linux-kernel - Re: RGB support prototype

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Date:   Mon, 14 Aug 2017 20:26:30 +0200
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     linux-kernel@...r.kernel.org, linux-leds@...r.kernel.org
Subject: Re: RGB support prototype

Hi,

On 08/13/2017 01:41 PM, Pavel Machek wrote:
> Hi!
> 
>>> You mentioned you was working on RGB support prototype. Could you post
>>> copy of the patches (even if unfinished)?
>>
>> Unfortunately it is at the stage of unfinished proof of concept and
>> I haven't managed yet to try how it fits to all API use cases we have.
>> Nor is it in a shape ready to post to the lists.
>>
>> I think we could try to discuss the design here. I'll list all the
>> issues I encountered during the implementation:
>>
>> Currently we set LED brightness with following API:
>>
>> void led_set_brightness(struct led_classdev *led_cdev,
>>                         enum led_brightness brightness);
>>
>> In case of RGB LED we could have something like this:
>>
>> struct led_color_triplet {
>>         enum led_brightness red;
>>         enum led_brightness green;
>>         enum led_brightness blue;
>> };
>>
>> void led_rgb_set_brightness(struct led_rgb_classdev *led_rgb_cdev,
>>                         struct led_color_triplet *color);
>>
>> We've agreed that LED RGB class device color could be set by writing
>> space separated list of "r g b" color values to a sysfs "color" file.
>>
>> While the above itself shouldn't raise too many doubts, they
>> arise quickly while trying to adapt it to the internal LED core
>> facilities:
>>
>> - led_base_timer_function()
>> - set_brightness_delayed()
>> - led_blink_* API family
>>
>> All these introduce problems especially with brightness/color type.
>> I tried to add a new abstraction layer by introducing
>> struct led_base_cdev with a set of generic ops that could be initialized
>> by particular type of LED but still the problem with brightness type
>> generalization remains. One solution could be an union with fields
>> mapping to either single or three brightness components.
>>
>> Other option could be void *brightness type which could be then
>> cast to the required brightness type basing on the LED flag.
> 
> Void *brightness is not really a good option.
> 
> We could pass triplet even in case of single-color LEDs, and then use
> just one component.
> 
> Another option would be to store color in HSV colorspace (not RGB). Then existing
> functions would get brightness (== value in HSV), and RGB-aware functions would
> operate on all 3 components. Triggers/blinking/etc. would then continue operating,
> without modifications.
> 
> We could even export (read-only) hue/saturation for single-color LEDs...

Related patches from Heiner Kallweit are still sitting on devel branch
of linux-leds.git:

https://git.kernel.org/pub/scm/linux/kernel/git/j.anaszewski/linux-leds.git/log/?h=devel

Possibly it can serve as a basis for further development.

I liked that approach because it was compatible with monochrome
LEDs and triggers.

-- 
Best regards,
Jacek Anaszewski