How-to solve black screen on iMac with ATI Radeon HD 5700 video chipset and Ubuntu 12.04.3 LTS

After an Kernel update my alu iMac 27" running Ubuntu 12.04.3 LTS boots with a total black screen. Even the backlight from the video panel is off.

To solve this login in over ssh an edit the file /etc/default/grub

sudo vi /etc/default/grub

Find the line that start with GRUB_CMDLINE_LINUX_DEFAULT= and edit is to:

iMac brightness setting script

I wrote a simple script to adjust the brightness om my iMac 27" running Ubuntu 11.10 oneiric. The script is using the gsd-backlight-helper that also available in Ubuntu precis 12.10 so I suppose this also works in 12.10. (let me know)

My solution is to add two brightness icons on the gnome-panel. One for brightness-up and one for brightness-down. (see screenshot)