drm/i915: paper over a pipe-enable vs pageflip race

I've discovered this on my ivb machine while stress-testing the new
flip_tests. Only harmful effect observed is that the timestamp is a
bit bogus.

Note that this is empirical duct-tape: I've noticed that we seem to
only ever miss the very first vblank irq right after enabling the
pipe. And with this hack applied I couldn't reproduce the failure case
anywhere else any more.

Tested-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 67912fe..9cecfd7 100644 (file)
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -3253,6 +3253,16 @@ static void ironlake_crtc_enable(struct drm_crtc *crtc)
 
        if (HAS_PCH_CPT(dev))
                intel_cpt_verify_modeset(dev, intel_crtc->pipe);
+
+       /*
+        * There seems to be a race in PCH platform hw (at least on some
+        * outputs) where an enabled pipe still completes any pageflip right
+        * away (as if the pipe is off) instead of waiting for vblank. As soon
+        * as the first vblank happend, everything works as expected. Hence just
+        * wait for one vblank before returning to avoid strange things
+        * happening.
+        */
+       intel_wait_for_vblank(dev, intel_crtc->pipe);
 }
 
 static void ironlake_crtc_disable(struct drm_crtc *crtc)