net: dsa: felix: create a template for the DSA tags on xmit

With this patch we try to kill 2 birds with 1 stone.

First of all, some switches that use tag_ocelot.c don't have the exact
same bitfield layout for the DSA tags. The destination ports field is
different for Seville VSC9953 for example. So the choices are to either
duplicate tag_ocelot.c into a new tag_seville.c (sub-optimal) or somehow
take into account a supposed ocelot->dest_ports_offset when packing this
field into the DSA injection header (again not ideal).

Secondly, tag_ocelot.c already needs to memset a 128-bit area to zero
and call some packing() functions of dubious performance in the
fastpath. And most of the values it needs to pack are pretty much
constant (BYPASS=1, SRC_PORT=CPU, DEST=port index). So it would be good
if we could improve that.

The proposed solution is to allocate a memory area per port at probe
time, initialize that with the statically defined bits as per chip
hardware revision, and just perform a simpler memcpy in the fastpath.

Other alternatives have been analyzed, such as:
- Create a separate tag_seville.c: too much code duplication for just 1
  bit field difference.
- Create a separate DSA_TAG_PROTO_SEVILLE under tag_ocelot.c, just like
  tag_brcm.c, which would have a separate .xmit function. Again, too
  much code duplication for just 1 bit field difference.
- Allocate the template from the init function of the tag_ocelot.c
  module, instead of from the driver: couldn't figure out a method of
  accessing the correct port template corresponding to the correct
  tagger in the .xmit function.

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

@@ -137,11 +137,10 @@ static struct sk_buff *ocelot_xmit(struct sk_buff *skb,
 				   struct net_device *netdev)
 {
 	struct dsa_port *dp = dsa_slave_to_port(netdev);
-	u64 bypass, dest, src, qos_class, rew_op;
 	struct dsa_switch *ds = dp->ds;
-	int port = dp->index;
 	struct ocelot *ocelot = ds->priv;
-	struct ocelot_port *ocelot_port = ocelot->ports[port];
+	struct ocelot_port *ocelot_port;
+	u64 qos_class, rew_op;
 	u8 *injection;
 
 	if (unlikely(skb_cow_head(skb, OCELOT_TAG_LEN) < 0)) {
@@ -149,19 +148,15 @@ static struct sk_buff *ocelot_xmit(struct sk_buff *skb,
 		return NULL;
 	}
 
+	ocelot_port = ocelot->ports[dp->index];
+
 	injection = skb_push(skb, OCELOT_TAG_LEN);
 
-	memset(injection, 0, OCELOT_TAG_LEN);
-
-	/* Set the source port as the CPU port module and not the NPI port */
-	src = ocelot->num_phys_ports;
-	dest = BIT(port);
-	bypass = true;
+	memcpy(injection, ocelot_port->xmit_template, OCELOT_TAG_LEN);
+	/* Fix up the fields which are not statically determined
+	 * in the template
+	 */
 	qos_class = skb->priority;
-
-	packing(injection, &bypass,   127, 127, OCELOT_TAG_LEN, PACK, 0);
-	packing(injection, &dest,      68,  56, OCELOT_TAG_LEN, PACK, 0);
-	packing(injection, &src,       46,  43, OCELOT_TAG_LEN, PACK, 0);
 	packing(injection, &qos_class, 19,  17, OCELOT_TAG_LEN, PACK, 0);
 
 	if (ocelot->ptp && (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)) {