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

Clemens Gruber
2021-02-14T14:47:50Z

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@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.
>
> 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