Version:  2.0.40 2.2.26 2.4.37 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 4.0 4.1 4.2 4.3 4.4 4.5

Linux/drivers/tty/tty_ldisc.c

  1 #include <linux/types.h>
  2 #include <linux/errno.h>
  3 #include <linux/kmod.h>
  4 #include <linux/sched.h>
  5 #include <linux/interrupt.h>
  6 #include <linux/tty.h>
  7 #include <linux/tty_driver.h>
  8 #include <linux/file.h>
  9 #include <linux/mm.h>
 10 #include <linux/string.h>
 11 #include <linux/slab.h>
 12 #include <linux/poll.h>
 13 #include <linux/proc_fs.h>
 14 #include <linux/module.h>
 15 #include <linux/device.h>
 16 #include <linux/wait.h>
 17 #include <linux/bitops.h>
 18 #include <linux/seq_file.h>
 19 #include <linux/uaccess.h>
 20 #include <linux/ratelimit.h>
 21 
 22 #undef LDISC_DEBUG_HANGUP
 23 
 24 #ifdef LDISC_DEBUG_HANGUP
 25 #define tty_ldisc_debug(tty, f, args...)        tty_debug(tty, f, ##args)
 26 #else
 27 #define tty_ldisc_debug(tty, f, args...)
 28 #endif
 29 
 30 /* lockdep nested classes for tty->ldisc_sem */
 31 enum {
 32         LDISC_SEM_NORMAL,
 33         LDISC_SEM_OTHER,
 34 };
 35 
 36 
 37 /*
 38  *      This guards the refcounted line discipline lists. The lock
 39  *      must be taken with irqs off because there are hangup path
 40  *      callers who will do ldisc lookups and cannot sleep.
 41  */
 42 
 43 static DEFINE_RAW_SPINLOCK(tty_ldiscs_lock);
 44 /* Line disc dispatch table */
 45 static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
 46 
 47 /**
 48  *      tty_register_ldisc      -       install a line discipline
 49  *      @disc: ldisc number
 50  *      @new_ldisc: pointer to the ldisc object
 51  *
 52  *      Installs a new line discipline into the kernel. The discipline
 53  *      is set up as unreferenced and then made available to the kernel
 54  *      from this point onwards.
 55  *
 56  *      Locking:
 57  *              takes tty_ldiscs_lock to guard against ldisc races
 58  */
 59 
 60 int tty_register_ldisc(int disc, struct tty_ldisc_ops *new_ldisc)
 61 {
 62         unsigned long flags;
 63         int ret = 0;
 64 
 65         if (disc < N_TTY || disc >= NR_LDISCS)
 66                 return -EINVAL;
 67 
 68         raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
 69         tty_ldiscs[disc] = new_ldisc;
 70         new_ldisc->num = disc;
 71         new_ldisc->refcount = 0;
 72         raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
 73 
 74         return ret;
 75 }
 76 EXPORT_SYMBOL(tty_register_ldisc);
 77 
 78 /**
 79  *      tty_unregister_ldisc    -       unload a line discipline
 80  *      @disc: ldisc number
 81  *      @new_ldisc: pointer to the ldisc object
 82  *
 83  *      Remove a line discipline from the kernel providing it is not
 84  *      currently in use.
