[PATCH v2 4/4] drm/vc4: Enable background color fill when necessary

[Stefan Schake]

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 at gmail.com>
---
 drivers/gpu/drm/vc4/vc4_crtc.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/drivers/gpu/drm/vc4/vc4_crtc.c b/drivers/gpu/drm/vc4/vc4_crtc.c
index ce1e3b9..bf46674 100644
--- a/drivers/gpu/drm/vc4/vc4_crtc.c
+++ b/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
-- 
2.7.4