I have Intel Imac 11,3 (27" LED, purchased in 2009) with Radeon HD 5750 graphic card. When I installed Ubuntu 12.04 LTS, everything works fine except brigthness control. I want to mention that I didn't install any additional driver.
I tried "sudo update-grub" after changing /etc/default/grub by acpi_backlight=vendor with/without acpi_osi=Linux. Nothing changes.
What can I do to fix this issue?