diff mbox series

[net,1/2] net: dsa: don't error out when drivers return ETH_DATA_LEN in .port_max_mtu()

Message ID 20230314182405.2449898-2-vladimir.oltean@nxp.com
State New
Headers
Series Fix MTU reporting for Marvell DSA switches where we can't change it |

Commit Message

Vladimir Oltean March 14, 2023, 6:24 p.m. UTC 
  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 b9c587fed61c ("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: bfcb813203e6 ("net: dsa: configure the MTU for switch ports")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
 net/dsa/slave.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

Comments

Simon Horman March 15, 2023, 9:53 a.m. UTC | #1 
On Tue, Mar 14, 2023 at 08:24:04PM +0200, Vladimir Oltean wrote:
> 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 b9c587fed61c ("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: bfcb813203e6 ("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>
Florian Fainelli March 15, 2023, 6:34 p.m. UTC | #2 
On 3/14/23 11:24, Vladimir Oltean wrote:
> 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 b9c587fed61c ("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: bfcb813203e6 ("net: dsa: configure the MTU for switch ports")
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
diff mbox series

Patch

diff --git a/net/dsa/slave.c b/net/dsa/slave.c
index 6957971c2db2..cac17183589f 100644
--- a/net/dsa/slave.c
+++ b/net/dsa/slave.c
@@ -1933,6 +1933,7 @@  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;
 
@@ -1961,9 +1962,10 @@  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;
 
@@ -1998,8 +2000,7 @@  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);