 85  *
 86  *      Locking:
 87  *              takes tty_ldiscs_lock to guard against ldisc races
 88  */
 89 
 90 int tty_unregister_ldisc(int disc)
 91 {
 92         unsigned long flags;
 93         int ret = 0;
 94 
 95         if (disc < N_TTY || disc >= NR_LDISCS)
 96                 return -EINVAL;
 97 
 98         raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
 99         if (tty_ldiscs[disc]->refcount)
100                 ret = -EBUSY;
101         else
102                 tty_ldiscs[disc] = NULL;
103         raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
104 
105         return ret;
106 }
107 EXPORT_SYMBOL(tty_unregister_ldisc);
108 
109 static struct tty_ldisc_ops *get_ldops(int disc)
110 {
111         unsigned long flags;
112         struct tty_ldisc_ops *ldops, *ret;
113 
114         raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
115         ret = ERR_PTR(-EINVAL);
116         ldops = tty_ldiscs[disc];
117         if (ldops) {
118                 ret = ERR_PTR(-EAGAIN);
119                 if (try_module_get(ldops->owner)) {
120                         ldops->refcount++;
121                         ret = ldops;
122                 }
123         }
124         raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
125         return ret;
126 }
127 
128 static void put_ldops(struct tty_ldisc_ops *ldops)
129 {
130         unsigned long flags;
131 
132         raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
133         ldops->refcount--;
134         module_put(ldops->owner);
135         raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
136 }
137 
138 /**
139  *      tty_ldisc_get           -       take a reference to an ldisc
140  *      @disc: ldisc number
141  *
142  *      Takes a reference to a line discipline. Deals with refcounts and
143  *      module locking counts. Returns NULL if the discipline is not available.
144  *      Returns a pointer to the discipline and bumps the ref count if it is
145  *      available
146  *
147  *      Locking:
148  *              takes tty_ldiscs_lock to guard against ldisc races
149  */
150 
151 static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
152 {
153         struct tty_ldisc *ld;
154         struct tty_ldisc_ops *ldops;
155 
156         if (disc < N_TTY || disc >= NR_LDISCS)
157                 return ERR_PTR(-EINVAL);
158 
159         /*
160          * Get the ldisc ops - we may need to request them to be loaded
161          * dynamically and try again.
162          */
163         ldops = get_ldops(disc);
164         if (IS_ERR(ldops)) {
165                 request_module("tty-ldisc-%d", disc);
166                 ldops = get_ldops(disc);
167                 if (IS_ERR(ldops))
168                         return ERR_CAST(ldops);
169         }
170 
171         ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL);
172         if (ld == NULL) {
173                 put_ldops(ldops);
174                 return ERR_PTR(-ENOMEM);
175         }
176 
177         ld->ops = ldops;
178         ld->tty = tty;
179 
180         return ld;
181 }
182 
183 /**
184  *      tty_ldisc_put           -       release the ldisc
185  *
186  *      Complement of tty_ldisc_get().
187  */
188 static void tty_ldisc_put(struct tty_ldisc *ld)
189 {
190         if (WARN_ON_ONCE(!ld))
191                 return;
192 
193         put_ldops(ld->ops);
194         kfree(ld);
195 }
196 
197 static void *tty_ldiscs_seq_start(struct seq_file *m, loff_t *pos)
198 {
199         return (*pos < NR_LDISCS) ? pos : NULL;
200 }
201 
202 static void *tty_ldiscs_seq_next(struct seq_file *m, void *v, loff_t *pos)
203 {
204         (*pos)++;
205         return (*pos < NR_LDISCS) ? pos : NULL;
206 }
207 
208 static void tty_ldiscs_seq_stop(struct seq_file *m, void *v)
209 {
210 }
211 
212 static int tty_ldiscs_seq_show(struct seq_file *m, void *v)
213 {
214         int i = *(loff_t *)v;
215         struct tty_ldisc_ops *ldops;
216 
217         ldops = get_ldops(i);
218         if (IS_ERR(ldops))
219                 return 0;
220         seq_printf(m, "%-10s %2d\n", ldops->name ? ldops->name : "???", i);
221         put_ldops(ldops);
222         return 0;
223 }
224 
225 static const struct seq_operations tty_ldiscs_seq_ops = {
226         .start  = tty_ldiscs_seq_start,
227         .next   = tty_ldiscs_seq_next,
228         .stop   = tty_ldiscs_seq_stop,
229         .show   = tty_ldiscs_seq_show,
230 };
231 
232 static int proc_tty_ldiscs_open(struct inode *inode, struct file *file)
233 {
234         return seq_open(file, &tty_ldiscs_seq_ops);
235 }
236 
237 const struct file_operations tty_ldiscs_proc_fops = {
238         .owner          = THIS_MODULE,
239         .open           = proc_tty_ldiscs_open,
240         .read           = seq_read,
241         .llseek         = seq_lseek,
242         .release        = seq_release,
243 };
244 
245 /**
246  *      tty_ldisc_ref_wait      -       wait for the tty ldisc
247  *      @tty: tty device
248  *
249  *      Dereference the line discipline for the terminal and take a
250  *      reference to it. If the line discipline is in flux then
251  *      wait patiently until it changes.
252  *
253  *      Note: Must not be called from an IRQ/timer context. The caller
254  *      must also be careful not to hold other locks that will deadlock
255  *      against a discipline change, such as an existing ldisc reference
256  *      (which we check for)
257  *
258  *      Note: only callable from a file_operations routine (which
259  *      guarantees tty->ldisc != NULL when the lock is acquired).
260  */
261 
262 struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
263 {
264         ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
265         WARN_ON(!tty->ldisc);
266         return tty->ldisc;
267 }
268 EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);
269 
270 /**
271  *      tty_ldisc_ref           -       get the tty ldisc
272  *      @tty: tty device
273  *
274  *      Dereference the line discipline for the terminal and take a
275  *      reference to it. If the line discipline is in flux then
276  *      return NULL. Can be called from IRQ and timer functions.
277  */
278 
279 struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
280 {
281         struct tty_ldisc *ld = NULL;
282 
283         if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
284                 ld = tty->ldisc;
285                 if (!ld)
286                         ldsem_up_read(&tty->ldisc_sem);
287         }
288         return ld;
289 }
290 EXPORT_SYMBOL_GPL(tty_ldisc_ref);
291 
292 /**
293  *      tty_ldisc_deref         -       free a tty ldisc reference
294  *      @ld: reference to free up
295  *
296  *      Undoes the effect of tty_ldisc_ref or tty_ldisc_ref_wait. May
297  *      be called in IRQ context.
298  */
299 
300 void tty_ldisc_deref(struct tty_ldisc *ld)
301 {
302         ldsem_up_read(&ld->tty->ldisc_sem);
303 }
304 EXPORT_SYMBOL_GPL(tty_ldisc_deref);
305 
306 
307 static inline int __lockfunc
308 __tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
309 {
310         return ldsem_down_write(&tty->ldisc_sem, timeout);
311 }
312 
313 static inline int __lockfunc
314 __tty_ldisc_lock_nested(struct tty_struct *tty, unsigned long timeout)
315 {
316         return ldsem_down_write_nested(&tty->ldisc_sem,
317                                        LDISC_SEM_OTHER, timeout);
318 }
319 
320 static inline void __tty_ldisc_unlock(struct tty_struct *tty)
321 {
322         ldsem_up_write(&tty->ldisc_sem);
323 }
324 
325 static int __lockfunc
326 tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
327 {
328         int ret;
329 
330         ret = __tty_ldisc_lock(tty, timeout);
331         if (!ret)
332                 return -EBUSY;
333         set_bit(TTY_LDISC_HALTED, &tty->flags);
334         return 0;
335 }
336 
337 static void tty_ldisc_unlock(struct tty_struct *tty)
338 {
339         clear_bit(TTY_LDISC_HALTED, &tty->flags);
340         __tty_ldisc_unlock(tty);
341 }
342 
343 static int __lockfunc
344 tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
345                             unsigned long timeout)
346 {
347         int ret;
348 
349         if (tty < tty2) {
350                 ret = __tty_ldisc_lock(tty, timeout);
351                 if (ret) {
352                         ret = __tty_ldisc_lock_nested(tty2, timeout);
353                         if (!ret)
354                                 __tty_ldisc_unlock(tty);
355                 }
356         } else {
357                 /* if this is possible, it has lots of implications */
358                 WARN_ON_ONCE(tty == tty2);
359                 if (tty2 && tty != tty2) {
360                         ret = __tty_ldisc_lock(tty2, timeout);
