summaryrefslogtreecommitdiffstats
path: root/drivers/tty/n_hdlc.c
blob: b0f33e8ac819ac1ad33af23cd753fad53dbc48fb (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
// SPDX-License-Identifier: GPL-1.0+
/* generic HDLC line discipline for Linux
 *
 * Written by Paul Fulghum paulkf@microgate.com
 * for Microgate Corporation
 *
 * Microgate and SyncLink are registered trademarks of Microgate Corporation
 *
 * Adapted from ppp.c, written by Michael Callahan <callahan@maths.ox.ac.uk>,
 *	Al Longyear <longyear@netcom.com>,
 *	Paul Mackerras <Paul.Mackerras@cs.anu.edu.au>
 *
 * Original release 01/11/99
 *
 * This module implements the tty line discipline N_HDLC for use with
 * tty device drivers that support bit-synchronous HDLC communications.
 *
 * All HDLC data is frame oriented which means:
 *
 * 1. tty write calls represent one complete transmit frame of data
 *    The device driver should accept the complete frame or none of
 *    the frame (busy) in the write method. Each write call should have
 *    a byte count in the range of 2-65535 bytes (2 is min HDLC frame
 *    with 1 addr byte and 1 ctrl byte). The max byte count of 65535
 *    should include any crc bytes required. For example, when using
 *    CCITT CRC32, 4 crc bytes are required, so the maximum size frame
 *    the application may transmit is limited to 65531 bytes. For CCITT
 *    CRC16, the maximum application frame size would be 65533.
 *
 *
 * 2. receive callbacks from the device driver represents
 *    one received frame. The device driver should bypass
 *    the tty flip buffer and call the line discipline receive
 *    callback directly to avoid fragmenting or concatenating
 *    multiple frames into a single receive callback.
 *
 *    The HDLC line discipline queues the receive frames in separate
 *    buffers so complete receive frames can be returned by the
 *    tty read calls.
 *
 * 3. tty read calls returns an entire frame of data or nothing.
 *
 * 4. all send and receive data is considered raw. No processing
 *    or translation is performed by the line discipline, regardless
 *    of the tty flags
 *
 * 5. When line discipline is queried for the amount of receive
 *    data available (FIOC), 0 is returned if no data available,
 *    otherwise the count of the next available frame is returned.
 *    (instead of the sum of all received frame counts).
 *
 * These conventions allow the standard tty programming interface
 * to be used for synchronous HDLC applications when used with
 * this line discipline (or another line discipline that is frame
 * oriented such as N_PPP).
 *
 * The SyncLink driver (synclink.c) implements both asynchronous
 * (using standard line discipline N_TTY) and synchronous HDLC
 * (using N_HDLC) communications, with the latter using the above
 * conventions.
 *
 * This implementation is very basic and does not maintain
 * any statistics. The main point is to enforce the raw data
 * and frame orientation of HDLC communications.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#define HDLC_MAGIC 0x239e

#include <linux/module.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/types.h>
#include <linux/fcntl.h>
#include <linux/interrupt.h>
#include <linux/ptrace.h>

#include <linux/poll.h>
#include <linux/in.h>
#include <linux/ioctl.h>
#include <linux/slab.h>
#include <linux/tty.h>
#include <linux/errno.h>
#include <linux/string.h>	/* used in new tty drivers */
#include <linux/signal.h>	/* used in new tty drivers */
#include <linux/if.h>
#include <linux/bitops.h>

#include <asm/termios.h>
#include <linux/uaccess.h>
#include "tty.h"

/*
 * Buffers for individual HDLC frames
 */
#define MAX_HDLC_FRAME_SIZE 65535
#define DEFAULT_RX_BUF_COUNT 10
#define MAX_RX_BUF_COUNT 60
#define DEFAULT_TX_BUF_COUNT 3

struct n_hdlc_buf {
	struct list_head  list_item;
	int		  count;
	char		  buf[];
};

struct n_hdlc_buf_list {
	struct list_head  list;
	int		  count;
	spinlock_t	  spinlock;
};

