Comment 10 for bug 847001

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:

 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.