361                         if (ret) {
362                                 ret = __tty_ldisc_lock_nested(tty, timeout);
363                                 if (!ret)
364                                         __tty_ldisc_unlock(tty2);
365                         }
366                 } else
367                         ret = __tty_ldisc_lock(tty, timeout);
368         }
369 
370         if (!ret)
371                 return -EBUSY;
372 
373         set_bit(TTY_LDISC_HALTED, &tty->flags);
374         if (tty2)
375                 set_bit(TTY_LDISC_HALTED, &tty2->flags);
376         return 0;
377 }
378 
379 static void __lockfunc
380 tty_ldisc_lock_pair(struct tty_struct *tty, struct tty_struct *tty2)
381 {
382         tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
383 }
384 
385 static void __lockfunc tty_ldisc_unlock_pair(struct tty_struct *tty,
386                                              struct tty_struct *tty2)
387 {
388         __tty_ldisc_unlock(tty);
389         if (tty2)
390                 __tty_ldisc_unlock(tty2);
391 }
392 
393 /**
394  *      tty_ldisc_flush -       flush line discipline queue
395  *      @tty: tty
396  *
397  *      Flush the line discipline queue (if any) and the tty flip buffers
398  *      for this tty.
399  */
400 
401 void tty_ldisc_flush(struct tty_struct *tty)
402 {
403         struct tty_ldisc *ld = tty_ldisc_ref(tty);
404 
405         tty_buffer_flush(tty, ld);
406         if (ld)
407                 tty_ldisc_deref(ld);
408 }
409 EXPORT_SYMBOL_GPL(tty_ldisc_flush);
410 
411 /**
412  *      tty_set_termios_ldisc           -       set ldisc field
413  *      @tty: tty structure
414  *      @num: line discipline number
415  *
416  *      This is probably overkill for real world processors but
417  *      they are not on hot paths so a little discipline won't do
418  *      any harm.
419  *
420  *      The line discipline-related tty_struct fields are reset to
421  *      prevent the ldisc driver from re-using stale information for
422  *      the new ldisc instance.
423  *
424  *      Locking: takes termios_rwsem
425  */
426 
427 static void tty_set_termios_ldisc(struct tty_struct *tty, int num)
428 {
429         down_write(&tty->termios_rwsem);
430         tty->termios.c_line = num;
431         up_write(&tty->termios_rwsem);
432 
433         tty->disc_data = NULL;
434         tty->receive_room = 0;
435 }
436 
437 /**
438  *      tty_ldisc_open          -       open a line discipline
439  *      @tty: tty we are opening the ldisc on
440  *      @ld: discipline to open
441  *
442  *      A helper opening method. Also a convenient debugging and check
443  *      point.
444  *
445  *      Locking: always called with BTM already held.
446  */
447 
448 static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
449 {
450         WARN_ON(test_and_set_bit(TTY_LDISC_OPEN, &tty->flags));
451         if (ld->ops->open) {
452                 int ret;
453                 /* BTM here locks versus a hangup event */
454                 ret = ld->ops->open(tty);
455                 if (ret)
456                         clear_bit(TTY_LDISC_OPEN, &tty->flags);
457 
458                 tty_ldisc_debug(tty, "%p: opened\n", tty->ldisc);
459                 return ret;
460         }
461         return 0;
462 }
463 
464 /**
465  *      tty_ldisc_close         -       close a line discipline
466  *      @tty: tty we are opening the ldisc on
467  *      @ld: discipline to close
468  *
469  *      A helper close method. Also a convenient debugging and check
470  *      point.
