linux-kernel - Re: [PATCH 0/1] pwm: meson: fix scheduling while atomic issue

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Date:   Mon, 25 Mar 2019 09:41:53 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     thierry.reding@...il.com, narmstrong@...libre.com,
        jbrunet@...libre.com, linux-pwm@...r.kernel.org,
        linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] pwm: meson: fix scheduling while atomic issue

Hello Martin,

On Sun, Mar 24, 2019 at 11:02:16PM +0100, Martin Blumenstingl wrote:
> Back in January a "BUG: scheduling while atomic" error showed up during
> boot on my Meson8b Odroid-C1 (which uses a PWM regulator as CPU supply).
> The call trace comes down to:
>   __mutex_lock
>   clk_prepare_lock
>   clk_core_get_rate
>   meson_pwm_apply
>   ..
>   dev_pm_opp_set_rate
>   ..
> 
> Jerome has also seen the same problem but from pwm-leds (instead of a
> pwm-regulator). He posted a patch which replaces the spinlock with a
> mutex. That works. I believe we can optimize this by reducing the time
> where the lock is held - that also allows to keep the spin-lock.
> 
> Analyzing this issue helped me understand the pwm-meson driver better.
> My plan is to send some cleanups (with the goal of re-using more of the
> goodies from the PWM core in the pwm-meson driver) after this single fix
> is merged (they can be found here: [1]).

I didn't look over these in detail, but I see an issue that according
to the shortlogs isn't addressed: In the .apply callback there is
(simplified):

	if (!state->enabled) {
		meson_pwm_disable(meson, pwm->hwpwm);
		return;
	}

This results in the wrong output after:

	pwm_apply_state(pwm, { .enabled = true, .polarity = PWM_POLARITY_NORMAL, ...});
	pwm_apply_state(pwm, { .enabled = false, .polarity = PWM_POLARITY_INVERTED, ...});

because the polarity isn't checked.

If you want to implement further cleanups, my questions and propositions
are:

 - Is there a publicly available manual for this hardware? If yes, you
   can add a link to it in the header of the driver.

 - Why do you handle reparenting of the PWM's clk in .request? Wouldn't
   this be more suitable in .apply?

 - Does stopping the PWM (i.e. clearing MISC_{A,B}_EN in the MISC_AB
   register) freeze the output, or is the currently running period
   completed first? (The latter is the right behaviour.)

 - Please point out in the header that for changing period/duty
   cycle/polarity the hardware must be stopped. (I suggest to apply the
   style used in https://www.spinics.net/lists/linux-pwm/msg09262.html
   for some consistency.)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |