output: refactor frame submission API

This is necessary for direct scan-out and other upcoming features. This patch
changes the output API to look like the wl_surface API.

Outputs now have some double-buffered state: the frame to be submitted
(currently only wlr_renderer frames are supported) and the damaged region.
To attach a pending frame, use wlr_output_attach_render. To set the pending
damaged region, use wlr_output_set_damage.

To submit the pending state, call wlr_output_commit. This will submit the
pending frame to the backend.

To migrate from the old API to the new one:

- Replace wlr_output_make_current calls by wlr_output_attach_render
- Replace wlr_output_swap_buffers calls by wlr_output_set_damage and
  wlr_output_commit

examples/fullscreen-shell.c

@@ -89,7 +89,7 @@ static void output_handle_frame(struct wl_listener *listener, void *data) {
 	int width, height;
 	wlr_output_effective_resolution(output->wlr_output, &width, &height);
 
-	if (!wlr_output_make_current(output->wlr_output, NULL)) {
+	if (!wlr_output_attach_render(output->wlr_output, NULL)) {
 		return;
 	}
 
@@ -108,7 +108,7 @@ static void output_handle_frame(struct wl_listener *listener, void *data) {
 	}
 
 	wlr_renderer_end(renderer);
-	wlr_output_swap_buffers(output->wlr_output, NULL, NULL);
+	wlr_output_commit(output->wlr_output);
 }
 
 static void output_set_surface(struct fullscreen_output *output,

examples/multi-pointer.c

@@ -94,14 +94,14 @@ void output_frame_notify(struct wl_listener *listener, void *data) {
 	struct sample_state *sample = output->sample;
 	struct wlr_output *wlr_output = output->output;
 
-	wlr_output_make_current(wlr_output, NULL);
+	wlr_output_attach_render(wlr_output, NULL);
 
 	glClearColor(sample->clear_color[0], sample->clear_color[1],
 		sample->clear_color[2], sample->clear_color[3]);
 	glClear(GL_COLOR_BUFFER_BIT);
 
 	wlr_output_render_software_cursors(wlr_output, NULL);
-	wlr_output_swap_buffers(wlr_output, NULL, NULL);
+	wlr_output_commit(wlr_output);
 }
 
 static void handle_cursor_motion(struct wl_listener *listener, void *data) {

examples/output-layout.c

@@ -115,7 +115,7 @@ void output_frame_notify(struct wl_listener *listener, void *data) {
 
 	struct wlr_output *wlr_output = output->output;
 
-	wlr_output_make_current(wlr_output, NULL);
+	wlr_output_attach_render(wlr_output, NULL);
 	wlr_renderer_begin(sample->renderer, wlr_output->width, wlr_output->height);
 	wlr_renderer_clear(sample->renderer, (float[]){0.25f, 0.25f, 0.25f, 1});
 
@@ -137,7 +137,7 @@ void output_frame_notify(struct wl_listener *listener, void *data) {
 	}
 
 	wlr_renderer_end(sample->renderer);
-	wlr_output_swap_buffers(wlr_output, NULL, NULL);
+	wlr_output_commit(wlr_output);
 }
 
 static void update_velocities(struct sample_state *sample,

examples/pointer.c

@@ -98,11 +98,11 @@ void output_frame_notify(struct wl_listener *listener, void *data) {
 	struct wlr_renderer *renderer = wlr_backend_get_renderer(wlr_output->backend);
 	assert(renderer);
 
-	wlr_output_make_current(wlr_output, NULL);
+	wlr_output_attach_render(wlr_output, NULL);
 	wlr_renderer_begin(renderer, wlr_output->width, wlr_output->height);
 	wlr_renderer_clear(renderer, state->clear_color);
 	wlr_output_render_software_cursors(wlr_output, NULL);
-	wlr_output_swap_buffers(wlr_output, NULL, NULL);
+	wlr_output_commit(wlr_output);
 	wlr_renderer_end(renderer);
 }
 

examples/rotation.c

@@ -60,7 +60,7 @@ static void output_frame_notify(struct wl_listener *listener, void *data) {
 	int32_t width, height;
 	wlr_output_effective_resolution(wlr_output, &width, &height);
 
-	wlr_output_make_current(wlr_output, NULL);
+	wlr_output_attach_render(wlr_output, NULL);
 	wlr_renderer_begin(sample->renderer, wlr_output->width, wlr_output->height);
 	wlr_renderer_clear(sample->renderer, (float[]){0.25f, 0.25f, 0.25f, 1});
 
