Re: [PATCH v7 1/5] leds: core: add generic support for RGB LED's

[Jacek Anaszewski]

Hi Karl,

On 03/06/2016 09:55 PM, Karl Palsson wrote:
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> Heiner Kallweit <hkallweit1@xxxxxxxxx> wrote:
> > Add generic support for RGB 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)
>
>
> I admit I hadn't seen this earlier, and I didn't spend all day
> looking at previous attempts, but what's the motivation for
> putting all this overloaded syntax into the "brightness" file? I
> thought the point was to have a single value in each file, one of
> the references I did find was

With single value per file there would be problems with colour
components setting synchronization.

> http://www.spinics.net/lists/linux-leds/msg02995.html Is there
> some thread where this was decided as advantageous? Surely 0-255
> for _brightness_ is what the brightness entry should do?

You can find a reference to the related discussion in [1].

> I'd like to set the rgb colour of a led (or hsv, that can be it's
> own file too) and separately ramp the brightness up and down. I
> also think it's substantially simpler and easier to use from the
> user's point of view, which is surely the place you want easy
> right?

I'm also not very keen on overloading the brightness attribute
semantics. Nonetheless it seems impossible to add support for
setting three colour components otherwise than through a single
syscall, if we want to make it synchronous and compatible with
LED triggers.

HSV color scheme is also very convenient for adapting monochrome
brightness semantics to the RGB realm.


[1] http://www.spinics.net/lists/linux-leds/msg05477.html

--
Best regards,
Jacek Anaszewski