/**
 * struct n_hdlc - per device instance data structure
 * @magic: magic value for structure
 * @tbusy: reentrancy flag for tx wakeup code
 * @woke_up: tx wakeup needs to be run again as it was called while @tbusy
 * @tx_buf_list: list of pending transmit frame buffers
 * @rx_buf_list: list of received frame buffers
 * @tx_free_buf_list: list unused transmit frame buffers
 * @rx_free_buf_list: list unused received frame buffers
 */
struct n_hdlc {
	int			magic;
	bool			tbusy;
	bool			woke_up;
	struct n_hdlc_buf_list	tx_buf_list;
	struct n_hdlc_buf_list	rx_buf_list;
	struct n_hdlc_buf_list	tx_free_buf_list;
	struct n_hdlc_buf_list	rx_free_buf_list;
};

/*
 * HDLC buffer list manipulation functions
 */
static void n_hdlc_buf_return(struct n_hdlc_buf_list *buf_list,
						struct n_hdlc_buf *buf);
static void n_hdlc_buf_put(struct n_hdlc_buf_list *list,
			   struct n_hdlc_buf *buf);
static struct n_hdlc_buf *n_hdlc_buf_get(struct n_hdlc_buf_list *list);

/* Local functions */

static struct n_hdlc *n_hdlc_alloc(void);

/* max frame size for memory allocations */
static int maxframe = 4096;

static void flush_rx_queue(struct tty_struct *tty)
{
	struct n_hdlc *n_hdlc = tty->disc_data;
	struct n_hdlc_buf *buf;

	while ((buf = n_hdlc_buf_get(&n_hdlc->rx_buf_list)))
		n_hdlc_buf_put(&n_hdlc->rx_free_buf_list, buf);
}

static void flush_tx_queue(struct tty_struct *tty)
{
	struct n_hdlc *n_hdlc = tty->disc_data;
	struct n_hdlc_buf *buf;

	while ((buf = n_hdlc_buf_get(&n_hdlc->tx_buf_list)))
		n_hdlc_buf_put(&n_hdlc->tx_free_buf_list, buf);
}

static void n_hdlc_free_buf_list(struct n_hdlc_buf_list *list)
{
	struct n_hdlc_buf *buf;

	do {
		buf = n_hdlc_buf_get(list);
		kfree(buf);
	} while (buf);
}

/**
 * n_hdlc_tty_close - line discipline close
 * @tty: pointer to tty info structure
 *
 * Called when the line discipline is changed to something
 * else, the tty is closed, or the tty detects a hangup.
 */
static void n_hdlc_tty_close(struct tty_struct *tty)
{
	struct n_hdlc *n_hdlc = tty->disc_data;

	if (n_hdlc->magic != HDLC_MAGIC) {
		pr_warn("n_hdlc: trying to close unopened tty!\n");
		return;
	}
#if defined(TTY_NO_WRITE_SPLIT)
	clear_bit(TTY_NO_WRITE_SPLIT, &tty->flags);
#endif
	tty->disc_data = NULL;

	/* Ensure that the n_hdlcd process is not hanging on select()/poll() */
	wake_up_interruptible(&tty->read_wait);
	wake_up_interruptible(&tty->write_wait);

	n_hdlc_free_buf_list(&n_hdlc->rx_free_buf_list);
	n_hdlc_free_buf_list(&n_hdlc->tx_free_buf_list);
	n_hdlc_free_buf_list(&n_hdlc->rx_buf_list);
	n_hdlc_free_buf_list(&n_hdlc->tx_buf_list);
	kfree(n_hdlc);
}	/* end of n_hdlc_tty_close() */

