Comment 45 for bug 615549

VGA switcheroo, race conditions, vesafb, oh my!

You say “I lose the video when Nouveau is loaded”, but nouveau isn't loaded until 20 seconds into your boot and doesn't start modesetting until 22 seconds. Did you mean “I lose the video right after GRUB starts loading the kernel”? If so, we have the first bug: vesafb being confused in some way by your multiple graphics cards.

In chronological order, then, we encounter the second bug: X starts at approximately +20seconds, and is probing DRM at 20.625. However, the nouveau kernel driver is busy trying to get the video bios at this point, and hasn't finished initialising. Nouveau finishes bringing up your 9200M GS (which looks like it should be driving the VGA output at this point) at +22 seconds, and then brings up the 9400M G at +25 seconds. By this time, the nouveau X driver has given up looking for DRM, the VESA X driver has loaded and started merrily stomping on card state of the 9400M, which would be my guess as to why the nouveau kernel module bails soon after with a bunch of
[ 25.465878] [drm] nouveau 0000:03:00.0: PFIFO_INTR 0x00000010 - Ch 1
errors.

I'd guess that this race condition is why it is reported by some that restarting gdm fixes things - by that point the nouveau kernel module will actually have initialised and be ready for the nouveau X driver.