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<!-- Date:        2001/01/11                                         -->
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<refentry id="devs-usb-sa11x0">
<refmeta>
<refentrytitle>SA11X0 USB Device Driver</refentrytitle>
</refmeta>
<refnamediv>
<refname>SA11X0 USB Support</refname>
<refpurpose>Device driver for the on-chip SA11X0 USB device</refpurpose>
</refnamediv>

<refsect1><title>SA11X0 USB Hardware</title>
<para>
The Intel StrongARM SA11x0 family of processors is supplied with an
on-chip USB slave device, the UDC (USB Device Controller). This
supports three endpoints. Endpoint 0 can only be used for control
messages. Endpoint 1 can only be used for bulk transfers from host to
peripheral. Endpoint 2 can only be used for bulk transfers from
peripheral to host. Isochronous and interrupt transfers are not
supported.
</para>
<caution>    
<para>
Different revisions of the SA11x0 silicon have had various problems
with the USB support. The device driver has been tested primarily
against stepping B4 of the SA1110 processor, and may not function as
expected with other revisions. Application developers should obtain
the manufacturer's current errata sheets and specification updates.
The B4 stepping still has a number of problems, but the device driver
can work around these. However there is a penalty in terms of extra
code, extra cpu cycles, and increased dispatch latency because extra
processing is needed at DSR level. Interrupt latency should not be
affected.
</para>
<para>
There is one specific problem inherent in the UDC design of which
application developers should be aware: the hardware cannot fully
implement the USB standard for bulk transfers. A bulk transfer
typically consists of some number of full-size 64-byte packets and is
terminated by a packet less than the full size. If the amount of data
transferred is an exact multiple of 64 bytes then this requires a
terminating packet of 0 bytes of data (plus header and checksum). The
SA11x0 USB hardware does not allow a 0-byte packet to be transmitted,
so the device driver is forced to substitute a 1-byte packet and the
host receives more data than expected. Protocol support is needed so
that the appropriate host-side device driver can allow buffer space
for the extra byte, detect when it gets sent, and discard it.
Consequently certain standard USB class protocols cannot be
implemented using the SA11x0, and therefore custom host-side device
drivers will generally have to be provided, rather than re-using
existing ones that understand the standard protocol.
</para>
</caution>
</refsect1>

<refsect1><title>Endpoint Data Structures</title>
<para>
The SA11x0 USB device driver can provide up to three data structures
corresponding to the three endpoints: a
<structname>usbs_control_endpoint</structname> structure
<literal>usbs_sa11x0_ep0</literal>; a
<structname>usbs_rx_endpoint</structname>
<literal>usbs_sa11x0_ep1</literal>; and a
<structname>usbs_tx_endpoint</structname>
<literal>usbs_sa11x0_ep2</literal>. The header file
<filename class="headerfile">cyg/io/usb/usbs_sa11x0.h</filename>
provides declarations for these.
</para>
<para>
Not all applications will require support for all the endpoints. For
example, if the intended use of the UDC only involves peripheral to
host transfers then <literal>usbs_sa11x0_ep1</literal> is redundant.
The device driver provides configuration options to control the
presence of each endpoint:
</para>
<orderedlist>
<listitem>
<para>
Endpoint 0 is controlled by
<literal>CYGFUN_DEVS_USB_SA11X0_EP0</literal>. This defaults to
enabled if there are any higher-level packages that require USB
hardware or if the global preference
<literal>CYGGLO_IO_USB_SLAVE_APPLICATION</literal> is enabled,
otherwise it is disabled. Usually this has the desired effect. It may
be necessary to override this in special circumstances, for example if
the target board uses an external USB chip in preference to the UDC
and it is that external chip's device driver that should be used
rather than the on-chip UDC. It is not possible to disable endpoint 0
and at the same time enable one or both of the other endpoints, since
a USB device is only usable if it can process the standard control
messages.
</para>
</listitem>
<listitem>
<para>
Endpoint 1 is controlled by
<literal>CYGPKG_DEVS_USB_SA11X0_EP1</literal>. By default it is
enabled whenever endpoint 0 is enabled, but it can be disabled
manually when not required.
</para>
</listitem>
<listitem>
<para>
Similarly endpoint 2 is controlled by
<literal>CYGPKG_DEVS_USB_SA11X0_EP2</literal>. This is also enabled by
default whenever endpoint 0 is enabled, but it can be disabled manually.
</para>
</listitem>
</orderedlist>
<para>
The SA11X0 USB device driver implements the interface specified by the
common eCos USB Slave Support package. The documentation for that
package should be consulted for further details. There is only one
major deviation: when there is a peripheral to host transfer on
endpoint 2 which is an exact multiple of the bulk transfer packet size
(usually 64 bytes) the device driver has to pad the transfer with one
extra byte. This is because of a hardware limitation: the UDC is
incapable of transmitting 0-byte packets as required by the USB
specification. Higher-level code, including the host-side device
driver, needs to be aware of this and adapt accordingly.
</para>
<para>
The device driver assumes a bulk packet size of 64 bytes, so this
value should be used in the endpoint descriptors in the enumeration
data provided by application code. There is experimental code
for running with <link linkend="usbs-sa11x0-dma">DMA disabled</link>,
in which case the packet size will be 16 bytes rather than 64.
</para>
</refsect1>

