drivers/net/mctp/mctp-serial.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * Management Component Transport Protocol (MCTP) - serial transport
  * binding. This driver is an implementation of the DMTF specificiation
  * "DSP0253 - Management Component Transport Protocol (MCTP) Serial Transport
  * Binding", available at:
  *
  *  https://www.dmtf.org/sites/default/files/standards/documents/DSP0253_1.0.0.pdf
  *
  * This driver provides DSP0253-type MCTP-over-serial transport using a Linux
  * tty device, by setting the N_MCTP line discipline on the tty.
  *
  * Copyright (c) 2021 Code Construct
  */

 #include <linux/idr.h>
 #include <linux/if_arp.h>
 #include <linux/module.h>
 #include <linux/skbuff.h>
 #include <linux/tty.h>
 #include <linux/workqueue.h>
 #include <linux/crc-ccitt.h>

 #include <linux/mctp.h>
 #include <net/mctp.h>
 #include <net/pkt_sched.h>

 #define MCTP_SERIAL_MTU		68 /* base mtu (64) + mctp header */
 #define MCTP_SERIAL_FRAME_MTU	(MCTP_SERIAL_MTU + 6) /* + serial framing */

 #define MCTP_SERIAL_VERSION	0x1 /* DSP0253 defines a single version: 1 */

 #define BUFSIZE			MCTP_SERIAL_FRAME_MTU

 #define BYTE_FRAME		0x7e
 #define BYTE_ESC		0x7d

 #define FCS_INIT		0xffff

 static DEFINE_IDA(mctp_serial_ida);

 enum mctp_serial_state {
 	STATE_IDLE,
 	STATE_START,
 	STATE_HEADER,
 	STATE_DATA,
 	STATE_ESCAPE,
 	STATE_TRAILER,
 	STATE_DONE,
 	STATE_ERR,
 };

 struct mctp_serial {
 	struct net_device	*netdev;
 	struct tty_struct	*tty;

 	int			idx;

 	/* protects our rx & tx state machines; held during both paths */
 	spinlock_t		lock;

 	struct work_struct	tx_work;
 	enum mctp_serial_state	txstate, rxstate;
 	u16			txfcs, rxfcs, rxfcs_rcvd;
 	unsigned int		txlen, rxlen;
 	unsigned int		txpos, rxpos;
 	unsigned char		txbuf[BUFSIZE],
 				rxbuf[BUFSIZE];
 };

 static bool needs_escape(unsigned char c)
 {
 	return c == BYTE_ESC || c == BYTE_FRAME;
 }

 static int next_chunk_len(struct mctp_serial *dev)
 {
 	int i;

 	/* either we have no bytes to send ... */
 	if (dev->txpos == dev->txlen)
 		return 0;

 	/* ... or the next byte to send is an escaped byte; requiring a
 	 * single-byte chunk...
 	 */
 	if (needs_escape(dev->txbuf[dev->txpos]))
 		return 1;

 	/* ... or we have one or more bytes up to the next escape - this chunk
 	 * will be those non-escaped bytes, and does not include the escaped
 	 * byte.
 	 */
 	for (i = 1; i + dev->txpos + 1 < dev->txlen; i++) {
 		if (needs_escape(dev->txbuf[dev->txpos + i + 1]))
 			break;
 	}

 	return i;
 }

 static int write_chunk(struct mctp_serial *dev, unsigned char *buf, int len)
 {
 	return dev->tty->ops->write(dev->tty, buf, len);
 }

 static void mctp_serial_tx_work(struct work_struct *work)
 {
 	struct mctp_serial *dev = container_of(work, struct mctp_serial,
 					       tx_work);
 	unsigned char c, buf[3];
 	unsigned long flags;
 	int len, txlen;

 	spin_lock_irqsave(&dev->lock, flags);

