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<part id="io-eth-drv-generic">
<title>Ethernet Device Drivers</title>
<chapter id="io-eth-drv-generic1">
<title>Generic Ethernet Device Driver</title>
<sect1 id="io-eth-drv-api">
<title>Generic Ethernet API</title>
<para>
This section provides a simple description of how to write a low-level,
hardware dependent ethernet driver.
</para>
<para>
There is a high-level driver (which is only code &mdash; with no state of
its own) that is part of the stack.  There will be one or more low-level
drivers tied to the actual network hardware.  Each of these drivers
contains one or more driver instances.  The intent is that the
low-level drivers know nothing of the details of the stack that will be
using them.  Thus, the same driver can be used by the
<productname>eCos</productname>
supported
<acronym>TCP/IP</acronym>
stack,
<productname>RedBoot</productname>,
or any other, with no changes.
</para>
<para>
A driver instance is contained within a
<type>struct eth_drv_sc</type>:
<programlisting>
struct eth_hwr_funs {
    // Initialize hardware (including startup)
    void (*start)(struct eth_drv_sc *sc,
                  unsigned char *enaddr,
                  int flags);
    // Shut down hardware
    void (*stop)(struct eth_drv_sc *sc);
    // Device control (ioctl pass-thru)
    int  (*control)(struct eth_drv_sc *sc,
                    unsigned long key,
                    void *data,
                    int   data_length);
    // Query - can a packet be sent?
    int  (*can_send)(struct eth_drv_sc *sc);
    // Send a packet of data
    void (*send)(struct eth_drv_sc *sc,
                 struct eth_drv_sg *sg_list,
                 int sg_len,
                 int total_len,
                 unsigned long key);
    // Receive [unload] a packet of data
    void (*recv)(struct eth_drv_sc *sc,
                 struct eth_drv_sg *sg_list,
                 int sg_len);
    // Deliver data to/from device from/to stack memory space
    // (moves lots of memcpy()s out of DSRs into thread)
    void (*deliver)(struct eth_drv_sc *sc);
    // Poll for interrupts/device service
    void (*poll)(struct eth_drv_sc *sc);
    // Get interrupt information from hardware driver
    int (*int_vector)(struct eth_drv_sc *sc);
    // Logical driver interface
    struct eth_drv_funs *eth_drv, *eth_drv_old;
};

struct eth_drv_sc {
    struct eth_hwr_funs *funs;
    void                *driver_private;
    const char          *dev_name;
    int                  state;
    struct arpcom        sc_arpcom; /* ethernet common */
};
</programlisting>
</para><note><para>
If you have two instances of the same hardware, you only need one
<type>struct eth_hwr_funs</type> shared between them.
</para></note><para>
There is another structure which is used to communicate with the rest of
the stack:
<programlisting>
struct eth_drv_funs {
    // Logical driver - initialization
    void (*init)(struct eth_drv_sc *sc, 
                 unsigned char *enaddr);
    // Logical driver - incoming packet notifier
    void (*recv)(struct eth_drv_sc *sc, 
                 int total_len);
    // Logical driver - outgoing packet notifier
    void (*tx_done)(struct eth_drv_sc *sc, 
                    CYG_ADDRESS key, 
                    int status);
};
</programlisting>
Your driver does <emphasis>not</emphasis> create an instance of this
structure.  It is provided for driver code to use in the
<type>eth_drv</type> member of the function record.
Its usage is described below in <xref linkend=io-eth-drv-upper-api>
</para><para>
One more function completes the API with which your driver communicates
with the rest of the stack:
<programlisting>
extern void eth_drv_dsr(cyg_vector_t vector,
                        cyg_ucount32 count,
                        cyg_addrword_t data);
</programlisting>
</para><para>
This function is designed so that it can be registered as the DSR for your
interrupt handler.  It will awaken the
&ldquo;Network Delivery Thread&rdquo;
to call your deliver routine.  See <xref linkend=io-eth-drv-api-deliver>.
