linux-kernel - Re: [PATCH] clocksource: arm_arch_timer: Don't assume clock runs in suspend

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Date:   Fri, 16 Sep 2016 09:06:55 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Brian Norris <briannorris@...omium.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Mark Rutland <mark.rutland@....com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        linux-rockchip@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clocksource: arm_arch_timer: Don't assume clock runs in
 suspend

Hi Brian,

On 16/09/16 06:49, Brian Norris wrote:
> Since commit 4fbcdc813fb9 ("clocksource: arm_arch_timer: Use clocksource
> for suspend timekeeping"), this driver assumes that the ARM architected
> timer keeps running in suspend. This is not the case for some ARM SoCs,
> depending on the HW state used for system suspend. Let's not assume that
> all SoCs support this, and instead only support this if the device tree
> explicitly tells us it's "always on". In all other cases, just fall back
> to the RTC. This should be relatively harmless.

I'm afraid you're confusing two things:
- the counter, which *must* carry on counting no matter what, as
(quoting the ARM ARM) "The system counter must be implemented in an
always-on power domain"
- the timer, which is allowed to be powered off, and can be tagged with
the "always-on" property to indicate that it is guaranteed to stay up
(which in practice only exists in virtual machines and never on real HW).

If your counter does stop counting when suspended, then this is starting
to either feel like a HW bug, or someone is killing the clock that feeds
this counter when entering suspend.

If this is the former, then we need a separate quirk to indicate the
non-standard behaviour. If it is the latter, don't do it! ;-)

> 
> It seems fair to key the system-suspend behavior off the same property
> used for C3STOP, since if the timer doesn't keep context for CPU sleep,
> it likely doesn't for system sleep either.
> 
> Signed-off-by: Brian Norris <briannorris@...omium.org>
> ---
>  drivers/clocksource/arm_arch_timer.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 57700541f951..e28677a34f02 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -490,7 +490,7 @@ static struct clocksource clocksource_counter = {
>  	.rating	= 400,
>  	.read	= arch_counter_read,
>  	.mask	= CLOCKSOURCE_MASK(56),
> -	.flags	= CLOCK_SOURCE_IS_CONTINUOUS | CLOCK_SOURCE_SUSPEND_NONSTOP,
> +	.flags	= CLOCK_SOURCE_IS_CONTINUOUS,
>  };
>  
>  static struct cyclecounter cyclecounter = {
> @@ -526,6 +526,8 @@ static void __init arch_counter_register(unsigned type)
>  		clocksource_counter.name = "arch_mem_counter";
>  	}
>  
> +	if (!arch_timer_c3stop)
> +		clocksource_counter.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
>  	start_count = arch_timer_read_counter();
>  	clocksource_register_hz(&clocksource_counter, arch_timer_rate);
>  	cyclecounter.mult = clocksource_counter.mult;
> 

Given the above, I don't think this patch is acceptable.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...