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>Calling graph for Transmission and Reception</TITLE
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><H1
CLASS="SECT1"
><A
NAME="IO-ETH-CALL-GRAPH">Calling graph for Transmission and Reception</H1
><P
>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 “foreground” when something asynchronous has occurred.</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="IO-ETH-CALL-GRAPH-TX">Transmission</H2
><P
></P
><OL
TYPE="1"
><LI
><P
>Some foreground task such as the application, SNMP “daemon”,
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 “ping” or <SPAN
CLASS="ACRONYM"
>ARP</SPAN
> request.</P
></LI
><LI
><P
>The driver calls the
<TT
CLASS="FUNCTION"
><TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_can_send()</TT
>
function in the hardware driver.</P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
><TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_can_send()</TT
>
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.</P
></LI
><LI
><P
>If it can send right now, <TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_send() is called.
<TT
CLASS="FUNCTION"
><TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_send()</TT
> copies the
data into special hardware buffers, or instructs the hardware to
“send that.” It also remembers the key that is associated with
this tx request.</P
></LI
><LI
><P
>These calls return … time passes …</P
></LI
><LI
><P
>Asynchronously, the hardware makes an interrupt to say
“transmit is done.”
The ISR quietens the interrupt source in the hardware and
requests that the associated DSR be run.</P
></LI
><LI
><P
>The DSR calls (or <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>is</I
></SPAN
>) the
<TT
CLASS="FUNCTION"
>eth_drv_dsr()</TT
> function in the generic driver.</P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>eth_drv_dsr()</TT
> in the generic driver awakens the
“Network Delivery Thread” which calls the deliver function
<TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_deliver() in the driver.</P
></LI
><LI
><P
>The deliver function realizes that a transmit request has completed,
and calls the callback tx-done function
<TT
CLASS="FUNCTION"
>(sc->funs->eth_drv->tx_done)()</TT
>
with the same key that it remembered for this tx.</P
></LI
><LI
><P
>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.</P
></LI
><LI
><P
>The callback tx-done function
also enquires whether <TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_can_send() now says
“yes, we can send”
and if so, dequeues a further transmit request
which may have been queued as described above. If so, then
<TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_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 “Network Delivery Thread”
which then sleeps, awaiting the next asynchronous event.</P
></LI
><LI
><P
>All done …</P
></LI
></OL
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="IO-ETH-CALL-GRAPH-RX">Receive</H2
><P
></P
><OL
TYPE="1"
><LI
><P
>Asynchronously, the hardware makes an interrupt to say
“there is ready data in a receive buffer.”
The ISR quietens the interrupt source in the hardware and
requests that the associated DSR be run.</P
></LI
><LI
><P
>The DSR calls (or <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>is</I
></SPAN
>) the
<TT
CLASS="FUNCTION"
>eth_drv_dsr()</TT
> function in the generic driver.</P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>eth_drv_dsr()</TT
> in the generic driver awakens the
“Network Delivery Thread” which calls the deliver function
<TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_deliver() in the driver.</P
></LI
><LI
><P
>The deliver function realizes that there is data ready and calls
the callback receive function
<TT
CLASS="FUNCTION"
>(sc->funs->eth_drv->recv)()</TT
>
to tell it how many bytes to prepare for.</P
></LI
><LI
><P
>The callback receive function allocates memory within the stack
(eg. <SPAN
CLASS="TYPE"
>MBUFs</SPAN
> in BSD/Unix style stacks) and prepares
a set of scatter-gather buffers that can
accommodate the packet.</P
></LI
><LI
><P
>It then calls back into the hardware driver routine
<TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_recv().
<TT
CLASS="REPLACEABLE"
><I
>HRDWR</I
></TT
>_recv() must copy the data from the
hardware's buffers into the scatter-gather buffers provided, and return.</P
></LI
><LI
><P
>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
“Network Delivery Thread”
sleeps once more, awaiting the next asynchronous event.</P
></LI
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