How to set the brightness of a screen#

It is possible to set the brightness of a LVDS screen using the interface in the sysfs. This interface consists mainly in 4 files:

  • /sys/class/backlight/<backlight>/bl_power, which is RW and gives the ability to control the backlight power.

  • /sys/class/backlight/<backlight>/brightness, which is RW and gives the ability to set the brightness for this backlight. The value must be between 0 and the max_brightness. If this file is read, it returns the value of the brightness stored in the driver.

  • /sys/class/backlight/<backlight>/actual_brightnes, which is RO and displays the value of the brightness by querying the hardware.

  • /sys/class/backlight/<backlight>/max_brightness, which is RO and returns the maximum value of the brightness we can set in brightness file.

The <backlight> directory name depends on the SOM you use. With an Intel SOM, it is intel_backlight, with an ARM SOM it depends on the LVDS device tree overlay which has been loaded. It could be lcd0_backlight or lcd1_backlight.

Set the brightness#

The process is simple:

  • read the maximum value you can set:

$ cat /sys/class/backlight/<backlight>/max_brightness

  • set the wanted value

$ echo 100 > /sys/class/backlight/<backlight>/brightness

By default the value of the brightness should be its maximum value. On Intel SOM, this value is 96000, on ARM SOM this value is 255.

Turn off/on the backlight#

It is possible to completely turn off the backlight. It is done with the following command:

$ echo 4 > /sys/class/backlight/<backlight>/bl_power

Then it is possible to turn it on again doing:

$ echo 0 > /sys/class/backlight/<backlight>/bl_power

The brightness of the screen will be the same that before turning off the backlight.

Note

bl_power file has no effect on Intel SOM.

Restore the brightness after a reboot#

It is automatically done by the systemd service systemd-backlight@.service, which stores the value of the brightness when the service is stopped, and loads this value when the service is started. This value is stored in the directory /var/lib/systemd/backlight/.

By default, the systemd-backlight@.service restores a value equal to at least 5% of the max_brightness value even if it has been set to a lower value. On Intel SOM, this value is 4800, on ARM SOM this value is 12.