net: dp83640: expire old TX-skb

During sendmsg() a cloned skb is saved via dp83640_txtstamp() in
->tx_queue. After the NIC sends this packet, the PHY will reply with a
timestamp for that TX packet. If the cable is pulled at the right time I
don't see that packet. It might gets flushed as part of queue shutdown
on NIC's side.
Once the link is up again then after the next sendmsg() we enqueue
another skb in dp83640_txtstamp() and have two on the list. Then the PHY
will send a reply and decode_txts() attaches it to the first skb on the
list.
No crash occurs since refcounting works but we are one packet behind.
linuxptp/ptp4l usually closes the socket and opens a new one (in such a
timeout case) so those "stale" replies never get there. However it does
not resume normal operation anymore.

Purge old skbs in decode_txts().

Fixes: cb646e2b ("ptp: Added a clock driver for the National Semiconductor PHYTER.")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/dp83640.c

@@ -898,14 +898,14 @@ static void decode_txts(struct dp83640_private *dp83640,
 			struct phy_txts *phy_txts)
 {
 	struct skb_shared_hwtstamps shhwtstamps;
+	struct dp83640_skb_info *skb_info;
 	struct sk_buff *skb;
-	u64 ns;
 	u8 overflow;
+	u64 ns;
 
 	/* We must already have the skb that triggered this. */
-
+again:
 	skb = skb_dequeue(&dp83640->tx_queue);
-
 	if (!skb) {
 		pr_debug("have timestamp but tx_queue empty\n");
 		return;
@@ -920,6 +920,11 @@ static void decode_txts(struct dp83640_private *dp83640,
 		}
 		return;
 	}
+	skb_info = (struct dp83640_skb_info *)skb->cb;
+	if (time_after(jiffies, skb_info->tmo)) {
+		kfree_skb(skb);
+		goto again;
+	}
 
 	ns = phy2txts(phy_txts);
 	memset(&shhwtstamps, 0, sizeof(shhwtstamps));
@@ -1472,6 +1477,7 @@ static bool dp83640_rxtstamp(struct phy_device *phydev,
 static void dp83640_txtstamp(struct phy_device *phydev,
 			     struct sk_buff *skb, int type)
 {
+	struct dp83640_skb_info *skb_info = (struct dp83640_skb_info *)skb->cb;
 	struct dp83640_private *dp83640 = phydev->priv;
 
 	switch (dp83640->hwts_tx_en) {
@@ -1484,6 +1490,7 @@ static void dp83640_txtstamp(struct phy_device *phydev,
 		/* fall through */
 	case HWTSTAMP_TX_ON:
 		skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
+		skb_info->tmo = jiffies + SKB_TIMESTAMP_TIMEOUT;
 		skb_queue_tail(&dp83640->tx_queue, skb);
 		break;