What happens when KMS fails?
closed as not a real question by Tom Brossman, Uri Herrera, Mitch♦, Takkat, heisenberg Sep 14 '12 at 14:28
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Mode setting is basically setting up the screen resolution and the depth mode for the graphics card.
If KMS were to fail, I would think that X would do its own modesetting; instead of having the kernel do it. But that might require you to first disable KMS. More information specifically for different graphics gards can be found on the Ubuntu Wiki article I linked to eariler.