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

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Message-ID: <a5628c87-0dcd-4992-a59a-15550a017766@linaro.org>
Date: Thu, 24 Jul 2025 12:04:27 +0200
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
 Thomas Gleixner <tglx@...utronix.de>
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>, x86@...nel.org
Subject: Re: [tip: timers/clocksource] clocksource/drivers/exynos_mct: Don't
 register as a sched_clock on arm64

On 24/07/2025 07:16, Ingo Molnar wrote:
> 
> * 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>

Yes, you are right. I should have pay more attention to author / sob, 
thanks for spotting it.

> 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.

Well my understanding of the SOB chain for these is the Signed-off-by 
from Youngmin is in the delivery path because it went first to the AOSP, 
then carried on to Linux by Will. Then Reviewed-by Youngmin letting us 
know the port from AOSP to Linux is ok.

> 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

In this specific case where the original commit is from AOSP, this chain 
seems to make sense. Souns like:

"I was in the original commit delivery path"
"I reviewed this patch carried to Linux"
"I tested it on Linux"

> ...
> 
> 4)
> 
> Why is the 'Link' tag just in the middle of the SOB chain, instead at the end of it?

I don't know. Link must be at the end  It is stated somewhere in the 
documentation?

I use git b4 -s <msg-id> and the tool adds the Link then my sign off.


> 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?








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