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Message-ID: <20090624104750.GA18486@hmsreliant.think-freely.org>
Date: Wed, 24 Jun 2009 06:47:50 -0400
From: Neil Horman <nhorman@...driver.com>
To: Nicholas Van Orton <turanammo@...il.com>
Cc: netdev@...r.kernel.org
Subject: Re: e100 skb allocation
On Wed, Jun 24, 2009 at 02:19:48PM +0530, Nicholas Van Orton wrote:
> Hello,
>
> For the Intel 8256 Ethernet Controller (e100.c), I notice that at the
> time of skb allocation using netdev_alloc_skb() and just before
> calling skb_reserve(), rx->skb->head and rx->skb->tail do not point to
> the same location. In fact they are already 16bytes apart. After
> calling skb_reserve(), which would add a 2 byte padding, head and
> tail would be 18 bytes apart. Is this behaviour correct? Shouldn't the
> ethernet header be 14+2 bytes after padding? Why aren't head and tail
> at the same location at the beginning?
>
Yes, its fine (although IIRC it should be 32, rather than 16 bytes).
netdev_alloc_skb, in its implementation, allocates an skb that is NET_SKB_PAD
bytes larger than the caller requested, then it calls skb_reserve internally to
add NET_SKB_PAD bytes of data to the skb's buffer.
Neil
>
> Thanks and regards,
> Nicholas
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