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

From: Uwe Kleine-König
Date: Mon Mar 25 2019 - 04:42:09 EST

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

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

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

- 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
for some consistency.)

Best regards

Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | |