[PATCH] drm/vc4: Ignore alpha on primary plane

[Stefan Schake <stschake@gmail.com>]

We allow alpha formats on the primary plane but a partially transparent
framebuffer will cause a corrupted display. With this change black pixels
are output instead, in line with the behavior for other DRM drivers.

Signed-off-by: Stefan Schake <stschake@gmail.com>
---
Test program is available at https://github.com/stschake/vc4-alpha-test

 drivers/gpu/drm/vc4/vc4_plane.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vc4/vc4_plane.c b/drivers/gpu/drm/vc4/vc4_plane.c
index 61ad955..8c829fa 100644
--- a/drivers/gpu/drm/vc4/vc4_plane.c
+++ b/drivers/gpu/drm/vc4/vc4_plane.c
@@ -521,6 +521,7 @@ 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 primary_plane = state->crtc->primary == plane;
 	u32 scl0, scl1, pitch0;
 	u32 lbm_size, tiling;
 	unsigned long irqflags;
@@ -620,8 +621,12 @@ static int vc4_plane_mode_set(struct drm_plane *plane,
 
 	/* Position Word 2: Source Image Size, Alpha Mode */
 	vc4_state->pos2_offset = vc4_state->dlist_count;
+	/* We do not enable the HVS background color fill so the primary plane
+	 * must be opaque to avoid display artifacts. Achieve this by always
+	 * using fixed alpha (initialized to 0xff above) on the primary plane.
+	 */
 	vc4_dlist_write(vc4_state,
-			VC4_SET_FIELD(fb->format->has_alpha ?
+			VC4_SET_FIELD(fb->format->has_alpha && !primary_plane ?
 				      SCALER_POS2_ALPHA_MODE_PIPELINE :
 				      SCALER_POS2_ALPHA_MODE_FIXED,
 				      SCALER_POS2_ALPHA_MODE) |
-- 
2.7.4

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