Re: [PATCH v2 1/3] leds: turris-omnia: change max brightness from 255 to 1

["Marek Behún" <kabel@kernel.org>]

On Sun, 16 Jul 2023 11:19:30 +0200
Pavel Machek <pavel@ucw.cz> wrote:

> Hi!
> 
> > Using binary brightness makes more sense for this controller, because
> > internally in the MCU it works that way: the LED has a color, and a
> > state whether it is ON or OFF.  
> 
> So, controller can do (1, 3, 5) but not (3, 3, 3)?
> 
> > The resulting brightness computation with led_mc_calc_color_components()
> > will now always result in either (0, 0, 0) or the multi_intensity value.  
> 
> Won't that limit you to 8 colors total?
> 
> I guess I`m confused how this hw works...

Hi Pavel.

No no no. That's not how it is.

The HW exposes color control for three channels (RGB), each channel with
range 0-255 (so 16M colors). The driver exposes this via the
multi_intensity sysfs file. This is communicated to the HW via
LED_SET_COLOR I2C command.

HW also exposes setting the LED ON and OFF, via the LED_SET_STATE
I2C command.

We currently have the following sysfs files via which we set LED state
and color:
  brightness
  multi_intensity

Because currently the driver sets max_brightness to 255, the actual
color that is sent to HW is recalculated via
led_mc_calc_color_components(). For example with

  $ echo 255 255 100 >multi_intensity
  $ echo 150 >brightness

the led_mc_calc_color_components() function recalculates the channel
intensities with formula
  brightness * channel_intensity / max_brightness
and so the (255, 255, 100) tuple is converted to (150, 150, 58) before
sending to HW.

What I think would make more sense is to make the two sysfs files
  brightness
  multi_intensity
correspond 1-to-1 with I2C commands LED_SET_STATE and LED_SET_COLOR.

This can be simply done by setting max_brightness to 1. The brightness
sysfs file then can simply control whether the LED is ON or OFF. The
multi_intensity file control the color.

I realize now that in the patch I should also make away with the call
to led_mc_calc_color_components()...

Marek