linux-kernel - Re: [PATCH 1/2] clocksource/drivers/exynos_mct: Prioritise Arm arch timer on arm64

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Date:   Mon, 21 Jun 2021 11:10:59 +0100
From:   Will Deacon <will@...nel.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Chanwoo Choi <cw00.choi@...sung.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] clocksource/drivers/exynos_mct: Prioritise Arm arch
 timer on arm64

On Mon, Jun 21, 2021 at 11:25:47AM +0200, Daniel Lezcano wrote:
> On 17/06/2021 23:47, Will Deacon wrote:
> > On Thu, Jun 17, 2021 at 09:58:35AM +0900, Chanwoo Choi wrote:
> >> On 6/17/21 12:25 AM, Daniel Lezcano wrote:
> >>> On 10/06/2021 03:03, Chanwoo Choi wrote:
> >>>> On 6/9/21 12:43 AM, Will Deacon wrote:
> >>>>> diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
> >>>>> index fabad79baafc..804d3e01c8f4 100644
> >>>>> --- a/drivers/clocksource/exynos_mct.c
> >>>>> +++ b/drivers/clocksource/exynos_mct.c
> >>>>> @@ -51,6 +51,15 @@
> >>>>>  
> >>>>>  #define TICK_BASE_CNT	1
> >>>>>  
> >>>>> +#ifdef CONFIG_ARM
> >>>>> +/* Use values higher than ARM arch timer. See 6282edb72bed. */
> >>>>> +#define MCT_CLKSOURCE_RATING		450
> >>>>> +#define MCT_CLKEVENTS_RATING		500
> >>>>> +#else
> >>>>> +#define MCT_CLKSOURCE_RATING		350
> >>>>> +#define MCT_CLKEVENTS_RATING		350
> >>>>> +#endif
> >>>>> +
> >>>>>  enum {
> >>>>>  	MCT_INT_SPI,
> >>>>>  	MCT_INT_PPI
> >>>>> @@ -206,7 +215,7 @@ static void exynos4_frc_resume(struct clocksource *cs)
> >>>>>  
> >>>>>  static struct clocksource mct_frc = {
> >>>>>  	.name		= "mct-frc",
> >>>>> -	.rating		= 450,	/* use value higher than ARM arch timer */
> >>>>> +	.rating		= MCT_CLKSOURCE_RATING,
> >>>>>  	.read		= exynos4_frc_read,
> >>>>>  	.mask		= CLOCKSOURCE_MASK(32),
> >>>>>  	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
> >>>>> @@ -457,7 +466,7 @@ static int exynos4_mct_starting_cpu(unsigned int cpu)
> >>>>>  	evt->set_state_oneshot_stopped = set_state_shutdown;
> >>>>>  	evt->tick_resume = set_state_shutdown;
> >>>>>  	evt->features = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT;
> >>>>> -	evt->rating = 500;	/* use value higher than ARM arch timer */
> >>>>> +	evt->rating = MCT_CLKEVENTS_RATING,
> >>>>>  
> >>>>>  	exynos4_mct_write(TICK_BASE_CNT, mevt->base + MCT_L_TCNTB_OFFSET);
> >>>>>  
> >>>>>
> >>>>
> >>>> I'm not sure that exynos mct is working without problem
> >>>> such as the case of 6282edb72bed.
> >>>> As described on On ,6282edb72bed the arch timer on exynos SoC
> >>>> depends on Exynos MCT device. the arch timer is not able to work
> >>>> without Exynos MCT because of using the common module.
> >>>
> >>> Is it possible to change the DT to have a phandle to the exynos_mct, so
> >>> it will be probed before the arch_arm_timer ?
> >>
> >> I think that DT changes is not proper way to keep the order between
> >> exynos_mct and arch timer.
> > 
> > exynos4_mct_frc_start() is called unconditionally from probe via
> > exynos4_clocksource_init() so as long as the mct probes first, then the
> > arch timer should work, no? The rating shouldn't affect that.
> 
> How do you ensure the exynos mct is probed before the arch timer ?
> 
> The Makefile provides the right order, but the dependency is implicit.

Currently, I think it's done by the order of the CPU hotplug notifiers (
see the hunk of 6282edb72bed which touches cpuhotplug.h).

Will