linux-kernel - Re: [PATCH] ARM: alignment: setup alignment handler earlier

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Date:	Wed, 07 Sep 2011 18:40:11 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Måns Rullgård <mans@...sr.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Dave Martin <dave.martin@...aro.org>,
	John Ogness <john.ogness@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	"linux-arm-kernel\@lists.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: alignment: setup alignment handler earlier

Russell King - ARM Linux <linux@....linux.org.uk> writes:

> On Wed, Sep 07, 2011 at 05:42:19PM +0100, Måns Rullgård wrote:
>> There are such instructions (ldrd, ldm), but gcc will not emit those
>> unless the address is known to be aligned.  For ARMv6 and later, gcc 4.6
>> *will* emit potentially unaligned ldr and ldrh since these very clearly
>> allow an unaligned address and are faster than the alternatives in all
>> implementations to date.  This is unless strict alignment checking is
>> explicitly enabled, which unfortunately the Linux kernel does for no
>> apparent reason at all.
>
> "no apparant reason at all" heh.  The reason is to keep the code
> simple and free from bugs.

Some people, myself included, consider the current behaviour a bug.

> To do otherwise means that each of the CPU files needs to be littered
> with ifdefs to deal with the alignment fault configuration, of which
> there are 16 of them (ignoring v6 and v7.)
>
> If you think code maintanence of the same thing in 16 places is efficient
> then I guess there is "no apparant reason".  I beg to differ, being one
> of those folk who have had to edit 18 different places several times.
>
> So no, I do not intend to move this:
>
> #ifdef CONFIG_ALIGNMENT_TRAP
>         orr     r0, r0, #CR_A
> #else
>         bic     r0, r0, #CR_A
> #endif
>
> into 16 separate places in the kernel.

So change that condition to also depend on !CPU_V6 && !CPU_V7 or
something equivalent.

-- 
Måns Rullgård
mans@...sr.com
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