@@ -72,7 +72,7 @@ static void output_frame_notify(struct wl_listener *listener, void *data) {
 	}
 
 	wlr_renderer_end(sample->renderer);
-	wlr_output_swap_buffers(wlr_output, NULL, NULL);
+	wlr_output_commit(wlr_output);
 
 	long ms = (now.tv_sec - sample->last_frame.tv_sec) * 1000 +
 		(now.tv_nsec - sample->last_frame.tv_nsec) / 1000000;

examples/simple.c

@@ -56,12 +56,12 @@ void output_frame_notify(struct wl_listener *listener, void *data) {
 		sample->dec = inc;
 	}
 
-	wlr_output_make_current(sample_output->output, NULL);
+	wlr_output_attach_render(sample_output->output, NULL);
 
 	glClearColor(sample->color[0], sample->color[1], sample->color[2], 1.0);
 	glClear(GL_COLOR_BUFFER_BIT);
 
-	wlr_output_swap_buffers(sample_output->output, NULL, NULL);
+	wlr_output_commit(sample_output->output);
 	sample->last_frame = now;
 }
 

examples/tablet.c

@@ -86,7 +86,7 @@ static void output_frame_notify(struct wl_listener *listener, void *data) {
 	int32_t width, height;
 	wlr_output_effective_resolution(wlr_output, &width, &height);
 
-	wlr_output_make_current(wlr_output, NULL);
+	wlr_output_attach_render(wlr_output, NULL);
 	wlr_renderer_begin(sample->renderer, wlr_output->width, wlr_output->height);
 	wlr_renderer_clear(sample->renderer, (float[]){0.25f, 0.25f, 0.25f, 1});
 
@@ -129,7 +129,7 @@ static void output_frame_notify(struct wl_listener *listener, void *data) {
 	}
 
 	wlr_renderer_end(sample->renderer);
-	wlr_output_swap_buffers(wlr_output, NULL, NULL);
+	wlr_output_commit(wlr_output);
 	sample->last_frame = now;
 }
 

examples/touch.c

@@ -73,7 +73,7 @@ static void output_frame_notify(struct wl_listener *listener, void *data) {
 	int32_t width, height;
 	wlr_output_effective_resolution(wlr_output, &width, &height);
 
-	wlr_output_make_current(wlr_output, NULL);
+	wlr_output_attach_render(wlr_output, NULL);
 	wlr_renderer_begin(sample->renderer, wlr_output->width, wlr_output->height);
 	wlr_renderer_clear(sample->renderer, (float[]){0.25f, 0.25f, 0.25f, 1});
 
@@ -89,7 +89,7 @@ static void output_frame_notify(struct wl_listener *listener, void *data) {
 	}
 
 	wlr_renderer_end(sample->renderer);
-	wlr_output_swap_buffers(wlr_output, NULL, NULL);
+	wlr_output_commit(wlr_output);
 	sample->last_frame = now;
 }
 

include/wlr/types/wlr_output.h

@@ -46,16 +46,31 @@ struct wlr_output_cursor {
 	} events;
 };
 
+enum wlr_output_state_field {
+	WLR_OUTPUT_STATE_BUFFER = 1 << 0,
+	WLR_OUTPUT_STATE_DAMAGE = 1 << 1,
+};
+
+/**
+ * Holds the double-buffered output state.
+ */
+struct wlr_output_state {
+	uint32_t committed; // enum wlr_output_state_field
+	pixman_region32_t damage;
+};
+
 struct wlr_output_impl;
 
 /**
  * A compositor output region. This typically corresponds to a monitor that
  * displays part of the compositor space.
  *
- * Compositors should listen to the `frame` event to render an output. They
- * should call `wlr_output_make_current`, render and then call
- * `wlr_output_swap_buffers`. No rendering should happen outside a `frame` event
- * handler.
+ * The `frame` event will be emitted when it is a good time for the compositor
+ * to submit a new frame.
+ *
+ * To render a new frame, compositors should call `wlr_output_attach_render`,
+ * render and call `wlr_output_commit`. No rendering should happen outside a
+ * `frame` event handler or before `wlr_output_attach_render`.
  */
 struct wlr_output {
 	const struct wlr_output_impl *impl;
@@ -88,6 +103,8 @@ struct wlr_output {
 	bool frame_pending;
 	float transform_matrix[9];
 
