On 02/20/2013 01:42 PM, Eric Dumazet wrote:
> On Wed, 2013-02-20 at 13:23 -0800, Alexander Duyck wrote:
>
>> NET_SKB_PAD is defined for the s390. It is already 32. If you look it
>> up we only have 2 definitions for NET_SKB_PAD, one specific to the s390
>> architecture and the other one in skbuff.h.
>>
> Andrew traces disagree, as they were :
>
>>>> s390 allmodconfig:
>>>>
>>>> bool __cond = !(!(((2048) - (((sizeof(struct skb_shared_info)) + (256 -
>>>> 1)) &
>>>> ~(256 - 1))) < (32 + 2 + 16 + 1514 + 4)));
>>>>
> So it might be only a cross-compile environment issue, I dont know.
Huh? I'm not seeing what you are saying. The NET_SKB_PAD is the value
that is in the last set of parenthesis since it was:
(NET_SKB_PAD + NET_IP_ALIGN + IGB_TS_HDR_LEN + ETH_FRAME_LEN + ETH_FCS_LEN)
that is the bit that became:
(32 + 2 + 16 + 1514 + 4)
The problem is the skb_shared_info bit rounds up to 512 reducing the
available space to 1536. If you add up all of the other bits ignoring
the NET_SKB_PAD value you end up with exactly 1536 meaning the only
value for NET_SKB_PAD that would work is 0.
>> From what I can tell we would have to drop the NET_SKB_PAD to 0 in order
>> to not trigger this error with igb since we still have to add 22 bytes
>> for igb's per packet timestamp header, IP alignment, and CRC.
>>
>> The simple fix is for us just to drop the BUILD_BUG_ON check for igb
>> since we already had a check for size check in igb_set_rx_buffer_len.
>> It just means that build_skb won't be available for standard MTU sizes
>> on s390.
> Yeah, probably nobody will notice ;)
Yeah, it is only a few percentage points difference and only really
impacts small packets anyway. :-)
Thanks,
Alex
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