[PATCH 4/5] drm/i915/skl: Don't try to update plane watermarks if they haven't changed

[Lyude]

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 <lyude at redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Fixes: 62e0fb880123 ("drm/i915/skl: Update plane watermarks atomically during plane updates")
Testcase: kms_plane
Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1472488288-27280-1-git-send-email-cpaul@redhat.com
Cc: drm-intel-fixes at lists.freedesktop.org
---
 drivers/gpu/drm/i915/intel_display.c | 7 ++++++-
 drivers/gpu/drm/i915/intel_sprite.c  | 8 ++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 8086e62..85d161a 100644
--- 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_plane(struct drm_plane *primary,
 	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);
diff --git a/drivers/gpu/drm/i915/intel_sprite.c b/drivers/gpu/drm/i915/intel_sprite.c
index d8bfff1..4178849 100644
--- 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 *dplane, struct drm_crtc *crtc)
 	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);
-- 
2.7.4