net: dsa: tag_brcm: use generic flow dissector procedure

There are 2 Broadcom tags in use, one places the DSA tag before the
Ethernet destination MAC address, and the other before the EtherType.
Nonetheless, both displace the rest of the headers, so this tagger can
use the generic flow dissector procedure which accounts for that.

The ASCII art drawing is a good reference though, so keep it but move it
somewhere else.

Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_brcm.c

@@ -107,6 +107,18 @@ static struct sk_buff *brcm_tag_xmit_ll(struct sk_buff *skb,
 	return skb;
 }
 
+/* Frames with this tag 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
+ * -----------------------------------
+ * In both cases, at receive time, 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. So the same low-level receive function can be
+ * used.
+ */
 static struct sk_buff *brcm_tag_rcv_ll(struct sk_buff *skb,
 				       struct net_device *dev,
 				       struct packet_type *pt,
@@ -149,26 +161,6 @@ static struct sk_buff *brcm_tag_rcv_ll(struct sk_buff *skb,
 
 	return skb;
 }
-
-static void 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];
-}
 #endif
 
 #if IS_ENABLED(CONFIG_NET_DSA_TAG_BRCM)
@@ -204,7 +196,6 @@ 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);
@@ -239,7 +230,6 @@ 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);