</para><para>
You create an instance of <type>struct eth_drv_sc</type>
using the
<function>ETH_DRV_SC()</function>
macro which
sets up the structure, including the prototypes for the functions, etc.
By doing things this way, if the internal design of the ethernet drivers
changes (e.g. we need to add a new low-level implementation function),
existing drivers will no longer compile until updated.  This is much
better than to have all of the definitions in the low-level drivers
themselves and have them be (quietly) broken if the interfaces change.
</para><para>
The &ldquo;magic&rdquo;
which gets the drivers started (and indeed, linked) is
similar to what is used for the I/O subsystem.
This is done using the
<function>NETDEVTAB_ENTRY()</function>
macro, which defines an initialization function
and the basic data structures for the low-level driver.
</para>
<para><programlisting>
  typedef struct cyg_netdevtab_entry {
      const char        *name;
      bool             (*init)(struct cyg_netdevtab_entry *tab);
      void              *device_instance;
      unsigned long     status;
  } cyg_netdevtab_entry_t;
</programlisting>
The <varname>device_instance</varname>
entry here would point to the <type>struct eth_drv_sc</type>
entry previously defined.  This allows the network driver
setup to work with any class of driver, not just ethernet drivers.  In
the future, there will surely be serial <acronym>PPP</acronym>
drivers, etc.  These will
use the <function>NETDEVTAB_ENTRY()</function>
setup to create the basic driver, but they will
most likely be built on top of other high-level device driver layers.
</para><para>
To instantiate itself, and connect it to the system,
a hardware driver will have a template
(boilerplate) which looks something like this:
<programlisting>
#include &lt;cyg/infra/cyg_type.h&gt;
#include &lt;cyg/hal/hal_arch.h&gt;
#include &lt;cyg/infra/diag.h&gt;
#include &lt;cyg/hal/drv_api.h&gt;
#include &lt;cyg/io/eth/netdev.h&gt;
#include &lt;cyg/io/eth/eth_drv.h&gt;

ETH_DRV_SC(<replaceable>DRV</replaceable>_sc,
           0,             // No driver specific data needed
           "eth0",        // Name for this interface
           <replaceable>HRDWR</replaceable>_start,
           <replaceable>HRDWR</replaceable>_stop,
           <replaceable>HRDWR</replaceable>_control,
           <replaceable>HRDWR</replaceable>_can_send
           <replaceable>HRDWR</replaceable>_send,
           <replaceable>HRDWR</replaceable>_recv,
           <replaceable>HRDWR</replaceable>_deliver,
           <replaceable>HRDWR</replaceable>_poll,
           <replaceable>HRDWR</replaceable>_int_vector
);

NETDEVTAB_ENTRY(<replaceable>DRV</replaceable>_netdev, 
                "<replaceable>DRV</replaceable>", 
                <replaceable>DRV_HRDWR</replaceable>_init, 
                &amp;<replaceable>DRV</replaceable>_sc);
</programlisting>
</para><para>
This, along with the referenced functions, completely define the driver.
</para><note><para>
If one needed the same low-level driver to handle
multiple similar hardware interfaces, you would need multiple invocations
of the
<function>ETH_DRV_SC()</function>/<function>NETDEVTAB_ENTRY()</function>
macros.  You would add a pointer
to some instance specific data, e.g. containing base addresses, interrupt
numbers, etc, where the
<programlisting>
	0, // No driver specific data
</programlisting>
is currently.
</para></note>
</sect1>
<sect1 id="io-eth-drv-api-funcs">
<title>Review of the functions</title>
<para>
Now a brief review of the functions.  This discussion will use generic
names for the functions &mdash; your driver should use hardware-specific
names to maintain uniqueness against any other drivers.
