netdev - Re: e100 skb allocation

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Date:	Wed, 24 Jun 2009 21:58:38 +0530
From:	Nicholas Van Orton <turanammo@...il.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	netdev@...r.kernel.org
Subject: Re: e100 skb allocation

On Wed, Jun 24, 2009 at 4:17 PM, Neil Horman<nhorman@...driver.com> wrote:
> 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.
>

Thanks a bunch for steering me in the right direction. Things are much
clearer now.

Regards,
Nicholas
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