net: dsa: Implement flow dissection for tag_brcm.c

Provide a flow_dissect callback which returns the network offset and
where to find the skb protocol, given the tags structure a common
function works for both tagging formats that are supported.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_brcm.c

@@ -142,6 +142,27 @@ static struct sk_buff *brcm_tag_rcv_ll(struct sk_buff *skb,
 
 	return skb;
 }
+
+static int brcm_tag_flow_dissect(const struct sk_buff *skb, __be16 *proto,
+				 int *offset)
+{
+	/* We have been called on the DSA master network device after
+	 * eth_type_trans() which pulled the Ethernet header already.
+	 * Frames have one of these two layouts:
+	 * -----------------------------------
+	 * | MAC DA | MAC SA | 4b tag | Type | DSA_TAG_PROTO_BRCM
+	 * -----------------------------------
+	 * -----------------------------------
+	 * | 4b tag | MAC DA | MAC SA | Type | DSA_TAG_PROTO_BRCM_PREPEND
+	 * -----------------------------------
+	 * skb->data points 2 bytes before the actual Ethernet type field and
+	 * we have an offset of 4bytes between where skb->data and where the
+	 * payload starts.
+	 */
+	*offset = BRCM_TAG_LEN;
+	*proto = ((__be16 *)skb->data)[1];
+	return 0;
+}
 #endif
 
 #if IS_ENABLED(CONFIG_NET_DSA_TAG_BRCM)
@@ -177,6 +198,7 @@ static const struct dsa_device_ops brcm_netdev_ops = {
 	.xmit	= brcm_tag_xmit,
 	.rcv	= brcm_tag_rcv,
 	.overhead = BRCM_TAG_LEN,
+	.flow_dissect = brcm_tag_flow_dissect,
 };
 
 DSA_TAG_DRIVER(brcm_netdev_ops);
@@ -205,6 +227,7 @@ static const struct dsa_device_ops brcm_prepend_netdev_ops = {
 	.xmit	= brcm_tag_xmit_prepend,
 	.rcv	= brcm_tag_rcv_prepend,
 	.overhead = BRCM_TAG_LEN,
+	.flow_dissect = brcm_tag_flow_dissect,
 };
 
 DSA_TAG_DRIVER(brcm_prepend_netdev_ops);