linux-kernel - Re: [PATCH 1/4] msm: scm: Mark inline asm as volatile

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Message-ID: <1298975439.7828.2.camel@e102144-lin.cambridge.arm.com>
Date:	Tue, 01 Mar 2011 10:30:39 +0000
From:	Will Deacon <will.deacon@....com>
To:	David Brown <davidb@...eaurora.org>
Cc:	Stephen Boyd <sboyd@...eaurora.org>, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/4] msm: scm: Mark inline asm as volatile

Hi David,

On Sun, 2011-02-27 at 17:38 +0000, David Brown wrote:
> Per the gcc manual:
> 
>     If your assembler instructions access memory in an unpredictable
>    fashion, add `memory' to the list of clobbered registers.  This will
>    cause GCC to not keep memory values cached in registers across the
>    assembler instruction and not optimize stores or loads to that
>    memory.  You will also want to add the `volatile' keyword if the
>    memory affected is not listed in the inputs or outputs of the `asm',
>    as the `memory' clobber does not count as a side-effect of the `asm'.
>    If you know how large the accessed memory is, you can add it as input
>    or output but if this is not known, you should add `memory'.  As an
>    example, if you access ten bytes of a string, you can use a memory
>    input like:
> 
Right, so if you neglected to check the output from the smc block then
it would be a candidate for removal even with a memory clobber. Now I
see why you want a volatile in there!

For what it's worth:

Acked-by: Will Deacon <will.deacon@....com>

Will

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