</para>
<sect2 id="io-eth-drv-api-init">
<title>Init function</title>
<para>
<programlisting>
static bool <replaceable>DRV_HDWR</replaceable>_init(struct cyg_netdevtab_entry *tab)
</programlisting>
This function is called as part of system initialization.  Its primary
function is to decide if the hardware (as indicated via
<type>tab-&gt;device_instance</type>)
is working and if the interface needs to be made
available in the system.  If this is the case, this function needs to
finish with a call to the ethernet driver function:
<programlisting>
    struct eth_drv_sc *sc = (struct eth_drv_sc *)tab->device_instance;
    <replaceable>....initialization code....</replaceable>
    // Initialize upper level driver
    (sc-&gt;funs-&gt;eth_drv-&gt;init)( sc, unsigned char *enaddr );
</programlisting>
where <parameter>enaddr</parameter>
is a pointer to the ethernet station address for this unit, to inform
the stack of this device's readiness and availability.
</para>
<note><para>The ethernet station address
(<acronym>ESA</acronym>)
is supposed to be a
world-unique, 48 bit address for this particular ethernet interface.
Typically it is provided by the board/hardware manufacturer in ROM.
</para>
<para>
In many packages it is possible for the
<acronym>ESA</acronym>
to be set from RedBoot,
(perhaps from 'fconfig' data), hard-coded from
<acronym>CDL</acronym>, or from an <acronym>EPROM</acronym>.
A driver should choose a run-time specified
<acronym>ESA</acronym>
(e.g. from RedBoot)
preferentially, otherwise (in order) it should use a <acronym>CDL</acronym> specified
<acronym>ESA</acronym>
if one has been set, otherwise an <acronym>EPROM</acronym> set
<acronym>ESA</acronym>, or otherwise
fail. See the <filename>cl/cs8900a</filename>
ethernet driver for an example.
</para></note>
</sect2>
<sect2 id="io-eth-drv-api-start">
<title>Start function</title>
<para>
<programlisting>
static void
<replaceable>HRDWR</replaceable>_start(struct eth_drv_sc *sc, unsigned char *enaddr, int flags)
</programlisting>
This function is called, perhaps much later than system initialization
time, when the system (an application) is ready for the interface to
become active.  The purpose of this function is to set up the hardware
interface to start accepting packets from the network and be able to
send packets out.  The receiver hardware should not be enabled prior to
this call.
</para><note><para>This function will be called whenever the
up/down state of the logical interface changes, e.g. when the IP address
changes, or when promiscuous mode is selected by means of an
<function>ioctl()</function> call in the application.
This may occur more than once, so this function needs to
be prepared for that case.
</para></note><note><para>
In future, the <parameter>flags</parameter>
field (currently unused) may be used to tell the
function how to start up, e.g. whether interrupts will be used,
alternate means of selecting promiscuous mode etc.
</para></note>
</sect2>
<sect2 id="io-eth-drv-api-stop">
<title>Stop function</title>
<para>
<programlisting>
static void <replaceable>HRDWR</replaceable>_stop(struct eth_drv_sc *sc)
</programlisting>
This function is the inverse of &ldquo;start.&rdquo;
It should shut down the hardware, disable the receiver, and keep it from
interacting with the physical network.
</para>
</sect2>
<sect2 id="io-eth-drv-api-control">
<title>Control function</title>
<para>
<programlisting>
static int
<replaceable>HRDWR</replaceable>_control(
	struct eth_drv_sc *sc, unsigned long key,
	void *data, int len)
</programlisting>
This function is used to perform low-level &ldquo;control&rdquo;
operations on the
interface.  These operations would typically be initiated via
<function>ioctl()</function> calls in the BSD
stack, and would be anything that might require the hardware setup to
change (i.e. cannot be performed totally by the
platform-independent layers).
</para><para>
The <parameter>key</parameter> parameter selects the operation, and the
<parameter>data</parameter> and <parameter>len</parameter> params point describe,
as required, some data for the operation in question.
</para>
<variablelist><title>Available Operations:</title>
<varlistentry><term>ETH_DRV_SET_MAC_ADDRESS</term>
<listitem><para>
This operation sets the ethernet station address (ESA or MAC) for the
device.  Normally this address is kept in non-volatile memory and is
unique in the world.  This function must at least set the interface to
use the new address.  It may also update the NVM as appropriate.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>ETH_DRV_GET_IF_STATS_UD</term>
<term>ETH_DRV_GET_IF_STATS</term>
<listitem><para>
These acquire a set of statistical counters from the interface, and write
the information into the memory pointed to by <parameter>data</parameter>.
