static netdev_tx_t ems_usb_start_xmit(struct sk_buff *skb, struct net_device *netdev)
{
	struct ems_usb *dev = netdev_priv(netdev);
	struct ems_tx_urb_context *context = NULL;
	struct net_device_stats *stats = &netdev->stats;
	struct can_frame *cf = (struct can_frame *)skb->data;
	struct ems_cpc_msg *msg;
	struct urb *urb;
	u8 *buf;
	int i, err;
	size_t size = CPC_HEADER_SIZE + CPC_MSG_HEADER_LEN
			+ sizeof(struct cpc_can_msg);

	if (can_dropped_invalid_skb(netdev, skb))
		return NETDEV_TX_OK;

	/* create a URB, and a buffer for it, and copy the data to the URB */
	urb = usb_alloc_urb(0, GFP_ATOMIC);
	if (!urb)
		goto nomem;

	buf = usb_alloc_coherent(dev->udev, size, GFP_ATOMIC, &urb->transfer_dma);
	if (!buf) {
		netdev_err(netdev, "No memory left for USB buffer\n");
		usb_free_urb(urb);
		goto nomem;
	}

	msg = (struct ems_cpc_msg *)&buf[CPC_HEADER_SIZE];

	msg->msg.can_msg.id = cpu_to_le32(cf->can_id & CAN_ERR_MASK);
	msg->msg.can_msg.length = cf->len;

	if (cf->can_id & CAN_RTR_FLAG) {
		msg->type = cf->can_id & CAN_EFF_FLAG ?
			CPC_CMD_TYPE_EXT_RTR_FRAME : CPC_CMD_TYPE_RTR_FRAME;

		msg->length = CPC_CAN_MSG_MIN_SIZE;
	} else {
		msg->type = cf->can_id & CAN_EFF_FLAG ?
			CPC_CMD_TYPE_EXT_CAN_FRAME : CPC_CMD_TYPE_CAN_FRAME;

		for (i = 0; i < cf->len; i++)
			msg->msg.can_msg.msg[i] = cf->data[i];

		msg->length = CPC_CAN_MSG_MIN_SIZE + cf->len;
	}

	for (i = 0; i < MAX_TX_URBS; i++) {
		if (dev->tx_contexts[i].echo_index == MAX_TX_URBS) {
			context = &dev->tx_contexts[i];
			break;
		}
	}

	/*
	 * May never happen! When this happens we'd more URBs in flight as
	 * allowed (MAX_TX_URBS).
	 */
	if (!context) {
		usb_free_coherent(dev->udev, size, buf, urb->transfer_dma);
		usb_free_urb(urb);

		netdev_warn(netdev, "couldn't find free context\n");

		return NETDEV_TX_BUSY;
	}

	context->dev = dev;
	context->echo_index = i;

	usb_fill_bulk_urb(urb, dev->udev, usb_sndbulkpipe(dev->udev, 2), buf,
			  size, ems_usb_write_bulk_callback, context);
	urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;
	usb_anchor_urb(urb, &dev->tx_submitted);

	can_put_echo_skb(skb, netdev, context->echo_index, 0);

	atomic_inc(&dev->active_tx_urbs);

	err = usb_submit_urb(urb, GFP_ATOMIC);
	if (unlikely(err)) {
		can_free_echo_skb(netdev, context->echo_index, NULL);

		usb_unanchor_urb(urb);
		usb_free_coherent(dev->udev, size, buf, urb->transfer_dma);
		dev_kfree_skb(skb);

		atomic_dec(&dev->active_tx_urbs);

		if (err == -ENODEV) {
			netif_device_detach(netdev);
		} else {
			netdev_warn(netdev, "failed tx_urb %d\n", err);

			stats->tx_dropped++;
		}
	} else {
		netif_trans_update(netdev);

		/* Slow down tx path */
		if (atomic_read(&dev->active_tx_urbs) >= MAX_TX_URBS ||
		    dev->free_slots < CPC_TX_QUEUE_TRIGGER_LOW) {
			netif_stop_queue(netdev);
		}
	}

