free electrons

Embedded Linux Experts

  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
#include <linux/types.h>
#include <linux/errno.h>
#include <linux/kmod.h>
#include <linux/sched.h>
#include <linux/interrupt.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/file.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/proc_fs.h>
#include <linux/module.h>
#include <linux/device.h>
#include <linux/wait.h>
#include <linux/bitops.h>
#include <linux/seq_file.h>
#include <linux/uaccess.h>
#include <linux/ratelimit.h>

#undef LDISC_DEBUG_HANGUP

#ifdef LDISC_DEBUG_HANGUP
#define tty_ldisc_debug(tty, f, args...)	tty_debug(tty, f, ##args)
#else
#define tty_ldisc_debug(tty, f, args...)
#endif

/* lockdep nested classes for tty->ldisc_sem */
enum {
	LDISC_SEM_NORMAL,
	LDISC_SEM_OTHER,
};


/*
 *	This guards the refcounted line discipline lists. The lock
 *	must be taken with irqs off because there are hangup path
 *	callers who will do ldisc lookups and cannot sleep.
 */

static DEFINE_RAW_SPINLOCK(tty_ldiscs_lock);
/* Line disc dispatch table */
static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];

/**
 *	tty_register_ldisc	-	install a line discipline
 *	@disc: ldisc number
 *	@new_ldisc: pointer to the ldisc object
 *
 *	Installs a new line discipline into the kernel. The discipline
 *	is set up as unreferenced and then made available to the kernel
 *	from this point onwards.
 *
 *	Locking:
 *		takes tty_ldiscs_lock to guard against ldisc races
 */

int tty_register_ldisc(int disc, struct tty_ldisc_ops *new_ldisc)
{
	unsigned long flags;
	int ret = 0;

	if (disc < N_TTY || disc >= NR_LDISCS)
		return -EINVAL;

	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
	tty_ldiscs[disc] = new_ldisc;
	new_ldisc->num = disc;
	new_ldisc->refcount = 0;
	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);

	return ret;
}
EXPORT_SYMBOL(tty_register_ldisc);

/**
 *	tty_unregister_ldisc	-	unload a line discipline
 *	@disc: ldisc number
 *	@new_ldisc: pointer to the ldisc object
 *
 *	Remove a line discipline from the kernel providing it is not
 *	currently in use.
 *
 *	Locking:
 *		takes tty_ldiscs_lock to guard against ldisc races
 */

int tty_unregister_ldisc(int disc)
{
	unsigned long flags;
	int ret = 0;

	if (disc < N_TTY || disc >= NR_LDISCS)
		return -EINVAL;

	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
	if (tty_ldiscs[disc]->refcount)
		ret = -EBUSY;
	else
		tty_ldiscs[disc] = NULL;
	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);

	return ret;
}
EXPORT_SYMBOL(tty_unregister_ldisc);

static struct tty_ldisc_ops *get_ldops(int disc)
{
	unsigned long flags;
	struct tty_ldisc_ops *ldops, *ret;

	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
	ret = ERR_PTR(-EINVAL);
	ldops = tty_ldiscs[disc];
	if (ldops) {
		ret = ERR_PTR(-EAGAIN);
		if (try_module_get(ldops->owner)) {
			ldops->refcount++;
			ret = ldops;
		}
	}
	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
	return ret;
}

static void put_ldops(struct tty_ldisc_ops *ldops)
{
	unsigned long flags;

	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
	ldops->refcount--;
	module_put(ldops->owner);
	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
}

/**
 *	tty_ldisc_get		-	take a reference to an ldisc
 *	@disc: ldisc number
 *
 *	Takes a reference to a line discipline. Deals with refcounts and
 *	module locking counts.
 *
 *	Returns: -EINVAL if the discipline index is not [N_TTY..NR_LDISCS] or
 *			 if the discipline is not registered
 *		 -EAGAIN if request_module() failed to load or register the
 *			 the discipline
 *		 -ENOMEM if allocation failure
 *
 *		 Otherwise, returns a pointer to the discipline and bumps the
 *		 ref count
 *
 *	Locking:
 *		takes tty_ldiscs_lock to guard against ldisc races
 */

static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
{
	struct tty_ldisc *ld;
	struct tty_ldisc_ops *ldops;

	if (disc < N_TTY || disc >= NR_LDISCS)
		return ERR_PTR(-EINVAL);

