usb: gadget: NCM: Add transmit multi-frame.

This adds multi-frame support to the NCM NTB's for
the gadget driver. This allows multiple network
packets to be put inside a single USB NTB with a
maximum size of 16kB.

It has a time out of 300ms to ensure that smaller
number of packets still maintain a normal latency.

Also the .fp_index and .next_fp_index have been
changed to .ndp_index and .next_ndp_index to
match the latest CDC-NCM specification and
help with maintenance.

Results transmitting from gadget to host.

Before the change:

TCP_STREAM Throughput (10^6bits/sec): 22.72
UDP_STREAM Throughput (10^6bits/sec): 25.94

Latency:
netperf -H 192.168.1.101 -v2 -l 50 -t TCP_RR -- -r 16384,16384
Trans.   RoundTrip  Throughput
Rate     Latency    10^6bits/s
per sec  usec/Tran  Outbound

100.83   9918.116   13.215

After the change:

TCP_STREAM Throughput (10^6bits/sec): 124.26
UDP_STREAM Throughput (10^6bits/sec): 227.48

Latency:
netperf -H 192.168.1.101 -v2 -l 50 -t TCP_RR -- -r 16384,16384
Trans.   RoundTrip  Throughput
Rate     Latency    10^6bits/s
per sec  usec/Tran  Outbound

156.80   6377.730   20.552
Signed-off-by: Jim Baxter <jim_baxter@mentor.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/u_ether.c

@@ -483,7 +483,7 @@ static netdev_tx_t eth_start_xmit(struct sk_buff *skb,
 					struct net_device *net)
 {
 	struct eth_dev		*dev = netdev_priv(net);
-	int			length = skb->len;
+	int			length = 0;
 	int			retval;
 	struct usb_request	*req = NULL;
 	unsigned long		flags;
@@ -500,13 +500,13 @@ static netdev_tx_t eth_start_xmit(struct sk_buff *skb,
 	}
 	spin_unlock_irqrestore(&dev->lock, flags);
 
-	if (!in) {
+	if (skb && !in) {
 		dev_kfree_skb_any(skb);
 		return NETDEV_TX_OK;
 	}
 
 	/* apply outgoing CDC or RNDIS filters */
-	if (!is_promisc(cdc_filter)) {
+	if (skb && !is_promisc(cdc_filter)) {
 		u8		*dest = skb->data;
 
 		if (is_multicast_ether_addr(dest)) {
@@ -557,11 +557,17 @@ static netdev_tx_t eth_start_xmit(struct sk_buff *skb,
 		if (dev->port_usb)
 			skb = dev->wrap(dev->port_usb, skb);
 		spin_unlock_irqrestore(&dev->lock, flags);
-		if (!skb)
+		if (!skb) {
+			/* Multi frame CDC protocols may store the frame for
+			 * later which is not a dropped frame.
+			 */
+			if (dev->port_usb->supports_multi_frame)
+				goto multiframe;
 			goto drop;
-
-		length = skb->len;
+		}
 	}
+
+	length = skb->len;
 	req->buf = skb->data;
 	req->context = skb;
 	req->complete = tx_complete;
@@ -604,6 +610,7 @@ static netdev_tx_t eth_start_xmit(struct sk_buff *skb,
 		dev_kfree_skb_any(skb);
 drop:
 		dev->net->stats.tx_dropped++;
+multiframe:
 		spin_lock_irqsave(&dev->req_lock, flags);
 		if (list_empty(&dev->tx_reqs))
 			netif_start_queue(net);

drivers/usb/gadget/u_ether.h

@@ -18,6 +18,7 @@
 #include <linux/if_ether.h>
 #include <linux/usb/composite.h>
 #include <linux/usb/cdc.h>
+#include <linux/netdevice.h>
 
 #include "gadget_chips.h"
 
@@ -74,6 +75,7 @@ struct gether {
 	bool				is_fixed;
 	u32				fixed_out_len;
 	u32				fixed_in_len;
+	bool				supports_multi_frame;
 	struct sk_buff			*(*wrap)(struct gether *port,
 						struct sk_buff *skb);
 	int				(*unwrap)(struct gether *port,