net: dsa: Prevent usage of NET_DSA_TAG_8021Q as tagging protocol

It is possible for a switch driver to use NET_DSA_TAG_8021Q as a valid
DSA tagging protocol since it registers itself as such, unfortunately
since there are not xmit or rcv functions provided, the lack of a xmit()
function will lead to a NPD in dsa_slave_xmit() to start with.

net/dsa/tag_8021q.c is only comprised of a set of helper functions at
the moment, but is not a fully autonomous or functional tagging "driver"
(though it could become later on). We do not have any users of
NET_DSA_TAG_8021Q so now is a good time to make sure there are not
issues being encountered by making this file strictly a place holder for
helper functions.
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/Kconfig

@@ -20,7 +20,7 @@ if NET_DSA
 
 # tagging formats
 config NET_DSA_TAG_8021Q
-	tristate "Tag driver for switches using custom 802.1Q VLAN headers"
+	tristate
 	select VLAN_8021Q
 	help
 	  Unlike the other tagging protocols, the 802.1Q config option simply

net/dsa/tag_8021q.c

@@ -341,13 +341,4 @@ struct sk_buff *dsa_8021q_remove_header(struct sk_buff *skb)
 }
 EXPORT_SYMBOL_GPL(dsa_8021q_remove_header);
 
-static const struct dsa_device_ops dsa_8021q_netdev_ops = {
-	.name		= "8021q",
-	.proto		= DSA_TAG_PROTO_8021Q,
-	.overhead	= VLAN_HLEN,
-};
-
 MODULE_LICENSE("GPL v2");
-MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_8021Q);
-
-module_dsa_tag_driver(dsa_8021q_netdev_ops);