net: dsa: create a helper for locating EtherType DSA headers on RX

It seems that protocol tagging driver writers are always surprised about
the formula they use to reach their EtherType header on RX, which
becomes apparent from the fact that there are comments in multiple
drivers that mention the same information.

Create a helper that returns a void pointer to skb->data - 2, as well as
centralize the explanation why that is the case.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 0 deletions

diff --git a/net/dsa/dsa_priv.h b/net/dsa/dsa_priv.h
index 28e1fbe64ee0..ee194df68902 100644
--- a/net/dsa/dsa_priv.h
+++ b/net/dsa/dsa_priv.h
@@ -507,6 +507,20 @@ static inline void dsa_alloc_etype_header(struct sk_buff *skb, int len)
 	memmove(skb->data, skb->data + len, 2 * ETH_ALEN);
 }
 
+/* On RX, eth_type_trans() on the DSA master pulls ETH_HLEN bytes starting from
+ * skb_mac_header(skb), which leaves skb->data pointing at the first byte after
+ * what the DSA master perceives as the EtherType (the beginning of the L3
+ * protocol). Since DSA EtherType header taggers treat the EtherType as part of
+ * the DSA tag itself, and the EtherType is 2 bytes in length, the DSA header
+ * is located 2 bytes behind skb->data. Note that EtherType in this context
+ * means the first 2 bytes of the DSA header, not the encapsulated EtherType
+ * that will become visible after the DSA header is stripped.
+ */
+static inline void *dsa_etype_header_pos_rx(struct sk_buff *skb)
+{
+	return skb->data - 2;
+}
+
 /* switch.c */
 int dsa_switch_register_notifier(struct dsa_switch *ds);
 void dsa_switch_unregister_notifier(struct dsa_switch *ds);