
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5010 system while this test runs. |
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5011 </para></listitem> |
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5012 </varlistentry> |
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5013 <varlistentry> |
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5014 <term>Trylock [unlocked] mutex</term> |
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5015 <listitem><para> |
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5016 This test measures the <function>cyg_mutex_trylock()</function> call. |
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5017 The purpose of this test is to measure the cost of locking a mutex |
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5018 which is currently unlocked. There are no other threads executing in |
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5019 the system while this test runs. |
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5020 </para></listitem> |
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5021 </varlistentry> |
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5022 <varlistentry> |
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5023 <term>Trylock [locked] mutex</term> |
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5024 <listitem><para> |
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5025 This test measures the <function>cyg_mutex_trylock()</function> call. |
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5026 The purpose of this test is to measure the cost of locking a mutex |
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5027 which is currently locked. There are no other threads executing in the |
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5028 system while this test runs. |
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5029 </para></listitem> |
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5030 </varlistentry> |
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5031 <varlistentry> |
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5032 <term>Destroy mutex</term> |
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5033 <listitem><para> |
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5034 This test measures the <function>cyg_mutex_destroy()</function> call. |
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5035 The purpose of this test is to measure the cost of deleting a mutex |
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5036 from the system. There are no other threads executing in the system |
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5037 while this test runs. |
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5038 </para></listitem> |
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5039 </varlistentry> |
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5040 <varlistentry> |
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5041 <term>Unlock/Lock mutex</term> |
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5042 <listitem><para> |
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5043 This test attempts to measure the cost of unlocking a mutex for which |
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5044 there is another higher priority thread waiting. When the mutex is |
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5045 unlocked, the higher priority waiting thread will immediately take the |
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5046 lock. The time from when the unlock is issued until after the lock |
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5047 succeeds in the second thread is measured, thus giving the round-trip |
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5048 or circuit time for this type of synchronizer. |
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5049 </para></listitem> |
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5050 </varlistentry> |
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5051 </variablelist> |
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5052 </refsect2> |
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5053 |
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5054 <refsect2 id="kernel-characterization-measure-mailbox"> |
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5055 <title>Mailbox Primitives</title> |
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5056 <variablelist> |
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5057 <varlistentry> |
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5058 <term>Create mbox</term> |
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5059 <listitem><para> |
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5060 This test measures the <function>cyg_mbox_create()</function> call. A |
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5061 number of separate mailboxes is created. The purpose of this test is |
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5062 to measure the cost of creating a new mailbox and introducing it to |
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5063 the system. |
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5064 </para></listitem> |
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5065 </varlistentry> |
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5066 <varlistentry> |
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5067 <term>Peek [empty] mbox</term> |
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5068 <listitem><para> |
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5069 This test measures the <function>cyg_mbox_peek()</function> call. An |
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5070 attempt is made to peek the value in each mailbox, which is currently |
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5071 empty. The purpose of this test is to measure the cost of checking a |
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5072 mailbox for a value without blocking. |
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5073 </para></listitem> |
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5074 </varlistentry> |
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5075 <varlistentry> |
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5076 <term>Put [first] mbox</term> |
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5077 <listitem><para> |
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5078 This test measures the <function>cyg_mbox_put()</function> call. One |
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5079 item is added to a currently empty mailbox. The purpose of this test |
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5080 is to measure the cost of adding an item to a mailbox. There are no |
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5081 other threads currently waiting for mailbox items to arrive. |
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5082 </para></listitem> |
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5083 </varlistentry> |
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5084 <varlistentry> |
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5085 <term>Peek [1 msg] mbox</term> |
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5086 <listitem><para> |
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5087 This test measures the <function>cyg_mbox_peek()</function> call. An |
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5088 attempt is made to peek the value in each mailbox, which contains a |
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5089 single item. The purpose of this test is to measure the cost of |
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5090 checking a mailbox which has data to deliver. |
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5091 </para></listitem> |
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5092 </varlistentry> |
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5093 <varlistentry> |
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5094 <term>Put [second] mbox</term> |
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5095 <listitem><para> |
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5096 This test measures the <function>cyg_mbox_put()</function> call. A |
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5097 second item is added to a mailbox. The purpose of this test is to |
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5098 measure the cost of adding an additional item to a mailbox. There are |
