leds: turris-omnia: Make set_brightness() more efficient

leds: turris-omnia: Make set_brightness() more efficient

Implement caching of the LED color and state values that are sent to MCU
in order to make the set_brightness() operation more efficient by
avoiding I2C transactions which are not needed.

On Turris Omnia's MCU, which acts as the RGB LED controller, each LED
has a RGB color, and a ON/OFF state, which are configurable via I2C
commands CMD_LED_COLOR and CMD_LED_STATE.

The CMD_LED_COLOR command sends 5 bytes and the CMD_LED_STATE command 2
bytes over the I2C bus, which operates at 100 kHz. With I2C overhead
this allows ~1670 color changing commands and ~3200 state changing
commands per second (or around 1000 color + state changes per second).
This may seem more than enough, but the issue is that the I2C bus is
shared with another peripheral, the MCU. The MCU exposes an interrupt
interface, and it can trigger hundreds of interrupts per second. Each
time, we need to read the interrupt state register over this I2C bus.
Whenever we are sending a LED color/state changing command, the
interrupt reading is waiting.

Currently, every time LED brightness or LED multi intensity is changed,
we send a CMD_LED_STATE command, and if the computed color (brightness
adjusted multi_intensity) is non-zero, we also send a CMD_LED_COLOR
command.

Consider for example the situation when we have a netdev trigger enabled
for a LED. The netdev trigger does not change the LED color, only the
brightness (either to 0 or to currently configured brightness), and so
there is no need to send the CMD_LED_COLOR command. But each change of
brightness to 0 sends one CMD_LED_STATE command, and each change of
brightness to max_brightness sends one CMD_LED_STATE command and one
CMD_LED_COLOR command:
    set_brightness(0)   ->  CMD_LED_STATE
    set_brightness(255) ->  CMD_LED_STATE + CMD_LED_COLOR
                                            (unnecessary)

We can avoid the unnecessary I2C transactions if we cache the values of
state and color that are sent to the controller. If the color does not
change from the one previously sent, there is no need to do the
CMD_LED_COLOR I2C transaction, and if the state does not change, there
is no need to do the CMD_LED_STATE transaction.

Because we need to make sure that our cached values are consistent with
the controller state, add explicit setting of the LED color to white at
probe time (this is the default setting when MCU resets, but does not
necessarily need to be the case, for example if U-Boot played with the
LED colors).

Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20230918161104.20860-3-kabel@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>