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>

1 files changed, 10 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 67912febb322..9cecfd73b0b1 100644
--- 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)