linux-kernel - Re: [PATCH v5 1/4] leds: core: add generic support for RGB Color LED's

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Date:	Tue, 5 Apr 2016 21:45:08 +0200
From:	Jacek Anaszewski <jacek.anaszewski@...il.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Jacek Anaszewski <j.anaszewski@...sung.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Greg KH <greg@...ah.com>, linux-leds@...r.kernel.org,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	pali.rohar@...il.com, sre@...nel.org, khilman@...nel.org,
	aaro.koskinen@....fi, ivo.g.dimitrov.75@...il.com,
	Patrik Bachan <patrikbachan@...il.com>, serge@...lyn.com
Subject: Re: [PATCH v5 1/4] leds: core: add generic support for RGB Color
 LED's

On 04/05/2016 11:01 AM, Pavel Machek wrote:
> Hi!
>
>>>>>> It would have the same downsides as in case of having r, g and b in
>>>>>> separate attributes, i.e. - problems with setting LED colour in
>>>>>> a consistent way. This way LED blinking in whatever colour couldn't
>>>>>> be supported reliably. It was one of your primary rationale standing
>>>>>> behind this design, if I remember correctly. Second - what about
>>>>>> triggers? We've had a long discussion about it and this design turned
>>>>>> out to be most fitting.
>>>>>
>>>>> Are on/off triggers really that useful for a LED that can produce 16
>>>>> million colors?
>>>>>
>>>>> I believe we should support patterns for RGB LEDs. Something like
>>>>> [ (time, r, g, b), ... ] . Ok, what about this one?
>>>>>
>>>>> Lets say we have
>>>>>
>>>>> /sys/class/pattern/lp5533::0
>>>>> /sys/class/pattern/software::0
>>>>>
>>>>> /sys/class/led/n900::red ; default trigger "lp5533::0:0"
>>>>> /sys/class/led/n900::green ; default trigger "lp5533::0:1"
>>>>> /sys/class/led/n900::blue ; default trigger "lp5533::0:2"
>>>>>
>>>>> Normally, pattern would correspond to one RGB LED. We could have
>>>>> attribute "/sys/class/pattern/lp5533::0/color" containing R,G,B for
>>>>> this pattern.
>>
>> Could you give an example on how to set a color for RGB LED using
>> this interface? Would it be compatible with LED triggers?
>> Where the "pattern" class would be implemented?
>
> Well, 'echo "50 60 70" > /sys/class/pattern/lp5533::0/color' should
> set the color for the led. 'echo "trigger-name" > trigger' would set
> the trigger, probably just toggling between LED off and set color for
> the old triggers.
>
> Where to implement the patterns is different question, but for example
> drivers/leds/pattern?

I'd rather leave the pattern issue for now, since it seems to be
different from the problem Heiner was trying to solve with his LED RGB
extension. Moreover, hardware patterns are device specific and it could
be hard to propose a generic interface.
Drivers can always expose their custom sysfs attributes for configuring
the patterns.

Regardless of the above, some of your considerations brought me an idea
on how to add generic and backwards compatible support for setting RGB
color at one go.

Currently LED class drivers of RGB LED controllers expose three LED
class devices - one per R, G and B color component. I propose that
such drivers set LED_DEV_CAP_RGB flag for each LED class device they
register. LED core, seeing the flag, would create a generic "color"
sysfs attribute for each of the three LED class devices.

The "color" attribute would contain "R G B" values. Setting the "color"
attribute of any of the three LED class devices would affect brightness
properties (i.e. constituent colors) of the remaining two ones.
It would result in disabling any active triggers and writing all the
three color settings to the RGB LED controller at one go.

We would probably need additional op in the LED core : color_set.

Having the color set to nonzero value would signify the the three LED
class devices are in sync and that setting a trigger on any of them
applies to the remaining two ones. It would have to be considered
whether existing triggers could be made compatible with synchronized
RGB LED class devices.

I'm curious what do you think about the idea.

Pavel, Heiner, others?

-- 
Best regards,
Jacek Anaszewski