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

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Date:   Sun, 14 Feb 2021 15:46:49 +0100
From:   Clemens Gruber <clemens.gruber@...ruber.com>
To:     linux-pwm@...r.kernel.org
Cc:     Sven Van Asbroeck <thesven73@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 2/7] pwm: pca9685: Support hardware readout

Hi all,

On Fri, Jan 29, 2021 at 09:37:47PM +0100, Clemens Gruber wrote:
> Hi Sven,
> 
> On Fri, Jan 29, 2021 at 01:05:14PM -0500, Sven Van Asbroeck wrote:
> > 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.
> 
> Is the driver really responsible for bootloaders that program the chip
> with invalid values?
> The chip comes out of PoR with sane default values. If the bootloader of
> a user messes them up, isn't that a bootloader problem instead of a
> Linux kernel driver problem?
> 
> > 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.
> 
> Yes, I agree. This would however make it impossible to do a flicker-free
> transition from bootloader to kernel, but that's not really a usecase I
> have so I can live without it.
> 
> Another point in favor of resetting is that the driver already does it.
> Removing the reset of the OFF register may break some boards who rely on
> that behaviour.
> My version only extended the reset to include the ON register.
> 
> > 
> > In short, it's a tradeoff between code complexity, and user friendliness/
> > features.
> > 
> > Sven
> 
> Thierry, Uwe, what's your take on this?
> 
> Thierry: Would you accept it if we continue to reset the registers in
> .probe?
> 
> Thanks,
> Clemens

I realize that it is a difficult time at the moment, but it is a little
bit frustrating not getting any response from the maintainer.

I think the best way forward is to just keep the register resets in
probe as they are. If this is to be changed, I think it should be done
in a separate patchset and by someone who has a usecase requiring it.

Best regards,
Clemens