IB/ipoib: When given an invalid UD MTU, give debug msg

In datagram mode, the IB UD (Unreliable Datagram) transport is used
so the MTU of the interface is equal to the IB L2 MTU minus the
IPoIB encapsulation header. Any request to change the MTU value
above the maximum range will change the MTU to the max allowed, but
will not show any warning message. An ipoib_warn is issued in such
cases, letting the user know that even though the value is legal,
it can't be currently applied.
Signed-off-by: NFeras Daoud <ferasda@mellanox.com>
Signed-off-by: NNoa Osherovich <noaos@mellanox.com>
Signed-off-by: NLeon Romanovsky <leon@kernel.org>
Signed-off-by: NDoug Ledford <dledford@redhat.com>

drivers/infiniband/ulp/ipoib/ipoib_main.c

@@ -229,6 +229,10 @@ static int ipoib_change_mtu(struct net_device *dev, int new_mtu)
 
 	priv->admin_mtu = new_mtu;
 
+	if (priv->mcast_mtu < priv->admin_mtu)
+		ipoib_dbg(priv, "MTU must be smaller than the underlying "
+				"link layer MTU - 4 (%u)\n", priv->mcast_mtu);
+
 	dev->mtu = min(priv->mcast_mtu, priv->admin_mtu);
 
 	return 0;