[PATCH net-next] mctp: Add MCTP-over-serial transport binding

[Jeremy Kerr <jk@codeconstruct.com.au>]

This change adds a MCTP Serial transport binding, as per DMTF DSP0253.
This is implemented as a new serial line discipline, and can be attached
to arbitrary serial devices.

The 'mctp' utility provides the ldisc magic to set up the serial link:

  # mctp link serial /dev/ttyS0 &
  # mctp link
  dev lo index 1 address 0x00:00:00:00:00:00 net 1 mtu 65536 up
  dev mctpserial0 index 5 address 0x(no-addr) net 1 mtu 68 down

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
---
 drivers/net/mctp/Kconfig       |  11 +
 drivers/net/mctp/Makefile      |   1 +
 drivers/net/mctp/mctp-serial.c | 494 +++++++++++++++++++++++++++++++++
 include/uapi/linux/tty.h       |   1 +
 4 files changed, 507 insertions(+)
 create mode 100644 drivers/net/mctp/mctp-serial.c

diff --git a/drivers/net/mctp/Kconfig b/drivers/net/mctp/Kconfig
index d8f966cedc89..02f3c2d600fd 100644
--- a/drivers/net/mctp/Kconfig
+++ b/drivers/net/mctp/Kconfig
@@ -3,6 +3,17 @@ if MCTP
 
 menu "MCTP Device Drivers"
 
+config MCTP_SERIAL
+	tristate "MCTP serial transport"
+	depends on TTY
+	select CRC_CCITT
+	help
+	  This driver provides an MCTP-over-serial interface, through a
+	  serial line-discipline. By attaching the ldisc to a serial device,
+	  we get a new net device to transport MCTP packets.
+
+	  Say y here if you need to connect to MCTP devices over serial.
+
 endmenu
 
 endif
diff --git a/drivers/net/mctp/Makefile b/drivers/net/mctp/Makefile
index e69de29bb2d1..d32622613ce4 100644
--- a/drivers/net/mctp/Makefile
+++ b/drivers/net/mctp/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_MCTP_SERIAL) += mctp-serial.o
diff --git a/drivers/net/mctp/mctp-serial.c b/drivers/net/mctp/mctp-serial.c
new file mode 100644
index 000000000000..30950f1ea6f4
--- /dev/null
+++ b/drivers/net/mctp/mctp-serial.c
@@ -0,0 +1,494 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Management Component Transport Protocol (MCTP) - serial transport
+ * binding.
+ *
+ * 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
+
+#define BUFSIZE			MCTP_SERIAL_FRAME_MTU
+
+#define BYTE_FRAME		0x7e
+#define BYTE_ESC		0x7d
+
+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(0, 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);
+		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 {
+				txlen = write_chunk(dev,
+						    dev->txbuf + dev->txpos,
+						    len);
+				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;
+		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;
+
+	rcu_read_lock();
+	dev = rcu_dereference(tty->disc_data);
+	schedule_work(&dev->tx_work);
+	rcu_read_unlock();
+}
+
+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_ni(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(0, 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 unsigned char *c,
+					const char *f, int 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 const struct net_device_ops mctp_serial_netdev_ops = {
+	.ndo_start_xmit = mctp_serial_tx,
+};
+
+static void mctp_serial_setup(struct net_device *ndev)
+{
+	ndev->type = ARPHRD_MCTP;
+	ndev->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;
+
+	if (tty->disc_data)
+		return -EEXIST;
+
+	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");
diff --git a/include/uapi/linux/tty.h b/include/uapi/linux/tty.h
index 376cccf397be..a58deb3061eb 100644
--- a/include/uapi/linux/tty.h
+++ b/include/uapi/linux/tty.h
@@ -38,5 +38,6 @@
 #define N_NCI		25	/* NFC NCI UART */
 #define N_SPEAKUP	26	/* Speakup communication with synths */
 #define N_NULL		27	/* Null ldisc used for error handling */
+#define N_MCTP		28	/* MCTP-over-serial */
 
 #endif /* _UAPI_LINUX_TTY_H */
-- 
2.30.2