net: dsa: mv88e6xxx: Default ports 9/10 6390X CMODE to 1000BaseX

The 6390X family has 8 SERDES interfaces. This allows ports 9 and 10
to support up to 10Gbps using 4 SERDES interfaces. However, when lower
speeds are used, which need fewer SERDES interfaces, the unused SERDES
interfaces can be used by ports 2-8.

The hardware defaults to ports 9 and 10 having all 4 SERDES interfaces
assigned to them. This only gets changed when the interface is
configured after what the SFP supports has been determined, or the 10G
PHY completes auto-neg.

For hardware designs which limit ports 9 and 10 to one or two SERDES
interfaces, and place SFPs on the lower interfaces, this is too
late. Those ports with SFP should not wait until ports 9/10 are up in
order to get access to the SERDES interface. So change the default
configuration when the driver is initialised. Configure ports 9 and 10
to 1000BaseX, so they use a single SERDES interface, freeing up the
others. They can steal them back if they need them.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

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;