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Date:   Thu, 31 Mar 2022 16:28:54 +0300
From:   Vladimir Oltean <>
Cc:     "David S . Miller" <>,
        Jakub Kicinski <>,
        Paolo Abeni <>,
        Florian Fainelli <>,
        Andrew Lunn <>,
        Vivien Didelot <>,
        Luiz Angelo Daros de Luca <>
Subject: [PATCH net] Revert "net: dsa: stop updating master MTU from master.c"

This reverts commit a1ff94c2973c43bc1e2677ac63ebb15b1d1ff846.

Switch drivers that don't implement ->port_change_mtu() will cause the
DSA master to remain with an MTU of 1500, since we've deleted the other
code path. In turn, this causes a regression for those systems, where
MTU-sized traffic can no longer be terminated.

Revert the change taking into account the fact that rtnl_lock() is now
taken top-level from the callers of dsa_master_setup() and
dsa_master_teardown(). Also add a comment in order for it to be
absolutely clear why it is still needed.

Fixes: a1ff94c2973c ("net: dsa: stop updating master MTU from master.c")
Reported-by: Luiz Angelo Daros de Luca <>
Signed-off-by: Vladimir Oltean <>
 net/dsa/master.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/net/dsa/master.c b/net/dsa/master.c
index 991c2930d631..2851e44c4cf0 100644
--- a/net/dsa/master.c
+++ b/net/dsa/master.c
@@ -335,11 +335,24 @@ static const struct attribute_group dsa_group = {
 	.attrs	= dsa_slave_attrs,
+static void dsa_master_reset_mtu(struct net_device *dev)
+	int err;
+	err = dev_set_mtu(dev, ETH_DATA_LEN);
+	if (err)
+		netdev_dbg(dev,
+			   "Unable to reset MTU to exclude DSA overheads\n");
 int dsa_master_setup(struct net_device *dev, struct dsa_port *cpu_dp)
+	const struct dsa_device_ops *tag_ops = cpu_dp->tag_ops;
 	struct dsa_switch *ds = cpu_dp->ds;
 	struct device_link *consumer_link;
-	int ret;
+	int mtu, ret;
+	mtu = ETH_DATA_LEN + dsa_tag_protocol_overhead(tag_ops);
 	/* The DSA master must use SET_NETDEV_DEV for this to work. */
 	consumer_link = device_link_add(ds->dev, dev->dev.parent,
@@ -349,6 +362,15 @@ int dsa_master_setup(struct net_device *dev, struct dsa_port *cpu_dp)
 			   "Failed to create a device link to DSA switch %s\n",
+	/* The switch driver may not implement ->port_change_mtu(), case in
+	 * which dsa_slave_change_mtu() will not update the master MTU either,
+	 * so we need to do that here.
+	 */
+	ret = dev_set_mtu(dev, mtu);
+	if (ret)
+		netdev_warn(dev, "error %d setting MTU to %d to include DSA overhead\n",
+			    ret, mtu);
 	/* 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.
@@ -384,6 +406,7 @@ void dsa_master_teardown(struct net_device *dev)
 	sysfs_remove_group(&dev->dev.kobj, &dsa_group);
 	dsa_netdev_ops_set(dev, NULL);
+	dsa_master_reset_mtu(dev);
 	dsa_master_set_promiscuity(dev, -1);
 	dev->dsa_ptr = NULL;

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