/**
 * n_hdlc_tty_open - called when line discipline changed to n_hdlc
 * @tty: pointer to tty info structure
 *
 * Returns 0 if success, otherwise error code
 */
static int n_hdlc_tty_open(struct tty_struct *tty)
{
	struct n_hdlc *n_hdlc = tty->disc_data;

	pr_debug("%s() called (device=%s)\n", __func__, tty->name);

	/* There should not be an existing table for this slot. */
	if (n_hdlc) {
		pr_err("%s: tty already associated!\n", __func__);
		return -EEXIST;
	}

	n_hdlc = n_hdlc_alloc();
	if (!n_hdlc) {
		pr_err("%s: n_hdlc_alloc failed\n", __func__);
		return -ENFILE;
	}

	tty->disc_data = n_hdlc;
	tty->receive_room = 65536;

	/* change tty_io write() to not split large writes into 8K chunks */
	set_bit(TTY_NO_WRITE_SPLIT, &tty->flags);

	/* flush receive data from driver */
	tty_driver_flush_buffer(tty);

	return 0;

}	/* end of n_tty_hdlc_open() */

/**
 * n_hdlc_send_frames - send frames on pending send buffer list
 * @n_hdlc: pointer to ldisc instance data
 * @tty: pointer to tty instance data
 *
 * Send frames on pending send buffer list until the driver does not accept a
 * frame (busy) this function is called after adding a frame to the send buffer
 * list and by the tty wakeup callback.
 */
static void n_hdlc_send_frames(struct n_hdlc *n_hdlc, struct tty_struct *tty)
{
	register int actual;
	unsigned long flags;
	struct n_hdlc_buf *tbuf;

check_again:

	spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
	if (n_hdlc->tbusy) {
		n_hdlc->woke_up = true;
		spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
		return;
	}
	n_hdlc->tbusy = true;
	n_hdlc->woke_up = false;
	spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);

	tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
	while (tbuf) {
		pr_debug("sending frame %p, count=%d\n", tbuf, tbuf->count);

		/* Send the next block of data to device */
		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
		actual = tty->ops->write(tty, tbuf->buf, tbuf->count);

		/* rollback was possible and has been done */
		if (actual == -ERESTARTSYS) {
			n_hdlc_buf_return(&n_hdlc->tx_buf_list, tbuf);
			break;
		}
		/* if transmit error, throw frame away by */
		/* pretending it was accepted by driver */
		if (actual < 0)
			actual = tbuf->count;

		if (actual == tbuf->count) {
			pr_debug("frame %p completed\n", tbuf);

			/* free current transmit buffer */
			n_hdlc_buf_put(&n_hdlc->tx_free_buf_list, tbuf);

			/* wait up sleeping writers */
			wake_up_interruptible(&tty->write_wait);

			/* get next pending transmit buffer */
			tbuf = n_hdlc_buf_get(&n_hdlc->tx_buf_list);
		} else {
			pr_debug("frame %p pending\n", tbuf);

			/*
			 * the buffer was not accepted by driver,
			 * return it back into tx queue
			 */
			n_hdlc_buf_return(&n_hdlc->tx_buf_list, tbuf);
			break;
		}
	}

	if (!tbuf)
		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);

	/* Clear the re-entry flag */
	spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
	n_hdlc->tbusy = false;
	spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);

	if (n_hdlc->woke_up)
		goto check_again;
}	/* end of n_hdlc_send_frames() */

/**
 * n_hdlc_tty_wakeup - Callback for transmit wakeup
 * @tty: pointer to associated tty instance data
 *
 * Called when low level device driver can accept more send data.
 */
static void n_hdlc_tty_wakeup(struct tty_struct *tty)
{
	struct n_hdlc *n_hdlc = tty->disc_data;

	n_hdlc_send_frames(n_hdlc, tty);
}	/* end of n_hdlc_tty_wakeup() */

/**
 * n_hdlc_tty_receive - Called by tty driver when receive data is available
 * @tty: pointer to tty instance data
 * @data: pointer to received data
 * @flags: pointer to flags for data
 * @count: count of received data in bytes
 *
 * Called by tty low level driver when receive data is available. Data is
 * interpreted as one HDLC frame.
 */
static void n_hdlc_tty_receive(struct tty_struct *tty, const __u8 *data,
			       char *flags, int count)
{
	register struct n_hdlc *n_hdlc = tty->disc_data;
	register struct n_hdlc_buf *buf;

	pr_debug("%s() called count=%d\n", __func__, count);

