Re: (EXT) Re: (EXT) Re: [PATCH v2 1/1] hwmon: pwm-fan: dynamically switch regulator

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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On Fri, May 06, 2022 at 10:35:21AM +0200, Alexander Stein wrote:
> Am Freitag, 6. Mai 2022, 10:20:01 CEST schrieb Uwe Kleine-König:
> > * PGP Signed by an unknown key
> > 
> > Hello,
> > 
> > On Fri, May 06, 2022 at 09:15:55AM +0200, Alexander Stein wrote:
> > > Am Freitag, 6. Mai 2022, 00:00:37 CEST schrieb Guenter Roeck:
> > > > On Wed, May 04, 2022 at 02:45:51PM +0200, Alexander Stein wrote:
> > > > > From: Markus Niebel <Markus.Niebel@ew.tq-group.com>
> > > > > 
> > > > > A pwm value equal to zero is meant to switch off the pwm
> > > > > hence also switching off the fan. Currently the optional
> > > > > regulator is always on. When using this driver on boards
> > > > > with an inverted pwm signal polarity this can cause running
> > > > > the fan at maximum speed when setting pwm to zero.
> > > > 
> > > > The appropriate solution in this case would be to tell the
> > > > software that the pwm is inverted. Turning off the regulator
> > > > in that situation is a bad idea since setting the pwm value to
> > > > 1 would set it to almost full speed. That does not really make
> > > > sense.
> > > 
> > > The pwm-fan driver is already configured for inverted PWM (ommited some
> > > properties for shortness):
> > > fan0: pwm-fan {
> > > 
> > > 	compatible = "pwm-fan";
> > > 	fan-supply = <&reg_pwm_fan>;
> > > 	pwms = <&pwm3 0 40000 PWM_POLARITY_INVERTED>;
> > > 	cooling-levels = <0 32 64 128 196 240>;
> > > 
> > > [...]
> > > };
> > > 
> > > The problem here is that the pwm-fan driver currently enables the
> > > regulator
> > > unconditionally, but the PWM only when the fan is enabled, refer to
> > > __set_pwm(). This results in a fan at full speed when pwm-fan is idle, as
> > > pwm state is not enabled.
> > 
> > Which PWM driver are you using?
> 
> It's pwm-imx27 on a imx8mp based board.

This is one of the known offenders.

> > There is an implicit assumption in some PWM consumers that a disabled
> > PWM emits the inactive level. However not all PWMs do this. Is this such
> > a case?
> 
> Oh, I was not aware of that assumption. As far I can tell, this assumption 
> might actually be violated in pwm-imx27.

The pwm-imx27 driver is a known offender.

IMHO the problem is that there is no properly documented definition what
"disabled" means for a PWM. I had some discussions about that in the
past with Thierry, but with no agreement. Either we have do define that
the output of a PWM is undefined when it's disabled (then the pwm-fan
needs fixing) or the output must be the inactive level (then the
pwm-imx27 must be fixed to not unset the EN bit when configured for an
inverted polarity). In my eyes the first is the sensible thing to do.

See
https://lore.kernel.org/linux-pwm/20180806155129.cjcc7okmwtaujf43@pengutronix.de/
for one of the previous discussions.

> If state->enabled==false then the EN Bit in PWMCR is not set which most 
> probably renders the output polarity in POUTC as inactive.

It drives the output to 0 which in the inverted polarity case is a 100%
relative duty.

Best regards
Uwe

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

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