Commit 95162d65 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

asix: avoid copies in tx path 



I noticed excess calls to skb_copy_expand() or memmove() in asix driver.

This driver needs to push 4 bytes in front of frame (packet_len)
and maybe add 4 bytes after the end (if padlen is 4)

So it should set needed_headroom & needed_tailroom to avoid
copies. But its not enough, because many packets are cloned
before entering asix_tx_fixup() and this driver use skb_cloned()
as a lazy way to check if it can push and put additional bytes in frame.

Avoid skb_copy_expand() expensive call, using following rules :

- We are allowed to push 4 bytes in headroom if skb_header_cloned()
  is false (and if we have 4 bytes of headroom)

- We are allowed to put 4 bytes at tail if skb_cloned()
  is false (and if we have 4 bytes of tailroom)

TCP packets for example are cloned, but skb_header_release()
was called in tcp stack, allowing us to use headroom for our needs.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Allan Chou <allan@asix.com.tw>
Cc: Trond Wuellner <trond@chromium.org>
Cc: Grant Grundler <grundler@chromium.org>
Cc: Paul Stewart <pstew@chromium.org>
Cc: Ming Lei <tom.leiming@gmail.com>
Tested-by: default avatarMing Lei <ming.lei@canonical.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent cabdc8ee

drivers/net/usb/asix.c

+24 −4
Original line number Diff line number Diff line
@@ -358,14 +358,30 @@ static struct sk_buff *asix_tx_fixup(struct usbnet *dev, struct sk_buff *skb, 


	padlen = ((skb->len + 4) & (dev->maxpacket - 1)) ? 0 : 4;



	if ((!skb_cloned(skb)) &&

	    ((headroom + tailroom) >= (4 + padlen))) {

		if ((headroom < 4) || (tailroom < padlen)) {

	/* We need to push 4 bytes in front of frame (packet_len)

	 * and maybe add 4 bytes after the end (if padlen is 4)

	 *

	 * Avoid skb_copy_expand() expensive call, using following rules :

	 * - We are allowed to push 4 bytes in headroom if skb_header_cloned()

	 *   is false (and if we have 4 bytes of headroom)

	 * - We are allowed to put 4 bytes at tail if skb_cloned()

	 *   is false (and if we have 4 bytes of tailroom)

	 *

	 * TCP packets for example are cloned, but skb_header_release()

	 * was called in tcp stack, allowing us to use headroom for our needs.

	 */

	if (!skb_header_cloned(skb) &&

	    !(padlen && skb_cloned(skb)) &&

	    headroom + tailroom >= 4 + padlen) {

		/* following should not happen, but better be safe */

		if (headroom < 4 ||

		    tailroom < padlen) {

			skb->data = memmove(skb->head + 4, skb->data, skb->len);

			skb_set_tail_pointer(skb, skb->len);

		}

	} else {

		struct sk_buff *skb2;



		skb2 = skb_copy_expand(skb, 4, padlen, flags);

		dev_kfree_skb_any(skb);

		skb = skb2;
@@ -373,8 +389,8 @@ static struct sk_buff *asix_tx_fixup(struct usbnet *dev, struct sk_buff *skb, 
			return NULL;

	}



	packet_len = ((skb->len ^ 0x0000ffff) << 16) + skb->len;

	skb_push(skb, 4);

	packet_len = (((skb->len - 4) ^ 0x0000ffff) << 16) + (skb->len - 4);

	cpu_to_le32s(&packet_len);

	skb_copy_to_linear_data(skb, &packet_len, sizeof(packet_len));
@@ -880,6 +896,8 @@ static int ax88172_bind(struct usbnet *dev, struct usb_interface *intf) 


	dev->net->netdev_ops = &ax88172_netdev_ops;

	dev->net->ethtool_ops = &ax88172_ethtool_ops;

	dev->net->needed_headroom = 4; /* cf asix_tx_fixup() */

	dev->net->needed_tailroom = 4; /* cf asix_tx_fixup() */



	asix_mdio_write(dev->net, dev->mii.phy_id, MII_BMCR, BMCR_RESET);

	asix_mdio_write(dev->net, dev->mii.phy_id, MII_ADVERTISE,
@@ -1075,6 +1093,8 @@ static int ax88772_bind(struct usbnet *dev, struct usb_interface *intf) 


	dev->net->netdev_ops = &ax88772_netdev_ops;

	dev->net->ethtool_ops = &ax88772_ethtool_ops;

	dev->net->needed_headroom = 4; /* cf asix_tx_fixup() */

	dev->net->needed_tailroom = 4; /* cf asix_tx_fixup() */



	embd_phy = ((dev->mii.phy_id & 0x1f) == 0x10 ? 1 : 0);

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