[PATCH 15/19] drm/sun4i: backend: Check for the number of alpha planes

Maxime Ripard
2018-01-09T10:56:34Z

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.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
---
drivers/gpu/drm/sun4i/sun4i_backend.c | 50 ++++++++++++++++++++++++++++-
drivers/gpu/drm/sun4i/sun4i_backend.h | 2 +-
drivers/gpu/drm/sun4i/sun4i_layer.c | 23 +-------------
3 files changed, 53 insertions(+), 22 deletions(-)

diff --git a/drivers/gpu/drm/sun4i/sun4i_backend.c b/drivers/gpu/drm/sun4i/sun4i_backend.c
index c4986054909b..dd995a6b8b12 100644
--- a/drivers/gpu/drm/sun4i/sun4i_backend.c
+++ b/drivers/gpu/drm/sun4i/sun4i_backend.c
@@ -329,6 +329,8 @@ 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");
@@ -341,6 +343,7 @@ 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;

if (sun4i_backend_plane_uses_frontend(plane_state)) {
DRM_DEBUG_DRIVER("Using the frontend for plane %d\n",
@@ -351,6 +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 (drm_format_has_alpha(fb->format->format))
+ 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).
+ *
+ * Since 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) {
@@ -358,6 +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;
}

diff --git a/drivers/gpu/drm/sun4i/sun4i_backend.h b/drivers/gpu/drm/sun4i/sun4i_backend.h
index 04a4f11b87a8..52e77591186a 100644
--- a/drivers/gpu/drm/sun4i/sun4i_backend.h
+++ b/drivers/gpu/drm/sun4i/sun4i_backend.h
@@ -146,6 +146,8 @@
#define SUN4I_BACKEND_HWCCOLORTAB_OFF 0x4c00
#define SUN4I_BACKEND_PIPE_OFF(p) (0x5000 + (0x400 * (p)))

+#define SUN4I_BACKEND_NUM_LAYERS 4
+#define SUN4I_BACKEND_NUM_ALPHA_LAYERS 1
#define SUN4I_BACKEND_NUM_FRONTEND_LAYERS 1

struct sun4i_backend {
diff --git a/drivers/gpu/drm/sun4i/sun4i_layer.c b/drivers/gpu/drm/sun4i/sun4i_layer.c
index fbf25d59cf88..900e716443b8 100644
--- a/drivers/gpu/drm/sun4i/sun4i_layer.c
+++ b/drivers/gpu/drm/sun4i/sun4i_layer.c
@@ -201,32 +201,11 @@ 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,
+ planes = devm_kcalloc(drm->dev, SUN4I_BACKEND_NUM_LAYERS,
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;
--
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