net: mscc: ocelot: avoid overflowing the PTP timestamp FIFO

PTP packets with 2-step TX timestamp requests are matched to packets
based on the egress port number and a 6-bit timestamp identifier.
All PTP timestamps are held in a common FIFO that is 128 entry deep.

This patch ensures that back-to-back timestamping requests cannot exceed
the hardware FIFO capacity. If that happens, simply send the packets
without requesting a TX timestamp to be taken (in the case of felix,
since the DSA API has a void return code in ds->ops->port_txtstamp) or
drop them (in the case of ocelot).

I've moved the ts_id_lock from a per-port basis to a per-switch basis,
because we need separate accounting for both numbers of PTP frames in
flight. And since we need locking to inc/dec the per-switch counter,
that also offers protection for the per-port counter and hence there is
no reason to have a per-port counter anymore.

Fixes: 4e3b0468e6d7 ("net: mscc: PTP Hardware Clock (PHC) support")
Signed-off-by: Vladimir Oltean <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/dsa/ocelot/felix.c

···
 	if (!ocelot->ptp)
 		return;
 
-	if (ocelot_port_txtstamp_request(ocelot, port, skb, &clone))
+	if (ocelot_port_txtstamp_request(ocelot, port, skb, &clone)) {
+		dev_err_ratelimited(ds->dev,
+				    "port %d delivering skb without TX timestamp\n",
+				    port);
 		return;
+	}
 
 	if (clone)
 		OCELOT_SKB_CB(skb)->clone = clone;

drivers/net/ethernet/mscc/ocelot.c

···
 }
 EXPORT_SYMBOL_GPL(ocelot_phylink_mac_link_up);
 
-static void ocelot_port_add_txtstamp_skb(struct ocelot *ocelot, int port,
-					 struct sk_buff *clone)
+static int ocelot_port_add_txtstamp_skb(struct ocelot *ocelot, int port,
+					struct sk_buff *clone)
 {
 	struct ocelot_port *ocelot_port = ocelot->ports[port];
+	unsigned long flags;
 
-	spin_lock(&ocelot_port->ts_id_lock);
+	spin_lock_irqsave(&ocelot->ts_id_lock, flags);
+
+	if (ocelot_port->ptp_skbs_in_flight == OCELOT_MAX_PTP_ID ||
+	    ocelot->ptp_skbs_in_flight == OCELOT_PTP_FIFO_SIZE) {
+		spin_unlock_irqrestore(&ocelot->ts_id_lock, flags);
+		return -EBUSY;
+	}
 
 	skb_shinfo(clone)->tx_flags |= SKBTX_IN_PROGRESS;
 	/* Store timestamp ID in OCELOT_SKB_CB(clone)->ts_id */
 	OCELOT_SKB_CB(clone)->ts_id = ocelot_port->ts_id;
+
 	ocelot_port->ts_id++;
 	if (ocelot_port->ts_id == OCELOT_MAX_PTP_ID)
 		ocelot_port->ts_id = 0;
+
+	ocelot_port->ptp_skbs_in_flight++;
+	ocelot->ptp_skbs_in_flight++;
+
 	skb_queue_tail(&ocelot_port->tx_skbs, clone);
 
-	spin_unlock(&ocelot_port->ts_id_lock);
+	spin_unlock_irqrestore(&ocelot->ts_id_lock, flags);
+
+	return 0;
 }
 
 u32 ocelot_ptp_rew_op(struct sk_buff *skb)
···
 {
 	struct ocelot_port *ocelot_port = ocelot->ports[port];
 	u8 ptp_cmd = ocelot_port->ptp_cmd;
+	int err;
 
 	/* Store ptp_cmd in OCELOT_SKB_CB(skb)->ptp_cmd */
 	if (ptp_cmd == IFH_REW_OP_ORIGIN_PTP) {
···
 		if (!(*clone))
 			return -ENOMEM;
 
-		ocelot_port_add_txtstamp_skb(ocelot, port, *clone);
+		err = ocelot_port_add_txtstamp_skb(ocelot, port, *clone);
+		if (err)
+			return err;
+
 		OCELOT_SKB_CB(skb)->ptp_cmd = ptp_cmd;
 	}
 
···
 		id = SYS_PTP_STATUS_PTP_MESS_ID_X(val);
 		txport = SYS_PTP_STATUS_PTP_MESS_TXPORT_X(val);
 
-		/* Retrieve its associated skb */
 		port = ocelot->ports[txport];
 
+		spin_lock(&ocelot->ts_id_lock);
+		port->ptp_skbs_in_flight--;
+		ocelot->ptp_skbs_in_flight--;
+		spin_unlock(&ocelot->ts_id_lock);
+
+		/* Retrieve its associated skb */
 		spin_lock_irqsave(&port->tx_skbs.lock, flags);
 
 		skb_queue_walk_safe(&port->tx_skbs, skb, skb_tmp) {
···
 	struct ocelot_port *ocelot_port = ocelot->ports[port];
 
 	skb_queue_head_init(&ocelot_port->tx_skbs);
-	spin_lock_init(&ocelot_port->ts_id_lock);
 
 	/* Basic L2 initialization */
 
···
 	mutex_init(&ocelot->stats_lock);
 	mutex_init(&ocelot->ptp_lock);
 	spin_lock_init(&ocelot->ptp_clock_lock);
+	spin_lock_init(&ocelot->ts_id_lock);
 	snprintf(queue_name, sizeof(queue_name), "%s-stats",
 		 dev_name(ocelot->dev));
 	ocelot->stats_queue = create_singlethread_workqueue(queue_name);

include/soc/mscc/ocelot.h

···
 	/* The VLAN ID that will be transmitted as untagged, on egress */
 	struct ocelot_vlan		native_vlan;
 
+	unsigned int			ptp_skbs_in_flight;
 	u8				ptp_cmd;
 	struct sk_buff_head		tx_skbs;
 	u8				ts_id;
-	spinlock_t			ts_id_lock;
 
 	phy_interface_t			phy_mode;
 
···
 	struct ptp_clock		*ptp_clock;
 	struct ptp_clock_info		ptp_info;
 	struct hwtstamp_config		hwtstamp_config;
+	unsigned int			ptp_skbs_in_flight;
+	/* Protects the 2-step TX timestamp ID logic */
+	spinlock_t			ts_id_lock;
 	/* Protects the PTP interface state */
 	struct mutex			ptp_lock;
 	/* Protects the PTP clock */

include/soc/mscc/ocelot_ptp.h

···
 #include <soc/mscc/ocelot.h>
 
 #define OCELOT_MAX_PTP_ID		63
+#define OCELOT_PTP_FIFO_SIZE		128
 
 #define PTP_PIN_CFG_RSZ			0x20
 #define PTP_PIN_TOD_SEC_MSB_RSZ		PTP_PIN_CFG_RSZ