	/* verify line is using HDLC discipline */
	if (n_hdlc->magic != HDLC_MAGIC) {
		pr_err("line not using HDLC discipline\n");
		return;
	}

	if (count > maxframe) {
		pr_debug("rx count>maxframesize, data discarded\n");
		return;
	}

	/* get a free HDLC buffer */
	buf = n_hdlc_buf_get(&n_hdlc->rx_free_buf_list);
	if (!buf) {
		/*
		 * no buffers in free list, attempt to allocate another rx
		 * buffer unless the maximum count has been reached
		 */
		if (n_hdlc->rx_buf_list.count < MAX_RX_BUF_COUNT)
			buf = kmalloc(struct_size(buf, buf, maxframe),
				      GFP_ATOMIC);
	}

	if (!buf) {
		pr_debug("no more rx buffers, data discarded\n");
		return;
	}

	/* copy received data to HDLC buffer */
	memcpy(buf->buf, data, count);
	buf->count = count;

	/* add HDLC buffer to list of received frames */
	n_hdlc_buf_put(&n_hdlc->rx_buf_list, buf);

	/* wake up any blocked reads and perform async signalling */
	wake_up_interruptible(&tty->read_wait);
	if (tty->fasync != NULL)
		kill_fasync(&tty->fasync, SIGIO, POLL_IN);

}	/* end of n_hdlc_tty_receive() */

/**
 * n_hdlc_tty_read - Called to retrieve one frame of data (if available)
 * @tty: pointer to tty instance data
 * @file: pointer to open file object
 * @buf: pointer to returned data buffer
 * @nr: size of returned data buffer
 *
 * Returns the number of bytes returned or error code.
 */
static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
			   __u8 *kbuf, size_t nr,
			   void **cookie, unsigned long offset)
{
	struct n_hdlc *n_hdlc = tty->disc_data;
	int ret = 0;
	struct n_hdlc_buf *rbuf;
	DECLARE_WAITQUEUE(wait, current);

	/* Is this a repeated call for an rbuf we already found earlier? */
	rbuf = *cookie;
	if (rbuf)
		goto have_rbuf;

	add_wait_queue(&tty->read_wait, &wait);

	for (;;) {
		if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
			ret = -EIO;
			break;
		}
		if (tty_hung_up_p(file))
			break;

		set_current_state(TASK_INTERRUPTIBLE);

		rbuf = n_hdlc_buf_get(&n_hdlc->rx_buf_list);
		if (rbuf)
			break;

		/* no data */
		if (tty_io_nonblock(tty, file)) {
			ret = -EAGAIN;
			break;
		}

		schedule();

		if (signal_pending(current)) {
			ret = -EINTR;
			break;
		}
	}

	remove_wait_queue(&tty->read_wait, &wait);
	__set_current_state(TASK_RUNNING);

	if (!rbuf)
		return ret;
	*cookie = rbuf;

have_rbuf:
	/* Have we used it up entirely? */
	if (offset >= rbuf->count)
		goto done_with_rbuf;

	/* More data to go, but can't copy any more? EOVERFLOW */
	ret = -EOVERFLOW;
	if (!nr)
		goto done_with_rbuf;

	/* Copy as much data as possible */
	ret = rbuf->count - offset;
	if (ret > nr)
		ret = nr;
	memcpy(kbuf, rbuf->buf+offset, ret);
	offset += ret;

