core: simplify the pacing logic a little bit

Make it simpler to stop requesting PresentCompleteNotify when there is
nothing to render.

Related: #1079

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/backend.c

@@ -82,13 +82,17 @@ void handle_device_reset(session_t *ps) {
 }
 
 /// paint all windows
-void paint_all_new(session_t *ps, struct managed_win *t) {
+///
+/// Returns if any render command is issued. IOW if nothing on the screen has changed,
+/// this1 function will return false.
+bool paint_all_new(session_t *ps, struct managed_win *const t) {
 	struct timespec now = get_time_timespec();
 	auto paint_all_start_us =
 	    (uint64_t)now.tv_sec * 1000000UL + (uint64_t)now.tv_nsec / 1000;
 	if (ps->backend_data->ops->device_status &&
 	    ps->backend_data->ops->device_status(ps->backend_data) != DEVICE_STATUS_NORMAL) {
-		return handle_device_reset(ps);
+		handle_device_reset(ps);
+		return false;
 	}
 	if (ps->o.xrender_sync_fence) {
 		if (ps->xsync_exists && !x_fence_sync(ps->c, ps->sync_fence)) {
@@ -114,7 +118,7 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 
 	if (!pixman_region32_not_empty(&reg_damage)) {
 		pixman_region32_fini(&reg_damage);
-		return;
+		return false;
 	}
 
 #ifdef DEBUG_REPAINT
@@ -199,7 +203,6 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 			ps->last_schedule_delay = after_damage_us - ps->next_render;
 		}
 	}
-	ps->did_render = true;
 
 	if (ps->backend_data->ops->prepare) {
 		ps->backend_data->ops->prepare(ps->backend_data, &reg_paint);
@@ -541,6 +544,7 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 	for (win *w = t; w; w = w->prev_trans)
 		log_trace(" %#010lx", w->id);
 #endif
+	return true;
 }
 
 // vim: set noet sw=8 ts=8 :

src/backend/backend.h

@@ -371,4 +371,8 @@ struct backend_operations {
 
 extern struct backend_operations *backend_list[];
 
-void paint_all_new(session_t *ps, struct managed_win *const t) attr_nonnull(1);
+/// paint all windows
+///
+/// Returns if any render command is issued. IOW if nothing on the screen has changed,
+/// this1 function will return false.
+bool paint_all_new(session_t *ps, struct managed_win *t) attr_nonnull(1);

src/common.h

@@ -146,6 +146,17 @@ struct shader_info {
 	UT_hash_handle hh;
 };
 
+enum render_progress {
+	/// Render is finished and presented to the screen.
+	RENDER_IDLE = 0,
+	/// Rendering is queued, but not started yet.
+	RENDER_QUEUED,
+	/// Backend has been called, render commands have been issued.
+	RENDER_STARTED,
+	/// Backend reported render commands have been finished. (not actually used).
+	RENDER_FINISHED,
+};
+
 /// Structure containing all necessary data for a session.
 typedef struct session {
 	// === Event handlers ===
@@ -249,17 +260,11 @@ typedef struct session {
 	uint64_t last_msc_instant;
 	/// The last MSC number
 	uint64_t last_msc;
-	/// When the currently rendered frame will be displayed.
-	/// 0 means there is no pending frame.
-	uint64_t target_msc;
 	/// The delay between when the last frame was scheduled to be rendered, and when
 	/// the render actually started.
 	uint64_t last_schedule_delay;
 	/// When do we want our next frame to start rendering.
 	uint64_t next_render;
-	/// Did we actually render the last frame. Sometimes redraw will be scheduled only
-	/// to find out nothing has changed. In which case this will be set to false.
-	bool did_render;
 	/// Whether we can perform frame pacing.
 	bool frame_pacing;
 
@@ -273,7 +278,13 @@ typedef struct session {
 	options_t o;
 	/// Whether we have hit unredirection timeout.
 	bool tmout_unredir_hit;
-	/// Whether we need to redraw the screen
+	/// Rendering is currently in progress. This means we are in any stage of
+	/// rendering a frame. The render could be queued but not yet started, or it could
+	/// have finished but not yet presented.
+	enum render_progress render_in_progress;
+	/// Whether there are changes pending for the next render. A render is currently
+	/// in progress, otherwise we would have started a new render instead of setting
+	/// this flag.
 	bool redraw_needed;
 
 	/// Cache a xfixes region so we don't need to allocate it every time.