The &ldquo;UD&rdquo; variant explicitly instructs the driver to acquire
up-to-date values.  This is a separate option because doing so may take
some time, depending on the hardware.
</para><para>
The definition of the data structure is in
<filename>cyg/io/eth/eth_drv_stats.h</filename>.
</para><para>
This call is typically made by SNMP, see <xref linkend=net-snmp-ecos-port>.
</para>
</listitem>
</varlistentry>
<varlistentry><term>ETH_DRV_SET_MC_LIST</term>
<listitem><para>
This entry instructs the device to set up multicast packet filtering
to receive only packets addressed to the multicast ESAs in the list pointed
to by <parameter>data</parameter>.
</para><para>
The format of the data is a 32-bit count of the ESAs in the list, followed
by packed bytes which are the ESAs themselves, thus:
<programlisting>
#define ETH_DRV_MAX_MC 8
struct eth_drv_mc_list {
    int len;
    unsigned char addrs[ETH_DRV_MAX_MC][ETHER_ADDR_LEN];
};
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry><term>ETH_DRV_SET_MC_ALL</term>
<listitem><para>
This entry instructs the device to receive all multicast packets, and
delete any explicit filtering which had been set up.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
This function should return zero if the specified operation was
completed successfully.  It should return non-zero if the operation
could not be performed, for any reason.
</para>
</sect2>
<sect2 id="io-eth-drv-api-can-send">
<title>Can-send function</title>
<para>
<programlisting>
static int <replaceable>HRDWR</replaceable>_can_send(struct eth_drv_sc *sc)
</programlisting>
This function is called to determine if it is possible to start the
transmission of a packet on the interface.  Some interfaces will allow
multiple packets to be "queued" and this function allows for the highest
possible utilization of that mode.
</para><para>
Return the number of packets which could be accepted at this time, zero
implies that the interface is saturated/busy.
</para>
</sect2>
<sect2 id="io-eth-drv-api-send">
<title>Send function</title>
<para>
<programlisting>
struct eth_drv_sg {
    CYG_ADDRESS  buf;
    CYG_ADDRWORD len;
};

static void
<replaceable>HRDWR</replaceable>_send(
	struct eth_drv_sc *sc,
	struct eth_drv_sg *sg_list, int sg_len,
        int total_len, unsigned long key)
</programlisting>
This function is used to send a packet of data to the network.  It is
the responsibility of this function to somehow hand the data over to the
hardware interface.  This will most likely require copying, but just the
address/length values could be used by smart hardware.
</para><note><para>
All data in/out of the driver is specified via a
&ldquo;scatter-gather&rdquo;
list.  This is just an array of address/length pairs which describe
sections of data to move (in the order given by the array), as in the
<type>struct eth_drv_sg</type> defined above and pointed to by
<parameter>sg_list</parameter>.
</para></note><para>
Once the data has been successfully sent by the interface (or if an
error occurs), the driver should call
<function>(sc->funs->eth_drv->tx_done)()</function>
(see <xref linkend=io-eth-drv-tx-done>)
using the specified <parameter>key</parameter>.
Only then will the upper layers release the resources
for that packet and start another transmission.
</para><note><para>
In future, this function may be extended so that the data need not be
copied by having the function return a &ldquo;disposition&rdquo; code
(done, send pending, etc).  At this point, you should move the data to some
&ldquo;safe&rdquo; location before returning.
</para></note>
</sect2>
<sect2 id="io-eth-drv-api-deliver">
<title>Deliver function</title>
<para>
<programlisting>
static void
<replaceable>HRDWR</replaceable>_deliver(struct eth_drv_sc *sc)
</programlisting>
This function is called from the &ldquo;Network Delivery Thread&rdquo; in
order to let the device driver do the time-consuming work associated with
receiving a packet &mdash; usually copying the entire packet from the
hardware or a special memory location into the network stack's memory.
