Re: [PATCH v8 4/9] clocksource/drivers/arm_arch_timer: use readq to get 64-bit CNTVCT

[Fu Wei]

Hi Will,

On 25 July 2016 at 17:02, Will Deacon <will.deacon@xxxxxxx> wrote:
> On Wed, Jul 20, 2016 at 02:17:59AM +0800, fu.wei@xxxxxxxxxx wrote:
>> From: Fu Wei <fu.wei@xxxxxxxxxx>
>>
>> This patch simplify arch_counter_get_cntvct_mem function by
>> using readq to get 64-bit CNTVCT value instead of readl_relaxed.
>>
>> Signed-off-by: Fu Wei <fu.wei@xxxxxxxxxx>
>> ---
>>  drivers/clocksource/arm_arch_timer.c | 10 +---------
>>  1 file changed, 1 insertion(+), 9 deletions(-)
>>
>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
>> index e6fd42d..483d2f9 100644
>> --- a/drivers/clocksource/arm_arch_timer.c
>> +++ b/drivers/clocksource/arm_arch_timer.c
>> @@ -418,15 +418,7 @@ u32 arch_timer_get_rate(void)
>>
>>  static u64 arch_counter_get_cntvct_mem(void)
>>  {
>> -     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;
>> +     return readq(arch_counter_base + CNTVCT_LO);
>

Sorry, right after posting v9, I got your comment,

> What's the benefit of doing this? If you use readq here, how can we

benefit:
1. simplify the code
2. from arch/arm64/include/asm/io.h, I guess readq is more efficient

> guarantee that (a) the endpoint won't generate a SLVERR or similar and
> (b) that we get ?

I think so, according to arch/arm64/include/asm/io.h.
readq Implement by "LDR" and "LDAR", So I think It is an atomic read.

Please correct me, If I misunderstand something, thanks

>
> "If it ain't broke, don't fix it"
>
> Will



-- 
Best regards,

Fu Wei
Software Engineer
Red Hat