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

From: Jacek Anaszewski
Date: Fri Apr 01 2016 - 10:27:58 EST


On 04/01/2016 04:07 PM, Pavel Machek wrote:
Hi!

pavel@duo:~$ ls -1 /sys/class/leds/
tpacpi:green:batt
tpacpi:orange:batt

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".

Well, that's how it is currently handled. But for the user, it looks
as a LED with multiple colors.

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
color.
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".

Why not?

What do you do if you want to display magenta on your LCD?

You compute RGB values, then display them.

The main drawback is that you can't set the colour at one go,
but have to set brightness of each LED class device (R,G,B)
separately. It incurs delays between setting each colour component.

Yeah. Well, on some hardware, that's just the way it is. If the leds
are separate devices on i2c, you can't really set them in one go.

Delays can occur even if the LEDs are controlled by the same device.
Brightness of each LED class device is set with separate system
call and there will be always some time shift between particular I2C
transmissions that set the brightness for each sub-LED.

If the three sub-LEDs were controlled by a single LED class device
then we could setup the brightness of each sub-LED with single I2C
transmission.

But some hardware has hardware pattern controls, and it can set them
atomically.

It is also impossible to set arbitrary colour from a trigger.
Similarly blinking with arbitrarily chosen colour from RGB palette
is impossible if separate colour components are treated as
separate LEDs.

Yes, see the proposal in the other mail. I believe we should have
separate R, G, B LED devices, and separate pattern controller.

Best regards,
Pavel



--
Best regards,
Jacek Anaszewski