I just upgraded to 2.6.37 (from http://kernel.ubuntu.com/~kernel-ppa/mainline/v2.6.37-natty/) and my display backlight can't be controlled any more after it's dimmed (dpms?).
It works just fine after booting with boot keys (Fn+Cursor Up/Down) and KDE Power Management Panel (Battery Settings - Screen Brightness Slider).
Both /sys/class/backlight/acpi_video0/(actual_brightness|brightness) return the current value. Setting acpi_video0/brightness shows no effect other than a changed value of actual_brightness and brightness.
Dell Latitude E6500
Just tested it: xset dpms force standy triggers it
I've attached my kern.log with drm.debug=0x07
Uploadsize too small ;)
75MB bzipped to 2MB but I can only upload 1MB
Created attachment 43232 [details]
Print more debugging info
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Can you try this debugging patch and without drm.debug and post the result? (Or just drm.debug=0x02.) This is just to see if setting and getting the backlight works as it's suposed to.
Seems to be fixed in 2.6.38 rc2