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

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Wed, Jul 20, 2016 at 02:17:59AM +0800, fu.wei-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org wrote:
> From: Fu Wei <fu.wei-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> 
> 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-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> ---
>  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);

What's the benefit of doing this? If you use readq here, how can we
guarantee that (a) the endpoint won't generate a SLVERR or similar and
(b) that we get an atomic read?

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

Will
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