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5099 no other threads currently waiting for mailbox items to arrive. |
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5100 </para></listitem> |
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5101 </varlistentry> |
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5102 <varlistentry> |
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5103 <term>Peek [2 msgs] mbox</term> |
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5104 <listitem><para> |
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5105 This test measures the <function>cyg_mbox_peek()</function> call. An |
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5106 attempt is made to peek the value in each mailbox, which contains two |
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5107 items. The purpose of this test is to measure the cost of checking a |
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5108 mailbox which has data to deliver. |
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5109 </para></listitem> |
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5110 </varlistentry> |
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5111 <varlistentry> |
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5112 <term>Get [first] mbox</term> |
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5113 <listitem><para> |
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5114 This test measures the <function>cyg_mbox_get()</function> call. The |
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5115 first item is removed from a mailbox that currently contains two |
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5116 items. The purpose of this test is to measure the cost of obtaining an |
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5117 item from a mailbox without blocking. |
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5118 </para></listitem> |
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5119 </varlistentry> |
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5120 <varlistentry> |
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5121 <term>Get [second] mbox</term> |
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5122 <listitem><para> |
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5123 This test measures the <function>cyg_mbox_get()</function> call. The |
|
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5124 last item is removed from a mailbox that currently contains one item. |
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5125 The purpose of this test is to measure the cost of obtaining an item |
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5126 from a mailbox without blocking. |
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5127 </para></listitem> |
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5128 </varlistentry> |
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5129 <varlistentry> |
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5130 <term>Tryput [first] mbox</term> |
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5131 <listitem><para> |
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5132 This test measures the <function>cyg_mbox_tryput()</function> call. A |
|
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5133 single item is added to a currently empty mailbox. The purpose of this |
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5134 test is to measure the cost of adding an item to a mailbox. |
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5135 </para></listitem> |
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5136 </varlistentry> |
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5137 <varlistentry> |
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5138 <term>Peek item [non-empty] mbox</term> |
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5139 <listitem><para> |
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5140 This test measures the <function>cyg_mbox_peek_item()</function> call. |
|
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|
5141 A single item is fetched from a mailbox that contains a single item. |
|
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5142 The purpose of this test is to measure the cost of obtaining an item |
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5143 without disturbing the mailbox. |
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5144 </para></listitem> |
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5145 </varlistentry> |
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5146 <varlistentry> |
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5147 <term>Tryget [non-empty] mbox</term> |
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5148 <listitem><para> |
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5149 This test measures the <function>cyg_mbox_tryget()</function> call. A |
|
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5150 single item is removed from a mailbox that contains exactly one item. |
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5151 The purpose of this test is to measure the cost of obtaining one item |
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5152 from a non-empty mailbox. |
|
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5153 </para></listitem> |
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5154 </varlistentry> |
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5155 <varlistentry> |
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5156 <term>Peek item [empty] mbox</term> |
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5157 <listitem><para> |
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5158 This test measures the <function>cyg_mbox_peek_item()</function> call. |
|
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5159 An attempt is made to fetch an item from a mailbox that is empty. The |
|
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5160 purpose of this test is to measure the cost of trying to obtain an |
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5161 item when the mailbox is empty. |
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5162 </para></listitem> |
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5163 </varlistentry> |
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5164 <varlistentry> |
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5165 <term>Tryget [empty] mbox</term> |
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5166 <listitem><para> |
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5167 This test measures the <function>cyg_mbox_tryget()</function> call. An |
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5168 attempt is made to fetch an item from a mailbox that is empty. The |
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5169 purpose of this test is to measure the cost of trying to obtain an |
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5170 item when the mailbox is empty. |
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5171 </para></listitem> |
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5172 </varlistentry> |
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5173 <varlistentry> |
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5174 <term>Waiting to get mbox</term> |
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5175 <listitem><para> |
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5176 This test measures the <function>cyg_mbox_waiting_to_get()</function> |
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5177 call. The purpose of this test is to measure the cost of determining |
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5178 how many threads are waiting to obtain a message from this mailbox. |
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5179 </para></listitem> |
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5180 </varlistentry> |
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5181 <varlistentry> |
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5182 <term>Waiting to put mbox</term> |
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5183 <listitem><para> |
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5184 This test measures the <function>cyg_mbox_waiting_to_put()</function> |
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5185 call. The purpose of this test is to measure the cost of determining |
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5186 how many threads are waiting to put a message into this mailbox. |
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5187 </para></listitem> |