</para><para>
After handling any outstanding incoming packets or pending transmission
status, it can unmask the device's interrupts, and free any relevant
resources so it can process further packets.
</para><para>
It will be called when the interrupt handler for the network device
has called
<programlisting>
    eth_drv_dsr( vector, count, (cyg_addrword_t)sc );
</programlisting>
to alert the system that &ldquo;something requires attention.&rdquo;
This <function>eth_drv_dsr()</function> call must occur from within the
interrupt handler's DSR (not the ISR) or actually <emphasis>be</emphasis>
the DSR, whenever it is determined that
the device needs attention from the foreground.  The third parameter
(<parameter>data</parameter> in the prototype of
<function>eth_drv_dsr()</function> <emphasis>must</emphasis>
be a valid <type>struct eth_drv_sc</type> pointer <varname>sc</varname>.
</para><para>
The reason for this slightly convoluted train of events is to keep the DSR
(and ISR) execution time as short as possible, so that other activities of
higher priority than network servicing are not denied the CPU by network
traffic.
</para><para>
To deliver a newly-received packet into the network stack, the deliver
routine must call
<programlisting>
(sc->funs->eth_drv->recv)(sc, len);
</programlisting>
which will in turn call the receive function, which we talk about next.
See also <xref linkend=io-eth-drv-upper-recv> below.
</para>
</sect2>
<sect2 id="io-eth-drv-api-recv">
<title>Receive function</title>
<para>
<programlisting>
static void
<replaceable>HRDWR</replaceable>_recv(
	struct eth_drv_sc *sc,
	struct eth_drv_sg *sg_list, int sg_len)
</programlisting>
This function is a call back, only invoked after the
upper-level function
<programlisting>
(sc->funs->eth_drv->recv)(struct eth_drv_sc *sc, int total_len)
</programlisting>
has been called itself from your deliver function when it knows that a
packet of data is available on the
interface.  The <function>(sc->funs->eth_drv->recv)()</function>
function then arranges network buffers
and structures for the data and then calls
<function><replaceable>HRDWR</replaceable>_recv()</function> to actually
move the data from the interface.
</para><para>
A scatter-gather list (<type>struct eth_drv_sg</type>) is used once more,
just like in the send case.
</para>
</sect2>
<sect2 id="io-eth-drv-api-poll">
<title>Poll function</title>
<para>
<programlisting>
static void
<replaceable>HRDWR</replaceable>_poll(struct eth_drv_sc *sc)
</programlisting>
This function is used when in a non-interrupt driven system, e.g. when
interrupts are completely disabled. This allows the driver time to check
whether anything needs doing either for transmission, or to check if
anything has been received, or if any other processing needs doing.
</para><para>
It is perfectly correct and acceptable for the poll function to look like
this:
<programlisting>
static void
<replaceable>HRDWR</replaceable>_poll(struct eth_drv_sc *sc)
{
   <replaceable>my_interrupt_ISR</replaceable>(sc);
   <replaceable>HRDWR</replaceable>_deliver(struct eth_drv_sc *sc);
}
</programlisting>
provided that both the ISR and the deliver functions are idempotent and
harmless if called when there is no attention needed by the hardware.  Some
devices might not need a call to the ISR here if the deliver function
contains all the &ldquo;intelligence.&rdquo;
</para>
</sect2>
<sect2 id="io-eth-drv-api-int-vector">
<title>Interrupt-vector function</title>
<para>
<programlisting>
static int
<replaceable>HRDWR</replaceable>_int_vector(struct eth_drv_sc *sc)
</programlisting>
This function returns the interrupt vector number used for receive
interrupts.
This is so that the common GDB stubs can detect when to check
for incoming &ldquo;CTRL-C&rdquo; packets (used to asynchronously
halt the application) when debugging over ethernet.
