I have Apple iMac 11,3 (27" LED, purchased in 2011) with a Radeon HD 5750 graphics card. When I installed Ubuntu 12.04 LTS, everything works fine except brightness control. I want to mention that I didn't install any additional driver.
sudo update-grub after changing
acpi_osi=Linux. Nothing changes.
What can I do to fix this issue? Help is appreciated.