backend/session: share is_drm_card implementation

backend/session/dev.c

@@ -0,0 +1,21 @@
+#define _POSIX_C_SOURCE 200809L
+#include <stdbool.h>
+#include <string.h>
+#include <xf86drm.h>
+#include "backend/session/dev.h"
+
+// TODO move to util?
+bool is_drm_card(const char *devname) {
+	const char prefix[] = DRM_PRIMARY_MINOR_NAME;
+	const char *name = strrchr(devname, '/');
+	name = name ? name + 1 : devname;
+	if (strncmp(name, prefix, strlen(prefix)) != 0) {
+		return false;
+	}
+	for (size_t i = strlen(prefix); name[i] != '\0'; i++) {
+		if (name[i] < '0' || name[i] > '9') {
+			return false;
+		}
+	}
+	return true;
+}