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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Message-ID: <aeccbb02-6e33-390c-d72d-2a580dd67ed8@redhat.com>
Date:   Tue, 15 Nov 2016 13:06:14 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Pavel Machek <pavel@....cz>
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>,
        Hans de Goede <hdegoede@...hat.com>
Subject: Re: LEDs that change brightness "itself" -- that's a trigger. Re: PM
 regression with LED changes in next-20161109

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
>