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5188 </varlistentry> |
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5189 <varlistentry> |
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5190 <term>Delete mbox</term> |
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5191 <listitem><para> |
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5192 This test measures the <function>cyg_mbox_delete()</function> call. |
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5193 The purpose of this test is to measure the cost of destroying a |
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5194 mailbox and removing it from the system. |
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5195 </para></listitem> |
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5196 </varlistentry> |
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5197 <varlistentry> |
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5198 <term>Put/Get mbox</term> |
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5199 <listitem><para> |
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5200 In this round-trip test, one thread is sending data to a mailbox that |
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5201 is being consumed by another thread. The time from when the data is |
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5202 put into the mailbox until it has been delivered to the waiting thread |
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5203 is measured. Note that this time will contain a thread switch. |
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5204 </para></listitem> |
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5205 </varlistentry> |
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5206 </variablelist> |
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5207 </refsect2> |
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5208 |
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5209 <refsect2 id="kernel-characterization-measure-semaphore"> |
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5210 <title>Semaphore Primitives</title> |
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5211 <variablelist> |
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5212 <varlistentry> |
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5213 <term>Init semaphore</term> |
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5214 <listitem><para> |
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5215 This test measures the <function>cyg_semaphore_init()</function> call. |
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5216 A number of separate semaphore objects are created and introduced to |
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5217 the system. The purpose of this test is to measure the cost of |
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5218 creating a new semaphore. |
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5219 </para></listitem> |
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5220 </varlistentry> |
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5221 <varlistentry> |
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5222 <term>Post [0] semaphore</term> |
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5223 <listitem><para> |
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5224 This test measures the <function>cyg_semaphore_post()</function> call. |
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5225 Each semaphore currently has a value of 0 and there are no other |
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5226 threads in the system. The purpose of this test is to measure the |
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5227 overhead cost of posting to a semaphore. This cost will differ if |
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5228 there is a thread waiting for the semaphore. |
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5229 </para></listitem> |
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5230 </varlistentry> |
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5231 <varlistentry> |
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5232 <term>Wait [1] semaphore</term> |
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5233 <listitem><para> |
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5234 This test measures the <function>cyg_semaphore_wait()</function> call. |
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5235 The semaphore has a current value of 1 so the call is non-blocking. |
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5236 The purpose of the test is to measure the overhead of |
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5237 “taking” a semaphore. |
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5238 </para></listitem> |
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5239 </varlistentry> |
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5240 <varlistentry> |
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5241 <term>Trywait [0] semaphore</term> |
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5242 <listitem><para> |
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5243 This test measures the <function>cyg_semaphore_trywait()</function> |
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5244 call. The semaphore has a value of 0 when the call is made. The |
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5245 purpose of this test is to measure the cost of seeing if a semaphore |
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5246 can be “taken” without blocking. In this case, the answer |
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5247 would be no. |
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5248 </para></listitem> |
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5249 </varlistentry> |
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5250 <varlistentry> |
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5251 <term>Trywait [1] semaphore</term> |
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5252 <listitem><para> |
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5253 This test measures the <function>cyg_semaphore_trywait()</function> |
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5254 call. The semaphore has a value of 1 when the call is made. The |
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5255 purpose of this test is to measure the cost of seeing if a semaphore |
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5256 can be “taken” without blocking. In this case, the answer |
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5257 would be yes. |
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5258 </para></listitem> |
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5259 </varlistentry> |
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5260 <varlistentry> |
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5261 <term>Peek semaphore</term> |
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5262 <listitem><para> |
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5263 This test measures the <function>cyg_semaphore_peek()</function> call. |
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5264 The purpose of this test is to measure the cost of obtaining the |
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5265 current semaphore count value. |
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5266 </para></listitem> |
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5267 </varlistentry> |
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5268 <varlistentry> |
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5269 <term>Destroy semaphore</term> |
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5270 <listitem><para> |
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5271 This test measures the <function>cyg_semaphore_destroy()</function> |
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5272 call. The purpose of this test is to measure the cost of deleting a |
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5273 semaphore from the system. |
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5274 </para></listitem> |
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5275 </varlistentry> |
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5276 <varlistentry> |
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5277 <term>Post/Wait semaphore</term> |
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5278 <listitem><para> |
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5279 In this round-trip test, two threads are passing control back and |
