net: mscc: ocelot: improve handling of TX timestamp for unknown skb

This condition, theoretically impossible to trigger, is not really
handled well. By "continuing", we are skipping the write to SYS_PTP_NXT
which advances the timestamp FIFO to the next entry. So we are reading
the same FIFO entry all over again, printing stack traces and eventually
killing the kernel.

No real problem has been observed here. This is part of a larger rework
of the timestamp IRQ procedure, with this logical change split out into
a patch of its own. We will need to "goto next_ts" for other conditions
as well.

Fixes: 9fde506 ("net: mscc: ocelot: warn when a PTP IRQ is raised for an unknown skb")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20241205145519.1236778-3-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

drivers/net/ethernet/mscc/ocelot_ptp.c

@@ -786,7 +786,7 @@ void ocelot_get_txtstamp(struct ocelot *ocelot)
 		spin_unlock_irqrestore(&port->tx_skbs.lock, flags);
 
 		if (WARN_ON(!skb_match))
-			continue;
+			goto next_ts;
 
 		if (!ocelot_validate_ptp_skb(skb_match, seqid)) {
 			dev_err_ratelimited(ocelot->dev,
@@ -804,7 +804,7 @@ void ocelot_get_txtstamp(struct ocelot *ocelot)
 		shhwtstamps.hwtstamp = ktime_set(ts.tv_sec, ts.tv_nsec);
 		skb_complete_tx_timestamp(skb_match, &shhwtstamps);
 
-		/* Next ts */
+next_ts:
 		ocelot_write(ocelot, SYS_PTP_NXT_PTP_NXT, SYS_PTP_NXT);
 	}
 }