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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Calling graph for Transmission and Reception</TITLE
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TITLE="Generic Ethernet Device Driver"
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TITLE="Upper Layer Functions"
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>eCos Reference Manual</TH
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><A
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HREF="io-eth-drv-upper-api.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 46. Generic Ethernet Device Driver</TD
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><TD
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ALIGN="right"
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VALIGN="bottom"
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="IO-ETH-CALL-GRAPH">Calling graph for Transmission and Reception</H1
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><P
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>It may be worth clarifying further the flow of control in the transmit and
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receive cases, where the hardware driver does use interrupts and so DSRs to
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tell the &#8220;foreground&#8221; when something asynchronous has occurred.</P
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="IO-ETH-CALL-GRAPH-TX">Transmission</H2
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><P
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></P
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><OL
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TYPE="1"
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><LI
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><P
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>Some foreground task such as the application, SNMP &#8220;daemon&#8221;,
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DHCP management thread or whatever, calls into network stack to send a
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packet, or the stack decides to send a packet in response to incoming
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traffic such as a &#8220;ping&#8221; or <SPAN
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CLASS="ACRONYM"
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>ARP</SPAN
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> request.</P
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></LI
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><LI
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><P
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>The driver calls the
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<TT
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CLASS="FUNCTION"
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><TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_can_send()</TT
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>
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function in the hardware driver.</P
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></LI
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><LI
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><P
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><TT
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CLASS="FUNCTION"
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><TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_can_send()</TT
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>
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returns the number of available "slots" in which it
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can store a pending transmit packet.
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If it cannot send at this time, the packet is queued outside the
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hardware driver for later; in this case, the hardware is already busy
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transmitting, so expect an interrupt as described below for completion
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of the packet currently outgoing.</P
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></LI
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><LI
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><P
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>If it can send right now, <TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_send() is called.
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<TT
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CLASS="FUNCTION"
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><TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_send()</TT
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> copies the
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data into special hardware buffers, or instructs the hardware to
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&#8220;send that.&#8221; It also remembers the key that is associated with
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this tx request.</P
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></LI
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><LI
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><P
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>These calls return &#8230; time passes &#8230;</P
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></LI
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><LI
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><P
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>Asynchronously, the hardware makes an interrupt to say
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&#8220;transmit is done.&#8221;
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The ISR quietens the interrupt source in the hardware and
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requests that the associated DSR be run.</P
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></LI
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><LI
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><P
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>The DSR calls (or <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>is</I
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></SPAN
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>) the
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<TT
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CLASS="FUNCTION"
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>eth_drv_dsr()</TT
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> function in the generic driver.</P
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></LI
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><LI
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><P
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><TT
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CLASS="FUNCTION"
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>eth_drv_dsr()</TT
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> in the generic driver awakens the
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&#8220;Network Delivery Thread&#8221; which calls the deliver function
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<TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_deliver() in the driver.</P
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></LI
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><LI
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><P
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>The deliver function realizes that a transmit request has completed,
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and calls the callback tx-done function
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<TT
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CLASS="FUNCTION"
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>(sc-&#62;funs-&#62;eth_drv-&#62;tx_done)()</TT
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>
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with the same key that it remembered for this tx.</P
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></LI
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><LI
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><P
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>The callback tx-done function
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uses the key to find the resources associated with
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this transmit request; thus the stack knows that the transmit has
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completed and its resources can be freed.</P
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></LI
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><LI
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><P
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>The callback tx-done function
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also enquires whether <TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_can_send() now says
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&#8220;yes, we can send&#8221;
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and if so, dequeues a further transmit request
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which may have been queued as described above.  If so, then
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<TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_send() copies the data into the hardware buffers, or
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instructs the hardware to "send that" and remembers the new key, as above.
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These calls then all return to the &#8220;Network Delivery Thread&#8221;
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which then sleeps, awaiting the next asynchronous event.</P
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></LI
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><LI
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><P
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>All done &#8230;</P
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></LI
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></OL
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="IO-ETH-CALL-GRAPH-RX">Receive</H2
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><P
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></P
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><OL
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TYPE="1"
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><LI
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><P
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>Asynchronously, the hardware makes an interrupt to say
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&#8220;there is ready data in a receive buffer.&#8221;
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The ISR quietens the interrupt source in the hardware and
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requests that the associated DSR be run.</P
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></LI
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><LI
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><P
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>The DSR calls (or <SPAN
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CLASS="emphasis"
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><I
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CLASS="EMPHASIS"
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>is</I
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></SPAN
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>) the
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<TT
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CLASS="FUNCTION"
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>eth_drv_dsr()</TT
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> function in the generic driver.</P
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></LI
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><LI
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><P
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><TT
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CLASS="FUNCTION"
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>eth_drv_dsr()</TT
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> in the generic driver awakens the
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&#8220;Network Delivery Thread&#8221; which calls the deliver function
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<TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_deliver() in the driver.</P
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></LI
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><LI
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><P
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>The deliver function realizes that there is data ready and calls
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the callback receive function
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<TT
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CLASS="FUNCTION"
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>(sc-&#62;funs-&#62;eth_drv-&#62;recv)()</TT
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>
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to tell it how many bytes to prepare for.</P
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></LI
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><LI
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><P
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>The callback receive function allocates memory within the stack
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(eg. <SPAN
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CLASS="TYPE"
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>MBUFs</SPAN
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> in BSD/Unix style stacks) and prepares
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a set of scatter-gather buffers that can
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accommodate the packet.</P
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></LI
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><LI
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><P
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>It then calls back into the hardware driver routine
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<TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_recv().
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<TT
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CLASS="REPLACEABLE"
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><I
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>HRDWR</I
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></TT
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>_recv() must copy the data from the
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hardware's buffers into the scatter-gather buffers provided, and return.</P
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></LI
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><LI
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><P
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>The network stack now has the data in-hand, and does with it what it will.
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This might include recursive calls to transmit a response packet.
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When this all is done, these calls return, and the
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&#8220;Network Delivery Thread&#8221;
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sleeps once more, awaiting the next asynchronous event.</P
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>Upper Layer Functions</TD
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>Up</A
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>SNMP</TD
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