The GDB stubs need to know which interrupt the ethernet device uses
so that they can mask or unmask that interrupt as required.
</para>
</sect2>
</sect1>
<sect1 id=io-eth-drv-upper-api>
<title>Upper Layer Functions</title>
<para>
Upper layer functions are called by drivers to deliver received packets
or transmission completion status back up into the network stack.
</para><para>
These functions are defined by the hardware independent upper layers of
the networking driver support.  They are present to hide the interfaces
to the actual networking stack so that the hardware drivers may
be used by different network stack implementations without change.
</para><para>
These functions require a pointer to a <type>struct eth_drv_sc</type>
which describes the interface at a logical level.  It is assumed that the
low level hardware driver will keep track of this pointer so
it may be passed &ldquo;up&rdquo; as appropriate.
</para>
<sect2 id="io-eth-drv-upper-init">
<title>Callback Init function</title>
<para>
<programlisting>
void (sc->funs->eth_drv->init)(
		struct eth_drv_sc *sc, unsigned char *enaddr)
</programlisting>
This function establishes the device at initialization time.
It should be called once per device instance only, from the
initialization function, if all is well
(see <xref linkend=io-eth-drv-api-init>).
The hardware should be totally initialized
(<emphasis>not</emphasis> &ldquo;started&rdquo;)
when this function is called.
</para>
</sect2>
<sect2 id="io-eth-drv-tx-done">
<title>Callback Tx-Done function</title>
<para>
<programlisting>
void (sc->funs->eth_drv->tx_done)(
		struct eth_drv_sc *sc,
		unsigned long key, int status)
</programlisting>
This function is called when a packet completes transmission on the
interface.  The <parameter>key</parameter>
value must be one of the keys provided to
<function><replaceable>HRDWR</replaceable>_send()</function>
above.  The value <parameter>status</parameter> should be non-zero
(details currently undefined) to indicate that an error occurred during the
transmission, and zero if all was well.
</para><para>
It should be called from the deliver function
(see <xref linkend=io-eth-drv-api-deliver>)
or poll function
(see <xref linkend=io-eth-drv-api-poll>).
</para>
</sect2>
<sect2 id="io-eth-drv-upper-recv">
<title>Callback Receive function</title>
<para>
<programlisting>
void (sc->funs->eth_drv->recv)(struct eth_drv_sc *sc, int len)
</programlisting>
This function is called to indicate that a packet of length
<parameter>len</parameter> has
arrived at the interface.
The callback
<function><replaceable>HRDWR</replaceable>_recv()</function> function
described above will be used to actually unload the data from the
interface into buffers used by the device independent layers.
</para><para>
It should be called from the deliver function
(see <xref linkend=io-eth-drv-api-deliver>)
or poll function
(see <xref linkend=io-eth-drv-api-poll>).
</para>
</sect2>
</sect1>
<sect1 id=io-eth-call-graph>
<title>Calling graph for Transmission and Reception</title>
<para>
It may be worth clarifying further the flow of control in the transmit and
receive cases, where the hardware driver does use interrupts and so DSRs to
tell the &ldquo;foreground&rdquo; when something asynchronous has occurred.
</para>
<sect2 id=io-eth-call-graph-tx>
<title>Transmission</title>
<orderedlist>
<listitem><para>
Some foreground task such as the application, SNMP &ldquo;daemon&rdquo;,
DHCP management thread or whatever, calls into network stack to send a
packet, or the stack decides to send a packet in response to incoming
traffic such as a &ldquo;ping&rdquo; or <acronym>ARP</acronym> request.
</para></listitem>
<listitem><para>
The driver calls the
<function><replaceable>HRDWR</replaceable>_can_send()</function>
function in the hardware driver.
</para></listitem>
<listitem><para>
<function><replaceable>HRDWR</replaceable>_can_send()</function>
returns the number of available "slots" in which it
can store a pending transmit packet.
If it cannot send at this time, the packet is queued outside the
hardware driver for later; in this case, the hardware is already busy
transmitting, so expect an interrupt as described below for completion
of the packet currently outgoing.
