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

[Clemens Gruber]

Hi Thierry,

On Mon, Mar 22, 2021 at 10:19:22AM +0100, Thierry Reding wrote:
> 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@xxxxxxxxxxxx> 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?
> 
> It is ultimately a problem of the bootloader and where possible the
> bootloader should be fixed. However, fixing bootloaders sometimes isn't
> possible, or impractical, so the kernel has to be able to deal with
> hardware that's been badly programmed by the bootloader. Within reason,
> of course. Sometimes this can't be done in any other way than forcing a
> hard reset of the chip, but it should always be a last resort.
> 
> > > 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?
> 
> Yes, I think it's fine to continue to reset the registers since that's
> basically what the driver already does. It'd be great if you could
> follow up with a patch that removes the reset and leaves the hardware in
> whatever state the bootloader has set up. Then we can take that patch
> for a ride and see if there are any complains about it breaking. If
> there are we can always try to fix them, but as a last resort we can
> also revert, which then may be something we have to live with. But I
> think we should at least try to make this consistent with how other
> drivers do this so that people don't stumble over this particular
> driver's behaviour.

Thanks for your input!

Sounds good to me. I am currently preparing a new revision of the
series. As soon as that is reviewed and good to go, I will look into
removing the resets.

Clemens