virtio: use an hrtimer for tx coalescing.

Not sure why hrtimer's cb_mode only exists with CONFIG_HIGH_RES_TIMERS; that
seems like a bug to me.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r 2a94425ac7d5 drivers/net/virtio_net.c
--- a/drivers/net/virtio_net.c	Thu Nov 15 13:47:28 2007 +1100
+++ b/drivers/net/virtio_net.c	Thu Nov 15 13:54:59 2007 +1100
@@ -23,6 +23,7 @@
 #include <linux/virtio.h>
 #include <linux/virtio_net.h>
 #include <linux/scatterlist.h>
+#include <linux/hrtimer.h>
 
 /* FIXME: MTU in config. */
 #define MAX_PACKET_LEN (ETH_HLEN+ETH_DATA_LEN)
@@ -34,8 +35,14 @@ struct virtnet_info
 	struct net_device *dev;
 	struct napi_struct napi;
 
+	/* TX coalescing. */
+	struct hrtimer tx_timer;
+
 	/* Number of input buffers, and max we've ever had. */
 	unsigned int num, max;
+
+	/* Number of queued output buffers, and max we've ever had. */
+	unsigned int out_num, out_max;
 
 	/* Receive & send queues. */
 	struct sk_buff_head recv;
@@ -223,6 +230,20 @@ static void free_old_xmit_skbs(struct vi
 	}
 }
 
+static enum hrtimer_restart kick_xmit(struct hrtimer *t)
+{
+	struct virtnet_info *vi = container_of(t,struct virtnet_info,tx_timer);
+
+	BUG_ON(!in_softirq());
+	BUG_ON(in_irq());
+	netif_tx_lock(vi->dev);
+	vi->svq->vq_ops->kick(vi->svq);
+	vi->out_num = 0;
+	netif_tx_unlock(vi->dev);
+
+	return HRTIMER_NORESTART;
+}
+
 static int start_xmit(struct sk_buff *skb, struct net_device *dev)
 {
 	struct virtnet_info *vi = netdev_priv(dev);
@@ -273,11 +294,25 @@ static int start_xmit(struct sk_buff *sk
 	if (err) {
 		pr_debug("%s: virtio not prepared to send\n", dev->name);
 		skb_unlink(skb, &vi->send);
+		if (vi->out_max != vi->out_num)
+			printk("%s: out_max changed from %u to %u\n",
+			       dev->name, vi->out_max, vi->out_num);
+		vi->out_max = vi->out_num;
+		vi->out_num = 0;
+		/* Kick off send immediately. */
+		hrtimer_cancel(&vi->tx_timer);
+		vi->svq->vq_ops->kick(vi->svq);
 		netif_stop_queue(dev);
 		return NETDEV_TX_BUSY;
 	}
-	vi->svq->vq_ops->kick(vi->svq);
 
+	if (++vi->out_num == vi->out_max) {
+		hrtimer_cancel(&vi->tx_timer);
+		vi->svq->vq_ops->kick(vi->svq);
+		vi->out_num = 0;
+	} else
+		hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
+			      HRTIMER_MODE_REL);
 	return 0;
 }
 
@@ -363,6 +398,10 @@ static int virtnet_probe(struct virtio_d
 	netif_napi_add(dev, &vi->napi, virtnet_poll, 16);
 	vi->dev = dev;
 	vi->vdev = vdev;
+	hrtimer_init(&vi->tx_timer, CLOCK_REALTIME, HRTIMER_MODE_REL);
+	vi->tx_timer.function = kick_xmit;
+	vi->tx_timer.cb_mode = HRTIMER_CB_SOFTIRQ;
+	vi->out_max = -1U;
 
 	/* We expect two virtqueues, receive then send. */
 	vi->rvq = vdev->config->find_vq(vdev, 0, skb_recv_done);