</para></listitem>
<listitem><para>
If it can send right now, <replaceable>HRDWR</replaceable>_send() is called.
<function><replaceable>HRDWR</replaceable>_send()</function> copies the
data into special hardware buffers, or instructs the hardware to
&ldquo;send that.&rdquo; It also remembers the key that is associated with
this tx request.
</para></listitem>
<listitem><para>
These calls return &hellip; time passes &hellip;
</para></listitem>
<listitem><para>
Asynchronously, the hardware makes an interrupt to say
&ldquo;transmit is done.&rdquo;
The ISR quietens the interrupt source in the hardware and
requests that the associated DSR be run.
</para></listitem>
<listitem><para>
The DSR calls (or <emphasis>is</emphasis>) the
<function>eth_drv_dsr()</function> function in the generic driver.
</para></listitem>
<listitem><para>
<function>eth_drv_dsr()</function> in the generic driver awakens the
&ldquo;Network Delivery Thread&rdquo; which calls the deliver function
<replaceable>HRDWR</replaceable>_deliver() in the driver.
</para></listitem>
<listitem><para>
The deliver function realizes that a transmit request has completed,
and calls the callback tx-done function
<function>(sc->funs->eth_drv->tx_done)()</function>
with the same key that it remembered for this tx.
</para></listitem>
<listitem><para>
The callback tx-done function
uses the key to find the resources associated with
this transmit request; thus the stack knows that the transmit has
completed and its resources can be freed.
</para></listitem>
<listitem><para>
The callback tx-done function
also enquires whether <replaceable>HRDWR</replaceable>_can_send() now says
&ldquo;yes, we can send&rdquo;
and if so, dequeues a further transmit request
which may have been queued as described above.  If so, then
<replaceable>HRDWR</replaceable>_send() copies the data into the hardware buffers, or
instructs the hardware to "send that" and remembers the new key, as above.
These calls then all return to the &ldquo;Network Delivery Thread&rdquo;
which then sleeps, awaiting the next asynchronous event.
</para></listitem>
<listitem><para>
All done &hellip;
</para></listitem>
</orderedlist>
</sect2>
<sect2 id=io-eth-call-graph-rx>
<title>Receive</title>
<orderedlist>
<listitem><para>
Asynchronously, the hardware makes an interrupt to say
&ldquo;there is ready data in a receive buffer.&rdquo;
The ISR quietens the interrupt source in the hardware and
requests that the associated DSR be run.
</para></listitem>
<listitem><para>
The DSR calls (or <emphasis>is</emphasis>) the
<function>eth_drv_dsr()</function> function in the generic driver.
</para></listitem>
<listitem><para>
<function>eth_drv_dsr()</function> in the generic driver awakens the
&ldquo;Network Delivery Thread&rdquo; which calls the deliver function
<replaceable>HRDWR</replaceable>_deliver() in the driver.
</para></listitem>
<listitem><para>
The deliver function realizes that there is data ready and calls
the callback receive function
<function>(sc->funs->eth_drv->recv)()</function>
to tell it how many bytes to prepare for.
</para></listitem>
<listitem><para>
The callback receive function allocates memory within the stack
(eg. <type>MBUFs</type> in BSD/Unix style stacks) and prepares
a set of scatter-gather buffers that can
accommodate the packet.
</para></listitem>
<listitem><para>
It then calls back into the hardware driver routine
<replaceable>HRDWR</replaceable>_recv().
<replaceable>HRDWR</replaceable>_recv() must copy the data from the
hardware's buffers into the scatter-gather buffers provided, and return.
</para></listitem>
<listitem><para>
The network stack now has the data in-hand, and does with it what it will.
This might include recursive calls to transmit a response packet.
When this all is done, these calls return, and the
&ldquo;Network Delivery Thread&rdquo;
sleeps once more, awaiting the next asynchronous event.
</para></listitem>
</orderedlist>
</sect2>
</sect1>
</chapter>
</part>