netdev - Re: alignment faults in 3.6

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Message-Id: <201210111228.25995.arnd@arndb.de>
Date:	Thu, 11 Oct 2012 12:28:25 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"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>,
	Rob Herring <robherring2@...il.com>
Subject: Re: alignment faults in 3.6

On Thursday 11 October 2012, Eric Dumazet wrote:
> On Thu, 2012-10-11 at 11:32 +0100, Russell King - ARM Linux wrote:
> > On Thu, Oct 11, 2012 at 12:22:06PM +0200, Eric Dumazet wrote:
> > > I took a look, and I dont see why/how gcc could use a ldm instruction
> > > 
> > > Doing so assumed the alignment of the structure was 8 bytes, but its
> > > not.
> > > 
> > > Networking stack mandates that IP headers are aligned on 4 bytes
> > > boundaries, not 8 bytes.
> > 
> > Err, no.  ldm is "load multiple" not "load double".  It loads multiple
> > 32-bit registers, and its requirement for non-faulting behaviour is for
> > the pointer to be 4 byte aligned.  However, "load double" requires 8
> > byte alignment.
> 
> So if you have an alignment fault, thats because IP header is not
> aligned on 4 bytes ?
> 
> If so a driver is buggy and must be fixed.
> 
> Please send us full stack trace

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.

	Arnd
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