Re: [RCF PATCH,v2,2/2] pwm: imx: Configure output to GPIO in disabled state

Uwe Kleine-König
2018-11-14T09:10:03Z

Hello Michal,

On Fri, Nov 09, 2018 at 05:55:55PM +0100, Uwe Kleine-König wrote:
> On Fri, Nov 09, 2018 at 02:24:42PM +0000, Vokáč Michal wrote:
> > On 8.11.2018 20:18, Uwe Kleine-König wrote:
> > > On Thu, Nov 08, 2018 at 03:21:44PM +0000, Vokáč Michal wrote:
> > >> Hi Uwe,
> > >>
> > >> On 7.11.2018 16:01, Uwe Kleine-König wrote:
> > >>>> Interesting idea. I just wonder why nobody else did not come up with such
> > >>>> a simple solution before.
> > >>>
> > >>> I think I mentioned it already in this thread, but it went unnoticed :-)
> > >>
> > >> I meant it like "How happened this was not invented years ago, when people
> > >> first noticed the issue with using inverted PWM for backlight on i.MX6."
> > >> In our project, this issue dates back to 2015 :(
> > >>
> > >>> Then the patch isn't correct yet. The idea is always keep the hardware
> > >>> running and only disable it if it's uninverted.
> > >>
> > >> OK, I got the point.
> > >>
> > >>> In imx_pwm_probe it's not yet known what the polarity is supposed to be,
> > >>> right?
> > >>
> > >> Not really. It can already be known but currently there is no way how to
> > >> pass the information to the probe function. I, as a creator of the device
> > >> (and author of a DTS file) know that the circuit needs inverted PWM signal.
> > >> And I know that the circuit needs to be disabled until I say it can be
> > >> enabled. How I say that can warry. It may be default brightness level > 0
> > >> in DTS file or from a userspace program using PWM sysfs interface.
> > >>
> > >>> So the right thing to do there is to not touch the configuration
> > >>> of the pwm. I think all states that are problematic then are also
> > >>> problematic with the gpio/pinmux approach.
> > >>
> > >> I think my use-case I already presented before is an example where
> > >> involving pinctrl solves the problem while the "leave PWM enabled
> > >> for inverted users" does not. That is all the time between
> > >> imx_pwm_probe() and imx_pwm_apply_v2().
> > >
> > > You're doing in probe:
> > >
> > > if (pwm_is_running()):
> > > mux(pin, function=pwm)
> > > else:
> > > gpio_set_value(gpio, 0)
> > > mux(pin, function=gpio)
> > >
> > > This gives you the right level assuming the gpio specification uses the
> > > right flag (GPIO_ACTIVE_HIGH or GPIO_ACTIVE_LOW).
> >
> > Agree.
> >
> > > Taking your example with the backlight device you specify an "init" and
> > > a "default" pinctrl and only "default" contains the muxing for the PWM
> > > pin everything should be as smooth as necessary: The pwm is only muxed
> > > when the backlight device is successfully bound.
> >
> > Have you tried that Uwe? The bad news is I tested that before and now
> > again and it does not work like that. We already discussed that earlier.
>
> The key is that the pinmux setting for the PWM pin should be part of the
> bl pinctrl, not the pwm pinctrl. Then "default" is only setup when the
> bl device is successfully bound which is after the bl's .probe callback
> called pwm_apply().

Did my mail clear up my suggestion? Do you agree it should work like
this (or even did you test that, as I did not)?

Best regards
Uwe

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