output: don't trigger a frame immediately in schedule_frame

This desynchronizes our rendering loop with the vblank cycle.

In case a compositor doesn't swap buffers but schedules a frame,
emitting a frame event immediately enters a busy-loop.

Instead, ask the backend to send a frame when appropriate. On
Wayland we can just register a frame callback on our surface. On
DRM we can do a no-op pageflip.

Fixes #617
Fixes swaywm/sway#2748

backend/drm/drm.c

@@ -730,6 +730,43 @@ static bool drm_connector_move_cursor(struct wlr_output *output,
 	return ok;
 }
 
+static void drm_connector_schedule_frame(struct wlr_output *output) {
+	struct wlr_drm_connector *conn = get_drm_connector_from_output(output);
+	struct wlr_drm_backend *drm = get_drm_backend_from_backend(output->backend);
+	if (!drm->session->active) {
+		return;
+	}
+
+	// We need to figure out where we are in the vblank cycle
+	// TODO: try using drmWaitVBlank and fallback to pageflipping
+
+	struct wlr_drm_crtc *crtc = conn->crtc;
+	if (!crtc) {
+		return;
+	}
+	struct wlr_drm_plane *plane = crtc->primary;
+	struct gbm_bo *bo = plane->surf.back;
+	if (!bo) {
+		// We haven't swapped buffers yet -- can't do a pageflip
+		wlr_output_send_frame(output);
+		return;
+	}
+	uint32_t fb_id = get_fb_for_bo(bo);
+
+	if (conn->pageflip_pending) {
+		wlr_log(WLR_ERROR, "Skipping pageflip on output '%s'",
+			conn->output.name);
+		return;
+	}
+
+	if (!drm->iface->crtc_pageflip(drm, conn, crtc, fb_id, NULL)) {
+		return;
+	}
+
+	conn->pageflip_pending = true;
+	wlr_output_update_enabled(output, true);
+}
+
 static void drm_connector_destroy(struct wlr_output *output) {
 	struct wlr_drm_connector *conn = get_drm_connector_from_output(output);
 	drm_connector_cleanup(conn);
@@ -751,6 +788,7 @@ static const struct wlr_output_impl output_impl = {
 	.set_gamma = set_drm_connector_gamma,
 	.get_gamma_size = drm_connector_get_gamma_size,
 	.export_dmabuf = drm_connector_export_dmabuf,
+	.schedule_frame = drm_connector_schedule_frame,
 };
 
 bool wlr_output_is_drm(struct wlr_output *output) {

backend/wayland/output.c

@@ -192,11 +192,24 @@ void update_wl_output_cursor(struct wlr_wl_output *output) {
 	}
 }
 
-bool output_move_cursor(struct wlr_output *_output, int x, int y) {
+static bool output_move_cursor(struct wlr_output *_output, int x, int y) {
 	// TODO: only return true if x == current x and y == current y
 	return true;
 }
 
+static void output_schedule_frame(struct wlr_output *wlr_output) {
+	struct wlr_wl_output *output = get_wl_output_from_output(wlr_output);
+
+	if (output->frame_callback != NULL) {
+		wlr_log(WLR_ERROR, "Skipping frame scheduling");
+		return;
+	}
+
+	output->frame_callback = wl_surface_frame(output->surface);
+	wl_callback_add_listener(output->frame_callback, &frame_listener, output);
+	wl_surface_commit(output->surface);
+}
+
 static const struct wlr_output_impl output_impl = {
 	.set_custom_mode = output_set_custom_mode,
 	.transform = output_transform,
@@ -205,6 +218,7 @@ static const struct wlr_output_impl output_impl = {
 	.swap_buffers = output_swap_buffers,
 	.set_cursor = output_set_cursor,
 	.move_cursor = output_move_cursor,
+	.schedule_frame = output_schedule_frame,
 };
 
 bool wlr_output_is_wl(struct wlr_output *wlr_output) {

include/wlr/interfaces/wlr_output.h

@@ -33,6 +33,7 @@ struct wlr_output_impl {
 	size_t (*get_gamma_size)(struct wlr_output *output);
 	bool (*export_dmabuf)(struct wlr_output *output,
 		struct wlr_dmabuf_attributes *attribs);
+	void (*schedule_frame)(struct wlr_output *output);
 };
 
 void wlr_output_init(struct wlr_output *output, struct wlr_backend *backend,

types/wlr_output.c

@@ -546,16 +546,23 @@ static void schedule_frame_handle_idle_timer(void *data) {
 	struct wlr_output *output = data;
 	output->idle_frame = NULL;
 	if (!output->frame_pending) {
+		if (output->impl->schedule_frame) {
+			// Ask the backend to send a frame event when appropriate
+			output->frame_pending = true;
+			output->impl->schedule_frame(output);
+		} else {
 			wlr_output_send_frame(output);
 		}
 	}
+}
 
 void wlr_output_schedule_frame(struct wlr_output *output) {
 	if (output->frame_pending || output->idle_frame != NULL) {
 		return;
 	}
 
-	// TODO: ask the backend to send a frame event when appropriate instead
+	// We're using an idle timer here in case a buffer swap happens right after
+	// this function is called
 	struct wl_event_loop *ev = wl_display_get_event_loop(output->display);
 	output->idle_frame =
 		wl_event_loop_add_idle(ev, schedule_frame_handle_idle_timer, output);