Re: [PATCH v9 4/9] clocksource/drivers/arm_arch_timer: use readq to get 64-bit CNTVCT

[Jisheng Zhang]

+1

On Tue, 26 Jul 2016 09:11:49 -0500 Timur Tabi  wrote:

> Will Deacon wrote:
> > The kernel really needs to support both of those platforms :/
> >
> > For the memory-mapped counter registers, the architecture says:
> >
> >    `If the implementation supports 64-bit atomic accesses, then the
> >     CNTV_CVAL register must be accessible as an atomic 64-bit value.'
> >
> > which is borderline tautological. If we take the generous reading that
> > this means AArch64 CPUs can use readq (and I'm not completely
> > comfortable with that assertion, particularly as you say that it breaks
> > the model), then you still need to use readq_relaxed here to avoid a
> > DSB. Furthermore, what are you going to do for AArch32? readq doesn't
> > exist over there, and if you use the generic implementation then it's
> > not atomic. In which case, we end up with the current code, as well as a
> > readq_relaxed guarded by a questionable #ifdef that is known to break a
> > supported platform for an unknown performance improvement. Hardly a big
> > win.  
> 
> I know Fu dropped this patch, and I don't want to kick a dead horse, but 
> I was wondering if it would be okay to do this:
> 
> static u64 arch_counter_get_cntvct_mem(void)
> {
> #ifdef readq_relaxed
> 	return readq_relaxed(arch_counter_base + CNTVCT_LO);
> #else
> 	u32 vct_lo, vct_hi, tmp_hi;
> 
> 	do {
> 		vct_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);
> 		vct_lo = readl_relaxed(arch_counter_base + CNTVCT_LO);
> 		tmp_hi = readl_relaxed(arch_counter_base + CNTVCT_HI);
> 	} while (vct_hi != tmp_hi);
> 
> 	return ((u64) vct_hi << 32) | vct_lo;
> #endif
> }
> 
> readq and readq_relaxed are defined in arch/arm64/include/asm/io.h.  Why 
> would the function exist if AArch64 CPUs can't use it?

+1

I measured the performance on berlin arm64 platforms:

compared with original version, using readq_relaxed could reduce
time of arch_counter_get_cntvct_mem() by about 42%!

Thanks,
Jisheng