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

[Fu Wei]

Hi all,

On 27 July 2016 at 11:33, Jisheng Zhang <jszhang at marvell.com> wrote:
> +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?

yes, that is a good idea. Thanks Timur! :-)

>
> +1

I like this idea too, but please allow me to upstream this patch separately,
because this GTDT patchset can work without it, this readq support is
a  optimizing.

I also can see another arm-related driver are using readq in this way(
#ifdef readq): bus/arm-ccn.c
And some other drivers are also doing this.

>
> 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%!

Great thanks for your data, :-)

>
> Thanks,
> Jisheng



-- 
Best regards,

Fu Wei
Software Engineer
Red Hat