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 13:11:01 +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

On Tue 2016-11-15 13:06:14, Hans de Goede wrote:
> 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.

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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