Also of possible interest. On my system I have three devices of class "backlight":
> ls -l /sys/class/backlight/
total 0
lrwxrwxrwx 1 root root 0 Jun 12 14:17 acpi_video0 -> ../../devices/pci0000:00/0000:00:01.0/0000:01:00.0/backlight/acpi_video0
lrwxrwxrwx 1 root root 0 Jun 12 13:39 acpi_video1 -> ../../devices/pci0000:00/0000:00:02.0/backlight/acpi_video1
lrwxrwxrwx 1 root root 0 Jun 12 14:17 intel_backlight -> ../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight
Each of these devices has a file inside called "brightness". The way these files change is interesting and suggests there is a probelem/confusion/interaction among acpi_video0 and acpi_video1
I could see what happens with the following command:
then watching how the values change in reaction to brightness keypresses.
The intel_backlight/brightness value stands fixed at 497
The acpi_video0/brightness and acpi_video1/brightness values are normally not the same.
e.g. the following happens:
Values are:
6 -- 7
then I do one Fn+ArrowUP keypress for increasing brighness. After 1s, values become (but brightness does not change):
8 -- 7
Then after 1s and at 0.5s intervals, they change to
8 -- 9
8 -- 10
8 -- 11
One thing it always happens is that after a single keypress of brightness up (down), the first value acpi_video0/brightness gets the value of acpi_video1/brightness plus (minus) one, respectively.
Then acpi_video1/brightness continues to change in the same direction.
Also of possible interest. On my system I have three devices of class "backlight":
> ls -l /sys/class/ backlight/ pci0000: 00/0000: 00:01.0/ 0000:01: 00.0/backlight/ acpi_video0 pci0000: 00/0000: 00:02.0/ backlight/ acpi_video1 pci0000: 00/0000: 00:02.0/ drm/card0/ card0-LVDS- 1/intel_ backlight
total 0
lrwxrwxrwx 1 root root 0 Jun 12 14:17 acpi_video0 -> ../../devices/
lrwxrwxrwx 1 root root 0 Jun 12 13:39 acpi_video1 -> ../../devices/
lrwxrwxrwx 1 root root 0 Jun 12 14:17 intel_backlight -> ../../devices/
Each of these devices has a file inside called "brightness". The way these files change is interesting and suggests there is a probelem/ confusion/ interaction among acpi_video0 and acpi_video1
I could see what happens with the following command:
watch -d -n 0.1 "cat /sys/class/ backlight/ acpi_video0/ brightness && cat /sys/class/ backlight/ acpi_video1/ brightness && cat /sys/class/ backlight/ intel_backlight /brightness"
then watching how the values change in reaction to brightness keypresses.
The intel_backlight /brightness value stands fixed at 497
The acpi_video0/ brightness and acpi_video1/ brightness values are normally not the same.
e.g. the following happens:
Values are:
6 -- 7
then I do one Fn+ArrowUP keypress for increasing brighness. After 1s, values become (but brightness does not change):
8 -- 7
Then after 1s and at 0.5s intervals, they change to
8 -- 9
8 -- 10
8 -- 11
One thing it always happens is that after a single keypress of brightness up (down), the first value acpi_video0/ brightness gets the value of acpi_video1/ brightness plus (minus) one, respectively.
Then acpi_video1/ brightness continues to change in the same direction.