	/*
	 * Get the ldisc ops - we may need to request them to be loaded
	 * dynamically and try again.
	 */
	ldops = get_ldops(disc);
	if (IS_ERR(ldops)) {
		request_module("tty-ldisc-%d", disc);
		ldops = get_ldops(disc);
		if (IS_ERR(ldops))
			return ERR_CAST(ldops);
	}

	ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL);
	if (ld == NULL) {
		put_ldops(ldops);
		return ERR_PTR(-ENOMEM);
	}

	ld->ops = ldops;
	ld->tty = tty;

	return ld;
}

/**
 *	tty_ldisc_put		-	release the ldisc
 *
 *	Complement of tty_ldisc_get().
 */
static void tty_ldisc_put(struct tty_ldisc *ld)
{
	if (WARN_ON_ONCE(!ld))
		return;

	put_ldops(ld->ops);
	kfree(ld);
}

static void *tty_ldiscs_seq_start(struct seq_file *m, loff_t *pos)
{
	return (*pos < NR_LDISCS) ? pos : NULL;
}

static void *tty_ldiscs_seq_next(struct seq_file *m, void *v, loff_t *pos)
{
	(*pos)++;
	return (*pos < NR_LDISCS) ? pos : NULL;
}

static void tty_ldiscs_seq_stop(struct seq_file *m, void *v)
{
}

static int tty_ldiscs_seq_show(struct seq_file *m, void *v)
{
	int i = *(loff_t *)v;
	struct tty_ldisc_ops *ldops;

	ldops = get_ldops(i);
	if (IS_ERR(ldops))
		return 0;
	seq_printf(m, "%-10s %2d\n", ldops->name ? ldops->name : "???", i);
	put_ldops(ldops);
	return 0;
}

static const struct seq_operations tty_ldiscs_seq_ops = {
	.start	= tty_ldiscs_seq_start,
	.next	= tty_ldiscs_seq_next,
	.stop	= tty_ldiscs_seq_stop,
	.show	= tty_ldiscs_seq_show,
};

static int proc_tty_ldiscs_open(struct inode *inode, struct file *file)
{
	return seq_open(file, &tty_ldiscs_seq_ops);
}

const struct file_operations tty_ldiscs_proc_fops = {
	.owner		= THIS_MODULE,
	.open		= proc_tty_ldiscs_open,
	.read		= seq_read,
	.llseek		= seq_lseek,
	.release	= seq_release,
};

/**
 *	tty_ldisc_ref_wait	-	wait for the tty ldisc
 *	@tty: tty device
 *
 *	Dereference the line discipline for the terminal and take a
 *	reference to it. If the line discipline is in flux then
 *	wait patiently until it changes.
 *
 *	Returns: NULL if the tty has been hungup and not re-opened with
 *		 a new file descriptor, otherwise valid ldisc reference
 *
 *	Note: Must not be called from an IRQ/timer context. The caller
 *	must also be careful not to hold other locks that will deadlock
 *	against a discipline change, such as an existing ldisc reference
 *	(which we check for)
 *
 *	Note: a file_operations routine (read/poll/write) should use this
 *	function to wait for any ldisc lifetime events to finish.
 */

struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
{
	struct tty_ldisc *ld;

	ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
	ld = tty->ldisc;
	if (!ld)
		ldsem_up_read(&tty->ldisc_sem);
	return ld;
}
EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);

