linux-dmabuf-v1: ignore scanout output for secondary DRM backends

We don't support direct scanout on such backends yet.

types/wlr_linux_dmabuf_v1.c

@@ -6,6 +6,7 @@
 #include <sys/mman.h>
 #include <unistd.h>
 #include <wlr/backend.h>
+#include <wlr/config.h>
 #include <wlr/interfaces/wlr_buffer.h>
 #include <wlr/render/wlr_renderer.h>
 #include <wlr/types/wlr_compositor.h>
@@ -17,6 +18,10 @@
 #include "render/drm_format_set.h"
 #include "util/shm.h"
 
+#if WLR_HAS_DRM_BACKEND
+#include <wlr/backend/drm.h>
+#endif
+
 #define LINUX_DMABUF_VERSION 4
 
 struct wlr_linux_buffer_params_v1 {
@@ -1054,6 +1059,15 @@ static bool devid_from_fd(int fd, dev_t *devid) {
 	return true;
 }
 
+static bool is_secondary_drm_backend(struct wlr_backend *backend) {
+#if WLR_HAS_DRM_BACKEND
+	return wlr_backend_is_drm(backend) &&
+		wlr_drm_backend_get_parent(backend) != NULL;
+#else
+	return false;
+#endif
+}
+
 bool wlr_linux_dmabuf_feedback_v1_init_with_options(struct wlr_linux_dmabuf_feedback_v1 *feedback,
 		const struct wlr_linux_dmabuf_feedback_v1_init_options *options) {
 	assert(options->main_renderer != NULL);
@@ -1096,7 +1110,8 @@ bool wlr_linux_dmabuf_feedback_v1_init_with_options(struct wlr_linux_dmabuf_feed
 			wlr_log(WLR_ERROR, "Failed to intersect renderer and scanout formats");
 			goto error;
 		}
-	} else if (options->scanout_primary_output != NULL) {
+	} else if (options->scanout_primary_output != NULL &&
+			!is_secondary_drm_backend(options->scanout_primary_output->backend)) {
 		int backend_drm_fd = wlr_backend_get_drm_fd(options->scanout_primary_output->backend);
 		if (backend_drm_fd < 0) {
 			wlr_log(WLR_ERROR, "Failed to get backend DRM FD");