Pierre Ossman a écrit :
On Mon, 22 Oct 2007 02:05:38 -0700 (PDT)
David Miller <[EMAIL PROTECTED]> wrote:
From: Pierre Ossman <[EMAIL PROTECTED]>
Date: Mon, 22 Oct 2007 10:42:08 +0200
This seems like a rather evil layering violation.
This has a 10+ year precedence and it's why the Linux networking stack
is so fast. If you read any other driver you would have seen the
skb_reserve() call every one of them do to align the headers.
The norm seems to be to not comment this call. It's hardly obvious.
This is not obvious you are right, but documented in
include/linux/skbuff.h (lines 1150 ...)
/*
* CPUs often take a performance hit when accessing unaligned memory
* locations. The actual performance hit varies, it can be small if the
* hardware handles it or large if we have to take an exception and fix it
* in software.
*
* Since an ethernet header is 14 bytes network drivers often end up with
* the IP header at an unaligned offset. The IP header can be aligned by
* shifting the start of the packet by 2 bytes. Drivers should do this
* with:
*
* skb_reserve(NET_IP_ALIGN);
*
* The downside to this alignment of the IP header is that the DMA is now
* unaligned. On some architectures the cost of an unaligned DMA is high
* and this cost outweighs the gains made by aligning the IP header.
*
* Since this trade off varies between architectures, we allow NET_IP_ALIGN
* to be overridden.
*/
#ifndef NET_IP_ALIGN
#define NET_IP_ALIGN 2
#endif
But then, many drivers dont use NET_IP_ALIGN but a hardcoded 2
drivers/net/pcnet32.c:608: skb_reserve(rx_skbuff, 2);
drivers/net/pcnet32.c:1214: skb_reserve(newskb, 2);
drivers/net/pcnet32.c:1245: skb_reserve(skb, 2); /* 16
byte align */
drivers/net/pcnet32.c:2396: skb_reserve(rx_skbuff, 2);
drivers/net/sundance.c:989: skb_reserve(skb, 2); /* 16
byte align the IP header. */
drivers/net/sundance.c:1312: skb_reserve(skb,
2); /* 16 byte align the IP header */
drivers/net/sundance.c:1375: skb_reserve(skb, 2); /*
Align IP on 16 byte boundaries */
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