linux-kernel - Re: [RFC PATCH] regulator: pwm-regulator: Make assumptions about disabled PWM consistent

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Date: Sun, 16 Jun 2024 01:10:32 +0200
From: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To: Uwe Kleine-König <u.kleine-koenig@...libre.com>
Cc: Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>, 
	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [RFC PATCH] regulator: pwm-regulator: Make assumptions about
 disabled PWM consistent

Hello Uwe,

On Mon, Jun 10, 2024 at 12:46 PM Uwe Kleine-König
<u.kleine-koenig@...libre.com> wrote:
[...]
> this is my suggestion to fix the concerns I expressed in
> https://lore.kernel.org/all/hf24mrplr76xtziztkqiscowkh2f3vmceuarecqcwnr6udggs6@uiof2rvvqq5v/
> .
>
> It's only compile tested as I don't have a machine with a pwm-regulator.
> So getting test feedback before applying it would be great.
Unfortunately this approach breaks my Odroid-C1 board again with the
same symptoms as before the mentioned commits: random memory
corruption, preventing the board from booting to userspace.

The cause also seems to be the same as before my commits:
- period (3666ns) and duty cycle (3333ns) in the hardware registers
the PWM controller when Linux boots, but the PWM output is disabled
- with a duty cycle of 0 or PWM output being disabled the output of
the voltage regulator is 1140mV, which is the only allowed voltage for
that rail (even though the regulator can achieve other voltages)
- pwm_regulator_enable() calls pwm_enable() which simply takes the
period and and duty cycle that was read back from the hardware and
enables the output, resulting in undervolting of a main voltage rail
of the SoC

I hope that this (especially the last item) also clarifies the
question you had in the linked mail on whether updating
pwm_regulator_init_state() would help/work.

Regarding your alternative and preferred approach from the other mail:
> Alternatively (and IMHO nicer) just keep the pwm_state around and don't
> use the (mis) feature of the PWM core that pwm_get_state only returns
> the last set state.
I tried this to see if it would work also for my Odroid-C1 board and
I'm happy to report it does - see the attached diff.
In case you are happy with this approach I can submit it as a proper patch.


Best regards,
Martin

View attachment "pwm-regulator-keep-pwm-state-around.diff" of type "text/x-patch" (6607 bytes)