Re: LEDs that change brightness "itself" -- that's a trigger. Re: PM regression with LED changes in next-20161109

[Hans de Goede]

Hi,

On 15-11-16 12:48, Pavel Machek wrote:
> Hi!
>
> > > > > The LED you are talking about _has_ a trigger, implemented in
> > > > > hardware. That trigger can change LED brightness behind kernel's (and
> > > > > userspace's) back. Don't pretend the trigger does not exist, it does.
> > > > >
> > > > > And when you do that, you'll have nice place to report changes to
> > > > > userspace -- trigger can now export that information, and offer poll()
> > > > > interface.
> > > >
> > > > Well, that sounds interesting. It is logically justifiable.
> > >
> > > Thanks.
> > >
> > > > I initially proposed exactly this solution, with recently
> > > > added userspace LED being a trigger listener. It seems a bit
> > > > awkward though. How would you listen to the trigger events?
> > >
> > > Trigger exposes a file in sysfs, with poll() working on that file
> >
> > Hmm, a new file would give the advantage of making it easy for
> > userspace to see if the trigger is poll-able, this is likely
> > better then my own proposal I just send.
>
> Good.
>
> > > (and
> > > probably read exposing the current brightness).
> >
> > If we do this, can we please make it mirror brightness, iow
> > also make it writable, that will make it easier for userspace
> > to deal with it. We can simply re-use the existing show / store
> > methods for brightness for this.
>
> Actually, echo 0 > brightness disables the trigger, IIRC. I'd avoid
> that here, you want to be able to turn off the backlight but still
> keep the trigger (and be notified of future changes).

True, that is easy to do the store method will just need to call
led_set_brightness_nosleep instead of led_set_brightness, this
will skip the checks to stop blinking in led_set_brightness and
otherwise is equivalent.

> > I suggest we call it:
> >
> > trigger_brightness
> >
> > And only register it when a poll-able trigger is present.
>
> I'd call it 'current_brightness', but that's no big deal. Yes, only
> registering it for poll-able triggers makes sense.

current_brightness works for me. I will take a shot a patch-set
implementing this.

Regards,

Hans



> > > Key difference is that only triggers where this makes sense (keyboard
> > > backlight) expose it and carry the overhead. CPU trigger would
> > > definitely not do this.
> >
> > Ack only having some triggers pollable is important.
>
> Thanks,
> 									Pavel