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

From: Jacek Anaszewski
Date: Thu Mar 31 2016 - 04:17:54 EST

Hi Heiner,

On 03/30/2016 03:59 PM, Heiner Kallweit wrote:
On Wed, Mar 30, 2016 at 3:03 PM, Pavel Machek <pavel@xxxxxx> wrote:

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

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.

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...)

Please note that HSV colour scheme allows to neatly project monochrome
brightness semantics on the RGB realm. I.e. you can have fixed
hue and saturation, and by changing the brightness component a perceived
colour intensity can be altered.

I see HSV has some advantages. But we already have LEDs with multiple
colors, and kernel already handles them:

pavel@duo:~$ ls -1 /sys/class/leds/

This is physically 2 leds but hidden under one indicator, so you got
"off", "green", "orange" and "green+orange".

That's a good example. As long as you can recognize green+orange as
separate lights/colors
(w/o magnifying glass) I wouldn't call it "a LED with multiple colors"
but "multiple
LED devices".

In my use case we talk about RGB LEDs like the commonly used 5050 SMD RGB LEDs.
And it's not only about using a handful of discrete colors but about
displaying any arbitrary
So far the kernel exposes the physical RGB LEDs as separate LEDs only
and I can't use
a trigger to e.g. set "magenta with 50% brightness".

I think that we should consult more people before pushing the solution
upstream. Would you mind writing a message with an explanation of the
issue to linux-api list?

Please keep in mind also the information from the "Attributes" section
of Documentation/filesystems/sysfs.txt.

Best regards,
Jacek Anaszewski