 	/* txstate represents the next thing to send */
 	switch (dev->txstate) {
 	case STATE_START:
 		dev->txpos = 0;
 		fallthrough;
 	case STATE_HEADER:
 		buf[0] = BYTE_FRAME;
 		buf[1] = MCTP_SERIAL_VERSION;
 		buf[2] = dev->txlen;

 		if (!dev->txpos)
 			dev->txfcs = crc_ccitt(FCS_INIT, buf + 1, 2);

 		txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
 		if (txlen <= 0) {
 			dev->txstate = STATE_ERR;
 		} else {
 			dev->txpos += txlen;
 			if (dev->txpos == 3) {
 				dev->txstate = STATE_DATA;
 				dev->txpos = 0;
 			}
 		}
 		break;

 	case STATE_ESCAPE:
 		buf[0] = dev->txbuf[dev->txpos] & ~0x20;
 		txlen = write_chunk(dev, buf, 1);
 		if (txlen <= 0) {
 			dev->txstate = STATE_ERR;
 		} else {
 			dev->txpos += txlen;
 			if (dev->txpos == dev->txlen) {
 				dev->txstate = STATE_TRAILER;
 				dev->txpos = 0;
 			}
 		}

 		break;

 	case STATE_DATA:
 		len = next_chunk_len(dev);
 		if (len) {
 			c = dev->txbuf[dev->txpos];
 			if (len == 1 && needs_escape(c)) {
 				buf[0] = BYTE_ESC;
 				buf[1] = c & ~0x20;
 				dev->txfcs = crc_ccitt_byte(dev->txfcs, c);
 				txlen = write_chunk(dev, buf, 2);
 				if (txlen == 2)
 					dev->txpos++;
 				else if (txlen == 1)
 					dev->txstate = STATE_ESCAPE;
 				else
 					dev->txstate = STATE_ERR;
 			} else {
 				txlen = write_chunk(dev,
 						    dev->txbuf + dev->txpos,
 						    len);
 				if (txlen <= 0) {
 					dev->txstate = STATE_ERR;
 				} else {
 					dev->txfcs = crc_ccitt(dev->txfcs,
 							       dev->txbuf +
 							       dev->txpos,
 							       txlen);
 					dev->txpos += txlen;
 				}
 			}
 			if (dev->txstate == STATE_DATA &&
 			    dev->txpos == dev->txlen) {
 				dev->txstate = STATE_TRAILER;
 				dev->txpos = 0;
 			}
 			break;
 		}
 		dev->txstate = STATE_TRAILER;
 		dev->txpos = 0;
 		fallthrough;

 	case STATE_TRAILER:
 		buf[0] = dev->txfcs >> 8;
 		buf[1] = dev->txfcs & 0xff;
 		buf[2] = BYTE_FRAME;
 		txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
 		if (txlen <= 0) {
 			dev->txstate = STATE_ERR;
 		} else {
 			dev->txpos += txlen;
 			if (dev->txpos == 3) {
 				dev->txstate = STATE_DONE;
 				dev->txpos = 0;
 			}
 		}
 		break;
 	default:
 		netdev_err_once(dev->netdev, "invalid tx state %d\n",
 				dev->txstate);
 	}

 	if (dev->txstate == STATE_DONE) {
 		dev->netdev->stats.tx_packets++;
 		dev->netdev->stats.tx_bytes += dev->txlen;
 		dev->txlen = 0;
 		dev->txpos = 0;
 		clear_bit(TTY_DO_WRITE_WAKEUP, &dev->tty->flags);
 		dev->txstate = STATE_IDLE;
 		spin_unlock_irqrestore(&dev->lock, flags);

 		netif_wake_queue(dev->netdev);
 	} else {
 		spin_unlock_irqrestore(&dev->lock, flags);
 	}
 }

 static netdev_tx_t mctp_serial_tx(struct sk_buff *skb, struct net_device *ndev)
 {
 	struct mctp_serial *dev = netdev_priv(ndev);
 	unsigned long flags;

