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

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Date:	Wed, 7 Sep 2011 18:24:45 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Måns Rullgård <mans@...sr.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@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: alignment: setup alignment handler earlier

On Wed, Sep 07, 2011 at 06:04:52PM +0100, Russell King - ARM Linux wrote:
> 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.  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.

What about something like this (untested):

#if defined(CONFIG_ALIGNMENT_TRAP) && __LINUX_ARM_ARCH__ < 6
	orr	r0, r0, #CR_A
#else
	bic	r0, r0, #CR_A
#endif

-- 
Catalin
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