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

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Date:   Fri, 29 Jan 2021 13:05:14 -0500
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Clemens Gruber <clemens.gruber@...ruber.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 Clemens,

On Fri, Jan 29, 2021 at 11:31 AM Clemens Gruber
<clemens.gruber@...ruber.com> wrote:
>
> Ok, so you suggest we extend our get_state logic to deal with cases
> like the following:

Kind of. We can't control how other actors (bootloaders etc) program the
chip. As far as I know, there are many, many different register settings that
result in the same physical chip outputs. So if .probe() wants to preserve the
existing chip settings, .get_state() has to be able to deal with every possible
setting. Even invalid ones.

In addition, .apply() cannot make any assumptions as to which bits are
already set/cleared on the chip. Including preserved, invalid settings.

This might get quite complex.

However if we reset the chip in .probe() to a known state (a normalized state,
in the mathematical sense), then both .get_state() and .apply() become
much simpler. because they only need to deal with known, normalized states.

In short, it's a tradeoff between code complexity, and user friendliness/
features.

Sven