Re: [PATCH 1/2] hwmon: (pwm-fan) add option to stop fan on shutdown

[Rob Herring <robh@kernel.org>]

On Thu, Jan 9, 2020 at 11:07 PM Akinobu Mita <akinobu.mita@gmail.com> wrote:
>
> 2020年1月9日(木) 1:13 Rob Herring <robh@kernel.org>:
> >
> > On Sun, Dec 22, 2019 at 11:10:22PM +0900, Akinobu Mita wrote:
> > > The pwm-fan driver leaves the fun running when shutting down the system.
> > > (On the other hand the gpio-fan driver stops it.)
> >
> > Seemms like we should have consistent behavior independent of what the
> > underlying implementation uses. Is there actually a case you'd want to
> > leave the fan on? It seems strange to disable in suspend and leave on in
> > shutdown.
>
> I agree.  I was trying to keep the current behavior unchanged, so I added
> "disable-state-shutdown" property.  But I can't think of any case we want
> to leave the fun on in shutdown.
>
> So it's better to change the shutdown behavior and remove the option
> completely or add "retain-state-shutdown" property instead.
> (The "retain-state-shutdown" property is inspired by gpio-leds)

I would just turn off the fan in shutdown and see if anyone complains.

> > Wouldn't the shutdown state depend if the PWM off state is high or low?
> > IIRC, i.MX PWM has a quirk that the PWM disabled state is high. Doesn't
>
> It could be possible to affect the shutdown behavior for pwm-fan.
> There are three i.MX PWM drivers (pwm-imx1, pwm-imx27, and pwm-tpm).
> Do you remember which one actually have such limitation?

No. I believe the fix was to use pinctrl modes to force the state. And
I think the issue was for suspend rather than shutdown (but it seems
unlikely you'd want the fan off in suspend and on in shutdown).

>
> Maybe it should be handled by the PWM controller/chip driver and PWM core.
> From the perspective of PWM user drivers for now, we have nothing to do
> other than setting duty cycle zero and then disable PWM for stopping the
> pwm-fan.
>
> > it also depend on what the PWM driver does in shutdown?
>
> I think so.  But as far as I can see, no PWM drivers implement shutdown
> callback.

Do they need to? I'd assume most SoC's are powered off or put in reset
which makes the PWN pin go low.

Rob