virtio: wean net driver off NETDEV_TX_BUSY

Herbert tells me that returning NETDEV_TX_BUSY from hard_start_xmit is
seen as a poor thing to do; we should cache the packet and stop the queue.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index fc7eeaa..e441164 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -41,6 +41,9 @@ struct virtnet_info
        struct net_device *dev;
        struct napi_struct napi;
 
+       /* The skb we couldn't send because buffers were full. */
+       struct sk_buff *last_xmit_skb;
+
        /* Number of input buffers, and max we've ever had. */
        unsigned int num, max;
 
@@ -227,17 +230,16 @@ static void free_old_xmit_skbs(struct virtnet_info *vi)
        }
 }
 
-static int start_xmit(struct sk_buff *skb, struct net_device *dev)
+static int xmit_skb(struct virtnet_info *vi, struct sk_buff *skb)
 {
-       struct virtnet_info *vi = netdev_priv(dev);
-       int num, err;
+       int num;
        struct scatterlist sg[2+MAX_SKB_FRAGS];
        struct virtio_net_hdr *hdr;
        const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
 
        sg_init_table(sg, 2+MAX_SKB_FRAGS);
 
-       pr_debug("%s: xmit %p " MAC_FMT "\n", dev->name, skb,
+       pr_debug("%s: xmit %p " MAC_FMT "\n", vi->dev->name, skb,
                 dest[0], dest[1], dest[2],
                 dest[3], dest[4], dest[5]);
 
@@ -272,30 +274,51 @@ static int start_xmit(struct sk_buff *skb, struct net_device *dev)
 
        vnet_hdr_to_sg(sg, skb);
        num = skb_to_sgvec(skb, sg+1, 0, skb->len) + 1;
-       __skb_queue_head(&vi->send, skb);
+
+       return vi->svq->vq_ops->add_buf(vi->svq, sg, num, 0, skb);
+}
+
+static int start_xmit(struct sk_buff *skb, struct net_device *dev)
+{
+       struct virtnet_info *vi = netdev_priv(dev);
 
 again:
        /* Free up any pending old buffers before queueing new ones. */
        free_old_xmit_skbs(vi);
-       err = vi->svq->vq_ops->add_buf(vi->svq, sg, num, 0, skb);
-       if (err) {
-               pr_debug("%s: virtio not prepared to send\n", dev->name);
-               netif_stop_queue(dev);
-
-               /* Activate callback for using skbs: if this returns false it
-                * means some were used in the meantime. */
-               if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq))) {
-                       vi->svq->vq_ops->disable_cb(vi->svq);
-                       netif_start_queue(dev);
-                       goto again;
+
+       /* If we has a buffer left over from last time, send it now. */
+       if (vi->last_xmit_skb) {
+               if (xmit_skb(vi, vi->last_xmit_skb) != 0) {
+                       /* Drop this skb: we only queue one. */
+                       vi->dev->stats.tx_dropped++;
+                       kfree_skb(skb);
+                       goto stop_queue;
                }
-               __skb_unlink(skb, &vi->send);
+               vi->last_xmit_skb = NULL;
+       }
 
-               return NETDEV_TX_BUSY;
+       /* Put new one in send queue and do transmit */
+       __skb_queue_head(&vi->send, skb);
+       if (xmit_skb(vi, skb) != 0) {
+               vi->last_xmit_skb = skb;
+               goto stop_queue;
        }
+done:
        vi->svq->vq_ops->kick(vi->svq);
-
-       return 0;
+       return NETDEV_TX_OK;
+
+stop_queue:
+       pr_debug("%s: virtio not prepared to send\n", dev->name);
+       netif_stop_queue(dev);
+
+       /* Activate callback for using skbs: if this returns false it
+        * means some were used in the meantime. */
+       if (unlikely(!vi->svq->vq_ops->enable_cb(vi->svq))) {
+               vi->svq->vq_ops->disable_cb(vi->svq);
+               netif_start_queue(dev);
+               goto again;
+       }
+       goto done;
 }
 
 #ifdef CONFIG_NET_POLL_CONTROLLER