(In reply to comment #5)
> Created attachment 69113 [details] [review] [review]
> possible fix
> 
> (In reply to comment #4)
> > the bug appeared. So it seems blanking the display controllers with for(i =
> > 0; i < rdev->num_crtc; i++) is not equivalent to the code that it replaces.
> > The original code first wrote in the EVERGREEN_CRTC_UPDATE_LOCK registers,
> > before setting EVERGREEN_CRTC_CONTROL registers and writing again in the
> > EVERGREEN_CRTC_UPDATE_LOCK registers. On the other hand, the new code
> > doesn't write in the EVERGREEN_CRTC_UPDATE_LOCK neither before nor after
> > setting EVERGREEN_CRTC_CONTROL.
> 
> It should be equivalent.  CRTC_UPDATE_LOCK turns off double buffering in the
> crtc which makes register updates atomic.  The new code waits for the frame
> count to increase (the double buffered updates happen at vblank) so it
> should be equivalent.  That said, it shouldn't hurt to take the lock.  Does
> this patch help?

Sadly, it didn't help.