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

[Daniel Lezcano]

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.


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