net: ethool: Document struct ethtool_flow_ext

Add documentation for struct ethtool_flow_ext especially in regard
to what flags are needed for which fields.

Signed-off-by: Yan Burman <yanb@mellanox.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/ethtool.h

@@ -503,9 +503,20 @@ union ethtool_flow_union {
 	__u8					hdata[52];
 };
 
+/**
+ * struct ethtool_flow_ext - additional RX flow fields
+ * @h_dest: destination MAC address
+ * @vlan_etype: VLAN EtherType
+ * @vlan_tci: VLAN tag control information
+ * @data: user defined data
+ *
+ * Note, @vlan_etype, @vlan_tci, and @data are only valid if %FLOW_EXT
+ * is set in &struct ethtool_rx_flow_spec @flow_type.
+ * @h_dest is valid if %FLOW_MAC_EXT is set.
+ */
 struct ethtool_flow_ext {
 	__u8		padding[2];
-	unsigned char	h_dest[ETH_ALEN];	/* destination eth addr	*/
+	unsigned char	h_dest[ETH_ALEN];
 	__be16		vlan_etype;
 	__be16		vlan_tci;
 	__be32		data[2];
@@ -519,7 +530,8 @@ struct ethtool_flow_ext {
  * @m_u: Masks for flow field bits to be matched
  * @m_ext: Masks for additional field bits to be matched
  *	Note, all additional fields must be ignored unless @flow_type
- *	includes the %FLOW_EXT flag.
+ *	includes the %FLOW_EXT or %FLOW_MAC_EXT flag
+ *	(see &struct ethtool_flow_ext description).
  * @ring_cookie: RX ring/queue index to deliver to, or %RX_CLS_FLOW_DISC
  *	if packets should be discarded
  * @location: Location of rule in the table.  Locations must be