[PATCH 4.8 022/125] drm/i915/skl: Dont try to update plane watermarks if they havent changed

[Greg Kroah-Hartman]

4.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Lyude <cpaul@xxxxxxxxxx>

commit ccebc23b57c313229526dc76383ce82f5e0b9001 upstream.

i915 sometimes needs to disable planes in the middle of an atomic
commit, and then reenable them later in the same commit. Because of
this, we can't make the assumption that the state of the plane actually
changed. Since the state of the plane hasn't actually changed, neither
have it's watermarks. And if the watermarks hasn't changed then we
haven't populated skl_results with anything, which means we'll end up
zeroing out a plane's watermarks in the middle of the atomic commit
without restoring them later.

Simple reproduction recipe:
 - Get a SKL laptop, launch any kind of X session
 - Get two extra monitors
 - Keep hotplugging both displays (so that the display configuration
   jumps from 1 active pipe to 3 active pipes and back)
 - Eventually underrun

Changes since v1:
 - Fix incorrect use of "it's"
Changes since v2:
 - Add reproduction recipe

Signed-off-by: Lyude <cpaul@xxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Fixes: 62e0fb880123 ("drm/i915/skl: Update plane watermarks atomically during plane updates")
Signed-off-by: Lyude <cpaul@xxxxxxxxxx>
Testcase: kms_plane
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Link: http://patchwork.freedesktop.org/patch/msgid/1472488288-27280-1-git-send-email-cpaul@xxxxxxxxxx
Cc: drm-intel-fixes@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>


---
 drivers/gpu/drm/i915/intel_display.c |    7 ++++++-
 drivers/gpu/drm/i915/intel_sprite.c  |    8 ++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -3068,7 +3068,12 @@ static void skylake_disable_primary_plan
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
 	int pipe = intel_crtc->pipe;
 
-	skl_write_plane_wm(intel_crtc, &dev_priv->wm.skl_results, 0);
+	/*
+	 * We only populate skl_results on watermark updates, and if the
+	 * plane's visiblity isn't actually changing neither is its watermarks.
+	 */
+	if (!to_intel_plane_state(crtc->primary->state)->visible)
+		skl_write_plane_wm(intel_crtc, &dev_priv->wm.skl_results, 0);
 
 	I915_WRITE(PLANE_CTL(pipe, 0), 0);
 	I915_WRITE(PLANE_SURF(pipe, 0), 0);
--- a/drivers/gpu/drm/i915/intel_sprite.c
+++ b/drivers/gpu/drm/i915/intel_sprite.c
@@ -314,6 +314,14 @@ skl_disable_plane(struct drm_plane *dpla
 	const int pipe = intel_plane->pipe;
 	const int plane = intel_plane->plane + 1;
 
+	/*
+	 * We only populate skl_results on watermark updates, and if the
+	 * plane's visiblity isn't actually changing neither is its watermarks.
+	 */
+	if (!to_intel_plane_state(dplane->state)->visible)
+		skl_write_plane_wm(to_intel_crtc(crtc),
+				   &dev_priv->wm.skl_results, plane);
+
 	I915_WRITE(PLANE_CTL(pipe, plane), 0);
 
 	I915_WRITE(PLANE_SURF(pipe, plane), 0);