	/* If we still have data left, we leave the rbuf in the cookie */
	if (offset < rbuf->count)
		return ret;

done_with_rbuf:
	*cookie = NULL;

	if (n_hdlc->rx_free_buf_list.count > DEFAULT_RX_BUF_COUNT)
		kfree(rbuf);
	else
		n_hdlc_buf_put(&n_hdlc->rx_free_buf_list, rbuf);

	return ret;

}	/* end of n_hdlc_tty_read() */

/**
 * n_hdlc_tty_write - write a single frame of data to device
 * @tty: pointer to associated tty device instance data
 * @file: pointer to file object data
 * @data: pointer to transmit data (one frame)
 * @count: size of transmit frame in bytes
 *
 * Returns the number of bytes written (or error code).
 */
static ssize_t n_hdlc_tty_write(struct tty_struct *tty, struct file *file,
			    const unsigned char *data, size_t count)
{
	struct n_hdlc *n_hdlc = tty->disc_data;
	int error = 0;
	DECLARE_WAITQUEUE(wait, current);
	struct n_hdlc_buf *tbuf;

	pr_debug("%s() called count=%zd\n", __func__, count);

	if (n_hdlc->magic != HDLC_MAGIC)
		return -EIO;

	/* verify frame size */
	if (count > maxframe) {
		pr_debug("%s: truncating user packet from %zu to %d\n",
				__func__, count, maxframe);
		count = maxframe;
	}

	add_wait_queue(&tty->write_wait, &wait);

	for (;;) {
		set_current_state(TASK_INTERRUPTIBLE);

		tbuf = n_hdlc_buf_get(&n_hdlc->tx_free_buf_list);
		if (tbuf)
			break;

		if (tty_io_nonblock(tty, file)) {
			error = -EAGAIN;
			break;
		}
		schedule();

		if (signal_pending(current)) {
			error = -EINTR;
			break;
		}
	}

	__set_current_state(TASK_RUNNING);
	remove_wait_queue(&tty->write_wait, &wait);

	if (!error) {
		/* Retrieve the user's buffer */
		memcpy(tbuf->buf, data, count);

		/* Send the data */
		tbuf->count = error = count;
		n_hdlc_buf_put(&n_hdlc->tx_buf_list, tbuf);
		n_hdlc_send_frames(n_hdlc, tty);
	}

	return error;

}	/* end of n_hdlc_tty_write() */

/**
 * n_hdlc_tty_ioctl - process IOCTL system call for the tty device.
 * @tty: pointer to tty instance data
 * @file: pointer to open file object for device
 * @cmd: IOCTL command code
 * @arg: argument for IOCTL call (cmd dependent)
 *
 * Returns command dependent result.
 */
static int n_hdlc_tty_ioctl(struct tty_struct *tty, struct file *file,
			    unsigned int cmd, unsigned long arg)
{
	struct n_hdlc *n_hdlc = tty->disc_data;
	int error = 0;
	int count;
	unsigned long flags;
	struct n_hdlc_buf *buf = NULL;

	pr_debug("%s() called %d\n", __func__, cmd);

	/* Verify the status of the device */
	if (n_hdlc->magic != HDLC_MAGIC)
		return -EBADF;

	switch (cmd) {
	case FIONREAD:
		/* report count of read data available */
		/* in next available frame (if any) */
		spin_lock_irqsave(&n_hdlc->rx_buf_list.spinlock, flags);
		buf = list_first_entry_or_null(&n_hdlc->rx_buf_list.list,
						struct n_hdlc_buf, list_item);
		if (buf)
			count = buf->count;
		else
			count = 0;
		spin_unlock_irqrestore(&n_hdlc->rx_buf_list.spinlock, flags);
		error = put_user(count, (int __user *)arg);
		break;

	case TIOCOUTQ:
		/* get the pending tx byte count in the driver */
		count = tty_chars_in_buffer(tty);
		/* add size of next output frame in queue */
		spin_lock_irqsave(&n_hdlc->tx_buf_list.spinlock, flags);
		buf = list_first_entry_or_null(&n_hdlc->tx_buf_list.list,
						struct n_hdlc_buf, list_item);
		if (buf)
			count += buf->count;
		spin_unlock_irqrestore(&n_hdlc->tx_buf_list.spinlock, flags);
		error = put_user(count, (int __user *)arg);
		break;

	case TCFLSH:
		switch (arg) {
		case TCIOFLUSH:
		case TCOFLUSH:
			flush_tx_queue(tty);
		}
		fallthrough;	/* to default */

	default:
		error = n_tty_ioctl_helper(tty, file, cmd, arg);
		break;
	}
	return error;