 	WARN_ON(dev->txstate != STATE_IDLE);

 	if (skb->len > MCTP_SERIAL_MTU) {
 		dev->netdev->stats.tx_dropped++;
 		goto out;
 	}

 	spin_lock_irqsave(&dev->lock, flags);
 	netif_stop_queue(dev->netdev);
 	skb_copy_bits(skb, 0, dev->txbuf, skb->len);
 	dev->txpos = 0;
 	dev->txlen = skb->len;
 	dev->txstate = STATE_START;
 	spin_unlock_irqrestore(&dev->lock, flags);

 	set_bit(TTY_DO_WRITE_WAKEUP, &dev->tty->flags);
 	schedule_work(&dev->tx_work);

 out:
 	kfree_skb(skb);
 	return NETDEV_TX_OK;
 }

 static void mctp_serial_tty_write_wakeup(struct tty_struct *tty)
 {
 	struct mctp_serial *dev = tty->disc_data;

 	schedule_work(&dev->tx_work);
 }

 static void mctp_serial_rx(struct mctp_serial *dev)
 {
 	struct mctp_skb_cb *cb;
 	struct sk_buff *skb;

 	if (dev->rxfcs != dev->rxfcs_rcvd) {
 		dev->netdev->stats.rx_dropped++;
 		dev->netdev->stats.rx_crc_errors++;
 		return;
 	}

 	skb = netdev_alloc_skb(dev->netdev, dev->rxlen);
 	if (!skb) {
 		dev->netdev->stats.rx_dropped++;
 		return;
 	}

 	skb->protocol = htons(ETH_P_MCTP);
 	skb_put_data(skb, dev->rxbuf, dev->rxlen);
 	skb_reset_network_header(skb);

 	cb = __mctp_cb(skb);
 	cb->halen = 0;

 	netif_rx(skb);
 	dev->netdev->stats.rx_packets++;
 	dev->netdev->stats.rx_bytes += dev->rxlen;
 }

 static void mctp_serial_push_header(struct mctp_serial *dev, unsigned char c)
 {
 	switch (dev->rxpos) {
 	case 0:
 		if (c == BYTE_FRAME)
 			dev->rxpos++;
 		else
 			dev->rxstate = STATE_ERR;
 		break;
 	case 1:
 		if (c == MCTP_SERIAL_VERSION) {
 			dev->rxpos++;
 			dev->rxfcs = crc_ccitt_byte(FCS_INIT, c);
 		} else {
 			dev->rxstate = STATE_ERR;
 		}
 		break;
 	case 2:
 		if (c > MCTP_SERIAL_FRAME_MTU) {
 			dev->rxstate = STATE_ERR;
 		} else {
 			dev->rxlen = c;
 			dev->rxpos = 0;
 			dev->rxstate = STATE_DATA;
 			dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
 		}
 		break;
 	}
 }

 static void mctp_serial_push_trailer(struct mctp_serial *dev, unsigned char c)
 {
 	switch (dev->rxpos) {
 	case 0:
 		dev->rxfcs_rcvd = c << 8;
 		dev->rxpos++;
 		break;
 	case 1:
 		dev->rxfcs_rcvd |= c;
 		dev->rxpos++;
 		break;
 	case 2:
 		if (c != BYTE_FRAME) {
 			dev->rxstate = STATE_ERR;
 		} else {
 			mctp_serial_rx(dev);
 			dev->rxlen = 0;
 			dev->rxpos = 0;
 			dev->rxstate = STATE_IDLE;
 		}
 		break;
 	}
 }

 static void mctp_serial_push(struct mctp_serial *dev, unsigned char c)
 {
 	switch (dev->rxstate) {
 	case STATE_IDLE:
 		dev->rxstate = STATE_HEADER;
 		fallthrough;
 	case STATE_HEADER:
 		mctp_serial_push_header(dev, c);
 		break;

 	case STATE_ESCAPE:
 		c |= 0x20;
 		fallthrough;
 	case STATE_DATA:
 		if (dev->rxstate != STATE_ESCAPE && c == BYTE_ESC) {
 			dev->rxstate = STATE_ESCAPE;
 		} else {
 			dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
 			dev->rxbuf[dev->rxpos] = c;
 			dev->rxpos++;
 			dev->rxstate = STATE_DATA;
 			if (dev->rxpos == dev->rxlen) {
 				dev->rxpos = 0;
 				dev->rxstate = STATE_TRAILER;
 			}
 		}
 		break;

 	case STATE_TRAILER:
 		mctp_serial_push_trailer(dev, c);
 		break;

 	case STATE_ERR:
 		if (c == BYTE_FRAME)
 			dev->rxstate = STATE_IDLE;
 		break;

 	default:
 		netdev_err_once(dev->netdev, "invalid rx state %d\n",
 				dev->rxstate);
 	}
 }

 static void mctp_serial_tty_receive_buf(struct tty_struct *tty, const u8 *c,
 					const u8 *f, size_t len)
 {
 	struct mctp_serial *dev = tty->disc_data;
 	int i;

 	if (!netif_running(dev->netdev))
 		return;

