netdev - Re: [E1000-devel] [next:akpm 16/587] 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

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Date:	Wed, 20 Feb 2013 15:02:24 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	"Allan, Bruce W" <bruce.w.allan@...el.com>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
	Daniel Santos <daniel.santos@...ox.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Wu, Fengguang" <fengguang.wu@...el.com>
Subject: Re: [E1000-devel] [next:akpm 16/587]
 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...

On Wed, 2013-02-20 at 14:47 -0800, Alexander Duyck wrote:

> 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.

Oh well, for one moment I thought the SKB_DATA_ALIGN() was also using
NET_SKB_PAD, but its using SMP_CACHE_BYTES.



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