/**
 *	tty_ldisc_ref		-	get the tty ldisc
 *	@tty: tty device
 *
 *	Dereference the line discipline for the terminal and take a
 *	reference to it. If the line discipline is in flux then
 *	return NULL. Can be called from IRQ and timer functions.
 */

struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
{
	struct tty_ldisc *ld = NULL;

	if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
		ld = tty->ldisc;
		if (!ld)
			ldsem_up_read(&tty->ldisc_sem);
	}
	return ld;
}
EXPORT_SYMBOL_GPL(tty_ldisc_ref);

/**
 *	tty_ldisc_deref		-	free a tty ldisc reference
 *	@ld: reference to free up
 *
 *	Undoes the effect of tty_ldisc_ref or tty_ldisc_ref_wait. May
 *	be called in IRQ context.
 */

void tty_ldisc_deref(struct tty_ldisc *ld)
{
	ldsem_up_read(&ld->tty->ldisc_sem);
}
EXPORT_SYMBOL_GPL(tty_ldisc_deref);


static inline int
__tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
{
	return ldsem_down_write(&tty->ldisc_sem, timeout);
}

static inline int
__tty_ldisc_lock_nested(struct tty_struct *tty, unsigned long timeout)
{
	return ldsem_down_write_nested(&tty->ldisc_sem,
				       LDISC_SEM_OTHER, timeout);
}

static inline void __tty_ldisc_unlock(struct tty_struct *tty)
{
	ldsem_up_write(&tty->ldisc_sem);
}

static int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
{
	int ret;

	ret = __tty_ldisc_lock(tty, timeout);
	if (!ret)
		return -EBUSY;
	set_bit(TTY_LDISC_HALTED, &tty->flags);
	return 0;
}

static void tty_ldisc_unlock(struct tty_struct *tty)
{
	clear_bit(TTY_LDISC_HALTED, &tty->flags);
	__tty_ldisc_unlock(tty);
}

static int
tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
			    unsigned long timeout)
{
	int ret;

	if (tty < tty2) {
		ret = __tty_ldisc_lock(tty, timeout);
		if (ret) {
			ret = __tty_ldisc_lock_nested(tty2, timeout);
			if (!ret)
				__tty_ldisc_unlock(tty);
		}
	} else {
		/* if this is possible, it has lots of implications */
		WARN_ON_ONCE(tty == tty2);
		if (tty2 && tty != tty2) {
			ret = __tty_ldisc_lock(tty2, timeout);
			if (ret) {
				ret = __tty_ldisc_lock_nested(tty, timeout);
				if (!ret)
					__tty_ldisc_unlock(tty2);
			}
		} else
			ret = __tty_ldisc_lock(tty, timeout);
	}

	if (!ret)
		return -EBUSY;

	set_bit(TTY_LDISC_HALTED, &tty->flags);
	if (tty2)
		set_bit(TTY_LDISC_HALTED, &tty2->flags);
	return 0;
}

static void tty_ldisc_lock_pair(struct tty_struct *tty, struct tty_struct *tty2)
{
	tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
}

static void tty_ldisc_unlock_pair(struct tty_struct *tty,
				  struct tty_struct *tty2)
{
	__tty_ldisc_unlock(tty);
	if (tty2)
		__tty_ldisc_unlock(tty2);
}

/**
 *	tty_ldisc_flush	-	flush line discipline queue
 *	@tty: tty
 *
 *	Flush the line discipline queue (if any) and the tty flip buffers
 *	for this tty.
 */

void tty_ldisc_flush(struct tty_struct *tty)
{
	struct tty_ldisc *ld = tty_ldisc_ref(tty);

	tty_buffer_flush(tty, ld);
	if (ld)
		tty_ldisc_deref(ld);
}
EXPORT_SYMBOL_GPL(tty_ldisc_flush);