}	/* end of n_hdlc_tty_ioctl() */

/**
 * n_hdlc_tty_poll - TTY callback for poll system call
 * @tty: pointer to tty instance data
 * @filp: pointer to open file object for device
 * @wait: wait queue for operations
 *
 * Determine which operations (read/write) will not block and return info
 * to caller.
 * Returns a bit mask containing info on which ops will not block.
 */
static __poll_t n_hdlc_tty_poll(struct tty_struct *tty, struct file *filp,
				    poll_table *wait)
{
	struct n_hdlc *n_hdlc = tty->disc_data;
	__poll_t mask = 0;

	if (n_hdlc->magic != HDLC_MAGIC)
		return 0;

	/*
	 * queue the current process into any wait queue that may awaken in the
	 * future (read and write)
	 */
	poll_wait(filp, &tty->read_wait, wait);
	poll_wait(filp, &tty->write_wait, wait);

	/* set bits for operations that won't block */
	if (!list_empty(&n_hdlc->rx_buf_list.list))
		mask |= EPOLLIN | EPOLLRDNORM;	/* readable */
	if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
		mask |= EPOLLHUP;
	if (tty_hung_up_p(filp))
		mask |= EPOLLHUP;
	if (!tty_is_writelocked(tty) &&
			!list_empty(&n_hdlc->tx_free_buf_list.list))
		mask |= EPOLLOUT | EPOLLWRNORM;	/* writable */

	return mask;
}	/* end of n_hdlc_tty_poll() */

static void n_hdlc_alloc_buf(struct n_hdlc_buf_list *list, unsigned int count,
		const char *name)
{
	struct n_hdlc_buf *buf;
	unsigned int i;

	for (i = 0; i < count; i++) {
		buf = kmalloc(struct_size(buf, buf, maxframe), GFP_KERNEL);
		if (!buf) {
			pr_debug("%s(), kmalloc() failed for %s buffer %u\n",
					__func__, name, i);
			return;
		}
		n_hdlc_buf_put(list, buf);
	}
}

/**
 * n_hdlc_alloc - allocate an n_hdlc instance data structure
 *
 * Returns a pointer to newly created structure if success, otherwise %NULL
 */
static struct n_hdlc *n_hdlc_alloc(void)
{
	struct n_hdlc *n_hdlc = kzalloc(sizeof(*n_hdlc), GFP_KERNEL);

	if (!n_hdlc)
		return NULL;

	spin_lock_init(&n_hdlc->rx_free_buf_list.spinlock);
	spin_lock_init(&n_hdlc->tx_free_buf_list.spinlock);
	spin_lock_init(&n_hdlc->rx_buf_list.spinlock);
	spin_lock_init(&n_hdlc->tx_buf_list.spinlock);

	INIT_LIST_HEAD(&n_hdlc->rx_free_buf_list.list);
	INIT_LIST_HEAD(&n_hdlc->tx_free_buf_list.list);
	INIT_LIST_HEAD(&n_hdlc->rx_buf_list.list);
	INIT_LIST_HEAD(&n_hdlc->tx_buf_list.list);

	n_hdlc_alloc_buf(&n_hdlc->rx_free_buf_list, DEFAULT_RX_BUF_COUNT, "rx");
	n_hdlc_alloc_buf(&n_hdlc->tx_free_buf_list, DEFAULT_TX_BUF_COUNT, "tx");

