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As the title says, in 12.04, when I hold FN and press the up and down arrow keys to adjust my screen brightness, the screen starts to flicker, and the OS freezes. I have to hold the power button to force it to shut down, and start it back up. I can't seem to adjust the brightness.

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What is your laptop model? –  Goddard Sep 5 '12 at 19:23

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I had this bug on my Samsung laptop. The trick that fixed everything for me was to add a boot parameter to grub during loading. You can test to see if it works first by rebooting the machine and pressing e when the boot menu appears. Then edit the kernel line to add acpi_osi= after the quiet splash part. Then execute grub (I forget the exact key combo, cntl-x or something). This basically tells the kernel to "lie" to the bios that you're not running Linux.

If it works, you can make it permanent by editing /etc/default/grub to add it to the GRUB_CMDLINE_LINUX_DEFAULT parameter. Update grub by running sudo update-grub

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