How to modify the panel backlight

Applicable for

STM32MP13x lines, STM32MP15x lines, STM32MP21x lines, STM32MP23x lines, STM32MP25x lines

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First identify the backlight device linked to your panel, thanks to the sysfs:

ls /sys/class/backlight
5a000000.dsi.0

In the above example, there is a single entry named "5a000000.dsi.0", which is replaced by "*" in the below command.

To adjust the panel backlight, enter the following command:

echo 32 > /sys/class/backlight/*/brightness    # set the dsi panel backlight to the value 32

Note: Only values between 0 and max_brightness must be used.

To get the maximum panel backlight value, enter:

cat /sys/class/backlight/*/max_brightness
255

Information

If the max_brightness value is 1, the backlight can only be switched on and off. If the max_brightness value is above 1, the backlight has several levels on luminosity (pwm-based backlight for instance).

To get the actual panel backlight value, enter:

cat /sys/class/backlight/*/actual_brightness
32

Use one of the following commands to quickly test the panel backlight:

export bl=/sys/class/backlight/5a000000.dsi.0/brightness
export max=/sys/class/backlight/5a000000.dsi.0/max_brightness

# switch on/off test
(while true; do echo 0 > $bl; sleep 0.5; echo $max > $bl; sleep 0.5; done)

# slow 32-step tests (only for pwm-based backlights)
(i=0; while true; do i=$(((i+32)%256)); echo $i > $bl; sleep 0.5; done)

# fast 1-step test  (only for pwm-based backlights)
(i=0; while true; do i=$(((i+1)%256)); echo $i > $bl; done)

Information

Be sure the panel is running (power on) before modifying its backlight.