output: add adaptive_sync_enabled property

include/wlr/types/wlr_output.h

@@ -46,6 +46,12 @@ struct wlr_output_cursor {
 	} events;
 };
 
+enum wlr_output_adaptive_sync_status {
+	WLR_OUTPUT_ADAPTIVE_SYNC_DISABLED,
+	WLR_OUTPUT_ADAPTIVE_SYNC_ENABLED,
+	WLR_OUTPUT_ADAPTIVE_SYNC_UNKNOWN, // requested, but maybe disabled
+};
+
 enum wlr_output_state_field {
 	WLR_OUTPUT_STATE_BUFFER = 1 << 0,
 	WLR_OUTPUT_STATE_DAMAGE = 1 << 1,
@@ -53,6 +59,7 @@ enum wlr_output_state_field {
 	WLR_OUTPUT_STATE_ENABLED = 1 << 3,
 	WLR_OUTPUT_STATE_SCALE = 1 << 4,
 	WLR_OUTPUT_STATE_TRANSFORM = 1 << 5,
+	WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED = 1 << 6,
 };
 
 enum wlr_output_state_buffer_type {
@@ -74,6 +81,7 @@ struct wlr_output_state {
 	bool enabled;
 	float scale;
 	enum wl_output_transform transform;
+	bool adaptive_sync_enabled;
 
 	// only valid if WLR_OUTPUT_STATE_BUFFER
 	enum wlr_output_state_buffer_type buffer_type;
@@ -126,6 +134,7 @@ struct wlr_output {
 	float scale;
 	enum wl_output_subpixel subpixel;
 	enum wl_output_transform transform;
+	enum wlr_output_adaptive_sync_status adaptive_sync_status;
 
 	bool needs_frame;
 	// damage for cursors and fullscreen surface, in output-local coordinates
@@ -246,6 +255,16 @@ void wlr_output_set_custom_mode(struct wlr_output *output, int32_t width,
  */
 void wlr_output_set_transform(struct wlr_output *output,
 	enum wl_output_transform transform);
+/**
+ * Enables or disables adaptive sync (ie. variable refresh rate) on this
+ * output. This is just a hint, the backend is free to ignore this setting.
+ *
+ * When enabled, compositors can submit frames a little bit later than the
+ * deadline without dropping a frame.
+ *
+ * Adaptive sync is double-buffered state, see `wlr_output_commit`.
+ */
+void wlr_output_enable_adaptive_sync(struct wlr_output *output, bool enabled);
 /**
  * Sets a scale for the output.
  *

types/wlr_output.c

@@ -240,6 +240,18 @@ void wlr_output_set_scale(struct wlr_output *output, float scale) {
 	output->pending.scale = scale;
 }
 
+void wlr_output_enable_adaptive_sync(struct wlr_output *output, bool enabled) {
+	bool currently_enabled =
+		output->adaptive_sync_status != WLR_OUTPUT_ADAPTIVE_SYNC_DISABLED;
+	if (currently_enabled == enabled) {
+		output->pending.committed &= ~WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED;
+		return;
+	}
+
+	output->pending.committed |= WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED;
+	output->pending.adaptive_sync_enabled = enabled;
+}
+
 void wlr_output_set_subpixel(struct wlr_output *output,
 		enum wl_output_subpixel subpixel) {
 	if (output->subpixel == subpixel) {
@@ -487,6 +499,10 @@ bool wlr_output_commit(struct wlr_output *output) {
 		wlr_log(WLR_ERROR, "Tried to modeset a disabled output");
 		goto error;
 	}
+	if (!enabled && output->pending.committed & WLR_OUTPUT_STATE_ADAPTIVE_SYNC_ENABLED) {
+		wlr_log(WLR_ERROR, "Tried to enable adaptive sync on a disabled output");
+		goto error;
+	}
 
 	struct timespec now;
 	clock_gettime(CLOCK_MONOTONIC, &now);