linux-kernel - Re: [PATCH 2/4] msm: scm: Fix improper register assignment

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Date:	Fri, 25 Feb 2011 13:23:39 +0000
From:	Will Deacon <will.deacon@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	David Brown <davidb@...eaurora.org>, linux-arm-msm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/4] msm: scm: Fix improper register assignment

On Thu, 2011-02-24 at 18:44 +0000, Stephen Boyd wrote:
> Assign the registers used in the inline assembly immediately
> before the inline assembly block. This ensures the compiler
> doesn't optimize away dead register assignments when it
> shouldn't.
> 
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> ---
>  arch/arm/mach-msm/scm.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm/mach-msm/scm.c b/arch/arm/mach-msm/scm.c
> index ba57b5a..5eddf54 100644
> --- a/arch/arm/mach-msm/scm.c
> +++ b/arch/arm/mach-msm/scm.c
> @@ -264,13 +264,16 @@ u32 scm_get_version(void)
>  {
>         int context_id;
>         static u32 version = -1;
> -       register u32 r0 asm("r0") = 0x1 << 8;
> -       register u32 r1 asm("r1") = (u32)&context_id;
> +       register u32 r0 asm("r0");
> +       register u32 r1 asm("r1");
> 
>         if (version != -1)
>                 return version;
> 
>         mutex_lock(&scm_lock);
> +
> +       r0 = 0x1 << 8;
> +       r1 = (u32)&context_id;
>         asm volatile(
>                 __asmeq("%0", "r1")
>                 __asmeq("%1", "r0")


Whoa, have you seen the compiler `optimise' the original assignments
away? Since there is a use in the asm block, the definition shouldn't
be omitted. What toolchain are you using?

Will

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