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 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
or an on-screen size of 0 pixels to avoid the incorrect rendering.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
---
 drivers/gpu/drm/vc4/vc4_plane.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/gpu/drm/vc4/vc4_plane.c b/drivers/gpu/drm/vc4/vc4_plane.c
index c084967d3527..94737c587f20 100644
--- a/drivers/gpu/drm/vc4/vc4_plane.c
+++ b/drivers/gpu/drm/vc4/vc4_plane.c
@@ -1230,6 +1230,13 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 	if (ret)
 		return ret;
 
+	if (!vc4_state->src_w[0] || !vc4_state->src_h[0] ||
+	    !vc4_state->crtc_w || !vc4_state->crtc_h) {
+		/* 0 source size probably means the plane is offscreen */
+		vc4_state->dlist_initialized = 1;
+		return 0;
+	}
+
 	width = vc4_state->src_w[0] >> 16;
 	height = vc4_state->src_h[0] >> 16;
 
@@ -1753,6 +1760,15 @@ static int vc6_plane_mode_set(struct drm_plane *plane,
 	if (ret)
 		return ret;
 
+	if (!vc4_state->src_w[0] || !vc4_state->src_h[0] ||
+	    !vc4_state->crtc_w || !vc4_state->crtc_h) {
+		/* 0 source size probably means the plane is offscreen.
+		 * 0 destination size is a redundant plane.
+		 */
+		vc4_state->dlist_initialized = 1;
+		return 0;
+	}
+
 	width = vc4_state->src_w[0] >> 16;
 	height = vc4_state->src_h[0] >> 16;
 
@@ -2135,6 +2151,10 @@ static int vc4_plane_atomic_check(struct drm_plane *plane,
 	if (ret)
 		return ret;
 
+	if (!vc4_state->src_w[0] || !vc4_state->src_h[0] ||
+	    !vc4_state->crtc_w || !vc4_state->crtc_h)
+		return 0;
+
 	ret = vc4_plane_allocate_lbm(new_plane_state);
 	if (ret)
 		return ret;

-- 
2.34.1