Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout

[Thierry Reding]

On Thu, Jan 14, 2021 at 06:16:22PM +0100, Clemens Gruber wrote:
> Hi,
> 
> On Mon, Jan 11, 2021 at 09:35:32PM +0100, Uwe Kleine-König wrote:
> > Hello,
> > 
> > On Thu, Dec 17, 2020 at 06:43:04PM +0100, Clemens Gruber wrote:
> > > On Wed, Dec 16, 2020 at 11:00:59PM -0500, Sven Van Asbroeck wrote:
> > > > On Wed, Dec 16, 2020 at 7:53 AM Clemens Gruber
> > > > <clemens.gruber@xxxxxxxxxxxx> wrote:
> > > > >
> > > > > Implements .get_state to read-out the current hardware state.
> > > > >
> > > > 
> > > > I am not convinced that we actually need this.
> > > > 
> > > > Looking at the pwm core, .get_state() is only called right after .request(),
> > > > to initialize the cached value of the state. The core then uses the cached
> > > > value throughout, it'll never read out the h/w again, until the next .request().
> > > > 
> > > > In our case, we know that the state right after request is always disabled,
> > > > because:
> > > > - we disable all pwm channels on probe (in PATCH v5 4/7)
> > > > - .free() disables the pwm channel
> > > > 
> > > > Conclusion: .get_state() will always return "pwm disabled", so why do we
> > > > bother reading out the h/w?
> > > 
> > > If there are no plans for the PWM core to call .get_state more often in
> > > the future, we could just read out the period and return 0 duty and
> > > disabled.
> > > 
> > > Thierry, Uwe, what's your take on this?
> > 
> > I have some plans here. In the past I tried to implement them (see
> > commit 01ccf903edd65f6421612321648fa5a7f4b7cb10), but this failed
> > (commit 40a6b9a00930fd6b59aa2eb6135abc2efe5440c3).
> > 
> > > > Of course, if we choose to leave the pwm enabled after .free(), then
> > > > .get_state() can even be left out! Do we want that? Genuine question, I do
> > > > not know the answer.
> > > 
> > > I do not think we should leave it enabled after free. It is less
> > > complicated if we know that unrequested channels are not in use.
> > 
> > My position here is: A consumer should disable a PWM before calling
> > pwm_put. The driver should however not enforce this and so should not
> > modify the hardware state in .free().
> > 
> > Also .probe should not change the PWM configuration.
> 
> I see. This would also allow PWMs initialized in the bootloader (e.g.
> backlights) to stay on between the bootloader and Linux and avoid
> flickering.

Yes, that's precisely one of the reasons why we introduced the atomic
API. One of the use-cases that led to the current design was that the
kernel pwm-regulator on some platforms was causing devices to crash
because the driver would reprogram the PWM and cause a glitch on the
power supply for the CPUs.

So it's crucial in some cases that the PWM driver don't touch the
hardware state in ->probe(). If some drivers currently do so, that's
something we should eventually change, but given that there haven't been
any complaints yet, it likely means nothing breaks because of this, so
we do have the luxury of not having to rush things.

> If no one objects, I would then no longer reset period and duty cycles
> in the driver (and for our projects, reset them in the bootloader code
> to avoid leaving PWMs on after a kernel panic and watchdog reset, etc.)

This isn't strictly necessary, but it's obviously something that's up to
board designers/maintainers to decide.

Thierry

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