1 // SPDX-License-Identifier: GPL-2.0
3 * Management Component Transport Protocol (MCTP) - serial transport
4 * binding. This driver is an implementation of the DMTF specificiation
5 * "DSP0253 - Management Component Transport Protocol (MCTP) Serial Transport
6 * Binding", available at:
8 * https://www.dmtf.org/sites/default/files/standards/documents/DSP0253_1.0.0.pdf
10 * This driver provides DSP0253-type MCTP-over-serial transport using a Linux
11 * tty device, by setting the N_MCTP line discipline on the tty.
13 * Copyright (c) 2021 Code Construct
16 #include <linux/idr.h>
17 #include <linux/if_arp.h>
18 #include <linux/module.h>
19 #include <linux/skbuff.h>
20 #include <linux/tty.h>
21 #include <linux/workqueue.h>
22 #include <linux/crc-ccitt.h>
24 #include <linux/mctp.h>
26 #include <net/pkt_sched.h>
28 #define MCTP_SERIAL_MTU 68 /* base mtu (64) + mctp header */
29 #define MCTP_SERIAL_FRAME_MTU (MCTP_SERIAL_MTU + 6) /* + serial framing */
31 #define MCTP_SERIAL_VERSION 0x1 /* DSP0253 defines a single version: 1 */
33 #define BUFSIZE MCTP_SERIAL_FRAME_MTU
35 #define BYTE_FRAME 0x7e
38 static DEFINE_IDA(mctp_serial_ida);
40 enum mctp_serial_state {
52 struct net_device *netdev;
53 struct tty_struct *tty;
57 /* protects our rx & tx state machines; held during both paths */
60 struct work_struct tx_work;
61 enum mctp_serial_state txstate, rxstate;
62 u16 txfcs, rxfcs, rxfcs_rcvd;
63 unsigned int txlen, rxlen;
64 unsigned int txpos, rxpos;
65 unsigned char txbuf[BUFSIZE],
69 static bool needs_escape(unsigned char c)
71 return c == BYTE_ESC || c == BYTE_FRAME;
74 static int next_chunk_len(struct mctp_serial *dev)
78 /* either we have no bytes to send ... */
79 if (dev->txpos == dev->txlen)
82 /* ... or the next byte to send is an escaped byte; requiring a
83 * single-byte chunk...
85 if (needs_escape(dev->txbuf[dev->txpos]))
88 /* ... or we have one or more bytes up to the next escape - this chunk
89 * will be those non-escaped bytes, and does not include the escaped
92 for (i = 1; i + dev->txpos + 1 < dev->txlen; i++) {
93 if (needs_escape(dev->txbuf[dev->txpos + i + 1]))
100 static int write_chunk(struct mctp_serial *dev, unsigned char *buf, int len)
102 return dev->tty->ops->write(dev->tty, buf, len);
105 static void mctp_serial_tx_work(struct work_struct *work)
107 struct mctp_serial *dev = container_of(work, struct mctp_serial,
109 unsigned char c, buf[3];
113 spin_lock_irqsave(&dev->lock, flags);
115 /* txstate represents the next thing to send */
116 switch (dev->txstate) {
122 buf[1] = MCTP_SERIAL_VERSION;
126 dev->txfcs = crc_ccitt(0, buf + 1, 2);
128 txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
130 dev->txstate = STATE_ERR;
133 if (dev->txpos == 3) {
134 dev->txstate = STATE_DATA;
141 buf[0] = dev->txbuf[dev->txpos] & ~0x20;
142 txlen = write_chunk(dev, buf, 1);
144 dev->txstate = STATE_ERR;
147 if (dev->txpos == dev->txlen) {
148 dev->txstate = STATE_TRAILER;
156 len = next_chunk_len(dev);
158 c = dev->txbuf[dev->txpos];
