Re: [PATCH v6 2/4] leds: Add driver for Qualcomm LPG

From: Pavel Machek
Date: Tue May 04 2021 - 11:43:10 EST


> > > One such example is the laptop in front of me, which has 3 channels
> > > wired to an RGB LED and 1 channel wired as a backlight control signal
> > > (i.e. using pwm-backlight). Another example is a devboard where the
> > > 4 channels are wired to 4 LEDs.
> >
> > Ok, so this is actually important. In this case you should have PWM
> > layer, exporting PWMs, and then rgb-LED driver that takes three of
> > those PWMs and turns them into LED, no?
> >
> > And ... surprise ... that is likely to help other people, as LEDs
> > connected to PWMs are quite common.
> >
> > Hmm.?
> >
> > If you can't do this for some reason, you should probably explain in
> > the changelog, because this is going to be FAQ.
> >
> This is exactly what the downstream implementation does and in the case
> of a solid color LED this works fine.
> But the hardware has a shared chunk of memory where you can write
> duty-cycle values, then for each PWM channel you can specify the
> start/stop index and pace for the PWM to read and update the configured
> duty-cycle. This is how the hardware implements pattern support.


> So downstream they have (last time I looked at the code) an addition in
> the PWM API where the LED driver can inform the PWM driver part about
> the indices to use. Naturally I don't think that's a good idea.

Dunno. Is it bad idea?

pattern support for other PWMs (vibration?) seems useful, too. Yes, it
means more discussion and extending PWMs properly..

> Additionally, representing this as individual PWM channels means we're
> loosing the grouping that now comes from the description of multicolor
> LEDs, which serves the basis for synchronizing the pattern traversal
> between the involved channels.

Yes, keeping grouping would be nice, but perhaps pattern API for PWMs
can do that too?

You can have solid-color-only driver now, with patterns being added
as discussion with PWM people progresses...

Best regards,