On Sun, 27 Feb 2011, David Brown wrote: > On Sat, Feb 26 2011, Nicolas Pitre wrote: > > > 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@xxxxxxxxxxxxxx> > >> >> --- > >> >> 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. > > Would this be better as a comment by the assembly or for the whole file > or function? The entire purpose of this file is to communicate with > another logical processor, so it's all about producing side effects > other than just modifying the registers or the memory. Maybe a file > comment briefly explaining that SCM runs in TrustZone and a short > comment by each asm stating that it traps to the other logical cpu? Now that I've looked more closely at the actual code, I think it is obvious enough that the volatile is needed and no extra comment should be required. Nicolas -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html