Comment 5 for bug 428910

I had a look at the data provided. It seems that before the patch, brightness was controlled by the acer-wmi driver (as the definition provides only _DOD and would therefor have been ignored). Now the acpi video driver looks at the video device but seems to get an invalid brightness value (which strikingly looks like a date to me).

[ 3.698698] [Firmware Bug]: ACPI(Z00P) defines _DOD but not _DOS
[ 4.232085] ACPI Error: Current brightness invalid 20090521 video-538
[ 4.232263] input: Video Bus as /devices/LNXSYSTM:00/device:00/PNP0A08:00/device:0f/device:10/input/input6
[ 4.232354] ACPI: Video Device [Z00P] (multi-head: yes rom: no post: no)
[ 13.325165] acer-wmi: Brightness must be controlled by generic video driver

The methods in the BIOS look incorrect to me (it stores x=((b/10)-1) but reads b=((x*10)+1), where b is the brightness value and x the one stored on the ec). But that should normally not result in a value seen in the error message. This needs more investigation.

For now, can you try the following: add "acpi_backlight=vendor" to the kernel command line either on boot (which is a bit hard with grub timeout being 0, one needs to press shift quickly) or in /etc/default/grub and calling "sudo update-grub". Does this restore the old functionality?