471  */
472 
473 static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
474 {
475         WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
476         clear_bit(TTY_LDISC_OPEN, &tty->flags);
477         if (ld->ops->close)
478                 ld->ops->close(tty);
479         tty_ldisc_debug(tty, "%p: closed\n", tty->ldisc);
480 }
481 
482 /**
483  *      tty_ldisc_restore       -       helper for tty ldisc change
484  *      @tty: tty to recover
485  *      @old: previous ldisc
486  *
487  *      Restore the previous line discipline or N_TTY when a line discipline
488  *      change fails due to an open error
489  */
490 
491 static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
492 {
493         struct tty_ldisc *new_ldisc;
494         int r;
495 
496         /* There is an outstanding reference here so this is safe */
497         old = tty_ldisc_get(tty, old->ops->num);
498         WARN_ON(IS_ERR(old));
499         tty->ldisc = old;
500         tty_set_termios_ldisc(tty, old->ops->num);
501         if (tty_ldisc_open(tty, old) < 0) {
502                 tty_ldisc_put(old);
503                 /* This driver is always present */
504                 new_ldisc = tty_ldisc_get(tty, N_TTY);
505                 if (IS_ERR(new_ldisc))
506                         panic("n_tty: get");
507                 tty->ldisc = new_ldisc;
508                 tty_set_termios_ldisc(tty, N_TTY);
509                 r = tty_ldisc_open(tty, new_ldisc);
510                 if (r < 0)
511                         panic("Couldn't open N_TTY ldisc for "
512                               "%s --- error %d.",
513                               tty_name(tty), r);
514         }
515 }
516 
517 /**
518  *      tty_set_ldisc           -       set line discipline
519  *      @tty: the terminal to set
520  *      @ldisc: the line discipline
521  *
522  *      Set the discipline of a tty line. Must be called from a process
523  *      context. The ldisc change logic has to protect itself against any
524  *      overlapping ldisc change (including on the other end of pty pairs),
525  *      the close of one side of a tty/pty pair, and eventually hangup.
526  */
527 
528 int tty_set_ldisc(struct tty_struct *tty, int ldisc)
529 {
530         int retval;
531         struct tty_ldisc *old_ldisc, *new_ldisc;
532 
533         new_ldisc = tty_ldisc_get(tty, ldisc);
534         if (IS_ERR(new_ldisc))
535                 return PTR_ERR(new_ldisc);
536 
537         tty_lock(tty);
538         retval = tty_ldisc_lock(tty, 5 * HZ);
539         if (retval)
540                 goto err;
541 
542         /* Check the no-op case */
543         if (tty->ldisc->ops->num == ldisc)
544                 goto out;
545 
546         if (test_bit(TTY_HUPPED, &tty->flags)) {
547                 /* We were raced by hangup */
548                 retval = -EIO;
549                 goto out;
550         }
551 
552         old_ldisc = tty->ldisc;
553 
554         /* Shutdown the old discipline. */
555         tty_ldisc_close(tty, old_ldisc);
556 
557         /* Now set up the new line discipline. */
558         tty->ldisc = new_ldisc;
559         tty_set_termios_ldisc(tty, ldisc);
560 
561         retval = tty_ldisc_open(tty, new_ldisc);
562         if (retval < 0) {
563                 /* Back to the old one or N_TTY if we can't */
564                 tty_ldisc_put(new_ldisc);
565                 tty_ldisc_restore(tty, old_ldisc);
566         }
567 
568         if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
569                 down_read(&tty->termios_rwsem);
570                 tty->ops->set_ldisc(tty);
571                 up_read(&tty->termios_rwsem);
572         }
573 
574         /* At this point we hold a reference to the new ldisc and a
575            reference to the old ldisc, or we hold two references to
576            the old ldisc (if it was restored as part of error cleanup
577            above). In either case, releasing a single reference from
578            the old ldisc is correct. */
579         new_ldisc = old_ldisc;
580 out:
581         tty_ldisc_unlock(tty);
582 
583         /* Restart the work queue in case no characters kick it off. Safe if
584            already running */
585         tty_buffer_restart_work(tty->port);
586 err:
587         tty_ldisc_put(new_ldisc);       /* drop the extra reference */
588         tty_unlock(tty);
589         return retval;
590 }
591 
592 /**
593  *      tty_reset_termios       -       reset terminal state
594  *      @tty: tty to reset
595  *
596  *      Restore a terminal to the driver default state.
