http://bugs.freedesktop.org/show_bug.cgi?id=24611





--- Comment #11 from Pierre Ossman <[email protected]>  2009-11-26 
13:12:33 PST ---
More investigation has turned up the following:

vblank ints are enabled on disabled CRTCs as a side effect of keeping vblank
counters sane for userspace. That's why vblank ints are enabled for the unused
CRTC.

Prohibiting the vblank int interrupt has no effect on the hang, so it isn't the
triggering condition. What is though is reading the current vblank counter from
the second CRTC. This is where the lockup occurs.

One theory was that reading the vblank counter from a disabled CRTC is
dangerous, but investigating that theory turned up something even stranger: the
CRTC refuses to disable. The _EN bit keeps being active even though the driver
tries to disable it.

It is not known right now why the _EN bit refuses to turn off, but it might be
related to the hanging.


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