Re: [PATCH 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile

[David Brown]

On Sun, Jan 02 2011, Pavel Machek wrote:

>> > diff --git a/drivers/char/hvc_dcc.c b/drivers/char/hvc_dcc.c
>> > index 6470f63..155ec10 100644
>> > --- a/drivers/char/hvc_dcc.c
>> > +++ b/drivers/char/hvc_dcc.c
>> > @@ -33,8 +33,7 @@
>> >  static inline u32 __dcc_getstatus(void)
>> >  {
>> >  	u32 __ret;
>> > -
>> > -	asm("mrc p14, 0, %0, c0, c1, 0	@ read comms ctrl reg"
>> > +	asm volatile("mrc p14, 0, %0, c0, c1, 0	@ read comms ctrl reg"
>> >  		: "=r" (__ret) : : "cc");
>> >  
>> >  	return __ret;
>
> Is volatile needed here? If __dcc_getstatus() return value is
> discarded, we want assembly discarded, right?

That's not really the issue being fixed.  Without the volatile, the
compiler is free to cache and reuse a previously loaded status value.
It is important that the status be read each time.

I don't think there is a way of indicating that assembly needs to happen
for each use, but that it is OK to discard if the value isn't used.
'volatile' is a bit overloaded.

-- 
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