[PATCH] ARM: fix cpu_relax() in case of doing dmb

[Jon Medhurst (Tixy)]

On Thu, 2012-08-23 at 21:58 +0800, Shawn Guo wrote:
> On Thu, Aug 23, 2012 at 11:43:56AM +0100, Will Deacon wrote:
> > On Wed, Aug 22, 2012 at 03:52:18PM +0100, Shawn Guo wrote:
> > > diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
> > > index 99afa74..7cc67ce 100644
> > > --- a/arch/arm/include/asm/processor.h
> > > +++ b/arch/arm/include/asm/processor.h
> > > @@ -80,7 +80,14 @@ extern void release_thread(struct task_struct *);
> > >  unsigned long get_wchan(struct task_struct *p);
> > >  
> > >  #if __LINUX_ARM_ARCH__ == 6 || defined(CONFIG_ARM_ERRATA_754327)
> > > -#define cpu_relax()			smp_mb()
> > > +#define cpu_relax()		do {					\
> > > +					asm("nop");			\
> > > +					asm("nop");			\
> > > +					asm("nop");			\
> > > +					asm("nop");			\
> > > +					asm("nop");			\
> > 
> > Can you use nop() instead of the explicit asm?
> 
> Yes.  I just tried, and it works too.
> 
> > Also, I think we should try
> > and use some methodology on deciding the number of nops to insert. Without
> > having a full handle on the problem at the moment, it would seem that we
> > need at least NR_CPUS worth (since the number of spinning secondaries is
> > NR_CPUS-1 and they may execute their barriers in lock-step).
> > 
> I'm not sure we get something like that.  In my testing here, I need
> at least 5 nops to get rid of the issue.

Doesn't A9 do dual issue? If so, the maths for your 4 core iMX6Q might
match up with Will's hypothesis. You could try the theory by building
say with CONFIG_NR_CPUS == 3.

-- 
Tixy