drm/i915: fix vblank wait test condition

When converting this to the new wait_for macro I inverted the wait condition, which causes all sorts of problems. So correct it to fix several failures caused by the bad wait (flickering, bad output detection, tearing, etc.).

---
 drivers/gpu/drm/i915/intel_display.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

index 6ccb797..dd90b80 100644
@@ -992,7 +992,7 @@ void intel_wait_for_vblank(struct drm_device *dev, int pipe)
 
 	/* Wait for vblank interrupt bit to set */
 	if (wait_for((I915_READ(pipestat_reg) &
-		      PIPE_VBLANK_INTERRUPT_STATUS) == 0,
+		      PIPE_VBLANK_INTERRUPT_STATUS),
 		     50, 0))
 		DRM_DEBUG_KMS("vblank wait timed out\n");
 }

1.6.3.3

Further testing has shown this fixed one set of cases where strange behaviour happened on my EeePC but there are other triggers. I've found the following:

With this patch booting without a battery in the machine will lead to a stable system without lots of errors appearing in dmesg (just one "Insufficient FIFO" at boot). Doing "watch --differences=cumulative xrandr" always shows the same outputs. VT switching occasionally causes the occasional "*ERROR* vblank wait timed out" message. However doing a suspend/resume brings back all the problems, dmesg starts filling up with FIFO errors and xrandr starts reporting different outputs every time it is called. Additionally, booting with a battery inside the machine at boot will always lead to the quirky behaviour too.

Sitsofe | http://sucs.org/~sits/