<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"> <HTML ><HEAD ><TITLE >Driver Operation</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="Tulip Driver User's Guide" HREF="book1.html"><LINK REL="PREVIOUS" TITLE="Board-specific Settings" HREF="c35.html"><LINK REL="NEXT" TITLE="Synchronization" HREF="x45.html"></HEAD ><BODY CLASS="CHAPTER" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >Tulip Driver User's Guide</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="c35.html" ACCESSKEY="P" ><<< Previous</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="x45.html" ACCESSKEY="N" >Next >>></A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="CHAPTER" ><H1 ><A NAME="DRIVER-OPERATION" ></A >Driver Operation</H1 ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="AEN41" ></A >Ring buffers</H1 ><P >The Tulip can use either ring buffers or lists of Tx and Rx descriptors. This driver uses statically allocated rings of Rx and Tx descriptors, set at compile time by RX/TX_RING_SIZE. This version of the driver allocates skbuffs for the Rx ring buffers at open() time and passes the skb->data field to the Tulip as receive data buffers. When an incoming frame is less than RX_COPYBREAK bytes long, a fresh skbuff is allocated and the frame is copied to the new skbuff. When the incoming frame is larger, the skbuff is passed directly up the protocol stack and replaced by a newly allocated skbuff.</P ><P >The RX_COPYBREAK value is chosen to trade-off the memory wasted by using a full-sized skbuff for small frames vs. the copying costs of larger frames. For small frames the copying cost is negligible (esp. considering that we are pre-loading the cache with immediately useful header information). For large frames the copying cost is non-trivial, and the larger copy might flush the cache of useful data. A subtle aspect of this choice is that the Tulip only receives into longword aligned buffers, thus the IP header at offset 14 isn't longword aligned for further processing. Copied frames are put into the new skbuff at an offset of "+2", thus copying has the beneficial effect of aligning the IP header and preloading the cache.</P ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="c35.html" ACCESSKEY="P" ><<< Previous</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="book1.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="x45.html" ACCESSKEY="N" >Next >>></A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Board-specific Settings</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" > </TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Synchronization</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
