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

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Date:   Tue, 15 Nov 2016 12:48:59 +0100
From:   Pavel Machek <pavel@....cz>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Jacek Anaszewski <j.anaszewski@...sung.com>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Tony Lindgren <tony@...mide.com>, linux-leds@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Darren Hart <dvhart@...radead.org>
Subject: Re: LEDs that change brightness "itself" -- that's a trigger. Re: PM
 regression with LED changes in next-20161109

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).

> 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.

> >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

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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