drm/vc4: Enable background color fill when necessary

Using the hint from the plane state, we turn on the background color
to avoid display corruption from planes blending with the background.

Changes from v1:
 - Use needs_bg_fill from plane state
Signed-off-by: Stefan Schake <stschake@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1520556817-97297-5-git-send-email-stschake@gmail.com

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -643,9 +643,12 @@ static void vc4_crtc_atomic_flush(struct drm_crtc *crtc,
 {
 	struct drm_device *dev = crtc->dev;
 	struct vc4_dev *vc4 = to_vc4_dev(dev);
+	struct vc4_crtc *vc4_crtc = to_vc4_crtc(crtc);
 	struct vc4_crtc_state *vc4_state = to_vc4_crtc_state(crtc->state);
 	struct drm_plane *plane;
+	struct vc4_plane_state *vc4_plane_state;
 	bool debug_dump_regs = false;
+	bool enable_bg_fill = false;
 	u32 __iomem *dlist_start = vc4->hvs->dlist + vc4_state->mm.start;
 	u32 __iomem *dlist_next = dlist_start;
 
@@ -656,6 +659,20 @@ static void vc4_crtc_atomic_flush(struct drm_crtc *crtc,
 
 	/* Copy all the active planes' dlist contents to the hardware dlist. */
 	drm_atomic_crtc_for_each_plane(plane, crtc) {
+		/* Is this the first active plane? */
+		if (dlist_next == dlist_start) {
+			/* We need to enable background fill when a plane
+			 * could be alpha blending from the background, i.e.
+			 * where no other plane is underneath. It suffices to
+			 * consider the first active plane here since we set
+			 * needs_bg_fill such that either the first plane
+			 * already needs it or all planes on top blend from
+			 * the first or a lower plane.
+			 */
+			vc4_plane_state = to_vc4_plane_state(plane->state);
+			enable_bg_fill = vc4_plane_state->needs_bg_fill;
+		}
+
 		dlist_next += vc4_plane_write_dlist(plane, dlist_next);
 	}
 
@@ -664,6 +681,14 @@ static void vc4_crtc_atomic_flush(struct drm_crtc *crtc,
 
 	WARN_ON_ONCE(dlist_next - dlist_start != vc4_state->mm.size);
 
+	if (enable_bg_fill)
+		/* This sets a black background color fill, as is the case
+		 * with other DRM drivers.
+		 */
+		HVS_WRITE(SCALER_DISPBKGNDX(vc4_crtc->channel),
+			  HVS_READ(SCALER_DISPBKGNDX(vc4_crtc->channel)) |
+			  SCALER_DISPBKGND_FILL);
+
 	/* Only update DISPLIST if the CRTC was already running and is not
 	 * being disabled.
 	 * vc4_crtc_enable() takes care of updating the dlist just after