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

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Message-ID: <YAB8ZmtOxRV1QN4l@workstation.tuxnet>
Date:   Thu, 14 Jan 2021 18:16:22 +0100
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Sven Van Asbroeck <thesven73@...il.com>,
        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,

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@...ruber.com> 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.

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.)

And if there is no pre-known state of the registers, we actually need
the .get_state function fully implemented.

Thanks,
Clemens