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

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Date:   Mon, 11 Jan 2021 21:35:32 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
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

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.

Best regards
Uwe

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

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