net: dsa: Expose tagging protocol to user-space

There is no way for user-space to know what a given DSA network device's
tagging protocol is. Expose this information through a dsa/tagging
attribute which reflects the tagging protocol currently in use.

This is helpful for configuration (e.g: none behaves dramatically
different wrt. bridges) as well as for packet capture tools when there
is not a proper Ethernet type available.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/dsa/dsa_priv.h b/net/dsa/dsa_priv.h
index 3964c6f..2868b5b 100644
--- a/net/dsa/dsa_priv.h
+++ b/net/dsa/dsa_priv.h
@@ -86,6 +86,7 @@ struct dsa_slave_priv {
 /* dsa.c */
 const struct dsa_device_ops *dsa_resolve_tag_protocol(int tag_protocol);
 bool dsa_schedule_work(struct work_struct *work);
+const char *dsa_tag_protocol_to_str(const struct dsa_device_ops *ops);
 
 /* legacy.c */
 #if IS_ENABLED(CONFIG_NET_DSA_LEGACY)