diff --git a/drivers/usb/serial/metro-usb.c b/drivers/usb/serial/metro-usb.c
index 40ccf6e5e318b..2b648c41f4e79 100644
--- a/drivers/usb/serial/metro-usb.c
+++ b/drivers/usb/serial/metro-usb.c
@@ -54,7 +54,7 @@ MODULE_DEVICE_TABLE(usb, id_table);
 #define UNI_CMD_OPEN	0x80
 #define UNI_CMD_CLOSE	0xFF
 
-inline int metrousb_is_unidirectional_mode(struct usb_serial_port *port)
+static inline int metrousb_is_unidirectional_mode(struct usb_serial_port *port)
 {
 	__u16 product_id = le16_to_cpu(
 		port->serial->dev->descriptor.idProduct);