USBNET: support DMA SG

This patch introduces support of DMA SG if the USB host controller
which usbnet device is attached to is capable of building packet from
discontinuous buffers.

The patch supports passing the skb fragment buffers to usb stack directly
via urb->sg.

Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: Grant Grundler <grundler@google.com>
Cc: Freddy Xin <freddy@asix.com.tw>
Cc: Alan Stern <stern@rowland.harvard.edu>
Acked-by: NOliver Neukum <oneukum@suse.de>
Signed-off-by: NMing Lei <ming.lei@canonical.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/net/usb/usbnet.c

@@ -1197,6 +1197,37 @@ EXPORT_SYMBOL_GPL(usbnet_tx_timeout);
 
 /*-------------------------------------------------------------------------*/
 
+static int build_dma_sg(const struct sk_buff *skb, struct urb *urb)
+{
+	unsigned num_sgs, total_len = 0;
+	int i, s = 0;
+
+	num_sgs = skb_shinfo(skb)->nr_frags + 1;
+	if (num_sgs == 1)
+		return 0;
+
+	urb->sg = kmalloc(num_sgs * sizeof(struct scatterlist), GFP_ATOMIC);
+	if (!urb->sg)
+		return -ENOMEM;
+
+	urb->num_sgs = num_sgs;
+	sg_init_table(urb->sg, urb->num_sgs);
+
+	sg_set_buf(&urb->sg[s++], skb->data, skb_headlen(skb));
+	total_len += skb_headlen(skb);
+
+	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
+		struct skb_frag_struct *f = &skb_shinfo(skb)->frags[i];
+
+		total_len += skb_frag_size(f);
+		sg_set_page(&urb->sg[i + s], f->page.p, f->size,
+				f->page_offset);
+	}
+	urb->transfer_buffer_length = total_len;
+
+	return 1;
+}
+
 netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
 				     struct net_device *net)
 {
@@ -1223,7 +1254,6 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
 			goto drop;
 		}
 	}
-	length = skb->len;
 
 	if (!(urb = usb_alloc_urb (0, GFP_ATOMIC))) {
 		netif_dbg(dev, tx_err, dev->net, "no urb\n");
@@ -1233,10 +1263,14 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
 	entry = (struct skb_data *) skb->cb;
 	entry->urb = urb;
 	entry->dev = dev;
-	entry->length = length;
 
 	usb_fill_bulk_urb (urb, dev->udev, dev->out,
 			skb->data, skb->len, tx_complete, skb);
+	if (dev->can_dma_sg) {
+		if (build_dma_sg(skb, urb) < 0)
+			goto drop;
+	}
+	entry->length = length = urb->transfer_buffer_length;
 
 	/* don't assume the hardware handles USB_ZERO_PACKET
 	 * NOTE:  strictly conforming cdc-ether devices should expect
@@ -1305,7 +1339,10 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
 not_drop:
 		if (skb)
 			dev_kfree_skb_any (skb);
-		usb_free_urb (urb);
+		if (urb) {
+			kfree(urb->sg);
+			usb_free_urb(urb);
+		}
 	} else
 		netif_dbg(dev, tx_queued, dev->net,
 			  "> tx, len %d, type 0x%x\n", length, skb->protocol);
@@ -1356,6 +1393,7 @@ static void usbnet_bh (unsigned long param)
 			rx_process (dev, skb);
 			continue;
 		case tx_done:
+			kfree(entry->urb->sg);
 		case rx_cleanup:
 			usb_free_urb (entry->urb);
 			dev_kfree_skb (skb);
@@ -1689,6 +1727,7 @@ int usbnet_resume (struct usb_interface *intf)
 			retval = usb_submit_urb(res, GFP_ATOMIC);
 			if (retval < 0) {
 				dev_kfree_skb_any(skb);
+				kfree(res->sg);
 				usb_free_urb(res);
 				usb_autopm_put_interface_async(dev->intf);
 			} else {

include/linux/usb/usbnet.h

@@ -34,6 +34,7 @@ struct usbnet {
 	struct mutex		phy_mutex;
 	unsigned char		suspend_count;
 	unsigned char		pkt_cnt, pkt_err;
+	unsigned		can_dma_sg:1;
 
 	/* i/o info: pipes etc */
 	unsigned		in, out;