net: dsa: Set the master device's MTU to account for DSA overheads

DSA tagging of frames sent over the master interface to the switch
increases the size of the frame. Such frames can then be bigger than
the normal MTU of the master interface, and it may drop them. Use the
overhead information from the tagger to set the MTU of the master
device to include this overhead.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/master.c

@@ -158,8 +158,24 @@ static void dsa_master_ethtool_teardown(struct net_device *dev)
 	cpu_dp->orig_ethtool_ops = NULL;
 }
 
+void dsa_master_set_mtu(struct net_device *dev, struct dsa_port *cpu_dp)
+{
+	unsigned int mtu = ETH_DATA_LEN + cpu_dp->tag_ops->overhead;
+	int err;
+
+	rtnl_lock();
+	if (mtu <= dev->max_mtu) {
+		err = dev_set_mtu(dev, mtu);
+		if (err)
+			netdev_dbg(dev, "Unable to set MTU to include for DSA overheads\n");
+	}
+	rtnl_unlock();
+}
+
 int dsa_master_setup(struct net_device *dev, struct dsa_port *cpu_dp)
 {
+	dsa_master_set_mtu(dev,  cpu_dp);
+
 	/* If we use a tagging format that doesn't have an ethertype
 	 * field, make sure that all packets from this point on get
 	 * sent to the tag format's receive function.