<refsect1><title>Devtab Entries</title>
<para>
In addition to the endpoint data structures the SA11X0 USB device
driver can also provide devtab entries for each endpoint. This allows
higher-level code to use traditional I/O operations such as
<function>open</function>/<function>read</function>/<function>write</function>
rather than the USB-specific non-blocking functions like
<function>usbs_start_rx_buffer</function>. These devtab entries are
optional since they are not always required. The relevant
configuration options are
<literal>CYGVAR_DEVS_USB_SA11X0_EP0_DEVTAB_ENTRY</literal>,
<literal>CYGVAR_DEVS_USB_SA11X0_EP1_DEVTAB_ENTRY</literal> and
<literal>CYGVAR_DEVS_USB_SA11X0_EP2_DEVTAB_ENTRY</literal>. By default
these devtab entries are provided if the global preference
<literal>CYGGLO_USB_SLAVE_PROVIDE_DEVTAB_ENTRIES</literal> is enabled,
which is usually the case. Obviously a devtab entry for a given
endpoint will only be provided if the underlying endpoint is enabled.
For example, there will not be a devtab entry for endpoint 1 if
<literal>CYGPKG_DEVS_USB_SA11X0_EP1</literal> is disabled.
</para>
<para>
The names for the three devtab entries are determined by using a
configurable base name and appending <literal>0c</literal>,
<literal>1r</literal> or <literal>2w</literal>. The base name is
determined by the configuration option
<literal>CYGDAT_DEVS_USB_SA11X0_DEVTAB_BASENAME</literal> and has a
default value of <literal>/dev/usbs</literal>, so the devtab entry for
endpoint 1 would default to <literal>/dev/usbs1r</literal>. If the
target hardware involves multiple USB devices then application
developers may have to change the base name to prevent a name clash.
</para>
</refsect1>

<refsect1><title id="usbs-sa11x0-dma">DMA Engines</title>
<para>
The SA11X0 UDC provides only limited fifos for bulk transfers on
endpoints 1 and 2; smaller than the normal 64-byte bulk packet size.
Therefore a typical transfer requires the use of DMA engines. The
SA11x0 provides six DMA engines that can be used for this, and the
endpoints require one each (assuming both endpoints are enabled). At
the time of writing there is no arbitration mechanism to control
access to the DMA engines. By default the device driver will use
DMA engine 4 for endpoint 1 and DMA engine 5 for endpoint 2, and it
assumes that no other code uses these particular engines.
</para>
<para>
The exact DMA engines that will be used are determined by the
configuration options
<literal>CYGNUM_DEVS_USB_SA11X0_EP1_DMA_CHANNEL</literal> and
<literal>CYGNUM_DEVS_USB_SA11X0_EP2_DMA_CHANNEL</literal>. These
options have the booldata flavor, allowing the use of DMA to be
disabled completely in addition to controlling which DMA engines are
used. If DMA is disabled then the device driver will attempt to
work purely using the fifos, and the packet size will be limited to
only 16 bytes. This limit should be reflected in the appropriate
endpoint descriptors in the enumeration data. The code for driving the
endpoints without DMA should be considered experimental. At best it
will be suitable only for applications where the amount of data
transferred is relatively small, because four times as many interrupts
will be raised and performance will suffer accordingly.
</para>
</refsect1>

</refentry>
