net: mscc: ocelot: fix hardware timestamp dequeue logic

The next hw timestamp should be snapshoot to the read registers
only once the current timestamp has been read.
If none of the pending skbs matches the current HW timestamp
just gracefully flush the available timestamp by reading it.
Signed-off-by: laurent brando <laurent.brando@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mscc/ocelot.c

@@ -748,21 +748,21 @@ void ocelot_get_txtstamp(struct ocelot *ocelot)
 
 		spin_unlock_irqrestore(&port->tx_skbs.lock, flags);
 
-		/* Next ts */
-		ocelot_write(ocelot, SYS_PTP_NXT_PTP_NXT, SYS_PTP_NXT);
+		/* Get the h/w timestamp */
+		ocelot_get_hwtimestamp(ocelot, &ts);
 
 		if (unlikely(!skb_match))
 			continue;
 
-		/* Get the h/w timestamp */
-		ocelot_get_hwtimestamp(ocelot, &ts);
-
 		/* Set the timestamp into the skb */
 		memset(&shhwtstamps, 0, sizeof(shhwtstamps));
 		shhwtstamps.hwtstamp = ktime_set(ts.tv_sec, ts.tv_nsec);
 		skb_tstamp_tx(skb_match, &shhwtstamps);
 
 		dev_kfree_skb_any(skb_match);
+
+		/* Next ts */
+		ocelot_write(ocelot, SYS_PTP_NXT_PTP_NXT, SYS_PTP_NXT);
 	}
 }
 EXPORT_SYMBOL(ocelot_get_txtstamp);