backend/drm: add explicit sync support to libliftoff interface

backend/drm/drm.c

@@ -1646,8 +1646,7 @@ static bool connect_drm_connector(struct wlr_drm_connector *wlr_conn,
 		output->non_desktop = non_desktop;
 	}
 
-	// TODO: support sync timelines with libliftoff
-	output->timeline = drm->iface == &atomic_iface;
+	output->timeline = drm->iface != &legacy_iface;
 	if (drm->parent) {
 		output->timeline = output->timeline && drm->mgpu_renderer.wlr_rend->features.timeline;
 	}

backend/drm/libliftoff.c

@@ -339,6 +339,17 @@ static bool add_connector(drmModeAtomicReq *req,
 		liftoff_layer_set_property(crtc->liftoff_composition_layer,
 			"FB_DAMAGE_CLIPS", state->fb_damage_clips);
 
+		if (state->primary_in_fence_fd >= 0) {
+			liftoff_layer_set_property(crtc->primary->liftoff_layer,
+				"IN_FENCE_FD", state->primary_in_fence_fd);
+			liftoff_layer_set_property(crtc->liftoff_composition_layer,
+				"IN_FENCE_FD", state->primary_in_fence_fd);
+		}
+		if (state->base->committed & WLR_OUTPUT_STATE_SIGNAL_TIMELINE) {
+			ok = ok && add_prop(req, crtc->id, crtc->props.out_fence_ptr,
+				(uintptr_t)&state->out_fence_fd);
+		}
+
 		if (state->base->committed & WLR_OUTPUT_STATE_LAYERS) {
 			for (size_t i = 0; i < state->base->layers_len; i++) {
 				const struct wlr_output_layer_state *layer_state = &state->base->layers[i];