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>

1 files changed, 43 insertions, 20 deletions

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index fc7eeaa1f1b6..e44116452b7b 100644
--- 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