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

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Message-id: <alpine.LFD.2.00.1102261439590.22034@xanadu.home>
Date:	Sat, 26 Feb 2011 14:43:16 -0500 (EST)
From:	Nicolas Pitre <nico@...xnic.net>
To:	David Brown <davidb@...eaurora.org>
Cc:	Will Deacon <will.deacon@....com>,
	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

On Sat, 26 Feb 2011, David Brown wrote:

> On Fri, Feb 25 2011, Will Deacon wrote:
> 
> > On Thu, 2011-02-24 at 18:44 +0000, Stephen Boyd wrote:
> >> We don't want the compiler to remove these asm statements or
> >> reorder them in any way. Mark them as volatile to be sure.
> >> 
> >> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
> >> ---
> >>  arch/arm/mach-msm/scm.c |    4 ++--
> >>  1 files changed, 2 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/arch/arm/mach-msm/scm.c b/arch/arm/mach-msm/scm.c
> >> index f4b9bc9..ba57b5a 100644
> >> --- a/arch/arm/mach-msm/scm.c
> >> +++ b/arch/arm/mach-msm/scm.c
> >> @@ -174,7 +174,7 @@ static u32 smc(u32 cmd_addr)
> >>         register u32 r0 asm("r0") = 1;
> >>         register u32 r1 asm("r1") = (u32)&context_id;
> >>         register u32 r2 asm("r2") = cmd_addr;
> >> -       asm(
> >> +       asm volatile(
> >>                 __asmeq("%0", "r0")
> >>                 __asmeq("%1", "r0")
> >>                 __asmeq("%2", "r1")
> >> @@ -271,7 +271,7 @@ u32 scm_get_version(void)
> >>                 return version;
> >> 
> >>         mutex_lock(&scm_lock);
> >> -       asm(
> >> +       asm volatile(
> >>                 __asmeq("%0", "r1")
> >>                 __asmeq("%1", "r0")
> >>                 __asmeq("%2", "r1")
> >
> > These asm blocks all have sensible looking output constraints. Why
> > do they need to be marked volatile?
> 
> Without the volatile, the compiler is free to assume the only side
> effects of the asm are to modify the output registers.  The volatile is
> needed to indicate to the compiler that the asm has other side effects.
> There isn't enough optimization, yet, in gcc to change the generated
> code in this case, so it happens to generate the correct code without
> it.
> 
> The second probably doesn't need it, unless we are expecting the version
> to change dynamically.  The volatile makes the scm_get_version()
> function clearly a call to scm, though, so is probably useful to
> document the intent.

If the inline asm does have side effects which are not obvious other 
than producing a result for the output operand then it is a good idea to 
add a comment to that effect.  Otherwise it is always best to omit the 
volatile and let gcc move the inline asm around or even delete it 
entirely when possible.


Nicolas
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