net: dsa: tag_ocelot: single out PTP-related transmit tag processing

There is one place where we cannot avoid accessing driver data, and that
is 2-step PTP TX timestamping, since the switch wants us to provide a
timestamp request ID through the injection header, which naturally must
come from a sequence number kept by the driver (it is generated by the
.port_txtstamp method prior to the tagger's xmit).

However, since other drivers like dsa_loop do not claim PTP support
anyway, the DSA_SKB_CB(skb)->clone will always be NULL anyway, so if we
move all PTP-related dereferences of struct ocelot and struct ocelot_port
into a separate function, we can effectively ensure that this is dead
code when the ocelot tagger is attached to non-ocelot switches, and the
stateful portion of the tagger is more self-contained.

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

net/dsa/tag_ocelot.c

@@ -5,6 +5,25 @@
 #include <soc/mscc/ocelot.h>
 #include "dsa_priv.h"
 
+static void ocelot_xmit_ptp(struct dsa_port *dp, void *injection,
+			    struct sk_buff *clone)
+{
+	struct ocelot *ocelot = dp->ds->priv;
+	struct ocelot_port *ocelot_port;
+	u64 rew_op;
+
+	ocelot_port = ocelot->ports[dp->index];
+	rew_op = ocelot_port->ptp_cmd;
+
+	/* Retrieve timestamp ID populated inside skb->cb[0] of the
+	 * clone by ocelot_port_add_txtstamp_skb
+	 */
+	if (ocelot_port->ptp_cmd == IFH_REW_OP_TWO_STEP_PTP)
+		rew_op |= clone->cb[0] << 3;
+
+	ocelot_ifh_set_rew_op(injection, rew_op);
+}
+
 static struct sk_buff *ocelot_xmit(struct sk_buff *skb,
 				   struct net_device *netdev)
 {
@@ -29,17 +48,8 @@ static struct sk_buff *ocelot_xmit(struct sk_buff *skb,
 	ocelot_ifh_set_qos_class(injection, skb->priority);
 
 	/* TX timestamping was requested */
-	if (clone) {
-		u64 rew_op = ocelot_port->ptp_cmd;
-
-		/* Retrieve timestamp ID populated inside skb->cb[0] of the
-		 * clone by ocelot_port_add_txtstamp_skb
-		 */
-		if (ocelot_port->ptp_cmd == IFH_REW_OP_TWO_STEP_PTP)
-			rew_op |= clone->cb[0] << 3;
-
-		ocelot_ifh_set_rew_op(injection, rew_op);
-	}
+	if (clone)
+		ocelot_xmit_ptp(dp, injection, clone);
 
 	return skb;
 }