netdev - Re: igb driver triggers BUILD_BUG

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Date:	Mon, 11 Mar 2013 08:47:12 -0700
From:	Alexander Duyck <alexander.h.duyck@...el.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
CC:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: igb driver triggers BUILD_BUG_ON on s390

On 03/09/2013 02:00 AM, Heiko Carstens wrote:
> git commit 74e238ead "igb: Support using build_skb in the case that
> jumbo frames are disabled" added a BUILD_BUG_ON() to the igb drivers
> which triggers on s390:
>
> drivers/net/ethernet/intel/igb/igb_main.c:6231:2:
> error: call to ‘__compiletime_assert_6235’ declared with attribute error:
>   BUILD_BUG_ON failed: SKB_WITH_OVERHEAD(IGB_RX_BUFSZ) <
>     (NET_SKB_PAD + NET_IP_ALIGN + IGB_TS_HDR_LEN + ETH_FRAME_LEN + ETH_FCS_LEN)
>
> This happens mainly because s390 has an unusual large L1_CACHE_BYTES aka
> SMP_CACHE_BYTES define with 256 Bytes.
>
> The BUILD_BUG_ON(..) translates to (taken from .i file):
>
> __cond = !(!(((2048) - (((sizeof(struct skb_shared_info)) + (256 - 1)) & ~(256 - 1))) < (32 + 2 + 16 + 1514 + 4)));
>
> With sizeof(struct skb_shared_info) == 320 we end up with
>
> BUILD_BUG_ON((2048 - 512 = 1536) < 1568)
>
> One possible solution would be to simply disable the driver on s390.
> Any opinions?
>

A fix for this has already been submitted. As it turns out the
BUILD_BUG_ON is mostly redundant anyway since we used a similar
calculation to determine if we were going to enable the use of build_skb
per ring.

Thanks,

Alex
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