backend/session: use drmIsKMS

This moves the magic incantation into libdrm and is clearer. See
[1] for details.

While at it, fixup the doc comment and improve logging.

[1]: 523b3658aa

backend/session/session.c

@@ -337,9 +337,8 @@ bool wlr_session_change_vt(struct wlr_session *session, unsigned vt) {
 	return libseat_switch_session(session->seat_handle, vt) == 0;
 }
 
-/* Tests if 'path' is KMS compatible by trying to open it.
- * It leaves the open device in *fd_out it it succeeds.
- */
+/* Tests if 'path' is KMS compatible by trying to open it. Returns the opened
+ * device on success. */
 static struct wlr_device *open_if_kms(struct wlr_session *restrict session,
 		const char *restrict path) {
 	if (!path) {
@@ -351,25 +350,13 @@ static struct wlr_device *open_if_kms(struct wlr_session *restrict session,
 		return NULL;
 	}
 
-	// The kernel errors out with EOPNOTSUPP if DRIVER_MODESET isn't set
-	drmModeRes *res = drmModeGetResources(dev->fd);
-	if (!res) {
-		if (errno != EOPNOTSUPP) {
-			wlr_log_errno(WLR_ERROR, "drmModeGetResources(%s) failed", path);
-		}
-		goto out_dev;
-	}
-	if (res->count_crtcs == 0) {
-		drmModeFreeResources(res);
-		goto out_dev;
+	if (!drmIsKMS(dev->fd)) {
+		wlr_log(WLR_DEBUG, "Ignoring '%s': not a KMS device", path);
+		wlr_session_close_file(session, dev);
+		return NULL;
 	}
-	drmModeFreeResources(res);
 
 	return dev;
-
-out_dev:
-	wlr_session_close_file(session, dev);
-	return NULL;
 }
 
 static ssize_t explicit_find_gpus(struct wlr_session *session,

meson.build

@@ -96,7 +96,7 @@ internal_features = {
 
 wayland_server = dependency('wayland-server', version: '>=1.19')
 wayland_client = dependency('wayland-client')
-drm = dependency('libdrm', version: '>=2.4.95')
+drm = dependency('libdrm', version: '>=2.4.105')
 gbm = dependency('gbm', version: '>=17.1.0')
 libinput = dependency('libinput', version: '>=1.14.0')
 xkbcommon = dependency('xkbcommon')