linux-kernel - Re: [PATCH v2 1/2] clocksource/drivers/exynos_mct_v2: introduce Exynos MCT version 2 driver for next Exynos SoC

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Message-ID: <20211102102802.GA16545@C02TD0UTHF1T.local>
Date:   Tue, 2 Nov 2021 10:28:10 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Youngmin Nam <youngmin.nam@...sung.com>
Cc:     krzysztof.kozlowski@...onical.com, will@...nel.org,
        daniel.lezcano@...aro.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, pullip.cho@...sung.com,
        hoony.yu@...sung.com, hajun.sung@...sung.com,
        myung-su.cha@...sung.com, kgene@...nel.org
Subject: Re: [PATCH v2 1/2] clocksource/drivers/exynos_mct_v2: introduce
 Exynos MCT version 2 driver for next Exynos SoC

On Tue, Nov 02, 2021 at 09:11:21AM +0900, Youngmin Nam wrote:
> Exynos MCT version 2 is composed of 1 FRC and 12 comparators.
> There are no global timer and local timer anymore.
> The 1 of 64bit FRC serves as "up-counter"(not "comparators").
> The 12 comaprators(not "counter") can be used as per-cpu event timer
> so that it can support upto 12 cores.
> And a RTC source can be used as backup clock source.

[...]

> +static int exynos_mct_starting_cpu(unsigned int cpu)
> +{
> +	struct mct_clock_event_device *mevt = per_cpu_ptr(&percpu_mct_tick, cpu);
> +	struct clock_event_device *evt = &mevt->evt;
> +
> +	snprintf(mevt->name, sizeof(mevt->name), "mct_comp%d", cpu);
> +
> +	evt->name = mevt->name;
> +	evt->cpumask = cpumask_of(cpu);
> +	evt->set_next_event = exynos_comp_set_next_event;
> +	evt->set_state_periodic = mct_set_state_periodic;
> +	evt->set_state_shutdown = mct_set_state_shutdown;
> +	evt->set_state_oneshot = mct_set_state_shutdown;
> +	evt->set_state_oneshot_stopped = mct_set_state_shutdown;
> +	evt->tick_resume = mct_set_state_shutdown;
> +	evt->features = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT;
> +	evt->rating = 500;	/* use value higher than ARM arch timer */

Previously Will asked you to try CLOCK_EVT_FEAT_PERCPU here, and to set
the C3STOP flag on the arch timer via the DT when necessary, rather than
trying to override the arch timer like this:

  https://lore.kernel.org/r/20211027073458.GA22231@willie-the-truck

There are a bunch of things that depend on the architected timer working
as a clocksource (e.g. vdso, kvm), and it *should* work as a lock
clockevent_device if configured correctly, and it's much more consistent
with *everyone else* to use the arhcitected timer by default.

Please try as Will suggested above, so that this works from day one.

Thanks,
Mark.