/**
 *	tty_set_termios_ldisc		-	set ldisc field
 *	@tty: tty structure
 *	@disc: line discipline number
 *
 *	This is probably overkill for real world processors but
 *	they are not on hot paths so a little discipline won't do
 *	any harm.
 *
 *	The line discipline-related tty_struct fields are reset to
 *	prevent the ldisc driver from re-using stale information for
 *	the new ldisc instance.
 *
 *	Locking: takes termios_rwsem
 */

static void tty_set_termios_ldisc(struct tty_struct *tty, int disc)
{
	down_write(&tty->termios_rwsem);
	tty->termios.c_line = disc;
	up_write(&tty->termios_rwsem);

	tty->disc_data = NULL;
	tty->receive_room = 0;
}

/**
 *	tty_ldisc_open		-	open a line discipline
 *	@tty: tty we are opening the ldisc on
 *	@ld: discipline to open
 *
 *	A helper opening method. Also a convenient debugging and check
 *	point.
 *
 *	Locking: always called with BTM already held.
 */

static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
{
	WARN_ON(test_and_set_bit(TTY_LDISC_OPEN, &tty->flags));
	if (ld->ops->open) {
		int ret;
                /* BTM here locks versus a hangup event */
		ret = ld->ops->open(tty);
		if (ret)
			clear_bit(TTY_LDISC_OPEN, &tty->flags);

		tty_ldisc_debug(tty, "%p: opened\n", ld);
		return ret;
	}
	return 0;
}

/**
 *	tty_ldisc_close		-	close a line discipline
 *	@tty: tty we are opening the ldisc on
 *	@ld: discipline to close
 *
 *	A helper close method. Also a convenient debugging and check
 *	point.
 */

static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
{
	WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
	clear_bit(TTY_LDISC_OPEN, &tty->flags);
	if (ld->ops->close)
		ld->ops->close(tty);
	tty_ldisc_debug(tty, "%p: closed\n", ld);
}

/**
 *	tty_ldisc_restore	-	helper for tty ldisc change
 *	@tty: tty to recover
 *	@old: previous ldisc
 *
 *	Restore the previous line discipline or N_TTY when a line discipline
 *	change fails due to an open error
 */

static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
{
	struct tty_ldisc *new_ldisc;
	int r;

	/* There is an outstanding reference here so this is safe */
	old = tty_ldisc_get(tty, old->ops->num);
	WARN_ON(IS_ERR(old));
	tty->ldisc = old;
	tty_set_termios_ldisc(tty, old->ops->num);
	if (tty_ldisc_open(tty, old) < 0) {
		tty_ldisc_put(old);
		/* This driver is always present */
		new_ldisc = tty_ldisc_get(tty, N_TTY);
		if (IS_ERR(new_ldisc))
			panic("n_tty: get");
		tty->ldisc = new_ldisc;
		tty_set_termios_ldisc(tty, N_TTY);
		r = tty_ldisc_open(tty, new_ldisc);
		if (r < 0)
			panic("Couldn't open N_TTY ldisc for "
			      "%s --- error %d.",
			      tty_name(tty), r);
	}
}

/**
 *	tty_set_ldisc		-	set line discipline
 *	@tty: the terminal to set
 *	@ldisc: the line discipline
 *
 *	Set the discipline of a tty line. Must be called from a process
 *	context. The ldisc change logic has to protect itself against any
 *	overlapping ldisc change (including on the other end of pty pairs),
 *	the close of one side of a tty/pty pair, and eventually hangup.
 */

int tty_set_ldisc(struct tty_struct *tty, int disc)
{
	int retval;
	struct tty_ldisc *old_ldisc, *new_ldisc;

	new_ldisc = tty_ldisc_get(tty, disc);
	if (IS_ERR(new_ldisc))
		return PTR_ERR(new_ldisc);

	tty_lock(tty);
	retval = tty_ldisc_lock(tty, 5 * HZ);
	if (retval)
		goto err;

	if (!tty->ldisc) {
		retval = -EIO;
		goto out;
	}

	/* Check the no-op case */
	if (tty->ldisc->ops->num == disc)
		goto out;

	if (test_bit(TTY_HUPPED, &tty->flags)) {
		/* We were raced by hangup */
		retval = -EIO;
		goto out;
	}

	old_ldisc = tty->ldisc;