 	/* we don't (currently) use the flag bytes, just data. */
 	for (i = 0; i < len; i++)
 		mctp_serial_push(dev, c[i]);
 }

 static void mctp_serial_uninit(struct net_device *ndev)
 {
 	struct mctp_serial *dev = netdev_priv(ndev);

 	cancel_work_sync(&dev->tx_work);
 }

 static const struct net_device_ops mctp_serial_netdev_ops = {
 	.ndo_start_xmit = mctp_serial_tx,
 	.ndo_uninit = mctp_serial_uninit,
 };

 static void mctp_serial_setup(struct net_device *ndev)
 {
 	ndev->type = ARPHRD_MCTP;

 	/* we limit at the fixed MTU, which is also the MCTP-standard
 	 * baseline MTU, so is also our minimum
 	 */
 	ndev->mtu = MCTP_SERIAL_MTU;
 	ndev->max_mtu = MCTP_SERIAL_MTU;
 	ndev->min_mtu = MCTP_SERIAL_MTU;

 	ndev->hard_header_len = 0;
 	ndev->addr_len = 0;
 	ndev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;
 	ndev->flags = IFF_NOARP;
 	ndev->netdev_ops = &mctp_serial_netdev_ops;
 	ndev->needs_free_netdev = true;
 }

 static int mctp_serial_open(struct tty_struct *tty)
 {
 	struct mctp_serial *dev;
 	struct net_device *ndev;
 	char name[32];
 	int idx, rc;

 	if (!capable(CAP_NET_ADMIN))
 		return -EPERM;

 	if (!tty->ops->write)
 		return -EOPNOTSUPP;

 	idx = ida_alloc(&mctp_serial_ida, GFP_KERNEL);
 	if (idx < 0)
 		return idx;

 	snprintf(name, sizeof(name), "mctpserial%d", idx);
 	ndev = alloc_netdev(sizeof(*dev), name, NET_NAME_ENUM,
 			    mctp_serial_setup);
 	if (!ndev) {
 		rc = -ENOMEM;
 		goto free_ida;
 	}

 	dev = netdev_priv(ndev);
 	dev->idx = idx;
 	dev->tty = tty;
 	dev->netdev = ndev;
 	dev->txstate = STATE_IDLE;
 	dev->rxstate = STATE_IDLE;
 	spin_lock_init(&dev->lock);
 	INIT_WORK(&dev->tx_work, mctp_serial_tx_work);

 	rc = register_netdev(ndev);
 	if (rc)
 		goto free_netdev;

 	tty->receive_room = 64 * 1024;
 	tty->disc_data = dev;

 	return 0;

 free_netdev:
 	free_netdev(ndev);

 free_ida:
 	ida_free(&mctp_serial_ida, idx);
 	return rc;
 }

 static void mctp_serial_close(struct tty_struct *tty)
 {
 	struct mctp_serial *dev = tty->disc_data;
 	int idx = dev->idx;

 	unregister_netdev(dev->netdev);
 	ida_free(&mctp_serial_ida, idx);
 }

 static struct tty_ldisc_ops mctp_ldisc = {
 	.owner		= THIS_MODULE,
 	.num		= N_MCTP,
 	.name		= "mctp",
 	.open		= mctp_serial_open,
 	.close		= mctp_serial_close,
 	.receive_buf	= mctp_serial_tty_receive_buf,
 	.write_wakeup	= mctp_serial_tty_write_wakeup,
 };

 static int __init mctp_serial_init(void)
 {
 	return tty_register_ldisc(&mctp_ldisc);
 }

 static void __exit mctp_serial_exit(void)
 {
 	tty_unregister_ldisc(&mctp_ldisc);
 }

 module_init(mctp_serial_init);
 module_exit(mctp_serial_exit);