	/* Initialize the control block */
	n_hdlc->magic  = HDLC_MAGIC;

	return n_hdlc;

}	/* end of n_hdlc_alloc() */

/**
 * n_hdlc_buf_return - put the HDLC buffer after the head of the specified list
 * @buf_list: pointer to the buffer list
 * @buf: pointer to the buffer
 */
static void n_hdlc_buf_return(struct n_hdlc_buf_list *buf_list,
						struct n_hdlc_buf *buf)
{
	unsigned long flags;

	spin_lock_irqsave(&buf_list->spinlock, flags);

	list_add(&buf->list_item, &buf_list->list);
	buf_list->count++;

	spin_unlock_irqrestore(&buf_list->spinlock, flags);
}

/**
 * n_hdlc_buf_put - add specified HDLC buffer to tail of specified list
 * @buf_list: pointer to buffer list
 * @buf: pointer to buffer
 */
static void n_hdlc_buf_put(struct n_hdlc_buf_list *buf_list,
			   struct n_hdlc_buf *buf)
{
	unsigned long flags;

	spin_lock_irqsave(&buf_list->spinlock, flags);

	list_add_tail(&buf->list_item, &buf_list->list);
	buf_list->count++;

	spin_unlock_irqrestore(&buf_list->spinlock, flags);
}	/* end of n_hdlc_buf_put() */

/**
 * n_hdlc_buf_get - remove and return an HDLC buffer from list
 * @buf_list: pointer to HDLC buffer list
 *
 * Remove and return an HDLC buffer from the head of the specified HDLC buffer
 * list.
 * Returns a pointer to HDLC buffer if available, otherwise %NULL.
 */
static struct n_hdlc_buf *n_hdlc_buf_get(struct n_hdlc_buf_list *buf_list)
{
	unsigned long flags;
	struct n_hdlc_buf *buf;

	spin_lock_irqsave(&buf_list->spinlock, flags);

	buf = list_first_entry_or_null(&buf_list->list,
						struct n_hdlc_buf, list_item);
	if (buf) {
		list_del(&buf->list_item);
		buf_list->count--;
	}

	spin_unlock_irqrestore(&buf_list->spinlock, flags);
	return buf;
}	/* end of n_hdlc_buf_get() */

static struct tty_ldisc_ops n_hdlc_ldisc = {
	.owner		= THIS_MODULE,
	.name		= "hdlc",
	.open		= n_hdlc_tty_open,
	.close		= n_hdlc_tty_close,
	.read		= n_hdlc_tty_read,
	.write		= n_hdlc_tty_write,
	.ioctl		= n_hdlc_tty_ioctl,
	.poll		= n_hdlc_tty_poll,
	.receive_buf	= n_hdlc_tty_receive,
	.write_wakeup	= n_hdlc_tty_wakeup,
	.flush_buffer   = flush_rx_queue,
};

static int __init n_hdlc_init(void)
{
	int status;

	/* range check maxframe arg */
	maxframe = clamp(maxframe, 4096, MAX_HDLC_FRAME_SIZE);

	status = tty_register_ldisc(N_HDLC, &n_hdlc_ldisc);
	if (!status)
		pr_info("N_HDLC line discipline registered with maxframe=%d\n",
				maxframe);
	else
		pr_err("N_HDLC: error registering line discipline: %d\n",
				status);

	return status;

}	/* end of init_module() */

static void __exit n_hdlc_exit(void)
{
	/* Release tty registration of line discipline */
	int status = tty_unregister_ldisc(N_HDLC);

	if (status)
		pr_err("N_HDLC: can't unregister line discipline (err = %d)\n",
				status);
	else
		pr_info("N_HDLC: line discipline unregistered\n");
}

module_init(n_hdlc_init);
module_exit(n_hdlc_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Paul Fulghum paulkf@microgate.com");
module_param(maxframe, int, 0);
MODULE_ALIAS_LDISC(N_HDLC);