597  */
598 
599 static void tty_reset_termios(struct tty_struct *tty)
600 {
601         down_write(&tty->termios_rwsem);
602         tty->termios = tty->driver->init_termios;
603         tty->termios.c_ispeed = tty_termios_input_baud_rate(&tty->termios);
604         tty->termios.c_ospeed = tty_termios_baud_rate(&tty->termios);
605         up_write(&tty->termios_rwsem);
606 }
607 
608 
609 /**
610  *      tty_ldisc_reinit        -       reinitialise the tty ldisc
611  *      @tty: tty to reinit
612  *      @ldisc: line discipline to reinitialize
613  *
614  *      Switch the tty to a line discipline and leave the ldisc
615  *      state closed
616  */
617 
618 static int tty_ldisc_reinit(struct tty_struct *tty, int ldisc)
619 {
620         struct tty_ldisc *ld = tty_ldisc_get(tty, ldisc);
621 
622         if (IS_ERR(ld))
623                 return -1;
624 
625         tty_ldisc_close(tty, tty->ldisc);
626         tty_ldisc_put(tty->ldisc);
627         /*
628          *      Switch the line discipline back
629          */
630         tty->ldisc = ld;
631         tty_set_termios_ldisc(tty, ldisc);
632 
633         return 0;
634 }
635 
636 /**
637  *      tty_ldisc_hangup                -       hangup ldisc reset
638  *      @tty: tty being hung up
639  *
640  *      Some tty devices reset their termios when they receive a hangup
641  *      event. In that situation we must also switch back to N_TTY properly
642  *      before we reset the termios data.
643  *
644  *      Locking: We can take the ldisc mutex as the rest of the code is
645  *      careful to allow for this.
646  *
647  *      In the pty pair case this occurs in the close() path of the
648  *      tty itself so we must be careful about locking rules.
649  */
650 
651 void tty_ldisc_hangup(struct tty_struct *tty)
652 {
653         struct tty_ldisc *ld;
654         int reset = tty->driver->flags & TTY_DRIVER_RESET_TERMIOS;
655         int err = 0;
656 
657         tty_ldisc_debug(tty, "%p: closing\n", tty->ldisc);
658 
659         ld = tty_ldisc_ref(tty);
660         if (ld != NULL) {
661                 if (ld->ops->flush_buffer)
662                         ld->ops->flush_buffer(tty);
663                 tty_driver_flush_buffer(tty);
664                 if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
665                     ld->ops->write_wakeup)
666                         ld->ops->write_wakeup(tty);
667                 if (ld->ops->hangup)
668                         ld->ops->hangup(tty);
669                 tty_ldisc_deref(ld);
670         }
671 
672         wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
673         wake_up_interruptible_poll(&tty->read_wait, POLLIN);
674 
675         /*
676          * Shutdown the current line discipline, and reset it to
677          * N_TTY if need be.