159 if (len == 1 && needs_escape(c)) {
162 dev->txfcs = crc_ccitt_byte(dev->txfcs, c);
163 txlen = write_chunk(dev, buf, 2);
167 dev->txstate = STATE_ESCAPE;
169 dev->txstate = STATE_ERR;
171 txlen = write_chunk(dev,
172 dev->txbuf + dev->txpos,
175 dev->txstate = STATE_ERR;
177 dev->txfcs = crc_ccitt(dev->txfcs,
184 if (dev->txstate == STATE_DATA &&
185 dev->txpos == dev->txlen) {
186 dev->txstate = STATE_TRAILER;
191 dev->txstate = STATE_TRAILER;
196 buf[0] = dev->txfcs >> 8;
197 buf[1] = dev->txfcs & 0xff;
199 txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
201 dev->txstate = STATE_ERR;
204 if (dev->txpos == 3) {
205 dev->txstate = STATE_DONE;
211 netdev_err_once(dev->netdev, "invalid tx state %d\n",
215 if (dev->txstate == STATE_DONE) {
216 dev->netdev->stats.tx_packets++;
217 dev->netdev->stats.tx_bytes += dev->txlen;
220 clear_bit(TTY_DO_WRITE_WAKEUP, &dev->tty->flags);
221 dev->txstate = STATE_IDLE;
222 spin_unlock_irqrestore(&dev->lock, flags);
224 netif_wake_queue(dev->netdev);
226 spin_unlock_irqrestore(&dev->lock, flags);
230 static netdev_tx_t mctp_serial_tx(struct sk_buff *skb, struct net_device *ndev)
232 struct mctp_serial *dev = netdev_priv(ndev);
235 WARN_ON(dev->txstate != STATE_IDLE);
237 if (skb->len > MCTP_SERIAL_MTU) {
238 dev->netdev->stats.tx_dropped++;
242 spin_lock_irqsave(&dev->lock, flags);
243 netif_stop_queue(dev->netdev);
244 skb_copy_bits(skb, 0, dev->txbuf, skb->len);
246 dev->txlen = skb->len;
247 dev->txstate = STATE_START;
248 spin_unlock_irqrestore(&dev->lock, flags);
250 set_bit(TTY_DO_WRITE_WAKEUP, &dev->tty->flags);
251 schedule_work(&dev->tx_work);
258 static void mctp_serial_tty_write_wakeup(struct tty_struct *tty)
260 struct mctp_serial *dev = tty->disc_data;
262 schedule_work(&dev->tx_work);
265 static void mctp_serial_rx(struct mctp_serial *dev)
267 struct mctp_skb_cb *cb;
270 if (dev->rxfcs != dev->rxfcs_rcvd) {
271 dev->netdev->stats.rx_dropped++;
272 dev->netdev->stats.rx_crc_errors++;
276 skb = netdev_alloc_skb(dev->netdev, dev->rxlen);
278 dev->netdev->stats.rx_dropped++;
282 skb->protocol = htons(ETH_P_MCTP);
283 skb_put_data(skb, dev->rxbuf, dev->rxlen);
284 skb_reset_network_header(skb);
290 dev->netdev->stats.rx_packets++;
291 dev->netdev->stats.rx_bytes += dev->rxlen;
294 static void mctp_serial_push_header(struct mctp_serial *dev, unsigned char c)
296 switch (dev->rxpos) {
301 dev->rxstate = STATE_ERR;
304 if (c == MCTP_SERIAL_VERSION) {
306 dev->rxfcs = crc_ccitt_byte(0, c);
308 dev->rxstate = STATE_ERR;
312 if (c > MCTP_SERIAL_FRAME_MTU) {
313 dev->rxstate = STATE_ERR;
317 dev->rxstate = STATE_DATA;
318 dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
324 static void mctp_serial_push_trailer(struct mctp_serial *dev, unsigned char c)
326 switch (dev->rxpos) {
328 dev->rxfcs_rcvd = c << 8;
332 dev->rxfcs_rcvd |= c;
336 if (c != BYTE_FRAME) {
337 dev->rxstate = STATE_ERR;
342 dev->rxstate = STATE_IDLE;
348 static void mctp_serial_push(struct mctp_serial *dev, unsigned char c)
350 switch (dev->rxstate) {
