diff --git a/drivers/net/dsa/mv88e6xxx/port.c b/drivers/net/dsa/mv88e6xxx/port.c
index e718404a5e03..ebd26b6a93e6 100644
--- a/drivers/net/dsa/mv88e6xxx/port.c
+++ b/drivers/net/dsa/mv88e6xxx/port.c
@@ -368,12 +368,15 @@ int mv88e6390x_port_set_cmode(struct mv88e6xxx_chip *chip, int port,
 	u16 reg;
 	int err;
 
-	if (mode == PHY_INTERFACE_MODE_NA)
-		return 0;
-
 	if (port != 9 && port != 10)
 		return -EOPNOTSUPP;
 
+	/* Default to a slow mode, so freeing up SERDES interfaces for
+	 * other ports which might use them for SFPs.
+	 */
+	if (mode == PHY_INTERFACE_MODE_NA)
+		mode = PHY_INTERFACE_MODE_1000BASEX;
+
 	switch (mode) {
 	case PHY_INTERFACE_MODE_1000BASEX:
 		cmode = MV88E6XXX_PORT_STS_CMODE_1000BASE_X;