netdev - Re: alignment faults in 3.6

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Message-Id: <201210111335.43177.arnd@arndb.de>
Date:	Thu, 11 Oct 2012 13:35:42 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Rob Herring <robherring2@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	linux-arm-kernel@...ts.infradead.org,
	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	Jon Masters <jonathan@...masters.org>, netdev@...r.kernel.org,
	Måns Rullgård <mans@...sr.com>,
	David Laight <David.Laight@...lab.com>
Subject: Re: alignment faults in 3.6

On Thursday 11 October 2012, Rob Herring wrote:
> 
> On 10/11/2012 07:40 AM, Eric Dumazet wrote:
> > On Thu, 2012-10-11 at 12:28 +0000, Arnd Bergmann wrote:
> > 
> >>
> >> Rob Herring as the original reporter has dropped off the Cc list, adding
> >> him back.
> >>
> >> I assume that the calxeda xgmac driver is the culprit then. It uses
> >> netdev_alloc_skb() rather than netdev_alloc_skb_ip_align() in
> >> xgmac_rx_refill but it is not clear whether it does so intentionally
> >> or by accident.
> 
> This in fact does work and eliminates the unaligned traps. However, not
> all h/w can do IP aligned DMA (i.MX FEC for example), so I still think
> this is a questionable optimization by the compiler. We're saving 1 load
> instruction here for data that is likely already in the cache. It may be
> legal per the ABI, but the downside of this optimization is much greater
> than the upside.
> 

What about the other approach that Eric suggested for such hardware
in http://www.spinics.net/lists/arm-kernel/msg200206.html ?

	Arnd
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