drm/sun4i: sun4i: Introduce a quirk for lowest plane alpha support

Not all sunxi platforms with the first version of the Display Engine
support an alpha component on the plane with the lowest z position
(as in: lowest z-pos), that gets blended with the background color.

In particular, the A13 is known to have this limitation. However, it was
recently discovered that the A20 and A33 are capable of having alpha on
their lowest plane.

Thus, this introduces a specific quirk to indicate such support,
per-platform. Since this was not tested on sun4i and sun6i platforms, a
conservative approach is kept and this feature is not supported.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180719080838.31598-2-paul.kocialkowski@bootlin.com

drivers/gpu/drm/sun4i/sun4i_backend.c

@@ -34,6 +34,9 @@
 struct sun4i_backend_quirks {
 	/* backend <-> TCON muxing selection done in backend */
 	bool needs_output_muxing;
+
+	/* alpha at the lowest z position is not always supported */
+	bool supports_lowest_plane_alpha;
 };
 
 static const u32 sunxi_rgb2yuv_coef[12] = {
@@ -457,12 +460,14 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
 				      struct drm_crtc_state *crtc_state)
 {
 	struct drm_plane_state *plane_states[SUN4I_BACKEND_NUM_LAYERS] = { 0 };
+	struct sun4i_backend *backend = engine_to_sun4i_backend(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;
+	unsigned int num_alpha_planes_max = 1;
 	unsigned int num_yuv_planes = 0;
 	unsigned int current_pipe = 0;
 	unsigned int i;
@@ -526,33 +531,40 @@ static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
 	 * 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
+	 * the two pipes as input, and uses the potential alpha
 	 * component to do the transparency between the two.
 	 *
-	 * This two steps scenario makes us unable to guarantee a
+	 * This two-step 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).
+	 * However, we cannot even do that on every platform, 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 data 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.
+	 * This means that on the affected platforms, 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) {
+
+	/* For platforms that are not affected by the issue described above. */
+	if (backend->quirks->supports_lowest_plane_alpha)
+		num_alpha_planes_max++;
+
+	if (num_alpha_planes > num_alpha_planes_max) {
 		DRM_DEBUG_DRIVER("Too many planes with alpha, rejecting...\n");
 		return -EINVAL;
 	}
 
 	/* We can't have an alpha plane at the lowest position */
-	if (plane_states[0]->fb->format->has_alpha ||
-	    (plane_states[0]->alpha != DRM_BLEND_ALPHA_OPAQUE))
+	if (!backend->quirks->supports_lowest_plane_alpha &&
+	    (plane_states[0]->fb->format->has_alpha ||
+	    (plane_states[0]->alpha != DRM_BLEND_ALPHA_OPAQUE)))
 		return -EINVAL;
 
 	for (i = 1; i < num_planes; i++) {
@@ -937,9 +949,11 @@ static const struct sun4i_backend_quirks sun6i_backend_quirks = {
 
 static const struct sun4i_backend_quirks sun7i_backend_quirks = {
 	.needs_output_muxing = true,
+	.supports_lowest_plane_alpha = true,
 };
 
 static const struct sun4i_backend_quirks sun8i_a33_backend_quirks = {
+	.supports_lowest_plane_alpha = true,
 };
 
 static const struct sun4i_backend_quirks sun9i_backend_quirks = {

drivers/gpu/drm/sun4i/sun4i_backend.h

@@ -167,7 +167,6 @@
 #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
 #define SUN4I_BACKEND_NUM_YUV_PLANES		1