 MODULE_LICENSE("GPL v2");
 MODULE_AUTHOR("Jeremy Kerr <jk@codeconstruct.com.au>");
 MODULE_DESCRIPTION("MCTP Serial transport");
	// SPDX-License-Identifier: GPL-2.0
	/*
	* Management Component Transport Protocol (MCTP) - serial transport
	* binding. This driver is an implementation of the DMTF specificiation
	* "DSP0253 - Management Component Transport Protocol (MCTP) Serial Transport
	* Binding", available at:
	*
	* https://www.dmtf.org/sites/default/files/standards/documents/DSP0253_1.0.0.pdf
	*
	* This driver provides DSP0253-type MCTP-over-serial transport using a Linux
	* tty device, by setting the N_MCTP line discipline on the tty.
	*
	* Copyright (c) 2021 Code Construct
	*/

	#include <linux/idr.h>
	#include <linux/if_arp.h>
	#include <linux/module.h>
	#include <linux/skbuff.h>
	#include <linux/tty.h>
	#include <linux/workqueue.h>
	#include <linux/crc-ccitt.h>

	#include <linux/mctp.h>
	#include <net/mctp.h>
	#include <net/pkt_sched.h>

	#define MCTP_SERIAL_MTU 68 /* base mtu (64) + mctp header */
	#define MCTP_SERIAL_FRAME_MTU (MCTP_SERIAL_MTU + 6) /* + serial framing */

	#define MCTP_SERIAL_VERSION 0x1 /* DSP0253 defines a single version: 1 */

	#define BUFSIZE MCTP_SERIAL_FRAME_MTU

	#define BYTE_FRAME 0x7e
	#define BYTE_ESC 0x7d

	#define FCS_INIT 0xffff

	static DEFINE_IDA(mctp_serial_ida);

	enum mctp_serial_state {
	STATE_IDLE,
	STATE_START,
	STATE_HEADER,
	STATE_DATA,
	STATE_ESCAPE,
	STATE_TRAILER,
	STATE_DONE,
	STATE_ERR,
	};

	struct mctp_serial {
	struct net_device *netdev;
	struct tty_struct *tty;

	int idx;

	/* protects our rx & tx state machines; held during both paths */
	spinlock_t lock;

	struct work_struct tx_work;
	enum mctp_serial_state txstate, rxstate;
	u16 txfcs, rxfcs, rxfcs_rcvd;
	unsigned int txlen, rxlen;
	unsigned int txpos, rxpos;
	unsigned char txbuf[BUFSIZE],
	rxbuf[BUFSIZE];
	};

	static bool needs_escape(unsigned char c)
	{
	return c == BYTE_ESC \|\| c == BYTE_FRAME;
	}

	static int next_chunk_len(struct mctp_serial *dev)
	{
	int i;

	/* either we have no bytes to send ... */
	if (dev->txpos == dev->txlen)
	return 0;

	/* ... or the next byte to send is an escaped byte; requiring a
	* single-byte chunk...
	*/
	if (needs_escape(dev->txbuf[dev->txpos]))
	return 1;

	/* ... or we have one or more bytes up to the next escape - this chunk
	* will be those non-escaped bytes, and does not include the escaped
	* byte.
	*/
	for (i = 1; i + dev->txpos + 1 < dev->txlen; i++) {
	if (needs_escape(dev->txbuf[dev->txpos + i + 1]))
	break;
	}

	return i;
	}

	static int write_chunk(struct mctp_serial dev, unsigned char buf, int len)
	{
	return dev->tty->ops->write(dev->tty, buf, len);
	}

	static void mctp_serial_tx_work(struct work_struct *work)
	{
	struct mctp_serial *dev = container_of(work, struct mctp_serial,
	tx_work);
	unsigned char c, buf[3];
	unsigned long flags;
	int len, txlen;

	spin_lock_irqsave(&dev->lock, flags);

