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/Documentation/ABI/testing/sysfs-class-net-dsa b/Documentation/ABI/testing/sysfs-class-net-dsa
new file mode 100644
index 0000000..f240221
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-net-dsa
@@ -0,0 +1,7 @@
+What:		/sys/class/net/<iface>/tagging
+Date:		August 2018
+KernelVersion:	4.20
+Contact:	netdev@vger.kernel.org
+Description:
+		String indicating the type of tagging protocol used by the
+		DSA slave network device.