Message-ID: <20251109143708.GD4126953@ragnatech.se>
Date: Sun, 9 Nov 2025 15:37:08 +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

Hi Daniel,

On 2025-11-07 10:53:41 +0100, Daniel Lezcano wrote:
> On 11/5/25 19:32, Niklas Söderlund wrote:
> > Hi Daniel,
> > 
> > Thanks for your feedback.
> > 
> > On 2025-11-05 17:39:14 +0100, Daniel Lezcano wrote:
> > > 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.
> > 
> > My understanding is that the core issue is that the clockevent core uses
> > raw spinlocks, so all operations done from the callbacks in drivers need
> > to use them too.
> > 
> > The Renesas CMT and TMU (which I intend to fix too once we find a way
> > forward for CMT) are the only clockenvet drivers attempting to do
> > runtime pm.
> > 
> >      $ git grep -l pm_runtime_get -- drivers/clocksource
> >      drivers/clocksource/sh_cmt.c
> >      drivers/clocksource/sh_mtu2.c
> >      drivers/clocksource/sh_tmu.c
> >      drivers/clocksource/timer-ti-dm.c
> > 
> > The timer-ti-dm.c driver do not register a clockevent device.
> > 
> > > 
> > > But if PREEMPT_RT is not set, then everything is running as usual.
> > 
> > I still get LOCKDEP warnings.
> 
> Ah ok, you get the LOCKDEP warning because it identifies the called code to
> be invalid in case the PREEMPT_RT is compiled-in but does not reflect a real
> problem with !PREEMPT_RT.
> 
> [ ... ]
> 
> > The problem is not really PREEMPT_RT. The problem is the clockevents
> > core require drivers to use raw spinlocks as itself uses them.
> > 
> > I would prefer to get the driver in a state without splats, warnings and
> > potential PREEMPT_RT issues. Especially if the cost is to disable
> > runtime pm.
> > 
> > As I understand it most systems where CMT exists, who don't use it, keep
> > them disabled in DT. And the devices that use them have
> > is_sh_early_platform_device() set so they already disable runtime pm.
> > 
> > Once we have that fixed we can work on enabling it without the quirks.
> > In your opinion am I missing something where this approach is a bad idea?
> 
> If you prefer to remove the PM in the driver, I'm fine with that.

I'm not super fond of removing it, but for now I think it is the best 
way to ensure correct operation in all cases. If anybody have ideas I'm 
all ears. Specially as I will need to do a similar fix for the TMU 
driver once we have found a acceptable way for CMT.

Once we I'm out of the woods and not worried about whats currently in 
tree can have issues I think it's time to start thinking, if and how we 
can improve tings in the core so we can enable some or all of the PM CMT 
and TMU.

I really appreciate you pushed back on this, I agree removing runtime PM 
support is kind of a last resort here. But I see no other path.
> 
> 
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Kind Regards,
Niklas Söderlund

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