	/* txstate represents the next thing to send */
	switch (dev->txstate) {
	case STATE_START:
	dev->txpos = 0;
	fallthrough;
	case STATE_HEADER:
	buf[0] = BYTE_FRAME;
	buf[1] = MCTP_SERIAL_VERSION;
	buf[2] = dev->txlen;

	if (!dev->txpos)
	dev->txfcs = crc_ccitt(FCS_INIT, buf + 1, 2);

	txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
	if (txlen <= 0) {
	dev->txstate = STATE_ERR;
	} else {
	dev->txpos += txlen;
	if (dev->txpos == 3) {
	dev->txstate = STATE_DATA;
	dev->txpos = 0;
	}
	}
	break;

	case STATE_ESCAPE:
	buf[0] = dev->txbuf[dev->txpos] & ~0x20;
	txlen = write_chunk(dev, buf, 1);
	if (txlen <= 0) {
	dev->txstate = STATE_ERR;
	} else {
	dev->txpos += txlen;
	if (dev->txpos == dev->txlen) {
	dev->txstate = STATE_TRAILER;
	dev->txpos = 0;
	}
	}

	break;

	case STATE_DATA:
	len = next_chunk_len(dev);
	if (len) {
	c = dev->txbuf[dev->txpos];
	if (len == 1 && needs_escape(c)) {
	buf[0] = BYTE_ESC;
	buf[1] = c & ~0x20;
	dev->txfcs = crc_ccitt_byte(dev->txfcs, c);
	txlen = write_chunk(dev, buf, 2);
	if (txlen == 2)
	dev->txpos++;
	else if (txlen == 1)
	dev->txstate = STATE_ESCAPE;
	else
	dev->txstate = STATE_ERR;
	} else {
	txlen = write_chunk(dev,
	dev->txbuf + dev->txpos,
	len);
	if (txlen <= 0) {
	dev->txstate = STATE_ERR;
	} else {
	dev->txfcs = crc_ccitt(dev->txfcs,
	dev->txbuf +
	dev->txpos,
	txlen);
	dev->txpos += txlen;
	}
	}
	if (dev->txstate == STATE_DATA &&
	dev->txpos == dev->txlen) {
	dev->txstate = STATE_TRAILER;
	dev->txpos = 0;
	}
	break;
	}
	dev->txstate = STATE_TRAILER;
	dev->txpos = 0;
	fallthrough;

	case STATE_TRAILER:
	buf[0] = dev->txfcs >> 8;
	buf[1] = dev->txfcs & 0xff;
	buf[2] = BYTE_FRAME;
	txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
	if (txlen <= 0) {
	dev->txstate = STATE_ERR;
	} else {
	dev->txpos += txlen;
	if (dev->txpos == 3) {
	dev->txstate = STATE_DONE;
	dev->txpos = 0;
	}
	}
	break;
	default:
	netdev_err_once(dev->netdev, "invalid tx state %d\n",
	dev->txstate);
	}

	if (dev->txstate == STATE_DONE) {
	dev->netdev->stats.tx_packets++;
	dev->netdev->stats.tx_bytes += dev->txlen;
	dev->txlen = 0;
	dev->txpos = 0;
	clear_bit(TTY_DO_WRITE_WAKEUP, &dev->tty->flags);
	dev->txstate = STATE_IDLE;
	spin_unlock_irqrestore(&dev->lock, flags);

	netif_wake_queue(dev->netdev);
	} else {
	spin_unlock_irqrestore(&dev->lock, flags);
	}
	}

	static netdev_tx_t mctp_serial_tx(struct sk_buff skb, struct net_device ndev)
	{
	struct mctp_serial *dev = netdev_priv(ndev);
	unsigned long flags;

