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

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Message-ID: <CAMuHMdXJfSoU-160BmD-obd7YZRdKT4z-P1YnrvvR1dq7XKaDg@mail.gmail.com>
Date: Wed, 15 Oct 2025 14:43:03 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>, Thomas Gleixner <tglx@...utronix.de>, 
	linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org
Subject: Re: [RFC] clocksource/drivers/sh_cmt: Always leave device running
 after probe

Hi Niklas,

Thanks for your patch!

On Sun, 12 Oct 2025 at 11:10, Niklas Söderlund
<niklas.soderlund+renesas@...natech.se> 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 have worked for a long time, but recent improvements in PREEMPT_RT

has

> and PROVE_LOCKING have highlighted an issue. As the CMT register itself

registers

> as a clockevent provided, clockevents_register_device(), it needs to use

provider

> 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.
>
> This mix of spinlock contexts trips a lockdep warning.
>
>     =============================
>     [ BUG: Invalid wait context ]
>     6.17.0-rc3-arm64-renesas-03071-gb3c4f4122b28-dirty #21 Not tainted
>     -----------------------------
>     swapper/1/0 is trying to lock:
>     ffff00000898d180 (&dev->power.lock){-...}-{3:3}, at: __pm_runtime_resume+0x38/0x88
>     ccree e6601000.crypto: ARM CryptoCell 630P Driver: HW version 0xAF400001/0xDCC63000, Driver version 5.0
>     other info that might help us debug this:
>     ccree e6601000.crypto: ARM ccree device initialized
>     context-{5:5}
>     2 locks held by swapper/1/0:
>      #0: ffff80008173c298 (tick_broadcast_lock){-...}-{2:2}, at: __tick_broadcast_oneshot_control+0xa4/0x3a8
>      #1: ffff0000089a5858 (&ch->lock){....}-{2:2}
>     usbcore: registered new interface driver usbhid
>     , at: sh_cmt_start+0x30/0x364
>     stack backtrace:
>     CPU: 1 UID: 0 PID: 0 Comm: swapper/1 Not tainted 6.17.0-rc3-arm64-renesas-03071-gb3c4f4122b28-dirty #21 PREEMPT
>     Hardware name: Renesas Salvator-X 2nd version board based on r8a77965 (DT)
>     Call trace:
>      show_stack+0x14/0x1c (C)
>      dump_stack_lvl+0x6c/0x90
>      dump_stack+0x14/0x1c
>      __lock_acquire+0x904/0x1584
>      lock_acquire+0x220/0x34c
>      _raw_spin_lock_irqsave+0x58/0x80
>      __pm_runtime_resume+0x38/0x88
>      sh_cmt_start+0x54/0x364
>      sh_cmt_clock_event_set_oneshot+0x64/0xb8
>      clockevents_switch_state+0xfc/0x13c
>      tick_broadcast_set_event+0x30/0xa4
>      __tick_broadcast_oneshot_control+0x1e0/0x3a8
>      tick_broadcast_oneshot_control+0x30/0x40
>      cpuidle_enter_state+0x40c/0x680
>      cpuidle_enter+0x30/0x40
>      do_idle+0x1f4/0x26c
>      cpu_startup_entry+0x34/0x40
>      secondary_start_kernel+0x11c/0x13c
>      __secondary_switched+0x74/0x78
>
> For non-PREEMPT_RT builds this is not really an issue, but for
> PREEMPT_RT builds where normal spinlocks can sleep this might be an
> issue. Be cautious and always leave the power and clock running after
> probe.
>
> Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>

Thanks, I gave this a try on various boards, no issues seen.
I also force-disabled the TMU on R-Mobile A1, to make sure its known
splat is not hiding any other locking problems.

Tested-by: Geert Uytterhoeven <geert+renesas@...der.be>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds