sunvnet: Avoid sending superfluous LDC messages.

When sending out a burst of packets across multiple descriptors,
it is sufficient to send one LDC "start" trigger for
the first descriptor, so do not send an LDC "start" for every
pass through vnet_start_xmit. Similarly, it is sufficient to send
one "DRING_STOPPED" trigger for the last dring (and if that
fails, hold off and send the trigger later).

Optimizations to the number of LDC messages helps avoid
filling up the LDC channel with superfluous LDC messages
that risk triggering flow-control on the channel,
and also boosts performance.
Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Raghuram Kothakota <raghuram.kothakota@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/sunvnet.c

@@ -37,6 +37,8 @@ MODULE_VERSION(DRV_MODULE_VERSION);
  */
 #define	VNET_MAX_RETRIES	10
 
+static int __vnet_tx_trigger(struct vnet_port *port, u32 start);
+
 /* Ordered from largest major to lowest */
 static struct vio_version vnet_versions[] = {
 	{ .major = 1, .minor = 0 },
@@ -283,10 +285,18 @@ static int vnet_send_ack(struct vnet_port *port, struct vio_dring_state *dr,
 				port->raddr[0], port->raddr[1],
 				port->raddr[2], port->raddr[3],
 				port->raddr[4], port->raddr[5]);
-			err = -ECONNRESET;
+			break;
 		}
 	} while (err == -EAGAIN);
 
+	if (err <= 0 && vio_dring_state == VIO_DRING_STOPPED) {
+		port->stop_rx_idx = end;
+		port->stop_rx = true;
+	} else {
+		port->stop_rx_idx = 0;
+		port->stop_rx = false;
+	}
+
 	return err;
 }
 
@@ -448,7 +458,7 @@ static int vnet_ack(struct vnet_port *port, void *msgbuf)
 	struct net_device *dev;
 	struct vnet *vp;
 	u32 end;
-
+	struct vio_net_desc *desc;
 	if (unlikely(pkt->tag.stype_env != VIO_DRING_DATA))
 		return 0;
 
@@ -456,7 +466,24 @@ static int vnet_ack(struct vnet_port *port, void *msgbuf)
 	if (unlikely(!idx_is_pending(dr, end)))
 		return 0;
 
+	/* sync for race conditions with vnet_start_xmit() and tell xmit it
+	 * is time to send a trigger.
+	 */
 	dr->cons = next_idx(end, dr);
+	desc = vio_dring_entry(dr, dr->cons);
+	if (desc->hdr.state == VIO_DESC_READY && port->start_cons) {
+		/* vnet_start_xmit() just populated this dring but missed
+		 * sending the "start" LDC message to the consumer.
+		 * Send a "start" trigger on its behalf.
+		 */
+		if (__vnet_tx_trigger(port, dr->cons) > 0)
+			port->start_cons = false;
+		else
+			port->start_cons = true;
+	} else {
+		port->start_cons = true;
+	}
+
 
 	vp = port->vp;
 	dev = vp->dev;
@@ -597,7 +624,7 @@ static void vnet_event(void *arg, int event)
 	local_irq_restore(flags);
 }
 
-static int __vnet_tx_trigger(struct vnet_port *port)
+static int __vnet_tx_trigger(struct vnet_port *port, u32 start)
 {
 	struct vio_dring_state *dr = &port->vio.drings[VIO_DRIVER_TX_RING];
 	struct vio_dring_data hdr = {
@@ -608,12 +635,21 @@ static int __vnet_tx_trigger(struct vnet_port *port)
 			.sid		= vio_send_sid(&port->vio),
 		},
 		.dring_ident		= dr->ident,
-		.start_idx		= dr->prod,
+		.start_idx		= start,
 		.end_idx		= (u32) -1,
 	};
 	int err, delay;
 	int retries = 0;
 
+	if (port->stop_rx) {
+		err = vnet_send_ack(port,
+				    &port->vio.drings[VIO_DRIVER_RX_RING],
+				    port->stop_rx_idx, -1,
+				    VIO_DRING_STOPPED);
+		if (err <= 0)
+			return err;
+	}
+
 	hdr.seq = dr->snd_nxt;
 	delay = 1;
 	do {
@@ -734,7 +770,30 @@ static int vnet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	d->hdr.state = VIO_DESC_READY;
 
-	err = __vnet_tx_trigger(port);
+	/* Exactly one ldc "start" trigger (for dr->cons) needs to be sent
+	 * to notify the consumer that some descriptors are READY.
+	 * After that "start" trigger, no additional triggers are needed until
+	 * a DRING_STOPPED is received from the consumer. The dr->cons field
+	 * (set up by vnet_ack()) has the value of the next dring index
+	 * that has not yet been ack-ed. We send a "start" trigger here
+	 * if, and only if, start_cons is true (reset it afterward). Conversely,
+	 * vnet_ack() should check if the dring corresponding to cons
+	 * is marked READY, but start_cons was false.
+	 * If so, vnet_ack() should send out the missed "start" trigger.
+	 *
+	 * Note that the wmb() above makes sure the cookies et al. are
+	 * not globally visible before the VIO_DESC_READY, and that the
+	 * stores are ordered correctly by the compiler. The consumer will
+	 * not proceed until the VIO_DESC_READY is visible assuring that
+	 * the consumer does not observe anything related to descriptors
+	 * out of order. The HV trap from the LDC start trigger is the
+	 * producer to consumer announcement that work is available to the
+	 * consumer
+	 */
+	if (!port->start_cons)
+		goto ldc_start_done; /* previous trigger suffices */
+
+	err = __vnet_tx_trigger(port, dr->cons);
 	if (unlikely(err < 0)) {
 		netdev_info(dev, "TX trigger error %d\n", err);
 		d->hdr.state = VIO_DESC_FREE;
@@ -742,6 +801,9 @@ static int vnet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		goto out_dropped_unlock;
 	}
 
+ldc_start_done:
+	port->start_cons = false;
+
 	dev->stats.tx_packets++;
 	dev->stats.tx_bytes += skb->len;
 
@@ -1035,6 +1097,7 @@ static int vnet_port_alloc_tx_bufs(struct vnet_port *port)
 			  (sizeof(struct ldc_trans_cookie) * 2));
 	dr->num_entries = VNET_TX_RING_SIZE;
 	dr->prod = dr->cons = 0;
+	port->start_cons  = true; /* need an initial trigger */
 	dr->pending = VNET_TX_RING_SIZE;
 	dr->ncookies = ncookies;
 

drivers/net/ethernet/sun/sunvnet.h

@@ -40,6 +40,10 @@ struct vnet_port {
 	struct vnet_tx_entry	tx_bufs[VNET_TX_RING_SIZE];
 
 	struct list_head	list;
+
+	u32			stop_rx_idx;
+	bool			stop_rx;
+	bool			start_cons;
 };
 
 static inline struct vnet_port *to_vnet_port(struct vio_driver_state *vio)