linux-kernel - Re: [PATCH v2] clocksource/drivers/sh_cmt: Always leave device running after probe

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ae167c7f-c32f-422b-9eb2-72889cbafef0@linaro.org>
Date: Wed, 5 Nov 2025 17:39:14 +0100
From: Daniel Lezcano <daniel.lezcano@...aro.org>
To: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
Cc: Thomas Gleixner <tglx@...utronix.de>,
 Geert Uytterhoeven <geert@...ux-m68k.org>, linux-kernel@...r.kernel.org,
 linux-renesas-soc@...r.kernel.org,
 Geert Uytterhoeven <geert+renesas@...der.be>
Subject: Re: [PATCH v2] clocksource/drivers/sh_cmt: Always leave device
 running after probe

On 11/5/25 17:06, Niklas Söderlund wrote:
> On 2025-11-05 16:36:15 +0100, Daniel Lezcano wrote:
>> On 10/16/25 20:20, Niklas Söderlund wrote:
>>> The CMT device can be used as both a clocksource and a clockevent
>>> provider. The driver tries to be smart and power itself on and off, as
>>> well as enabling and disabling its clock when it's not in operation.
>>> This behavior is slightly altered if the CMT is used as an early
>>> platform device in which case the device is left powered on after probe,
>>> but the clock is still enabled and disabled at runtime.
>>>
>>> This has worked for a long time, but recent improvements in PREEMPT_RT
>>> and PROVE_LOCKING have highlighted an issue. As the CMT registers itself
>>> as a clockevent provider, clockevents_register_device(), it needs to use
>>> raw spinlocks internally as this is the context of which the clockevent
>>> framework interacts with the CMT driver. However in the context of
>>> holding a raw spinlock the CMT driver can't really manage its power
>>> state or clock with calls to pm_runtime_*() and clk_*() as these calls
>>> end up in other platform drivers using regular spinlocks to control
>>> power and clocks.
>>
>> So the fix is to remove PM management in the driver ?
> 
> Yes. As I understand it we can't do runtime pm in these drivers as the
> core calls into the functions with the raw spinlock held. I hope we can
> improve this in future.


IIUC, the changes done for PREEMPT_RT prevent to use pm_runtime by 
functions running in atomic context because the spinlocks are actually 
mutexes.

But if PREEMPT_RT is not set, then everything is running as usual.

This change drops the PM while it should be working for kernel compiled 
without PREEMPT_RT.

I suggest to handle the case with/out PREEMPT_RT.

Hopefully pm_runtime will be fixed with PREEMPT_RT and you won't have to 
reintroduce pm_runtime in this driver but just remove the PREEMPT_RT case.


-- 
<http://www.linaro.org/> Linaro.org │ Open source software for ARM SoCs

Follow Linaro:  <http://www.facebook.com/pages/Linaro> Facebook |
<http://twitter.com/#!/linaroorg> Twitter |
<http://www.linaro.org/linaro-blog/> Blog