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

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Date:	Wed, 30 Mar 2016 07:58:27 +0200
From:	Heiner Kallweit <hkallweit1@...il.com>
To:	Pavel Machek <pavel@....cz>, pali.rohar@...il.com, sre@...nel.org,
	khilman@...nel.org, aaro.koskinen@....fi,
	ivo.g.dimitrov.75@...il.com, patrikbachan@...il.com,
	serge@...lyn.com
Cc:	Greg KH <greg@...ah.com>,
	Jacek Anaszewski <j.anaszewski@...sung.com>,
	linux-leds@...r.kernel.org,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org
Subject: Re: [PATCH v5 1/4] leds: core: add generic support for RGB Color
 LED's

Am 29.03.2016 um 23:43 schrieb Pavel Machek:
> Hi!
> 
>>> First, please Cc me on RGB color support.
>>>
>>>> Add generic support for RGB Color LED's.
>>>>
>>>> Basic idea is to use enum led_brightness also for the hue and saturation
>>>> color components.This allows to implement the color extension w/o
>>>> changes to struct led_classdev.
>>>>
>>>> Select LEDS_RGB to enable building drivers using the RGB extension.
>>>>
>>>> Flag LED_SET_HUE_SAT allows to specify that hue / saturation
>>>> should be overridden even if the provided values are zero.
>>>>
>>>> Some examples for writing values to /sys/class/leds/<xx>/brightness:
>>>> (now also hex notation can be used)
>>>>
>>>> 255 -> set full brightness and keep existing color if set
>>>> 0 -> switch LED off but keep existing color so that it can be restored
>>>>      if the LED is switched on again later
>>>> 0x1000000 -> switch LED off and set also hue and saturation to 0
>>>> 0x00ffff -> set full brightness, full saturation and set hue to 0
>>>> (red)
>>>
>>> Umm, that's rather strange interface -- and three values in single sysfs
>>> file is actually forbidden.
>>>
>>> Plus, it is very much unlike existing interfaces for RGB LEDs, which
>>> we already have supported in the tree. (At least nokia N900 and Sony
>>> motion controller already contain supported three-color LEDs).
>>>
>>> Now... yes, there's work to be done for the 3-color LEDs. Currently,
>>> they are treated as three different LEDs. (Which makes some sense, you
>>> can use "battery charging" trigger for LED, and CPU activity trigger
>>> for green, for example). It would be good to have some kind of
>>> grouping, so that userspace can tell "these 3 leds are actually
>>> combined into one light".
>>>
>> At first thanks for the review comments.
>> Treating the three physical LEDs of a RGB LED as separate LED devices
>> might have been implemented due to the lack of alternatives.
> 
> To be fair... they _are_ separate LED devices. In N900 case, you can
> even see light comming from slightly different places if you look closely.
> 
I mainly work with encapsulated USB HID LED devices like Thingm blink(1).
Due to the diffuse plastic cover you don't see the individual LEDs on the chip.

>> With one trigger controlling the red LED and another controlling the green
>> LED we may end up with a yellow light. Not sure whether this is what
>> we want.
> 
> Well, it should be understandable for most people.
> 
>> One driver for this extension was the idea of triggers using color
>> to visualize states etc.
>> Therefore it's not only about userspace controlling the color.
>> As a trigger is bound to a led_classdev we need a led_classdev
>> representing a RGB LED device.
>>
>> And ok: If required the sysfs interface can be splitted into separate
>> attributes for hue, saturation, and (existing) brightness.
> 
> Required.
> 
> Ok, so:
> 
> a) Do we want RGB leds to be handled by existing subsystem, or do we
> need separate layer on top of that?
> 
> b) Does RGB make sense, or HSV? RGB is quite widely used in graphics,
> and it is what hardware implements. (But we'd need to do gamma
> correction).
> 
HSV has the charme that the current monochrome V-only is a subset.
Therefore the current API can be used also with color LEDs.
However there might be good reasons for using RGB too.

> c) My hardware has "acceleration engine", LED is independend from
> CPU. That's rather big deal. Does yours? It seems to be quite common,
> at least in cellphones.
> 
Devices like blink(1) support storing and re-playing patterns, fading etc.

> Ideally, I'd like to have "triggers", but different ones. As in: if
> charging, do yellow " .xX" pattern. If fully charged, do green steady
> light. If message is waiting, do blue " x x" pattern. If none of
> above, do slow white blinking. (Plus priorities of events). But that's
> quite different from existing support...)
> 
I think for this a separate layer would be helpful.
Your primary intention is to e.g. display "charging" on the RGB LED
device. Most likely you don't want to split yellow into its red + green
component and then write to the respective (sub-)LEDs.
Also just think of the case that later you might decide that orange
is nicer than yellow.

> 								Pavel
>