net: dsa: don't error out when drivers return ETH_DATA_LEN in .port_max_mtu()

[ Upstream commit 1bebf9e9cdda753ae52b3a217236c1f594b85aee ]

Currently, when dsa_slave_change_mtu() is called on a user port where
dev->max_mtu is 1500 (as returned by ds->ops->port_max_mtu()), the code
will stumble upon this check:

	if (new_master_mtu > mtu_limit)
		return -ERANGE;

because new_master_mtu is adjusted for the tagger overhead but mtu_limit
is not.

But it would be good if the logic went through, for example if the DSA
master really depends on an MTU adjustment to accept DSA-tagged frames.

To make the code pass through the check, we need to adjust mtu_limit for
the overhead as well, if the minimum restriction was caused by the DSA
user port's MTU (dev->max_mtu). A DSA user port MTU and a DSA master MTU
are always offset by the protocol overhead.

Currently no drivers return 1500 .port_max_mtu(), but this is only
temporary and a bug in itself - mv88e6xxx should have done that, but
since commit ea2f9182a4cd ("dsa: mv88e6xxx: Include tagger overhead when
setting MTU for DSA and CPU ports") it no longer does. This is a
preparation for fixing that.

Fixes: b202c04f8d0d ("net: dsa: configure the MTU for switch ports")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/net/dsa/slave.c b/net/dsa/slave.c
index a9fde48cffd437bc8501512f1fb12521e61d0928..5fe075bf479ec5b3a5fadaf182b12897a82f725a 100644 (file)
--- a/net/dsa/slave.c
+++ b/net/dsa/slave.c
@@ -1852,6 +1852,7 @@ int dsa_slave_change_mtu(struct net_device *dev, int new_mtu)
        int new_master_mtu;
        int old_master_mtu;
        int mtu_limit;
+       int overhead;
        int cpu_mtu;
        int err;
 
@@ -1880,9 +1881,10 @@ int dsa_slave_change_mtu(struct net_device *dev, int new_mtu)
                        largest_mtu = slave_mtu;
        }
 
-       mtu_limit = min_t(int, master->max_mtu, dev->max_mtu);
+       overhead = dsa_tag_protocol_overhead(cpu_dp->tag_ops);
+       mtu_limit = min_t(int, master->max_mtu, dev->max_mtu + overhead);
        old_master_mtu = master->mtu;
-       new_master_mtu = largest_mtu + dsa_tag_protocol_overhead(cpu_dp->tag_ops);
+       new_master_mtu = largest_mtu + overhead;
        if (new_master_mtu > mtu_limit)
                return -ERANGE;
 
@@ -1917,8 +1919,7 @@ int dsa_slave_change_mtu(struct net_device *dev, int new_mtu)
 
 out_port_failed:
        if (new_master_mtu != old_master_mtu)
-               dsa_port_mtu_change(cpu_dp, old_master_mtu -
-                                   dsa_tag_protocol_overhead(cpu_dp->tag_ops));
+               dsa_port_mtu_change(cpu_dp, old_master_mtu - overhead);
 out_cpu_failed:
        if (new_master_mtu != old_master_mtu)
                dev_set_mtu(master, old_master_mtu);