[PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP

From: mans@mansr.com (Måns Rullgård)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] ARM: Do not allow unaligned accesses when CONFIG_ALIGNMENT_TRAP
Date: Mon, 23 May 2011 15:37:49 +0100	[thread overview]
Message-ID: <yw1xzkmdpjnm.fsf@unicorn.mansr.com> (raw)
In-Reply-To: 1306158690.7196.15.camel@e102109-lin.cambridge.arm.com

Catalin Marinas <catalin.marinas@arm.com> writes:

> On Mon, 2011-05-23 at 14:21 +0100, Russell King - ARM Linux wrote:
>> On Mon, May 23, 2011 at 12:16:48PM +0100, Catalin Marinas wrote:
>> > Newer versions of gcc generate unaligned accesses by default, causing
>> > kernel panics when CONFIG_ALIGNMENT_TRAP is enabled. This patch adds the
>> > -mno-unaligned-access option to gcc.
>> 
>> This description doesn't make sense.  If we have alignment traps enabled,
>> then we _expect_ to fix up unaligned loads and stores.
>> 
>> Therefore there should be no panic if CONFIG_ALIGNMENT_TRAP is enabled.
>> 
>> So what's the _actual_ problem that this is trying to address?  What's
>> the panic/oops look like?  And that information should be in the commit
>> description _anyway_.
>
> OK, not clear from my commit log. See below for details (I'll add them
> to the log):
>
> http://www.codesourcery.com/archives/arm-gnu/msg04202.html
>
> Basically the fault happens before we call alignment_init(). There are
> other ways to fix this issue (like initialising the alignment handler
> earlier).

The reset value of SCTLR.A is 0 on ARMv7.  Something must be setting it
to 1 early, before the alignment trap handler is initialised.  This
something is what needs to be changed.

-- 
M?ns Rullg?rd
mans at mansr.com