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

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Message-ID: <20251105160627.GA3684509@ragnatech.se>
Date: Wed, 5 Nov 2025 17:06:27 +0100
From: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
To: Daniel Lezcano <daniel.lezcano@...aro.org>
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 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.

> 
> 
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-- 
Kind Regards,
Niklas Söderlund