text: #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
   */
  
  #if defined(CONFIG_LDISC_AUTOLOAD)
  	#define INITIAL_AUTOLOAD_STATE	1
  #else
  	#define INITIAL_AUTOLOAD_STATE	0
  #endif
  static int tty_ldisc_autoload = INITIAL_AUTOLOAD_STATE;
  
  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)) {
  		if (!capable(CAP_SYS_MODULE) && !tty_ldisc_autoload)
  			return ERR_PTR(-EPERM);
  		request_module("tty-ldisc-%d", disc);
  		ldops = get_ldops(disc);
  		if (IS_ERR(ldops))
  			return ERR_CAST(ldops);
  	}
  
  	/*
  	 * There is no way to handle allocation failure of only 16 bytes.
  	 * Let's simplify error handling and save more memory.
  	 */
  	ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL | __GFP_NOFAIL);
  	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);
  }
  
  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;
  }
  
  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_failto	-	helper for ldisc failback
   *	@tty: tty to open the ldisc on
   *	@ld: ldisc we are trying to fail back to
   *
   *	Helper to try and recover a tty when switching back to the old
   *	ldisc fails and we need something attached.
   */
  
  static int tty_ldisc_failto(struct tty_struct *tty, int ld)
  {
  	struct tty_ldisc *disc = tty_ldisc_get(tty, ld);
  	int r;
  
  	if (IS_ERR(disc))
  		return PTR_ERR(disc);
  	tty->ldisc = disc;
  	tty_set_termios_ldisc(tty, ld);
  	if ((r = tty_ldisc_open(tty, disc)) < 0)
  		tty_ldisc_put(disc);
  	return r;
  }
  
  /**
   *	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)
  {
  	/* There is an outstanding reference here so this is safe */
  	if (tty_ldisc_failto(tty, old->ops->num) < 0) {
  		const char *name = tty_name(tty);
  
  		pr_warn("Falling back ldisc for %s.\n", name);
  		/* The traditional behaviour is to fall back to N_TTY, we
  		   want to avoid falling back to N_NULL unless we have no
  		   choice to avoid the risk of breaking anything */
  		if (tty_ldisc_failto(tty, N_TTY) < 0 &&
  		    tty_ldisc_failto(tty, N_NULL) < 0)
  			panic("Couldn't open N_NULL ldisc for %s.", name);
  	}
  }
  
  /**
   *	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;
  }
  EXPORT_SYMBOL_GPL(tty_set_ldisc);
  
  /**
   *	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) {
  		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) < 0)
  				WARN_ON(tty_ldisc_reinit(tty, N_NULL) < 0);
  		} 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");
  }
  EXPORT_SYMBOL_GPL(tty_ldisc_release);
  
  /**
   *	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.
   */
  
  int tty_ldisc_init(struct tty_struct *tty)
  {
  	struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
  	if (IS_ERR(ld))
  		return PTR_ERR(ld);
  	tty->ldisc = ld;
  	return 0;
  }
  
  /**
   *	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) {
  #if defined(CONFIG_TTY_FLUSH_LOCAL_ECHO)
  		if (tty->echo_delayed_work.work.func)
  			cancel_delayed_work_sync(&tty->echo_delayed_work);
  #endif
  		tty_ldisc_put(tty->ldisc);
  	}
  	tty->ldisc = NULL;
  }
  
  static int zero;
  static int one = 1;
  static struct ctl_table tty_table[] = {
  	{
  		.procname	= "ldisc_autoload",
  		.data		= &tty_ldisc_autoload,
  		.maxlen		= sizeof(tty_ldisc_autoload),
  		.mode		= 0644,
  		.proc_handler	= proc_dointvec,
  		.extra1		= &zero,
  		.extra2		= &one,
  	},
  	{ }
  };
  
  static struct ctl_table tty_dir_table[] = {
  	{
  		.procname	= "tty",
  		.mode		= 0555,
  		.child		= tty_table,
  	},
  	{ }
  };
  
  static struct ctl_table tty_root_table[] = {
  	{
  		.procname	= "dev",
  		.mode		= 0555,
  		.child		= tty_dir_table,
  	},
  	{ }
  };
  
  void tty_sysctl_init(void)
  {
  	register_sysctl_table(tty_root_table);
  }
  