drm/vc4: Drop planes that are completely off-screen

It is permitted for a plane to be configured such that none
of it is on-screen via either negative dest rectangle X,Y
offset, or just an offset that is greater than the crtc
dimensions.

These planes were resized via drm_atomic_helper_check_plane_state
such that the source rectangle had a zero width or height, but
they still created a dlist entry even though they contributed
no pixels. In the case of vc6_plane_mode_set, that it could result
in negative values being written into registers, which caused
incorrect behaviour.

Drop planes that result in a source width or height of 0 pixels
to avoid the incorrect rendering.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

Author: 6by9

drivers/gpu/drm/vc4/vc4_plane.c

@@ -1108,6 +1108,12 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 	width = vc4_state->src_w[0] >> 16;
 	height = vc4_state->src_h[0] >> 16;
 
+	if (!width || !height) {
+		/* 0 source size probably means the plane is offscreen */
+		vc4_state->dlist_initialized = 1;
+		return 0;
+	}
+
 	/* SCL1 is used for Cb/Cr scaling of planar formats.  For RGB
 	 * and 4:4:4, scl1 should be set to scl0 so both channels of
 	 * the scaler do the same thing.  For YUV, the Y plane needs
@@ -1623,6 +1629,12 @@ static int vc6_plane_mode_set(struct drm_plane *plane,
 	width = vc4_state->src_w[0] >> 16;
 	height = vc4_state->src_h[0] >> 16;
 
+	if (!width || !height) {
+		/* 0 source size probably means the plane is offscreen */
+		vc4_state->dlist_initialized = 1;
+		return 0;
+	}
+
 	/* SCL1 is used for Cb/Cr scaling of planar formats.  For RGB
 	 * and 4:4:4, scl1 should be set to scl0 so both channels of
 	 * the scaler do the same thing.  For YUV, the Y plane needs
@@ -1994,6 +2006,9 @@ int vc4_plane_atomic_check(struct drm_plane *plane,
 	if (ret)
 		return ret;
 
+	if (!vc4_state->src_w[0] || !vc4_state->src_h[0])
+		return 0;
+
 	ret = vc4_plane_allocate_lbm(new_plane_state);
 	if (ret)
 		return ret;