drm/vc4: Check if plane requires background fill

Considering a single plane only, we have to enable background color
when the plane has an alpha format and could be blending from the
background or when it doesn't cover the entire screen.

Changes from v1:
 - Drop unrelated change
 - Move needs_bg_fill to 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/1520556817-97297-3-git-send-email-stschake@gmail.com

drivers/gpu/drm/vc4/vc4_plane.c

@@ -73,6 +73,12 @@ struct vc4_plane_state {
 
 	/* Our allocation in LBM for temporary storage during scaling. */
 	struct drm_mm_node lbm;
+
+	/* Set when the plane has per-pixel alpha content or does not cover
+	 * the entire screen. This is a hint to the CRTC that it might need
+	 * to enable background color fill.
+	 */
+	bool needs_bg_fill;
 };
 
 static inline struct vc4_plane_state *
@@ -521,6 +527,7 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 	u32 ctl0_offset = vc4_state->dlist_count;
 	const struct hvs_format *format = vc4_get_hvs_format(fb->format->format);
 	int num_planes = drm_format_num_planes(format->drm);
+	bool covers_screen;
 	u32 scl0, scl1, pitch0;
 	u32 lbm_size, tiling;
 	unsigned long irqflags;
@@ -701,6 +708,16 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 	vc4_state->dlist[ctl0_offset] |=
 		VC4_SET_FIELD(vc4_state->dlist_count, SCALER_CTL0_SIZE);
 
+	/* crtc_* are already clipped coordinates. */
+	covers_screen = vc4_state->crtc_x == 0 && vc4_state->crtc_y == 0 &&
+			vc4_state->crtc_w == state->crtc->mode.hdisplay &&
+			vc4_state->crtc_h == state->crtc->mode.vdisplay;
+	/* Background fill might be necessary when the plane has per-pixel
+	 * alpha content and blends from the background or does not cover
+	 * the entire screen.
+	 */
+	vc4_state->needs_bg_fill = fb->format->has_alpha || !covers_screen;
+
 	return 0;
 }