usb: gadget: ether: Allow jumbo frames

USB network adapters support Jumbo frames. The only thing blocking
that feature is the code in the gadget driver that disposes of
packets larger than 1518 bytes, and the limit on the ioctl to set
the mtu.

This patch relaxes these limits, and allows up to 15k frames sizes.
The 15k value was chosen because 16k does not work on all platforms,
and usingclose to 16k will result in allocating 5 or 8 4k pages to
store the skb, wasting pages at no measurable performance gain.

On a topic-miami board (Zynq-7000), iperf3 performance reports:
MTU= 1500, PC-to-gadget: 139 Mbps, Gadget-to-PC: 116 Mbps
MTU=15000, PC-to-gadget: 239 Mbps, Gadget-to-PC: 361 Mbps

On boards with slower CPUs the performance improvement will be
relatively much larger, e.g. an OMAP-L138 increased from 40 to
220 Mbps using a similar patch on an  2.6.37 kernel.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/function/u_ether.c

@@ -48,6 +48,11 @@
 
 #define UETH__VERSION	"29-May-2008"
 
+/* Experiments show that both Linux and Windows hosts allow up to 16k
+ * frame sizes. Set the max size to 15k+52 to prevent allocating 32k
+ * blocks and still have efficient handling. */
+#define GETHER_MAX_ETH_FRAME_LEN 15412
+
 struct eth_dev {
 	/* lock is held while accessing port_usb
 	 */
@@ -146,7 +151,7 @@ static int ueth_change_mtu(struct net_device *net, int new_mtu)
 	spin_lock_irqsave(&dev->lock, flags);
 	if (dev->port_usb)
 		status = -EBUSY;
-	else if (new_mtu <= ETH_HLEN || new_mtu > ETH_FRAME_LEN)
+	else if (new_mtu <= ETH_HLEN || new_mtu > GETHER_MAX_ETH_FRAME_LEN)
 		status = -ERANGE;
 	else
 		net->mtu = new_mtu;
@@ -294,7 +299,7 @@ static void rx_complete(struct usb_ep *ep, struct usb_request *req)
 		while (skb2) {
 			if (status < 0
 					|| ETH_HLEN > skb2->len
-					|| skb2->len > VLAN_ETH_FRAME_LEN) {
+					|| skb2->len > GETHER_MAX_ETH_FRAME_LEN) {
 				dev->net->stats.rx_errors++;
 				dev->net->stats.rx_length_errors++;
 				DBG(dev, "rx length %d\n", skb2->len);