	/* Shutdown the old discipline. */
	tty_ldisc_close(tty, old_ldisc);

	/* Now set up the new line discipline. */
	tty->ldisc = new_ldisc;
	tty_set_termios_ldisc(tty, disc);

	retval = tty_ldisc_open(tty, new_ldisc);
	if (retval < 0) {
		/* Back to the old one or N_TTY if we can't */
		tty_ldisc_put(new_ldisc);
		tty_ldisc_restore(tty, old_ldisc);
	}

	if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
		down_read(&tty->termios_rwsem);
		tty->ops->set_ldisc(tty);
		up_read(&tty->termios_rwsem);
	}

	/* At this point we hold a reference to the new ldisc and a
	   reference to the old ldisc, or we hold two references to
	   the old ldisc (if it was restored as part of error cleanup
	   above). In either case, releasing a single reference from
	   the old ldisc is correct. */
	new_ldisc = old_ldisc;
out:
	tty_ldisc_unlock(tty);

	/* Restart the work queue in case no characters kick it off. Safe if
	   already running */
	tty_buffer_restart_work(tty->port);
err:
	tty_ldisc_put(new_ldisc);	/* drop the extra reference */
	tty_unlock(tty);
	return retval;
}

/**
 *	tty_ldisc_kill	-	teardown ldisc
 *	@tty: tty being released
 *
 *	Perform final close of the ldisc and reset tty->ldisc
 */
static void tty_ldisc_kill(struct tty_struct *tty)
{
	if (!tty->ldisc)
		return;
	/*
	 * Now kill off the ldisc
	 */
	tty_ldisc_close(tty, tty->ldisc);
	tty_ldisc_put(tty->ldisc);
	/* Force an oops if we mess this up */
	tty->ldisc = NULL;
}

/**
 *	tty_reset_termios	-	reset terminal state
 *	@tty: tty to reset
 *
 *	Restore a terminal to the driver default state.
 */

static void tty_reset_termios(struct tty_struct *tty)
{
	down_write(&tty->termios_rwsem);
	tty->termios = tty->driver->init_termios;
	tty->termios.c_ispeed = tty_termios_input_baud_rate(&tty->termios);
	tty->termios.c_ospeed = tty_termios_baud_rate(&tty->termios);
	up_write(&tty->termios_rwsem);
}


/**
 *	tty_ldisc_reinit	-	reinitialise the tty ldisc
 *	@tty: tty to reinit
 *	@disc: line discipline to reinitialize
 *
 *	Completely reinitialize the line discipline state, by closing the
 *	current instance, if there is one, and opening a new instance. If
 *	an error occurs opening the new non-N_TTY instance, the instance
 *	is dropped and tty->ldisc reset to NULL. The caller can then retry
 *	with N_TTY instead.
 *
 *	Returns 0 if successful, otherwise error code < 0
 */

int tty_ldisc_reinit(struct tty_struct *tty, int disc)
{
	struct tty_ldisc *ld;
	int retval;

	ld = tty_ldisc_get(tty, disc);
	if (IS_ERR(ld)) {
		BUG_ON(disc == N_TTY);
		return PTR_ERR(ld);
	}

	if (tty->ldisc) {
		tty_ldisc_close(tty, tty->ldisc);
		tty_ldisc_put(tty->ldisc);
	}

	/* switch the line discipline */
	tty->ldisc = ld;
	tty_set_termios_ldisc(tty, disc);
	retval = tty_ldisc_open(tty, tty->ldisc);
	if (retval) {
		if (!WARN_ON(disc == N_TTY)) {
			tty_ldisc_put(tty->ldisc);
			tty->ldisc = NULL;
		}
	}
	return retval;
}

