Okay, I've tested with your possible fix. But it did not work. Now /sys/class/backlight/actual_brightness reports valid values which is the same as /sys/class/backlight/brightness Now when I change the brightness both /sys/class/backlight/actual_brightness and /sys/class/backlight/brightness reports valid values but still the display brightness is at maximum and does not change. Seems like this is not a easy fix.