net: dsa: sja1105: Take PTP egress timestamp by port, not mgmt slot

The PTP egress timestamp N must be captured from register PTPEGR_TS[n],
where n = 2 * PORT + TSREG. There are 10 PTPEGR_TS registers, 2 per
port. We are only using TSREG=0.

As opposed to the management slots, which are 4 in number
(SJA1105_NUM_PORTS, minus the CPU port). Any management frame (which
includes PTP frames) can be sent to any non-CPU port through any
management slot. When the CPU port is not the last port (#4), there will
be a mismatch between the slot and the port number.

Luckily, the only mainline occurrence with this switch
(arch/arm/boot/dts/ls1021a-tsn.dts) does have the CPU port as #4, so the
issue did not manifest itself thus far.

Fixes: 47ed985 ("net: dsa: sja1105: Add logic for TX timestamping")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/sja1105/sja1105_main.c

@@ -1855,7 +1855,7 @@ static netdev_tx_t sja1105_port_deferred_xmit(struct dsa_switch *ds, int port,
 	if (!clone)
 		goto out;
 
-	sja1105_ptp_txtstamp_skb(ds, slot, clone);
+	sja1105_ptp_txtstamp_skb(ds, port, clone);
 
 out:
 	mutex_unlock(&priv->mgmt_lock);

drivers/net/dsa/sja1105/sja1105_ptp.c

@@ -659,7 +659,7 @@ void sja1105_ptp_clock_unregister(struct dsa_switch *ds)
 	ptp_data->clock = NULL;
 }
 
-void sja1105_ptp_txtstamp_skb(struct dsa_switch *ds, int slot,
+void sja1105_ptp_txtstamp_skb(struct dsa_switch *ds, int port,
 			      struct sk_buff *skb)
 {
 	struct sja1105_private *priv = ds->priv;
@@ -679,7 +679,7 @@ void sja1105_ptp_txtstamp_skb(struct dsa_switch *ds, int slot,
 		goto out;
 	}
 
-	rc = sja1105_ptpegr_ts_poll(ds, slot, &ts);
+	rc = sja1105_ptpegr_ts_poll(ds, port, &ts);
 	if (rc < 0) {
 		dev_err(ds->dev, "timed out polling for tstamp\n");
 		kfree_skb(skb);