	/*
	 * Release our reference to this URB, the USB core will eventually free
	 * it entirely.
	 */
	usb_free_urb(urb);

	return NETDEV_TX_OK;

nomem:
	dev_kfree_skb(skb);
	stats->tx_dropped++;

	return NETDEV_TX_OK;
}
static netdev_tx_t ems_usb_start_xmit(struct sk_buff *skb, struct net_device *netdev)
{
	struct ems_usb *dev = netdev_priv(netdev);
	struct ems_tx_urb_context *context = NULL;
	struct net_device_stats *stats = &netdev->stats;
	struct can_frame *cf = (struct can_frame *)skb->data;
	struct ems_cpc_msg *msg;
	struct urb *urb;
	u8 *buf;
	int i, err;
	size_t size = CPC_HEADER_SIZE + CPC_MSG_HEADER_LEN
			+ sizeof(struct cpc_can_msg);

	if (can_dropped_invalid_skb(netdev, skb))
		return NETDEV_TX_OK;

	 
	urb = usb_alloc_urb(0, GFP_ATOMIC);
	if (!urb)
		goto nomem;

	buf = usb_alloc_coherent(dev->udev, size, GFP_ATOMIC, &urb->transfer_dma);
	if (!buf) {
		netdev_err(netdev, "No memory left for USB buffer\n");
		usb_free_urb(urb);
		goto nomem;
	}

	msg = (struct ems_cpc_msg *)&buf[CPC_HEADER_SIZE];

	msg->msg.can_msg.id = cpu_to_le32(cf->can_id & CAN_ERR_MASK);
	msg->msg.can_msg.length = cf->len;

	if (cf->can_id & CAN_RTR_FLAG) {
		msg->type = cf->can_id & CAN_EFF_FLAG ?
			CPC_CMD_TYPE_EXT_RTR_FRAME : CPC_CMD_TYPE_RTR_FRAME;

		msg->length = CPC_CAN_MSG_MIN_SIZE;
	} else {
		msg->type = cf->can_id & CAN_EFF_FLAG ?
			CPC_CMD_TYPE_EXT_CAN_FRAME : CPC_CMD_TYPE_CAN_FRAME;

		for (i = 0; i < cf->len; i++)
			msg->msg.can_msg.msg[i] = cf->data[i];

		msg->length = CPC_CAN_MSG_MIN_SIZE + cf->len;
	}

	for (i = 0; i < MAX_TX_URBS; i++) {
		if (dev->tx_contexts[i].echo_index == MAX_TX_URBS) {
			context = &dev->tx_contexts[i];
			break;
		}
	}

	 
	if (!context) {
		usb_free_coherent(dev->udev, size, buf, urb->transfer_dma);
		usb_free_urb(urb);

		netdev_warn(netdev, "couldn't find free context\n");

		return NETDEV_TX_BUSY;
	}

	context->dev = dev;
	context->echo_index = i;

	usb_fill_bulk_urb(urb, dev->udev, usb_sndbulkpipe(dev->udev, 2), buf,
			  size, ems_usb_write_bulk_callback, context);
	urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;
	usb_anchor_urb(urb, &dev->tx_submitted);

	can_put_echo_skb(skb, netdev, context->echo_index, 0);

	atomic_inc(&dev->active_tx_urbs);

	err = usb_submit_urb(urb, GFP_ATOMIC);
	if (unlikely(err)) {
		can_free_echo_skb(netdev, context->echo_index, NULL);

		usb_unanchor_urb(urb);
		usb_free_coherent(dev->udev, size, buf, urb->transfer_dma);
		dev_kfree_skb(skb);

		atomic_dec(&dev->active_tx_urbs);

		if (err == -ENODEV) {
			netif_device_detach(netdev);
		} else {
			netdev_warn(netdev, "failed tx_urb %d\n", err);

			stats->tx_dropped++;
		}
	} else {
		netif_trans_update(netdev);

		 
		if (atomic_read(&dev->active_tx_urbs) >= MAX_TX_URBS ||
		    dev->free_slots < CPC_TX_QUEUE_TRIGGER_LOW) {
			netif_stop_queue(netdev);
		}
	}

	 
	usb_free_urb(urb);

	return NETDEV_TX_OK;

nomem:
	dev_kfree_skb(skb);
	stats->tx_dropped++;

	return NETDEV_TX_OK;
}