net: mscc: ocelot: adjust MTU on the CPU port in NPI mode

When using the NPI port, the DSA tag is passed through Ethernet, so the
switch's MAC needs to accept it as it comes from the DSA master. Increase
the MTU on the external CPU port to account for the length of the
injection header.

Without this patch, MTU-sized frames are dropped by the switch's CPU
port on xmit, which is especially obvious in TCP sessions.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mscc/ocelot.c

@@ -2230,9 +2230,18 @@ void ocelot_set_cpu_port(struct ocelot *ocelot, int cpu,
 	 * Only one port can be an NPI at the same time.
 	 */
 	if (cpu < ocelot->num_phys_ports) {
+		int mtu = VLAN_ETH_FRAME_LEN + OCELOT_TAG_LEN;
+
 		ocelot_write(ocelot, QSYS_EXT_CPU_CFG_EXT_CPUQ_MSK_M |
 			     QSYS_EXT_CPU_CFG_EXT_CPU_PORT(cpu),
 			     QSYS_EXT_CPU_CFG);
+
+		if (injection == OCELOT_TAG_PREFIX_SHORT)
+			mtu += OCELOT_SHORT_PREFIX_LEN;
+		else if (injection == OCELOT_TAG_PREFIX_LONG)
+			mtu += OCELOT_LONG_PREFIX_LEN;
+
+		ocelot_port_set_mtu(ocelot, cpu, mtu);
 	}
 
 	/* CPU port Injection/Extraction configuration */

drivers/net/ethernet/mscc/ocelot.h

@@ -65,6 +65,8 @@ struct frame_info {
 #define IFH_REW_OP_ORIGIN_PTP		0x5
 
 #define OCELOT_TAG_LEN			16
+#define OCELOT_SHORT_PREFIX_LEN		4
+#define OCELOT_LONG_PREFIX_LEN		16
 
 #define OCELOT_SPEED_2500 0
 #define OCELOT_SPEED_1000 1