352 dev->rxstate = STATE_HEADER;
355 mctp_serial_push_header(dev, c);
362 if (dev->rxstate != STATE_ESCAPE && c == BYTE_ESC) {
363 dev->rxstate = STATE_ESCAPE;
365 dev->rxfcs = crc_ccitt_byte(dev->rxfcs, c);
366 dev->rxbuf[dev->rxpos] = c;
368 dev->rxstate = STATE_DATA;
369 if (dev->rxpos == dev->rxlen) {
371 dev->rxstate = STATE_TRAILER;
377 mctp_serial_push_trailer(dev, c);
382 dev->rxstate = STATE_IDLE;
386 netdev_err_once(dev->netdev, "invalid rx state %d\n",
391 static void mctp_serial_tty_receive_buf(struct tty_struct *tty,
392 const unsigned char *c,
393 const char *f, int len)
395 struct mctp_serial *dev = tty->disc_data;
398 if (!netif_running(dev->netdev))
401 /* we don't (currently) use the flag bytes, just data. */
402 for (i = 0; i < len; i++)
403 mctp_serial_push(dev, c[i]);
406 static void mctp_serial_uninit(struct net_device *ndev)
408 struct mctp_serial *dev = netdev_priv(ndev);
410 cancel_work_sync(&dev->tx_work);
413 static const struct net_device_ops mctp_serial_netdev_ops = {
414 .ndo_start_xmit = mctp_serial_tx,
415 .ndo_uninit = mctp_serial_uninit,
418 static void mctp_serial_setup(struct net_device *ndev)
420 ndev->type = ARPHRD_MCTP;
422 /* we limit at the fixed MTU, which is also the MCTP-standard
423 * baseline MTU, so is also our minimum
425 ndev->mtu = MCTP_SERIAL_MTU;
426 ndev->max_mtu = MCTP_SERIAL_MTU;
427 ndev->min_mtu = MCTP_SERIAL_MTU;
429 ndev->hard_header_len = 0;
431 ndev->tx_queue_len = DEFAULT_TX_QUEUE_LEN;
432 ndev->flags = IFF_NOARP;
433 ndev->netdev_ops = &mctp_serial_netdev_ops;
434 ndev->needs_free_netdev = true;
437 static int mctp_serial_open(struct tty_struct *tty)
439 struct mctp_serial *dev;
440 struct net_device *ndev;
444 if (!capable(CAP_NET_ADMIN))
447 if (!tty->ops->write)
450 idx = ida_alloc(&mctp_serial_ida, GFP_KERNEL);
454 snprintf(name, sizeof(name), "mctpserial%d", idx);
455 ndev = alloc_netdev(sizeof(*dev), name, NET_NAME_ENUM,
462 dev = netdev_priv(ndev);
466 dev->txstate = STATE_IDLE;
467 dev->rxstate = STATE_IDLE;
468 spin_lock_init(&dev->lock);
469 INIT_WORK(&dev->tx_work, mctp_serial_tx_work);
471 rc = register_netdev(ndev);
475 tty->receive_room = 64 * 1024;
476 tty->disc_data = dev;
484 ida_free(&mctp_serial_ida, idx);
488 static void mctp_serial_close(struct tty_struct *tty)
490 struct mctp_serial *dev = tty->disc_data;
493 unregister_netdev(dev->netdev);
494 ida_free(&mctp_serial_ida, idx);
497 static struct tty_ldisc_ops mctp_ldisc = {
498 .owner = THIS_MODULE,
501 .open = mctp_serial_open,
502 .close = mctp_serial_close,
503 .receive_buf = mctp_serial_tty_receive_buf,
504 .write_wakeup = mctp_serial_tty_write_wakeup,
507 static int __init mctp_serial_init(void)
509 return tty_register_ldisc(&mctp_ldisc);
512 static void __exit mctp_serial_exit(void)
514 tty_unregister_ldisc(&mctp_ldisc);
517 module_init(mctp_serial_init);
518 module_exit(mctp_serial_exit);
520 MODULE_LICENSE("GPL v2");
521 MODULE_AUTHOR("Jeremy Kerr <jk@codeconstruct.com.au>");
522 MODULE_DESCRIPTION("MCTP Serial transport");