+	struct wlr_output_state pending;
+
 	struct {
 		// Request to render a frame
 		struct wl_signal frame;
@@ -189,12 +206,14 @@ void wlr_output_transformed_resolution(struct wlr_output *output,
 void wlr_output_effective_resolution(struct wlr_output *output,
 	int *width, int *height);
 /**
- * Makes the output rendering context current.
+ * Attach the renderer's buffer to the output. Compositors must call this
+ * function before rendering. After they are done rendering, they should call
+ * `wlr_output_commit` to submit the new frame.
  *
- * `buffer_age` is set to the drawing buffer age in number of frames or -1 if
- * unknown. This is useful for damage tracking.
+ * If non-NULL, `buffer_age` is set to the drawing buffer age in number of
+ * frames or -1 if unknown. This is useful for damage tracking.
  */
-bool wlr_output_make_current(struct wlr_output *output, int *buffer_age);
+bool wlr_output_attach_render(struct wlr_output *output, int *buffer_age);
 /**
  * Get the preferred format for reading pixels.
  * This function might change the current rendering context.
@@ -202,17 +221,21 @@ bool wlr_output_make_current(struct wlr_output *output, int *buffer_age);
 bool wlr_output_preferred_read_format(struct wlr_output *output,
 	enum wl_shm_format *fmt);
 /**
- * Swaps the output buffers. If the time of the frame isn't known, set `when` to
- * NULL. If the compositor doesn't support damage tracking, set `damage` to
- * NULL.
+ * Set the damage region for the frame to be submitted.
  *
- * Damage is given in output-buffer-local coordinates (ie. scaled and
+ * Compositors implementing damage tracking should call this function with the
+ * damaged region in output-buffer-local coordinates (ie. scaled and
  * transformed).
- *
- * Swapping buffers schedules a `frame` event.
  */
-bool wlr_output_swap_buffers(struct wlr_output *output, struct timespec *when,
+void wlr_output_set_damage(struct wlr_output *output,
 	pixman_region32_t *damage);
+/**
+ * Commit the pending output state. If `wlr_output_attach_render` has been
+ * called, the pending frame will be submitted for display.
+ *
+ * This function schedules a `frame` event.
+ */
+bool wlr_output_commit(struct wlr_output *output);
 /**
  * Manually schedules a `frame` event. If a `frame` event is already pending,
  * it is a no-op.

tinywl/tinywl.c

@@ -591,8 +591,8 @@ static void output_frame(struct wl_listener *listener, void *data) {
 	struct timespec now;
 	clock_gettime(CLOCK_MONOTONIC, &now);
 
-	/* wlr_output_make_current makes the OpenGL context current. */
-	if (!wlr_output_make_current(output->wlr_output, NULL)) {
+	/* wlr_output_attach_render makes the OpenGL context current. */
+	if (!wlr_output_attach_render(output->wlr_output, NULL)) {
 		return;
 	}
 	/* The "effective" resolution can change if you rotate your outputs. */
@@ -635,7 +635,7 @@ static void output_frame(struct wl_listener *listener, void *data) {
 	/* Conclude rendering and swap the buffers, showing the final frame
 	 * on-screen. */
 	wlr_renderer_end(renderer);
-	wlr_output_swap_buffers(output->wlr_output, NULL, NULL);
+	wlr_output_commit(output->wlr_output);
 }
 
 static void server_new_output(struct wl_listener *listener, void *data) {

types/wlr_output.c

@@ -282,6 +282,7 @@ void wlr_output_init(struct wlr_output *output, struct wlr_backend *backend,
 	wl_signal_init(&output->events.transform);
 	wl_signal_init(&output->events.destroy);
 	pixman_region32_init(&output->damage);
+	pixman_region32_init(&output->pending.damage);
 
 	const char *no_hardware_cursors = getenv("WLR_NO_HARDWARE_CURSORS");
 	if (no_hardware_cursors != NULL && strcmp(no_hardware_cursors, "1") == 0) {
@@ -317,6 +318,7 @@ void wlr_output_destroy(struct wlr_output *output) {
 		wl_event_source_remove(output->idle_frame);
 	}
 
+	pixman_region32_fini(&output->pending.damage);
 	pixman_region32_fini(&output->damage);
 
 	if (output->impl && output->impl->destroy) {
@@ -360,13 +362,18 @@ struct wlr_output_mode *wlr_output_preferred_mode(struct wlr_output *output) {
 	return mode;
 }
 