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5280 forth by using a semaphore. The time from when one thread calls |
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5281 <function>cyg_semaphore_post()</function> until the other thread |
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5282 completes its <function>cyg_semaphore_wait()</function> is measured. |
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5283 Note that each iteration of this test will involve a thread switch. |
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5284 </para></listitem> |
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5285 </varlistentry> |
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5286 </variablelist> |
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5287 </refsect2> |
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5288 |
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5289 <refsect2 id="kernel-characterization-measure-counters"> |
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5290 <title>Counters</title> |
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5291 <variablelist> |
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5292 <varlistentry> |
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5293 <term>Create counter</term> |
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5294 <listitem><para> |
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5295 This test measures the <function>cyg_counter_create()</function> call. |
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5296 A number of separate counters are created. The purpose of this test is |
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5297 to measure the cost of creating a new counter and introducing it to |
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5298 the system. |
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5299 </para></listitem> |
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5300 </varlistentry> |
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5301 <varlistentry> |
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5302 <term>Get counter value</term> |
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5303 <listitem><para> |
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5304 This test measures the |
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5305 <function>cyg_counter_current_value()</function> call. The current |
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5306 value of each counter is obtained. |
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5307 </para></listitem> |
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5308 </varlistentry> |
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5309 <varlistentry> |
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5310 <term>Set counter value</term> |
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5311 <listitem><para> |
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5312 This test measures the <function>cyg_counter_set_value()</function> |
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5313 call. Each counter is set to a new value. |
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5314 </para></listitem> |
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5315 </varlistentry> |
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5316 <varlistentry> |
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5317 <term>Tick counter</term> |
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5318 <listitem><para> |
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5319 This test measures the <function>cyg_counter_tick()</function> call. |
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5320 Each counter is “ticked” once. |
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5321 </para></listitem> |
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5322 </varlistentry> |
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5323 <varlistentry> |
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5324 <term>Delete counter</term> |
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5325 <listitem><para> |
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5326 This test measures the <function>cyg_counter_delete()</function> call. |
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5327 Each counter is deleted from the system. The purpose of this test is |
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5328 to measure the cost of deleting a counter object. |
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5329 </para></listitem> |
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5330 </varlistentry> |
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5331 </variablelist> |
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5332 </refsect2> |
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5333 |
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5334 <refsect2 id="kernel-characterization-measure-alarms"> |
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5335 <title>Alarms</title> |
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5336 <variablelist> |
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5337 <varlistentry> |
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5338 <term>Create alarm</term> |
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5339 <listitem><para> |
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5340 This test measures the <function>cyg_alarm_create()</function> call. A |
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5341 number of separate alarms are created, all attached to the same |
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5342 counter object. The purpose of this test is to measure the cost of |
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5343 creating a new counter and introducing it to the system. |
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5344 </para></listitem> |
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5345 </varlistentry> |
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5346 <varlistentry> |
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5347 <term>Initialize alarm</term> |
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5348 <listitem><para> |
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5349 This test measures the <function>cyg_alarm_initialize()</function> |
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5350 call. Each alarm is initialized to a small value. |
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5351 </para></listitem> |
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5352 </varlistentry> |
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5353 <varlistentry> |
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5354 <term>Disable alarm</term> |
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5355 <listitem><para> |
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5356 This test measures the <function>cyg_alarm_disable()</function> call. |
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5357 Each alarm is explicitly disabled. |
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5358 </para></listitem> |
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5359 </varlistentry> |
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5360 <varlistentry> |
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5361 <term>Enable alarm</term> |
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5362 <listitem><para> |
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5363 This test measures the <function>cyg_alarm_enable()</function> call. |
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5364 Each alarm is explicitly enabled. |
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5365 </para></listitem> |
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5366 </varlistentry> |
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5367 <varlistentry> |
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5368 <term>Delete alarm</term> |
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5369 <listitem><para> |
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5370 This test measures the <function>cyg_alarm_delete()</function> call. |
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5371 Each alarm is destroyed. The purpose of this test is to measure the |
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5372 cost of deleting an alarm and removing it from the system. |
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5373 </para></listitem> |
