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From:   Sven Van Asbroeck <thesven73@gmail.com>
Date:   Fri, 29 Jan 2021 13:05:14 -0500
Message-ID: <CAGngYiWd0u=+DPhvK+8v9FT8Y1Evn1brWRheMNDXWFVVL-wNFw@mail.gmail.com>
Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout
To:     Clemens Gruber <clemens.gruber@pqgruber.com>
Cc:     =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= 
        <u.kleine-koenig@pengutronix.de>,
        Thierry Reding <thierry.reding@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-pwm@vger.kernel.org
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Hi Clemens,

On Fri, Jan 29, 2021 at 11:31 AM Clemens Gruber
<clemens.gruber@pqgruber.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