linux-kernel - Re: [PATCH] nommu: Enable the strict alignment (CR

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Message-ID: <20121203165442.GJ20074@mudshark.cambridge.arm.com>
Date:	Mon, 3 Dec 2012 16:54:42 +0000
From:	Will Deacon <will.deacon@....com>
To:	Armando Visconti <armando.visconti@...com>
Cc:	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"kgene.kim@...sung.com" <kgene.kim@...sung.com>,
	"swarren@...dia.com" <swarren@...dia.com>,
	"nico@...aro.org" <nico@...aro.org>,
	spear-devel <spear-devel@...t.st.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hsweeten@...ionengravers.com" <hsweeten@...ionengravers.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] nommu: Enable the strict alignment (CR_A) bit only if
 ARCH < v6

On Mon, Dec 03, 2012 at 04:44:11PM +0000, Armando Visconti wrote:
> On 11/29/2012 12:04 PM, Armando VISCONTI wrote:
> > This patch keeps disabled the strict alignment CP15 bit for
> > all armv6 and armv7 processor without the mmu. This behaviour
> > is now same as in the mmu case.
> >
> > Signed-off-by: Armando Visconti<armando.visconti@...com>
> > ---
> >   arch/arm/kernel/head-nommu.S |    2 +-
> >   1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
> > index 278cfc1..2c228a0 100644
> > --- a/arch/arm/kernel/head-nommu.S
> > +++ b/arch/arm/kernel/head-nommu.S
> > @@ -68,7 +68,7 @@ __after_proc_init:
> >   	 * CP15 system control register value returned in r0 from
> >   	 * the CPU init function.
> >   	 */
> > -#ifdef CONFIG_ALIGNMENT_TRAP
> > +#if defined(CONFIG_ALIGNMENT_TRAP)&&  __LINUX_ARM_ARCH__<  6
> >   	orr	r0, r0, #CR_A
> >   #else
> >   	bic	r0, r0, #CR_A
> 
> Any feedback on this simple patch?

Although this is correct, you still need some extra patches to avoid
unaligned accesses to strongly-ordered memory (given that we don't
currently have mainline support for the MPU). Take a look at my nommu/fixes
branch:

  https://git.kernel.org/?p=linux/kernel/git/will/linux.git;a=shortlog;h=refs/heads/nommu/fixes

I'm happy to post some of that if it's useful to you.

Will
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