Messages in this thread | | | From | David Brown <> | Subject | Re: [PATCH 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile | Date | Sun, 02 Jan 2011 10:49:32 -0800 |
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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.
-- Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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