linux-kernel - Re: [tip: timers/clocksource] clocksource/drivers/exynos_mct: Don't register as a sched

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Message-ID: <aIHBnFESZwjpXzjr@gmail.com>
Date: Thu, 24 Jul 2025 07:16:12 +0200
From: Ingo Molnar <mingo@...nel.org>
To: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: linux-tip-commits@...r.kernel.org, Donghoon Yu <hoony.yu@...sung.com>,
	Youngmin Nam <youngmin.nam@...sung.com>,
	John Stultz <jstultz@...gle.com>,
	Will McVicker <willmcvicker@...gle.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>, x86@...nel.org
Subject: Re: [tip: timers/clocksource] clocksource/drivers/exynos_mct: Don't
 register as a sched_clock on arm64


* tip-bot2 for Will McVicker <tip-bot2@...utronix.de> wrote:

> The following commit has been merged into the timers/clocksource branch of tip:
> 
> Commit-ID:     394b981382e6198363cf513f6eb6be4c55b22e44
> Gitweb:        https://git.kernel.org/tip/394b981382e6198363cf513f6eb6be4c55b22e44
> Author:        Will McVicker <willmcvicker@...gle.com>
> AuthorDate:    Fri, 20 Jun 2025 11:17:05 -07:00
> Committer:     Daniel Lezcano <daniel.lezcano@...aro.org>
> CommitterDate: Tue, 15 Jul 2025 13:00:50 +02:00
> 
> clocksource/drivers/exynos_mct: Don't register as a sched_clock on arm64
> 
> 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].
> 
> [1] https://lore.kernel.org/linux-samsung-soc/1400188079-21832-1-git-send-email-chirantan@chromium.org/
> 
> [Original commit from https://android.googlesource.com/kernel/gs/+/630817f7080e92c5e0216095ff52f6eb8dd00727
> 
> Signed-off-by: Donghoon Yu <hoony.yu@...sung.com>
> Signed-off-by: Youngmin Nam <youngmin.nam@...sung.com>
> Reviewed-by: Youngmin Nam <youngmin.nam@...sung.com>
> Acked-by: John Stultz <jstultz@...gle.com>
> Tested-by: Youngmin Nam <youngmin.nam@...sung.com>
> Signed-off-by: Will McVicker <willmcvicker@...gle.com>
> Link: https://lore.kernel.org/r/20250620181719.1399856-3-willmcvicker@google.com
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>

The whole SOB chain of this commit is messy and has several serious 
problems:

1)

This commit has misattributed authorship: the first SOB is:

   Signed-off-by: Donghoon Yu <hoony.yu@...sung.com>

but the Author field is not Donghoon Yu:

   Author:        Will McVicker <willmcvicker@...gle.com>

2)

The Reviewed-by tag is misapplied:

> Signed-off-by: Youngmin Nam <youngmin.nam@...sung.com>
> Reviewed-by: Youngmin Nam <youngmin.nam@...sung.com>

When someone passes along a patch, it's implicit that they have 
reviewed it.

3)

There's also a stray Tested-by tag by one of the SOB entries:

> Signed-off-by: Youngmin Nam <youngmin.nam@...sung.com>
> Reviewed-by: Youngmin Nam <youngmin.nam@...sung.com>
> Tested-by: Youngmin Nam <youngmin.nam@...sung.com>

When someone passes along a patch, it's implicit that they not only 
have reviewed the patch, but have also tested it to a certain extent 
...

4)

Why is the 'Link' tag just in the middle of the SOB chain, instead at the end of it?


Presumably this is the proper SOB chain:

> Author:        Donghoon Yu <hoony.yu@...sung.com>

> Signed-off-by: Donghoon Yu <hoony.yu@...sung.com>
> Signed-off-by: Youngmin Nam <youngmin.nam@...sung.com>
> Signed-off-by: Will McVicker <willmcvicker@...gle.com>
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> Acked-by: John Stultz <jstultz@...gle.com>
> Link: https://lore.kernel.org/r/20250620181719.1399856-3-willmcvicker@google.com

Correct?

Thanks,

	Ingo