	WARN_ON(dev->txstate != STATE_IDLE);

	if (skb->len > MCTP_SERIAL_MTU) {
	dev->netdev->stats.tx_dropped++;
	goto out;
	}

	spin_lock_irqsave(&dev->lock, flags);
	netif_stop_queue(dev->netdev);
	skb_copy_bits(skb, 0, dev->txbuf, skb->len);
	dev->txpos = 0;
	dev->txlen = skb->len;
	dev->txstate = STATE_START;
	spin_unlock_irqrestore(&dev->lock, flags);

	set_bit(TTY_DO_WRITE_WAKEUP, &dev->tty->flags);
	schedule_work(&dev->tx_work);

	out:
	kfree_skb(skb);
	return NETDEV_TX_OK;
	}

	static void mctp_serial_tty_write_wakeup(struct tty_struct *tty)
	{
	struct mctp_serial *dev = tty->disc_data;

	schedule_work(&dev->tx_work);
	}

	static void mctp_serial_rx(struct mctp_serial *dev)
	{
	struct mctp_skb_cb *cb;
	struct sk_buff *skb;

	if (dev->rxfcs != dev->rxfcs_rcvd) {
	dev->netdev->stats.rx_dropped++;
	dev->netdev->stats.rx_crc_errors++;
	return;
	}

	skb = netdev_alloc_skb(dev->netdev, dev->rxlen);
	if (!skb) {
	dev->netdev->stats.rx_dropped++;
	return;
	}

	skb->protocol = htons(ETH_P_MCTP);
	skb_put_data(skb, dev->rxbuf, dev->rxlen);
	skb_reset_network_header(skb);

	cb = __mctp_cb(skb);
	cb->halen = 0;

	netif_rx(skb);
	dev->netdev->stats.rx_packets++;
	dev->netdev->stats.rx_bytes += dev->rxlen;
	}

	static void mctp_serial_push_header(struct mctp_serial *dev, unsigned char c)
	{
	switch (dev->rxpos) {
	case 0:
	if (c == BYTE_FRAME)
	dev->rxpos++;
	else
	dev->rxstate = STATE_ERR;
	break;
	case 1:
	if (c == MCTP_SERIAL_VERSION) {
	dev->rxpos++;
	dev->rxfcs = crc_ccitt_byte(FCS_INIT, c);
	} else {
	dev->rxstate = STATE_ERR;
	}
	break;
	case 2:
	if (c > MCTP_SERIAL_FRAME_MTU) {
	dev->rxstate = STATE_ERR;
	} else {
	dev->rxlen = c;
	dev->rxpos = 0;
	dev->rxstate = STATE_DATA;
	dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
	}
	break;
	}
	}

	static void mctp_serial_push_trailer(struct mctp_serial *dev, unsigned char c)
	{
	switch (dev->rxpos) {
	case 0:
	dev->rxfcs_rcvd = c << 8;
	dev->rxpos++;
	break;
	case 1:
	dev->rxfcs_rcvd \|= c;
	dev->rxpos++;
	break;
	case 2:
	if (c != BYTE_FRAME) {
	dev->rxstate = STATE_ERR;
	} else {
	mctp_serial_rx(dev);
	dev->rxlen = 0;
	dev->rxpos = 0;
	dev->rxstate = STATE_IDLE;
	}
	break;
	}
	}

	static void mctp_serial_push(struct mctp_serial *dev, unsigned char c)
	{
	switch (dev->rxstate) {
	case STATE_IDLE:
	dev->rxstate = STATE_HEADER;
	fallthrough;
	case STATE_HEADER:
	mctp_serial_push_header(dev, c);
	break;

	case STATE_ESCAPE:
	c \|= 0x20;
	fallthrough;
	case STATE_DATA:
	if (dev->rxstate != STATE_ESCAPE && c == BYTE_ESC) {
	dev->rxstate = STATE_ESCAPE;
	} else {
	dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
	dev->rxbuf[dev->rxpos] = c;
	dev->rxpos++;
	dev->rxstate = STATE_DATA;
	if (dev->rxpos == dev->rxlen) {
	dev->rxpos = 0;
	dev->rxstate = STATE_TRAILER;
	}
	}
	break;

	case STATE_TRAILER:
	mctp_serial_push_trailer(dev, c);
	break;

	case STATE_ERR:
	if (c == BYTE_FRAME)
	dev->rxstate = STATE_IDLE;
	break;

	default:
	netdev_err_once(dev->netdev, "invalid rx state %d\n",
	dev->rxstate);
	}
	}

	static void mctp_serial_tty_receive_buf(struct tty_struct tty, const u8 c,
	const u8 *f, size_t len)
	{
	struct mctp_serial *dev = tty->disc_data;
	int i;

	if (!netif_running(dev->netdev))
	return;

