backend: add timeline feature flag

The output feature flag has a flaw: it's not possible to check
whether the backend supports timelines during compositor
initialization when we need to figure out whether we want to enable
the linux-drm-syncobj-v1 protocol.

Introduce a backend-wide feature flag to indicate support for
timelines to address this defect.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3904

backend/drm/drm.c

@@ -126,6 +126,12 @@ bool check_drm_features(struct wlr_drm_backend *drm) {
 		drm->supports_tearing_page_flips = drmGetCap(drm->fd, DRM_CAP_ATOMIC_ASYNC_PAGE_FLIP, &cap) == 0 && cap == 1;
 	}
 
+	drm->backend.features.timeline = drm->iface != &legacy_iface;
+	if (drm->parent) {
+		drm->backend.features.timeline = drm->backend.features.timeline &&
+			drm->mgpu_renderer.wlr_rend->features.timeline;
+	}
+
 	if (env_parse_bool("WLR_DRM_NO_MODIFIERS")) {
 		wlr_log(WLR_DEBUG, "WLR_DRM_NO_MODIFIERS set, disabling modifiers");
 	} else {

backend/headless/backend.c

@@ -80,6 +80,8 @@ struct wlr_backend *wlr_headless_backend_create(struct wl_event_loop *loop) {
 	backend->event_loop_destroy.notify = handle_event_loop_destroy;
 	wl_event_loop_add_destroy_listener(loop, &backend->event_loop_destroy);
 
+	backend->backend.features.timeline = true;
+
 	return &backend->backend;
 }
 

backend/multi/backend.c

@@ -225,6 +225,16 @@ static struct subbackend_state *multi_backend_get_subbackend(struct wlr_multi_ba
 	return NULL;
 }
 
+static void multi_backend_refresh_features(struct wlr_multi_backend *multi) {
+	multi->backend.features.timeline = true;
+
+	struct subbackend_state *sub = NULL;
+	wl_list_for_each(sub, &multi->backends, link) {
+		multi->backend.features.timeline = multi->backend.features.timeline &&
+			sub->backend->features.timeline;
+	}
+}
+
 bool wlr_multi_backend_add(struct wlr_backend *_multi,
 		struct wlr_backend *backend) {
 	assert(_multi && backend);
@@ -256,6 +266,7 @@ bool wlr_multi_backend_add(struct wlr_backend *_multi,
 	wl_signal_add(&backend->events.new_output, &sub->new_output);
 	sub->new_output.notify = new_output_reemit;
 
+	multi_backend_refresh_features(multi);
 	wl_signal_emit_mutable(&multi->events.backend_add, backend);
 	return true;
 }
@@ -270,6 +281,7 @@ void wlr_multi_backend_remove(struct wlr_backend *_multi,
 	if (sub) {
 		wl_signal_emit_mutable(&multi->events.backend_remove, backend);
 		subbackend_state_destroy(sub);
+		multi_backend_refresh_features(multi);
 	}
 }
 

include/wlr/backend.h

@@ -29,6 +29,11 @@ struct wlr_backend_output_state {
 struct wlr_backend {
 	const struct wlr_backend_impl *impl;
 
+	struct {
+		// Whether wait/signal timelines are supported in output commits
+		bool timeline;
+	} features;
+
 	struct {
 		/** Raised when destroyed */
 		struct wl_signal destroy;