drm/i915: Prevent MI_DISPLAY_FLIP straddling two cachelines on IVB

commit f66fab8e upstream.

According to BSpec the entire MI_DISPLAY_FLIP packet must be contained
in a single cacheline. Make sure that happens.

v2: Use intel_ring_begin_cacheline_safe()
v3: Use intel_ring_cacheline_align() (Chris)

Cc: Bjoern C <lkml@call-home.ch>
Cc: Alexandru DAMIAN <alexandru.damian@intel.com>
Cc: Enrico Tagliavini <enrico.tagliavini@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74053Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -7252,6 +7252,20 @@ static int intel_gen7_queue_flip(struct drm_device *dev,
 		goto err_unpin;
 	}
 
+	/*
+	 * BSpec MI_DISPLAY_FLIP for IVB:
+	 * "The full packet must be contained within the same cache line."
+	 *
+	 * Currently the LRI+SRM+MI_DISPLAY_FLIP all fit within the same
+	 * cacheline, if we ever start emitting more commands before
+	 * the MI_DISPLAY_FLIP we may need to first emit everything else,
+	 * then do the cacheline alignment, and finally emit the
+	 * MI_DISPLAY_FLIP.
+	 */
+	ret = intel_ring_cacheline_align(ring);
+	if (ret)
+		goto err_unpin;
+
 	ret = intel_ring_begin(ring, 4);
 	if (ret)
 		goto err_unpin;