drm/i915: Warn if we run out of FIFO space for a mode

Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -2745,8 +2745,14 @@ static unsigned long intel_calculate_wm(unsigned long clock_in_khz,
 	/* Don't promote wm_size to unsigned... */
 	if (wm_size > (long)wm->max_wm)
 		wm_size = wm->max_wm;
-	if (wm_size <= 0)
+	if (wm_size <= 0) {
 		wm_size = wm->default_wm;
+		DRM_ERROR("Insufficient FIFO for plane, expect flickering:"
+			  " entries required = %ld, available = %lu.\n",
+			  entries_required + wm->guard_size,
+			  wm->fifo_size);
+	}
+
 	return wm_size;
 }