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

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Date:   Mon, 22 Mar 2021 14:25:06 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Clemens Gruber <clemens.gruber@...ruber.com>,
        Sven Van Asbroeck <thesven73@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout

Hello,

On Mon, Mar 22, 2021 at 02:15:08PM +0200, Andy Shevchenko wrote:
> On Mon, Mar 22, 2021 at 1:48 PM Uwe Kleine-König
> <u.kleine-koenig@...gutronix.de> wrote:
> > On Mon, Mar 22, 2021 at 01:40:57PM +0200, Andy Shevchenko wrote:
> > > On Mon, Mar 22, 2021 at 1:22 PM Uwe Kleine-König
> > > <u.kleine-koenig@...gutronix.de> wrote:
> > > > When the PWM driver is loaded and the PWM configuration is invalid, it
> > > > was already invalid for the time between power up (or warm start) and
> > > > PWM driver load time. Then it doesn't really hurt to keep the PWM
> > > > in this invalid state for a little moment longer until the consumer of
> > > > the PWM becomes active.
> > >
> > > But this won't work in the cases when we have a chip with a shared
> > > settings for period and/or duty cycle. You will never have a user come
> > > due to -EBUSY.
> >
> > That's wrong, the first consumer to enable the PWM (in software) is
> > supposed to be able to change the settings.
> 
> If it's a critical PWM, how can you be allowed to do that?

You seem to have a tight concept of a critical PWM. I don't, so I have
problems following you. What is your picture about what is to be
allowed/denied for a critical PWM?

> And if so, what is the difference between resetting the device in this
> case?

The difference is that we have a consumer that knows what to do with the
PWM then.

> You may consider it as a change to the settings by the first
> consumer.

.. but without knowing if the first consumer is a backlight driver or a
motor control it's hard to know if disabling the PWM is OK. So I like
the concept of not doing anything until a process comes along that knows
better.

Best regards
Uwe

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

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