Re: [PATCH v2 2/7] clocksource/drivers/exynos_mct: Don't register as a sched_clock on arm64

[William McVicker <willmcvicker@google.com>]

On 04/02/2025, John Stultz wrote:
> On Wed, Apr 2, 2025 at 4:34 PM 'Will McVicker' via kernel-team
> <kernel-team@android.com> wrote:
> >
> > To use the MCT as a sched_clock, the timer value has to be accessed vi
> > an MCT register which is extremely slow. To improve performance on Arm64
> > SoCs, use the Arm architected timer as the default clocksource. Note, we
> 
> Nit:  sched_clock is sort of separate from the "default clocksource",
> and after this patch we still register MCT as a clocksource, so this
> doesn't sound quite right.
> 
> I'd probably reword this slightly to:
> "The MCT register is unfortunately very slow to access, but importantly
> does not halt in the c2 idle state.  So for ARM64, we can improve
> performance by not registering the MCT for sched_clock, allowing the
> system to use the faster ARM architected timer for sched_clock instead.
> 
> The MCT is still registered as a clocksource, and a clockevent in order
> to be a wakeup source for the arch_timer to exit the "c2" idle state.
> 
> Since ARM32 SoCs don't have an architected timer, the MCT must continue
> to be used for sched_clock. Detailed discussion on this topic can be
> found at [1]. "

Thanks John for the suggestion! I'll give more time for review feedback before
sending the v3 with this update.

Regards,
Will

> 
> > can't completely disable the MCT on Arm64 since it needs to be used as
> > the wakeup source for the arch_timer to exit the "c2" idle state.
> >
> > Since ARM SoCs don't have an architectured timer, the MCT will continue
> > to be the default clocksource. Detailed discussion on this topic can be
> > found at [1].
> >
> > [1] https://lore.kernel.org/linux-samsung-soc/1400188079-21832-1-git-send-email-chirantan@chromium.org/
> >
> > Signed-off-by: Donghoon Yu <hoony.yu@samsung.com>
> > Signed-off-by: Youngmin Nam <youngmin.nam@samsung.com>
> > [Original commit from https://android.googlesource.com/kernel/gs/+/630817f7080e92c5e0216095ff52f6eb8dd00727
> > Signed-off-by: Will McVicker <willmcvicker@google.com>
> > Reviewed-by: Youngmin Nam <youngmin.nam@samsung.com>
> 
> Otherwise, looks good.
> Acked-by: John Stultz <jstultz@google.com>
> 
> thanks
> -john