linux-kernel - Re: [PATCH v2] timekeeping: Allow runtime PM from change

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Date:   Wed, 3 Mar 2021 12:04:13 +0100
From:   Wolfram Sang <wsa@...nel.org>
To:     Niklas Söderlund 
        <niklas.soderlund+renesas@...natech.se>
Cc:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH v2] timekeeping: Allow runtime PM from
 change_clocksource()

On Thu, Feb 11, 2021 at 02:43:18PM +0100, Niklas Söderlund wrote:
> The struct clocksource callbacks enable() and disable() are described as
> a way to allow clock sources to enter a power save mode [1]. But using
> runtime PM from these callbacks triggers a cyclic lockdep warning when
> switching clock source using change_clocksource().
> 
> This change allows the new clocksource to be enabled() and the old one
> to be disabled() without holding the timekeeper_lock. This solution is
> modeled on timekeeping_resume() and timekeeping_suspend() where the
> struct clocksource resume() and suspend() callbacks are called without
> holding the timekeeper_lock.
> 
> Triggering and log of the deadlock warning,
> 
>   # echo e60f0000.timer > /sys/devices/system/clocksource/clocksource0/current_clocksource

...

I had the same issue when running 'clocksource-switch' from the Kernel
selftests with a Renesas Falcon board using a R-Car V3U. This patch
fixed the deadlock warning, so:

Tested-by: Wolfram Sang <wsa+renesas@...g-engineering.com>


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