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

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Date:   Fri, 29 Jan 2021 17:31:47 +0100
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Sven Van Asbroeck <thesven73@...il.com>
Cc:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pwm@...r.kernel.org
Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout

Hi Sven,

On Fri, Jan 29, 2021 at 08:42:13AM -0500, Sven Van Asbroeck wrote:
> On Mon, Jan 11, 2021 at 3:35 PM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> >
> > 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 agree that this is the most user-friendly behaviour.
> 
> The problem however with the pca9685 is that it has many degrees of
> freedom: there are many possible register values which produce the same
> physical chip outputs.
> 
> This could lead to a situation where, if .probe() does not reset the register
> values, subsequent writes may lead to different outputs than expected.
> 
> One possible solution is to write .get_state() so that it always reads the
> correct state, even if "unconventional" register settings are present, i.e.
> those written by an outside entity, e.g. a bootloader. Then write that
> state back using driver conventions.
> 
> This may be trickier than it sounds - after all we've learnt that the pca9685
> looks simple on the surface, but is actually quite challenging to get right.
> 
> Clemens, Uwe, what do you think?

Ok, so you suggest we extend our get_state logic to deal with cases
like the following:
If neither full OFF nor full ON is set && ON == OFF, we should probably
set the full OFF bit to disable the PWM and log a warning message?
(e.g. "invalid register setting detected: pwm disabled" ?)
If the ON registers are set and the nxp,staggered-outputs property is
not, I'd calculate (off - on) & 4095, set the OFF register to that value
and clear the ON register.

And then call our get_state in .probe, followed by a write of the
resulting / fixed-up state?

This would definitely solve the problem of invalid/unconventional values
set by the bootloader and avoid inconsistencies.
Sounds good to me!

If Thierry and Uwe have no objections, I can send out a new round of
patches in the upcoming weeks.

My current goal is to get the changes into 5.13.

Thanks,
Clemens