openwrt/target/linux/generic/backport-5.15/765-v5.17-03-net-next-net-dsa-stop-updating-master-MTU-from-master.c.patch

From a1ff94c2973c43bc1e2677ac63ebb15b1d1ff846 Mon Sep 17 00:00:00 2001
From: Vladimir Oltean <[email protected]>
Date: Thu, 6 Jan 2022 01:11:14 +0200
Subject: [PATCH 3/6] net: dsa: stop updating master MTU from master.c

At present there are two paths for changing the MTU of the DSA master.

The first is:

dsa_tree_setup
-> dsa_tree_setup_ports
   -> dsa_port_setup
      -> dsa_slave_create
         -> dsa_slave_change_mtu
            -> dev_set_mtu(master)

The second is:

dsa_tree_setup
-> dsa_tree_setup_master
   -> dsa_master_setup
      -> dev_set_mtu(dev)

So the dev_set_mtu() call from dsa_master_setup() has been effectively
superseded by the dsa_slave_change_mtu(slave_dev, ETH_DATA_LEN) that is
done from dsa_slave_create() for each user port. The later function also
updates the master MTU according to the largest user port MTU from the
tree. Therefore, updating the master MTU through a separate code path
isn't needed.

Signed-off-by: Vladimir Oltean <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
---
 net/dsa/master.c | 25 +------------------------
 1 file changed, 1 insertion(+), 24 deletions(-)

--- a/net/dsa/master.c
+++ b/net/dsa/master.c
@@ -329,28 +329,13 @@ static const struct attribute_group dsa_
 	.attrs	= dsa_slave_attrs,
 };
 
-static void dsa_master_reset_mtu(struct net_device *dev)
-{
-	int err;
-
-	rtnl_lock();
-	err = dev_set_mtu(dev, ETH_DATA_LEN);
-	if (err)
-		netdev_dbg(dev,
-			   "Unable to reset MTU to exclude DSA overheads\n");
-	rtnl_unlock();
-}
-
 static struct lock_class_key dsa_master_addr_list_lock_key;
 
 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 mtu, ret;
-
-	mtu = ETH_DATA_LEN + dsa_tag_protocol_overhead(tag_ops);
+	int ret;
 
 	/* The DSA master must use SET_NETDEV_DEV for this to work. */
 	consumer_link = device_link_add(ds->dev, dev->dev.parent,
@@ -360,13 +345,6 @@ int dsa_master_setup(struct net_device *
 			   "Failed to create a device link to DSA switch %s\n",
 			   dev_name(ds->dev));
 
-	rtnl_lock();
-	ret = dev_set_mtu(dev, mtu);
-	rtnl_unlock();
-	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.
@@ -404,7 +382,6 @@ void dsa_master_teardown(struct net_devi
 	sysfs_remove_group(&dev->dev.kobj, &dsa_group);
 	dsa_netdev_ops_set(dev, NULL);
 	dsa_master_ethtool_teardown(dev);
-	dsa_master_reset_mtu(dev);
 	dsa_master_set_promiscuity(dev, -1);
 
 	dev->dsa_ptr = NULL;