[PATCH v3] drm/i915/skl: Don't try to update plane watermarks if they haven't changed

[Lyude]

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 at redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Fixes: 62e0fb880123 ("drm/i915/skl: Update plane watermarks atomically
during plane updates")

Signed-off-by: Lyude <cpaul at redhat.com>
---
 drivers/gpu/drm/i915/intel_display.c | 7 ++++++-
 drivers/gpu/drm/i915/intel_sprite.c  | 9 +++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index e4e6141..13e47a7 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -3448,7 +3448,12 @@ 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 (!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 0df783a..73a521f 100644
--- a/drivers/gpu/drm/i915/intel_sprite.c
+++ b/drivers/gpu/drm/i915/intel_sprite.c
@@ -292,8 +292,13 @@ skl_disable_plane(struct drm_plane *dplane, struct drm_crtc *crtc)
 	const int pipe = intel_plane->pipe;
 	const int plane = intel_plane->plane + 1;
 
-	skl_write_plane_wm(to_intel_crtc(crtc), &dev_priv->wm.skl_results,
-			   plane);
+	/*
+	 * We only populate skl_results on watermark updates, and if the
+	 * plane's visiblity isn't actually changing neither is its watermarks.
+	 */
+	if (!dplane->state->visible)
+		skl_write_plane_wm(to_intel_crtc(crtc),
+				   &dev_priv->wm.skl_results, plane);
 
 	I915_WRITE(PLANE_CTL(pipe, plane), 0);
 
-- 
2.7.4