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, 51 insertions, 0 deletions

diff --git a/drivers/gpu/drm/sun4i/sun4i_backend.c b/drivers/gpu/drm/sun4i/sun4i_backend.c
index 0cbe9f066f0c..245b189fc4d8 100644
--- a/drivers/gpu/drm/sun4i/sun4i_backend.c
+++ b/drivers/gpu/drm/sun4i/sun4i_backend.c
@@ -328,6 +328,8 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
         struct drm_atomic_state *state = crtc_state->state;
         struct drm_device *drm = state->dev;
         struct drm_plane *plane;
+        unsigned int num_planes = 0;
+        unsigned int num_alpha_planes = 0;
         unsigned int num_frontend_planes = 0;
 
         DRM_DEBUG_DRIVER("Starting checking our planes\n");
@@ -340,6 +342,8 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
                         drm_atomic_get_plane_state(state, plane);
                 struct sun4i_layer_state *layer_state =
                         state_to_sun4i_layer_state(plane_state);
+                struct drm_framebuffer *fb = plane_state->fb;
+                struct drm_format_name_buf format_name;
 
                 if (sun4i_backend_plane_uses_frontend(plane_state)) {
                         DRM_DEBUG_DRIVER("Using the frontend for plane %d\n",
@@ -350,6 +354,50 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
                 } else {
                         layer_state->uses_frontend = false;
                 }
+
+                DRM_DEBUG_DRIVER("Plane FB format is %s\n",
+                                 drm_get_format_name(fb->format->format,
+                                                     &format_name));
+                if (fb->format->has_alpha)
+                        num_alpha_planes++;
+
+                num_planes++;
+        }
+
+        /*
+         * 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, since this means that we need to have one layer
+         * with alpha at the lowest position of our two pipes.
+         *
+         * However, we cannot even do that, since the hardware has a
+         * bug where the lowest plane of the lowest pipe (pipe 0,
+         * priority 0), if it has any alpha, will discard the pixel
+         * entirely and just display the pixels in the background
+         * color (black by default).
+         *
+         * This means that we effectively have only three valid
+         * configurations with alpha, all of them with the alpha being
+         * on pipe1 with the lowest position, which can be 1, 2 or 3
+         * depending on the number of planes and their zpos.
+         */
+        if (num_alpha_planes > SUN4I_BACKEND_NUM_ALPHA_LAYERS) {
+                DRM_DEBUG_DRIVER("Too many planes with alpha, rejecting...\n");
+                return -EINVAL;
         }
 
         if (num_frontend_planes > SUN4I_BACKEND_NUM_FRONTEND_LAYERS) {
@@ -357,6 +405,9 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
                 return -EINVAL;
         }
 
+        DRM_DEBUG_DRIVER("State valid with %u planes, %u alpha, %u video\n",
+                         num_planes, num_alpha_planes, num_frontend_planes);
+
         return 0;
 }