[PATCH v1 2/6] clocksource/drivers/exynos_mct: Don't register as a sched_clock on arm64

[John Stultz]

On Mon, Mar 31, 2025 at 4:00 PM 'Will McVicker' via kernel-team
<kernel-team at android.com> wrote:
>
> When using the Exynos MCT as a sched_clock, accessing the timer value
> via the MCT register is extremely slow. To improve performance on Arm64
> SoCs, use the Arm architected timer instead for timekeeping.

This probably needs some further expansion to explain why we don't
want to use it for sched_clock but continue to register the MCT as a
clocksource (ie: why not disable MCT entirely?).

> Note, ARM32 SoCs don't have an architectured timer and therefore
> will continue to use the MCT timer. Detailed discussion on this topic
> can be found at [1].
>
> [1] https://lore.kernel.org/all/1400188079-21832-1-git-send-email-chirantan@chromium.org/

That's a pretty deep thread (more so with the duplicate messages, as
you used the "all" instead of a specific list). It might be good to
have a bit more of a summary here in the commit message, so folks
don't have to dig too deeply themselves.

> Signed-off-by: Donghoon Yu <hoony.yu at samsung.com>
> Signed-off-by: Youngmin Nam <youngmin.nam at samsung.com>
> [Original commit from https://android.googlesource.com/kernel/gs/+/630817f7080e92c5e0216095ff52f6eb8dd00727
> Signed-off-by: Will McVicker <willmcvicker at google.com>
> ---
>  drivers/clocksource/exynos_mct.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
> index da09f467a6bb..05c50f2f7a7e 100644
> --- a/drivers/clocksource/exynos_mct.c
> +++ b/drivers/clocksource/exynos_mct.c
> @@ -219,12 +219,12 @@ static struct clocksource mct_frc = {
>         .resume         = exynos4_frc_resume,
>  };
>
> +#if defined(CONFIG_ARM)

I'd probably suggest adding a comment here explaining why this is kept
on ARM and not on AARCH64 as well.

>  static u64 notrace exynos4_read_sched_clock(void)
>  {
>         return exynos4_read_count_32();
>  }
>
> -#if defined(CONFIG_ARM)
>  static struct delay_timer exynos4_delay_timer;
>
>  static cycles_t exynos4_read_current_timer(void)
> @@ -250,12 +250,13 @@ static int __init exynos4_clocksource_init(bool frc_shared)
>         exynos4_delay_timer.read_current_timer = &exynos4_read_current_timer;
>         exynos4_delay_timer.freq = clk_rate;
>         register_current_timer_delay(&exynos4_delay_timer);
> +
> +       sched_clock_register(exynos4_read_sched_clock, 32, clk_rate);
>  #endif
>
>         if (clocksource_register_hz(&mct_frc, clk_rate))
>                 panic("%s: can't register clocksource\n", mct_frc.name);
>
> -       sched_clock_register(exynos4_read_sched_clock, 32, clk_rate);
>
>         return 0;

Otherwise, this looks ok to me.

thanks
-john