render/allocator: use render node if available in reopen_drm_node

If we aren't trying to create a dumb buffer allocator, and if the
DRM device has a render node (ie, not a split render/display SoC),
then we can use the render node instead of the primary node. This
should allow wlroots to run under seatd when the current user
doesn't have the permission to open primary nodes (logind has a
quirk to allow physically logged in users to open primary nodes).

render/allocator/allocator.c

@@ -25,11 +25,19 @@ void wlr_allocator_init(struct wlr_allocator *alloc,
  * TODO: don't assume we have the permission to just open the DRM node,
  * find another way to re-open it.
  */
-static int reopen_drm_node(int drm_fd) {
-	char *name = drmGetDeviceNameFromFd2(drm_fd);
+static int reopen_drm_node(int drm_fd, bool allow_render_node) {
+	char *name = NULL;
+	if (allow_render_node) {
+		name = drmGetRenderDeviceNameFromFd(drm_fd);
+	}
 	if (name == NULL) {
-		wlr_log(WLR_ERROR, "drmGetDeviceNameFromFd2 failed");
-		return -1;
+		// Either the DRM device has no render node, either the caller wants
+		// a primary node
+		name = drmGetDeviceNameFromFd2(drm_fd);
+		if (name == NULL) {
+			wlr_log(WLR_ERROR, "drmGetDeviceNameFromFd2 failed");
+			return -1;
+		}
 	}
 
 	int new_fd = open(name, O_RDWR | O_CLOEXEC);
@@ -74,7 +82,7 @@ struct wlr_allocator *allocator_autocreate_with_drm_fd(
 	if ((backend_caps & gbm_caps) && (renderer_caps & gbm_caps)
 			&& drm_fd >= 0) {
 		wlr_log(WLR_DEBUG, "Trying to create gbm allocator");
-		int gbm_fd = reopen_drm_node(drm_fd);
+		int gbm_fd = reopen_drm_node(drm_fd, true);
 		if (gbm_fd < 0) {
 			return NULL;
 		}
@@ -98,7 +106,7 @@ struct wlr_allocator *allocator_autocreate_with_drm_fd(
 	if ((backend_caps & drm_caps) && (renderer_caps & drm_caps)
 			&& drm_fd >= 0 && drmIsMaster(drm_fd)) {
 		wlr_log(WLR_DEBUG, "Trying to create drm dumb allocator");
-		int dumb_fd = reopen_drm_node(drm_fd);
+		int dumb_fd = reopen_drm_node(drm_fd, false);
 		if (dumb_fd < 0) {
 			return NULL;
 		}