The issue:
Ever since I installed Ubuntu 10.04 Lucid, I have been unable to use an external monitor with my laptop. I tried several times to find a solution, but the problem was difficult to describe, and that made searching for answers difficult. Yesterday, I found a bug report which matched my symptoms and hardware. In the graphical boot screen and in X.org, the external monitor would have horizontal wavy patterns that severely distorted the image and would cause the monitor to lose sync for brief periods of time.
People who had Radeon Mobility X1400 graphics adapters were reporting the same issue with external monitors. To find out what graphics card you have in your system, you can use the lspci command to list all of the recognized PCI devices. When I run this, the relevant result is:
01:00.0 VGA compatible controller: ATI Technologies Inc Radeon Mobility X1400
I am on kernel version (cat /proc/version):
Linux version 2.6.32-22-generic (buildd@rothera) (gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) ) #33-Ubuntu SMP Wed Apr 28 13:27:30 UTC 2010
The explanation:
The comments to the bug report indicate that this is a problem with the KMS or the Kernel Mode Setting. This feature of Lucid promises to speed up boot times by shifting responsibility for determining the graphics mode from X.org (the windowing system) to the kernel. This fails to correctly detect with X1400 graphics cards however.
The solution:
The recommendation is to disable KMS. To see if this will solve the problem, add “radeon.modeset=0” to the end of the kernel boot line in grub when you boot. If this fixes it, you can make the fix permanent by running the following command from a root shell:
# echo options radeon modeset=0 > /etc/modprobe.d/radeon-kms.conf
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