Comment 16 for bug 432835

I've got stock kernel 2.6.32.3 and Debian (not Ubuntu), so all this FWIW.

On my 5810TZ, xrandr --prop does not show BACKLIGHT_CONTROL. Backlight keys do not work, but when booting *without* nomodeset (some posts on the Internets suggest that "nomodeset acpi_backlight=vendor" fixes this, but for me it doesn't), it does show "Backlight" and "BACKLIGHT" (if booting *with* acpi_backlight=vendor, even these settings disappear):

------------------------------------------------------------
$ xrandr --prop
Screen 0: minimum 320 x 200, current 1366 x 768, maximum 8192 x 8192
VGA1 disconnected (normal left inverted right x axis y axis)
LVDS1 connected 1366x768+0+0 (normal left inverted right x axis y axis) 345mm x 194mm
        EDID:
                00ffffffffffff0030e4020200000000
                00130103802313780a28659759548e27
                1e505400000001010101010101010101
                010101010101561d56c6500020303020
                350059c2100000190000000000000000
                00000000000000000000000000fe004c
                4720446973706c61790a2020000000fc
                004c503135365748332d544c41310039
        BACKLIGHT: 3 (0x00000003) range: (0,9)
        Backlight: 3 (0x00000003) range: (0,9)
        scaling mode: Full
                supported: None Full Center Full aspect
   1366x768 60.0*+
   1360x768 59.8
   1024x768 60.0
   800x600 60.3
   640x480 59.9
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
------------------------------------------------------------

Doing "xrandr --output LVDS1 --set Backlight 5" (or whatever number from 0..9) doesn't seem to change anything, but I noticed by accident that after setting the value using that command, pressing the backlight setting keys on the keyboard (Fn+arrow keys) sets then the backlight to the brightness set with xrandr.

The same actually can be done via /proc:

------------------------------------------------------------
root@lap:/proc/acpi/video/OVGA# ls -l
total 0
dr-xr-xr-x 2 root root 0 Jan 14 01:58 DD01
dr-xr-xr-x 2 root root 0 Jan 14 01:58 DD02
dr-xr-xr-x 2 root root 0 Jan 14 01:58 DD03
dr-xr-xr-x 2 root root 0 Jan 14 01:58 DD04
dr-xr-xr-x 2 root root 0 Jan 14 01:58 DD05
-rw-r--r-- 1 root root 0 Jan 14 01:58 DOS
-r--r--r-- 1 root root 0 Jan 14 01:58 info
-rw-r--r-- 1 root root 0 Jan 14 01:58 POST
-r--r--r-- 1 root root 0 Jan 14 01:58 POST_info
-r--r--r-- 1 root root 0 Jan 14 01:58 ROM
root@lap:/proc/acpi/video/OVGA# ls DD0*/brightness
DD01/brightness DD02/brightness DD03/brightness DD04/brightness DD05/brightness
root@lap:/proc/acpi/video/OVGA# cat DD*/brightness
<not supported>
levels: 10 20 30 40 50 60 70 80 90 100
current: 10
<not supported>
<not supported>
<not supported>
root@lap:/proc/acpi/video/OVGA# echo 50 >DD02/brightness
------------------------------------------------------------
[and now pressing Fn+brightness key immediately sets the display brightness to 50.]

/proc/acpi/video/DD02/brightness always reflects the brightness as set by xrandr, but it only seems to take effect when pressing the brightness adjustment keys on the keyboard (which have no effect other than that, although they do send XF86MonBrightnessUp/Down keycodes as shown by xev).