drm/i915: Reorder skl+ scaler vs. plane updates

When scanning out NV12 if we at any time have the plane enabled
while the scaler is disabled we get a pretty catastrophic
underrun.

Let's reorder the operations so that we try to avoid that happening
even if our vblank evade fails and the scaler enable/disable and
the plane enable/disable get latched during two diffent frames.

This takes care of the most common cases. I suppose there is still
at least a theoretical possibility of hitting this if one plane
takes the scaler away from another plane before the second plane
had a chance to set up another scaler for its use. But that
is starting to get a bit complicated, especially since the plane
commit order already has to be carefully sequenced to avoid any
dbuf overlaps. So plugging this 100% may prove somewhat hard...

Cc: Cooper Chiou <cooper.chiou@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210506073836.14848-1-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>

1 files changed, 8 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/display/skl_universal_plane.c b/drivers/gpu/drm/i915/display/skl_universal_plane.c
index 0d34a5ad4e2b..6ad85d7cb219 100644
--- a/drivers/gpu/drm/i915/display/skl_universal_plane.c
+++ b/drivers/gpu/drm/i915/display/skl_universal_plane.c
@@ -1033,6 +1033,14 @@ skl_program_plane(struct intel_plane *plane,
 		intel_psr2_program_plane_sel_fetch(plane, crtc_state, plane_state, color_plane);
 
 	/*
+	 * Enable the scaler before the plane so that we don't
+	 * get a catastrophic underrun even if the two operations
+	 * end up happening in two different frames.
+	 */
+	if (plane_state->scaler_id >= 0)
+		skl_program_plane_scaler(plane, crtc_state, plane_state);
+
+	/*
 	 * The control register self-arms if the plane was previously
 	 * disabled. Try to make the plane enable atomic by writing
 	 * the control register just before the surface register.
@@ -1041,9 +1049,6 @@ skl_program_plane(struct intel_plane *plane,
 	intel_de_write_fw(dev_priv, PLANE_SURF(pipe, plane_id),
 			  intel_plane_ggtt_offset(plane_state) + surf_addr);
 
-	if (plane_state->scaler_id >= 0)
-		skl_program_plane_scaler(plane, crtc_state, plane_state);
-
 	spin_unlock_irqrestore(&dev_priv->uncore.lock, irqflags);
 }