-bool wlr_output_make_current(struct wlr_output *output, int *buffer_age) {
-	return output->impl->make_current(output, buffer_age);
+bool wlr_output_attach_render(struct wlr_output *output, int *buffer_age) {
+	if (!output->impl->make_current(output, buffer_age)) {
+		return false;
+	}
+
+	output->pending.committed |= WLR_OUTPUT_STATE_BUFFER;
+	return true;
 }
 
 bool wlr_output_preferred_read_format(struct wlr_output *output,
 		enum wl_shm_format *fmt) {
-	if (!wlr_output_make_current(output, NULL)) {
+	if (!output->impl->make_current(output, NULL)) {
 		return false;
 	}
 
@@ -378,10 +385,21 @@ bool wlr_output_preferred_read_format(struct wlr_output *output,
 	return true;
 }
 
-bool wlr_output_swap_buffers(struct wlr_output *output, struct timespec *when,
+void wlr_output_set_damage(struct wlr_output *output,
 		pixman_region32_t *damage) {
+	pixman_region32_intersect_rect(&output->pending.damage, damage,
+		0, 0, output->width, output->height);
+	output->pending.committed |= WLR_OUTPUT_STATE_DAMAGE;
+}
+
+static void output_state_clear(struct wlr_output_state *state) {
+	state->committed = 0;
+	pixman_region32_clear(&state->damage);
+}
+
+bool wlr_output_commit(struct wlr_output *output) {
 	if (output->frame_pending) {
-		wlr_log(WLR_ERROR, "Tried to swap buffers when a frame is pending");
+		wlr_log(WLR_ERROR, "Tried to commit when a frame is pending");
 		return false;
 	}
 	if (output->idle_frame != NULL) {
@@ -389,46 +407,43 @@ bool wlr_output_swap_buffers(struct wlr_output *output, struct timespec *when,
 		output->idle_frame = NULL;
 	}
 
+	if (output->pending.committed & WLR_OUTPUT_STATE_BUFFER) {
 		struct timespec now;
-	if (when == NULL) {
 		clock_gettime(CLOCK_MONOTONIC, &now);
-		when = &now;
+
+		pixman_region32_t *damage = NULL;
+		if (output->pending.committed & WLR_OUTPUT_STATE_DAMAGE) {
+			damage = &output->pending.damage;
 		}
 
 		struct wlr_output_event_swap_buffers event = {
 			.output = output,
-		.when = when,
+			.when = &now,
 			.damage = damage,
 		};
 		wlr_signal_emit_safe(&output->events.swap_buffers, &event);
 
-	pixman_region32_t render_damage;
-	pixman_region32_init(&render_damage);
-	pixman_region32_union_rect(&render_damage, &render_damage, 0, 0,
-		output->width, output->height);
-	if (damage != NULL) {
-		// Damage tracking supported
-		pixman_region32_intersect(&render_damage, &render_damage, damage);
-	}
-
-	if (!output->impl->swap_buffers(output, damage ? &render_damage : NULL)) {
-		pixman_region32_fini(&render_damage);
+		if (!output->impl->swap_buffers(output, damage)) {
 			return false;
 		}
 
-	pixman_region32_fini(&render_damage);
-
 		struct wlr_output_cursor *cursor;
 		wl_list_for_each(cursor, &output->cursors, link) {
 			if (!cursor->enabled || !cursor->visible || cursor->surface == NULL) {
 				continue;
 			}
-		wlr_surface_send_frame_done(cursor->surface, when);
+			wlr_surface_send_frame_done(cursor->surface, &now);
 		}
 
 		output->frame_pending = true;
 		output->needs_swap = false;
-	pixman_region32_clear(&output->damage);
+		output_state_clear(&output->pending);
+		return true;
+	} else {
+		wlr_log(WLR_ERROR, "Tried to commit without attaching a buffer");
+		return false;
+	}
+
 	return true;
 }
 

types/wlr_output_damage.c

@@ -106,7 +106,7 @@ bool wlr_output_damage_make_current(struct wlr_output_damage *output_damage,
 	struct wlr_output *output = output_damage->output;
 
 	int buffer_age = -1;
-	if (!wlr_output_make_current(output, &buffer_age)) {
+	if (!wlr_output_attach_render(output, &buffer_age)) {
 		return false;
 	}
 
@@ -142,7 +142,10 @@ bool wlr_output_damage_make_current(struct wlr_output_damage *output_damage,
 
 bool wlr_output_damage_swap_buffers(struct wlr_output_damage *output_damage,
 		struct timespec *when, pixman_region32_t *damage) {
-	if (!wlr_output_swap_buffers(output_damage->output, when, damage)) {
+	if (damage != NULL) {
+		wlr_output_set_damage(output_damage->output, damage);
+	}
+	if (!wlr_output_commit(output_damage->output)) {
 		return false;
 	}