drm/sun4i: backend: Check for the number of alpha planes

Due to the way the composition is done in hardware, we can only have a
single alpha-enabled plane active at a time, placed in the second (highest
priority) pipe.

Make sure of that in our atomic_check to not end up in an impossible
scenario.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7371f62a1385f2cbe3ed75dfca2e746338eb2286.1516617243.git-series.maxime.ripard@free-electrons.com

1 files changed, 2 insertions, 22 deletions

diff --git a/drivers/gpu/drm/sun4i/sun4i_layer.c b/drivers/gpu/drm/sun4i/sun4i_layer.c
index fbf25d59cf88..19be798e4fac 100644
--- a/drivers/gpu/drm/sun4i/sun4i_layer.c
+++ b/drivers/gpu/drm/sun4i/sun4i_layer.c
@@ -201,32 +201,12 @@ struct drm_plane **sun4i_layers_init(struct drm_device *drm,
 	struct sun4i_backend *backend = engine_to_sun4i_backend(engine);
 	int i;
 
-	planes = devm_kcalloc(drm->dev, ARRAY_SIZE(sun4i_backend_planes) + 1,
+	/* We need to have a sentinel at the need, hence the overallocation */
+	planes = devm_kcalloc(drm->dev, SUN4I_BACKEND_NUM_LAYERS + 1,
 			      sizeof(*planes), GFP_KERNEL);
 	if (!planes)
 		return ERR_PTR(-ENOMEM);
 
-	/*
-	 * The hardware is a bit unusual here.
-	 *
-	 * Even though it supports 4 layers, it does the composition
-	 * in two separate steps.
-	 *
-	 * The first one is assigning a layer to one of its two
-	 * pipes. If more that 1 layer is assigned to the same pipe,
-	 * and if pixels overlaps, the pipe will take the pixel from
-	 * the layer with the highest priority.
-	 *
-	 * The second step is the actual alpha blending, that takes
-	 * the two pipes as input, and uses the eventual alpha
-	 * component to do the transparency between the two.
-	 *
-	 * This two steps scenario makes us unable to guarantee a
-	 * robust alpha blending between the 4 layers in all
-	 * situations. So we just expose two layers, one per pipe. On
-	 * SoCs that support it, sprites could fill the need for more
-	 * layers.
-	 */
 	for (i = 0; i < ARRAY_SIZE(sun4i_backend_planes); i++) {
 		const struct sun4i_plane_desc *plane = &sun4i_backend_planes[i];
 		struct sun4i_layer *layer;