678          *
679          * Avoid racing set_ldisc or tty_ldisc_release
680          */
681         tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
682 
683         if (tty->ldisc) {
684 
685                 /* At this point we have a halted ldisc; we want to close it and
686                    reopen a new ldisc. We could defer the reopen to the next
687                    open but it means auditing a lot of other paths so this is
688                    a FIXME */
689                 if (reset == 0) {
690 
691                         if (!tty_ldisc_reinit(tty, tty->termios.c_line))
692                                 err = tty_ldisc_open(tty, tty->ldisc);
693                         else
694                                 err = 1;
695                 }
696                 /* If the re-open fails or we reset then go to N_TTY. The
697                    N_TTY open cannot fail */
698                 if (reset || err) {
699                         BUG_ON(tty_ldisc_reinit(tty, N_TTY));
700                         WARN_ON(tty_ldisc_open(tty, tty->ldisc));
701                 }
702         }
703         tty_ldisc_unlock(tty);
704         if (reset)
705                 tty_reset_termios(tty);
706 
707         tty_ldisc_debug(tty, "%p: re-opened\n", tty->ldisc);
708 }
709 
710 /**
711  *      tty_ldisc_setup                 -       open line discipline
712  *      @tty: tty being shut down
713  *      @o_tty: pair tty for pty/tty pairs
714  *
715  *      Called during the initial open of a tty/pty pair in order to set up the
716  *      line disciplines and bind them to the tty. This has no locking issues
717  *      as the device isn't yet active.
718  */
719 
720 int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
721 {
722         struct tty_ldisc *ld = tty->ldisc;
723         int retval;
724 
725         retval = tty_ldisc_open(tty, ld);
726         if (retval)
727                 return retval;
728 
729         if (o_tty) {
730                 retval = tty_ldisc_open(o_tty, o_tty->ldisc);
731                 if (retval) {
732                         tty_ldisc_close(tty, ld);
733                         return retval;
734                 }
735         }
736         return 0;
737 }
738 
739 static void tty_ldisc_kill(struct tty_struct *tty)
740 {
741         /*
742          * Now kill off the ldisc
743          */
744         tty_ldisc_close(tty, tty->ldisc);
745         tty_ldisc_put(tty->ldisc);
746         /* Force an oops if we mess this up */
747         tty->ldisc = NULL;
748 
749         /* Ensure the next open requests the N_TTY ldisc */
750         tty_set_termios_ldisc(tty, N_TTY);
751 }
752 
753 /**
754  *      tty_ldisc_release               -       release line discipline
755  *      @tty: tty being shut down (or one end of pty pair)
756  *
757  *      Called during the final close of a tty or a pty pair in order to shut
758  *      down the line discpline layer. On exit, each ldisc assigned is N_TTY and
759  *      each ldisc has not been opened.
760  */
761 
762 void tty_ldisc_release(struct tty_struct *tty)
763 {
764         struct tty_struct *o_tty = tty->link;
765 
766         /*
767          * Shutdown this line discipline. As this is the final close,
768          * it does not race with the set_ldisc code path.
769          */
770 
771         tty_ldisc_lock_pair(tty, o_tty);
772         tty_ldisc_kill(tty);
773         if (o_tty)
774                 tty_ldisc_kill(o_tty);
775         tty_ldisc_unlock_pair(tty, o_tty);
776 
777         /* And the memory resources remaining (buffers, termios) will be
778            disposed of when the kref hits zero */
779 
780         tty_ldisc_debug(tty, "released\n");
781 }
782 
783 /**
784  *      tty_ldisc_init          -       ldisc setup for new tty
785  *      @tty: tty being allocated
786  *
787  *      Set up the line discipline objects for a newly allocated tty. Note that
788  *      the tty structure is not completely set up when this call is made.
789  */
790 
791 void tty_ldisc_init(struct tty_struct *tty)
792 {
793         struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
794         if (IS_ERR(ld))
795                 panic("n_tty: init_tty");
796         tty->ldisc = ld;
797 }
798 
799 /**
800  *      tty_ldisc_init          -       ldisc cleanup for new tty
801  *      @tty: tty that was allocated recently
802  *
803  *      The tty structure must not becompletely set up (tty_ldisc_setup) when
804  *      this call is made.
805  */
806 void tty_ldisc_deinit(struct tty_struct *tty)
807 {
808         tty_ldisc_put(tty->ldisc);
809         tty->ldisc = NULL;
810 }
811 
812 void tty_ldisc_begin(void)
813 {
814         /* Setup the default TTY line discipline. */
815         (void) tty_register_ldisc(N_TTY, &tty_ldisc_N_TTY);
816 }
817 

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