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

[Andy Shevchenko <andy.shevchenko@gmail.com>]

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On Monday, March 22, 2021, Thierry Reding <thierry.reding@gmail.com> 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@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?
>
> 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


I guess we may miss (a PCB / silicon design flaw or warm boot case) when
boot loader left device completely untouched and device either in wrong
state because if failed reset (saw this on PCA9555 which has a
corresponding errata), or simply we have done a warm reset of the system.
So, we also have to understand how to properly exit.

Another point, CCF has a bit “is critical”, and u guess PWM may get the
same and make the all assumptions much easier.


>
> Thierry
>


-- 
With Best Regards,
Andy Shevchenko

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