	/* we don't (currently) use the flag bytes, just data. */
	for (i = 0; i < len; i++)
	mctp_serial_push(dev, c[i]);
	}

	static void mctp_serial_uninit(struct net_device *ndev)
	{
	struct mctp_serial *dev = netdev_priv(ndev);

	cancel_work_sync(&dev->tx_work);
	}

	static const struct net_device_ops mctp_serial_netdev_ops = {
	.ndo_start_xmit = mctp_serial_tx,
	.ndo_uninit = mctp_serial_uninit,
	};

	static void mctp_serial_setup(struct net_device *ndev)
	{
	ndev->type = ARPHRD_MCTP;

	/* we limit at the fixed MTU, which is also the MCTP-standard
	* baseline MTU, so is also our minimum
	*/
	ndev->mtu = MCTP_SERIAL_MTU;
	ndev->max_mtu = MCTP_SERIAL_MTU;
	ndev->min_mtu = MCTP_SERIAL_MTU;

	ndev->hard_header_len = 0;
	ndev->addr_len = 0;
	ndev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;
	ndev->flags = IFF_NOARP;
	ndev->netdev_ops = &mctp_serial_netdev_ops;
	ndev->needs_free_netdev = true;
	}

	static int mctp_serial_open(struct tty_struct *tty)
	{
	struct mctp_serial *dev;
	struct net_device *ndev;
	char name[32];
	int idx, rc;

	if (!capable(CAP_NET_ADMIN))
	return -EPERM;

	if (!tty->ops->write)
	return -EOPNOTSUPP;

	idx = ida_alloc(&mctp_serial_ida, GFP_KERNEL);
	if (idx < 0)
	return idx;

	snprintf(name, sizeof(name), "mctpserial%d", idx);
	ndev = alloc_netdev(sizeof(*dev), name, NET_NAME_ENUM,
	mctp_serial_setup);
	if (!ndev) {
	rc = -ENOMEM;
	goto free_ida;
	}

	dev = netdev_priv(ndev);
	dev->idx = idx;
	dev->tty = tty;
	dev->netdev = ndev;
	dev->txstate = STATE_IDLE;
	dev->rxstate = STATE_IDLE;
	spin_lock_init(&dev->lock);
	INIT_WORK(&dev->tx_work, mctp_serial_tx_work);

	rc = register_netdev(ndev);
	if (rc)
	goto free_netdev;

	tty->receive_room = 64 * 1024;
	tty->disc_data = dev;

	return 0;

	free_netdev:
	free_netdev(ndev);

	free_ida:
	ida_free(&mctp_serial_ida, idx);
	return rc;
	}

	static void mctp_serial_close(struct tty_struct *tty)
	{
	struct mctp_serial *dev = tty->disc_data;
	int idx = dev->idx;

	unregister_netdev(dev->netdev);
	ida_free(&mctp_serial_ida, idx);
	}

	static struct tty_ldisc_ops mctp_ldisc = {
	.owner = THIS_MODULE,
	.num = N_MCTP,
	.name = "mctp",
	.open = mctp_serial_open,
	.close = mctp_serial_close,
	.receive_buf = mctp_serial_tty_receive_buf,
	.write_wakeup = mctp_serial_tty_write_wakeup,
	};

	static int __init mctp_serial_init(void)
	{
	return tty_register_ldisc(&mctp_ldisc);
	}

	static void __exit mctp_serial_exit(void)
	{
	tty_unregister_ldisc(&mctp_ldisc);
	}

	module_init(mctp_serial_init);
	module_exit(mctp_serial_exit);

	MODULE_LICENSE("GPL v2");
	MODULE_AUTHOR("Jeremy Kerr <jk@codeconstruct.com.au>");
	MODULE_DESCRIPTION("MCTP Serial transport");