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5374 </varlistentry> |
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5375 <varlistentry> |
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5376 <term>Tick counter [1 alarm]</term> |
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5377 <listitem><para> |
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5378 This test measures the <function>cyg_counter_tick()</function> call. A |
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5379 counter is created that has a single alarm attached to it. The purpose |
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5380 of this test is to measure the cost of “ticking” a counter |
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5381 when it has a single attached alarm. In this test, the alarm is not |
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5382 activated (fired). |
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5383 </para></listitem> |
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5384 </varlistentry> |
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5385 <varlistentry> |
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5386 <term>Tick counter [many alarms]</term> |
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5387 <listitem><para> |
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5388 This test measures the <function>cyg_counter_tick()</function> call. A |
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5389 counter is created that has multiple alarms attached to it. The |
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5390 purpose of this test is to measure the cost of “ticking” a |
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5391 counter when it has many attached alarms. In this test, the alarms are |
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5392 not activated (fired). |
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5393 </para></listitem> |
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5394 </varlistentry> |
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5395 <varlistentry> |
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5396 <term>Tick & fire counter [1 alarm]</term> |
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5397 <listitem><para> |
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5398 This test measures the <function>cyg_counter_tick()</function> call. A |
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5399 counter is created that has a single alarm attached to it. The purpose |
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5400 of this test is to measure the cost of “ticking” a counter |
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5401 when it has a single attached alarm. In this test, the alarm is |
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5402 activated (fired). Thus the measured time will include the overhead of |
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5403 calling the alarm callback function. |
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5404 </para></listitem> |
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5405 </varlistentry> |
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5406 <varlistentry> |
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5407 <term>Tick & fire counter [many alarms]</term> |
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5408 <listitem><para> |
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5409 This test measures the <function>cyg_counter_tick()</function> call. A |
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5410 counter is created that has multiple alarms attached to it. The |
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5411 purpose of this test is to measure the cost of “ticking” a |
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5412 counter when it has many attached alarms. In this test, the alarms are |
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5413 activated (fired). Thus the measured time will include the overhead of |
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5414 calling the alarm callback function. |
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5415 </para></listitem> |
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5416 </varlistentry> |
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5417 <varlistentry> |
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5418 <term>Alarm latency [0 threads]</term> |
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5419 <listitem><para> |
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5420 This test attempts to measure the latency in calling an alarm callback |
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5421 function. The time from the clock interrupt until the alarm function |
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5422 is called is measured. In this test, there are no threads that can be |
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5423 run, other than the system idle thread, when the clock interrupt |
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5424 occurs (all threads are suspended). |
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5425 </para></listitem> |
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5426 </varlistentry> |
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5427 <varlistentry> |
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5428 <term>Alarm latency [2 threads]</term> |
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5429 <listitem><para> |
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5430 This test attempts to measure the latency in calling an alarm callback |
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5431 function. The time from the clock interrupt until the alarm function |
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5432 is called is measured. In this test, there are exactly two threads |
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5433 which are running when the clock interrupt occurs. They are simply |
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5434 passing back and forth by way of the |
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5435 <function>cyg_thread_yield()</function> call. The purpose of this test |
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5436 is to measure the variations in the latency when there are executing |
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5437 threads. |
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5438 </para></listitem> |
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5439 </varlistentry> |
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5440 <varlistentry> |
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5441 <term>Alarm latency [many threads]</term> |
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5442 <listitem><para> |
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5443 This test attempts to measure the latency in calling an alarm callback |
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5444 function. The time from the clock interrupt until the alarm function |
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5445 is called is measured. In this test, there are a number of threads |
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5446 which are running when the clock interrupt occurs. They are simply |
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5447 passing back and forth by way of the |
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5448 <function>cyg_thread_yield()</function> call. The purpose of this test |
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5449 is to measure the variations in the latency when there are many |
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5450 executing threads. |
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5451 </para></listitem> |
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5452 </varlistentry> |
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5453 </variablelist> |
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5454 </refsect2> |
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5455 |
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5456 </refsect1> |
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5457 |
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5458 </refentry> |
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5459 |
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5460 <!-- }}} --> |
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5461 |
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5462 </part> |