linux-kernel - Re: [PATCH v8 4/9] clocksource/drivers/arm_arch

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Message-ID: <20160725090258.GA15864@arm.com>
Date:	Mon, 25 Jul 2016 10:02:59 +0100
From:	Will Deacon <will.deacon@....com>
To:	fu.wei@...aro.org
Cc:	rjw@...ysocki.net, lenb@...nel.org, daniel.lezcano@...aro.org,
	tglx@...utronix.de, marc.zyngier@....com,
	lorenzo.pieralisi@....com, sudeep.holla@....com,
	hanjun.guo@...aro.org, linux-arm-kernel@...ts.infradead.org,
	linaro-acpi@...ts.linaro.org, linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org, rruigrok@...eaurora.org,
	harba@...eaurora.org, cov@...eaurora.org, timur@...eaurora.org,
	graeme.gregory@...aro.org, al.stone@...aro.org, jcm@...hat.com,
	wei@...hat.com, arnd@...db.de, wim@...ana.be,
	catalin.marinas@....com, Suravee.Suthikulpanit@....com,
	leo.duran@....com, linux@...ck-us.net,
	linux-watchdog@...r.kernel.org
Subject: Re: [PATCH v8 4/9] clocksource/drivers/arm_arch_timer: use readq to
 get 64-bit CNTVCT

On Wed, Jul 20, 2016 at 02:17:59AM +0800, fu.wei@...aro.org wrote:
> From: Fu Wei <fu.wei@...aro.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@...aro.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