compositor/shells: Require shells to flag outputs as ready to allow repaint

We've added a curtain to the shells so at startup we have something to
render, but this causes a flicker if someone is trying to have a seamless
transition from boot to weston.

Add a ready flag that allows the shell to indicate repaints are safe, so
we can remove the curtains and have no wasted frames at startup.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

desktop-shell/shell.c

@@ -2662,6 +2662,7 @@ background_committed(struct weston_surface *es,
 				 sh_output->output->pos);
 	weston_view_move_to_layer(sh_output->background_view,
 				  &shell->background_layer.view_list);
+	weston_output_set_ready(sh_output->output);
 }
 
 static void

fullscreen-shell/fullscreen-shell.c

@@ -353,6 +353,8 @@ fs_output_create(struct fullscreen_shell *shell, struct weston_output *output)
 		}
 	}
 
+	weston_output_set_ready(output);
+
 	return fsout;
 }
 

include/libweston/libweston.h

@@ -541,6 +541,12 @@ struct weston_output {
 	enum weston_vrr_mode vrr_mode;
 
 	enum weston_color_format color_format;
+
+	/**
+	 * Set by the shell to indicate there's some content for this
+	 * output to display.
+	 */
+	bool ready;
 };
 
 enum weston_pointer_motion_mask {
@@ -2715,6 +2721,9 @@ weston_output_set_color_format(struct weston_output *output,
 uint32_t
 weston_output_get_supported_color_formats(struct weston_output *output);
 
+void
+weston_output_set_ready(struct weston_output *output);
+
 #ifdef  __cplusplus
 }
 #endif

ivi-shell/ivi-layout.c

@@ -353,6 +353,7 @@ create_ivi_screen(struct ivi_layout *layout, struct weston_output *output)
 				  &layout->layout_layer.view_list);
 	weston_view_set_output(iviscrn->temporary_curtain->view, output);
 
+	weston_output_set_ready(output);
 }
 
 static void

kiosk-shell/kiosk-shell.c

@@ -824,6 +824,7 @@ kiosk_shell_output_create(struct kiosk_shell *shell, struct weston_output *outpu
 
 	kiosk_shell_output_recreate_background(shoutput);
 	kiosk_shell_output_configure(shoutput);
+	weston_output_set_ready(output);
 
 	return shoutput;
 }

libweston/compositor.c

@@ -3904,6 +3904,9 @@ output_repaint_timer_handler(void *data)
 	compositor->last_repaint_start = now;
 
 	wl_list_for_each(output, &compositor->output_list, link) {
+		if (!output->ready)
+			continue;
+
 		if (!weston_output_check_repaint(output, &now)) {
 			output->will_repaint = false;
 			continue;
@@ -4336,6 +4339,9 @@ weston_output_schedule_repaint(struct weston_output *output)
 	struct weston_compositor *compositor = output->compositor;
 	struct wl_event_loop *loop;
 
+	if (!output->ready)
+		return;
+
 	if (compositor->state == WESTON_COMPOSITOR_SLEEPING ||
 	    compositor->state == WESTON_COMPOSITOR_OFFSCREEN)
 		return;
@@ -10495,3 +10501,30 @@ weston_get_backend_type(struct weston_backend *backend)
 	assert(backend);
 	return backend->backend_type;
 }
+
+/** Inform the compositor that an output is ready for its first repaint
+ *
+ * When a new output is added, it may not have any content to be
+ * displayed. This could lead to strange flickers or hangs at
+ * startup if uninitialized buffers become attached for
+ * scanout. To prevent this, we assert that the output has
+ * content to display.
+ *
+ * The shell must inform the compositor when content such as a
+ * background is in the scene graph and this output can safely
+ * be repainted.
+ *
+ * Only needs to be called once per output, and an output
+ * can never become unready.
+ *
+ * \param output the output that has become ready
+ * \ingroup output
+ */
+WL_EXPORT void
+weston_output_set_ready(struct weston_output *output)
+{
+	if (!output->ready) {
+		output->ready = true;
+		weston_output_schedule_repaint(output);
+	}
+}

lua-shell/lua-shell.c

@@ -921,6 +921,15 @@ lua_shell_env_output_is_enabled(struct lua_State *lua)
 	return 1;
 }
 
+static int
+lua_shell_env_output_set_ready(struct lua_State *lua)
+{
+	struct lua_shell_output *shoutput = get_output_from_arg(lua, 1);
+
+	weston_output_set_ready(shoutput->output);
+	return 0;
+}
+
 static int
 lua_shell_env_output_set_private(struct lua_State *lua)
 {
@@ -954,6 +963,7 @@ lua_shell_env_init_output(struct lua_shell *shell)
 		{ "is_enabled", lua_shell_env_output_is_enabled },
 		{ "set_private", lua_shell_env_output_set_private },
 		{ "get_private", lua_shell_env_output_get_private },
+		{ "set_ready", lua_shell_env_output_set_ready },
 		{ NULL, NULL }
 	};
 

lua-shell/shell.lua

@@ -95,6 +95,9 @@ function my_output_create(output)
 
   pd.background_view = bv
   output:set_private(pd)
+
+  -- Must set ready or no repaints will take place
+  output:set_ready()
 end
 
 function output_moved(output, move_x, move_y)

tests/weston-test-desktop-shell.c

@@ -338,6 +338,7 @@ wet_shell_init(struct weston_compositor *ec,
 	screenshooter_create(ec);
 
 	desktest_shell_add_bindings(dts);
+	weston_output_set_ready(output);
 
 	return 0;