/**
 *	tty_ldisc_hangup		-	hangup ldisc reset
 *	@tty: tty being hung up
 *
 *	Some tty devices reset their termios when they receive a hangup
 *	event. In that situation we must also switch back to N_TTY properly
 *	before we reset the termios data.
 *
 *	Locking: We can take the ldisc mutex as the rest of the code is
 *	careful to allow for this.
 *
 *	In the pty pair case this occurs in the close() path of the
 *	tty itself so we must be careful about locking rules.
 */

void tty_ldisc_hangup(struct tty_struct *tty, bool reinit)
{
	struct tty_ldisc *ld;

	tty_ldisc_debug(tty, "%p: hangup\n", tty->ldisc);

	ld = tty_ldisc_ref(tty);
	if (ld != NULL) {
		if (ld->ops->flush_buffer)
			ld->ops->flush_buffer(tty);
		tty_driver_flush_buffer(tty);
		if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
		    ld->ops->write_wakeup)
			ld->ops->write_wakeup(tty);
		if (ld->ops->hangup)
			ld->ops->hangup(tty);
		tty_ldisc_deref(ld);
	}

	wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
	wake_up_interruptible_poll(&tty->read_wait, POLLIN);

	/*
	 * Shutdown the current line discipline, and reset it to
	 * N_TTY if need be.
	 *
	 * Avoid racing set_ldisc or tty_ldisc_release
	 */
	tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);

	if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
		tty_reset_termios(tty);

	if (tty->ldisc) {
		if (reinit) {
			if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0)
				tty_ldisc_reinit(tty, N_TTY);
		} else
			tty_ldisc_kill(tty);
	}
	tty_ldisc_unlock(tty);
}

/**
 *	tty_ldisc_setup			-	open line discipline
 *	@tty: tty being shut down
 *	@o_tty: pair tty for pty/tty pairs
 *
 *	Called during the initial open of a tty/pty pair in order to set up the
 *	line disciplines and bind them to the tty. This has no locking issues
 *	as the device isn't yet active.
 */

int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
{
	int retval = tty_ldisc_open(tty, tty->ldisc);
	if (retval)
		return retval;

	if (o_tty) {
		retval = tty_ldisc_open(o_tty, o_tty->ldisc);
		if (retval) {
			tty_ldisc_close(tty, tty->ldisc);
			return retval;
		}
	}
	return 0;
}

/**
 *	tty_ldisc_release		-	release line discipline
 *	@tty: tty being shut down (or one end of pty pair)
 *
 *	Called during the final close of a tty or a pty pair in order to shut
 *	down the line discpline layer. On exit, each tty's ldisc is NULL.
 */

void tty_ldisc_release(struct tty_struct *tty)
{
	struct tty_struct *o_tty = tty->link;

	/*
	 * Shutdown this line discipline. As this is the final close,
	 * it does not race with the set_ldisc code path.
	 */

	tty_ldisc_lock_pair(tty, o_tty);
	tty_ldisc_kill(tty);
	if (o_tty)
		tty_ldisc_kill(o_tty);
	tty_ldisc_unlock_pair(tty, o_tty);

	/* And the memory resources remaining (buffers, termios) will be
	   disposed of when the kref hits zero */

	tty_ldisc_debug(tty, "released\n");
}

/**
 *	tty_ldisc_init		-	ldisc setup for new tty
 *	@tty: tty being allocated
 *
 *	Set up the line discipline objects for a newly allocated tty. Note that
 *	the tty structure is not completely set up when this call is made.
 */

void tty_ldisc_init(struct tty_struct *tty)
{
	struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
	if (IS_ERR(ld))
		panic("n_tty: init_tty");
	tty->ldisc = ld;
}

/**
 *	tty_ldisc_deinit	-	ldisc cleanup for new tty
 *	@tty: tty that was allocated recently
 *
 *	The tty structure must not becompletely set up (tty_ldisc_setup) when
 *      this call is made.
 */
void tty_ldisc_deinit(struct tty_struct *tty)
{
	if (tty->ldisc)
		tty_ldisc_put(tty->ldisc);
	tty->ldisc = NULL;
}