drm/sun4i: backend: Add a custom atomic_check for the frontend

Now that we have everything in place, we can start enabling the frontend.
This is more difficult than one would assume since there can only be one
plane using the frontend per-backend.

We therefore need to make sure that the userspace will not try to setup
multiple planes using it, since that would be impossible. In order to
prevent that, we can create an atomic_check callback that will check that
only one plane will effectively make use of the frontend in a given
configuration, and will toggle the switch in that plane state so that the
proper setup function can do their role.
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/278e6c514a8311750fe627c7f28d58b3e2cbd825.1516613040.git-series.maxime.ripard@free-electrons.com

drivers/gpu/drm/sun4i/sun4i_backend.c

@@ -11,6 +11,7 @@
  */
 
 #include <drm/drmP.h>
+#include <drm/drm_atomic.h>
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_crtc_helper.h>
@@ -270,6 +271,69 @@ int sun4i_backend_update_layer_buffer(struct sun4i_backend *backend,
 	return 0;
 }
 
+static bool sun4i_backend_plane_uses_scaler(struct drm_plane_state *state)
+{
+	u16 src_h = state->src_h >> 16;
+	u16 src_w = state->src_w >> 16;
+
+	DRM_DEBUG_DRIVER("Input size %dx%d, output size %dx%d\n",
+			 src_w, src_h, state->crtc_w, state->crtc_h);
+
+	if ((state->crtc_h != src_h) || (state->crtc_w != src_w))
+		return true;
+
+	return false;
+}
+
+static bool sun4i_backend_plane_uses_frontend(struct drm_plane_state *state)
+{
+	struct sun4i_layer *layer = plane_to_sun4i_layer(state->plane);
+	struct sun4i_backend *backend = layer->backend;
+
+	if (IS_ERR(backend->frontend))
+		return false;
+
+	return sun4i_backend_plane_uses_scaler(state);
+}
+
+static int sun4i_backend_atomic_check(struct sunxi_engine *engine,
+				      struct drm_crtc_state *crtc_state)
+{
+	struct drm_atomic_state *state = crtc_state->state;
+	struct drm_device *drm = state->dev;
+	struct drm_plane *plane;
+	unsigned int num_frontend_planes = 0;
+
+	DRM_DEBUG_DRIVER("Starting checking our planes\n");
+
+	if (!crtc_state->planes_changed)
+		return 0;
+
+	drm_for_each_plane_mask(plane, drm, crtc_state->plane_mask) {
+		struct drm_plane_state *plane_state =
+			drm_atomic_get_plane_state(state, plane);
+		struct sun4i_layer_state *layer_state =
+			state_to_sun4i_layer_state(plane_state);
+
+		if (sun4i_backend_plane_uses_frontend(plane_state)) {
+			DRM_DEBUG_DRIVER("Using the frontend for plane %d\n",
+					 plane->index);
+
+			layer_state->uses_frontend = true;
+			num_frontend_planes++;
+		} else {
+			layer_state->uses_frontend = false;
+		}
+	}
+
+	if (num_frontend_planes > SUN4I_BACKEND_NUM_FRONTEND_LAYERS) {
+		DRM_DEBUG_DRIVER("Too many planes going through the frontend, rejecting\n");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 static void sun4i_backend_vblank_quirk(struct sunxi_engine *engine)
 {
 	struct sun4i_backend *backend = engine_to_sun4i_backend(engine);
@@ -414,6 +478,7 @@ static struct sun4i_frontend *sun4i_backend_find_frontend(struct sun4i_drv *drv,
 }
 
 static const struct sunxi_engine_ops sun4i_backend_engine_ops = {
+	.atomic_check			= sun4i_backend_atomic_check,
 	.commit				= sun4i_backend_commit,
 	.layers_init			= sun4i_layers_init,
 	.apply_color_correction		= sun4i_backend_apply_color_correction,

drivers/gpu/drm/sun4i/sun4i_backend.h

@@ -144,6 +144,8 @@
 #define SUN4I_BACKEND_HWCCOLORTAB_OFF		0x4c00
 #define SUN4I_BACKEND_PIPE_OFF(p)		(0x5000 + (0x400 * (p)))
 
+#define SUN4I_BACKEND_NUM_FRONTEND_LAYERS	1
+
 struct sun4i_backend {
 	struct sunxi_engine	engine;
 	struct sun4i_frontend	*frontend;