text: /*
   * Copyright (C) 2001 Anton Blanchard <anton@au.ibm.com>, IBM
   * Copyright (C) 2001 Paul Mackerras <paulus@au.ibm.com>, IBM
   * Copyright (C) 2004 Benjamin Herrenschmidt <benh@kernel.crashing.org>, IBM Corp.
   * Copyright (C) 2004 IBM Corporation
   *
   * Additional Author(s):
   *  Ryan S. Arnold <rsa@us.ibm.com>
   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License as published by
   * the Free Software Foundation; either version 2 of the License, or
   * (at your option) any later version.
   * 
   * This program is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   * GNU General Public License for more details.
   * 
   * You should have received a copy of the GNU General Public License
   * along with this program; if not, write to the Free Software
   * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA
   */
  
  #include <linux/console.h>
  #include <linux/cpumask.h>
  #include <linux/init.h>
  #include <linux/kbd_kern.h>
  #include <linux/kernel.h>
  #include <linux/kthread.h>
  #include <linux/list.h>
  #include <linux/module.h>
  #include <linux/major.h>
  #include <linux/sysrq.h>
  #include <linux/tty.h>
  #include <linux/tty_flip.h>
  #include <linux/sched.h>
  #include <linux/spinlock.h>
  #include <linux/delay.h>
  #include <linux/freezer.h>
  #include <linux/slab.h>
  
  #include <asm/uaccess.h>
  
  #include "hvc_console.h"
  
  #define HVC_MAJOR	229
  #define HVC_MINOR	0
  
  /*
   * Wait this long per iteration while trying to push buffered data to the
   * hypervisor before allowing the tty to complete a close operation.
   */
  #define HVC_CLOSE_WAIT (HZ/100) /* 1/10 of a second */
  
  /*
   * These sizes are most efficient for vio, because they are the
   * native transfer size. We could make them selectable in the
   * future to better deal with backends that want other buffer sizes.
   */
  #define N_OUTBUF	16
  #define N_INBUF		16
  
  #define __ALIGNED__ __attribute__((__aligned__(sizeof(long))))
  
  static struct tty_driver *hvc_driver;
  static struct task_struct *hvc_task;
  
  /* Picks up late kicks after list walk but before schedule() */
  static int hvc_kicked;
  
  static int hvc_init(void);
  
  #ifdef CONFIG_MAGIC_SYSRQ
  static int sysrq_pressed;
  #endif
  
  /* dynamic list of hvc_struct instances */
  static LIST_HEAD(hvc_structs);
  
  /*
   * Protect the list of hvc_struct instances from inserts and removals during
   * list traversal.
   */
  static DEFINE_SPINLOCK(hvc_structs_lock);
  
  /*
   * This value is used to assign a tty->index value to a hvc_struct based
   * upon order of exposure via hvc_probe(), when we can not match it to
   * a console candidate registered with hvc_instantiate().
   */
  static int last_hvc = -1;
  
  /*
   * Do not call this function with either the hvc_structs_lock or the hvc_struct
   * lock held.  If successful, this function increments the kref reference
   * count against the target hvc_struct so it should be released when finished.
   */
  static struct hvc_struct *hvc_get_by_index(int index)
  {
  	struct hvc_struct *hp;
  	unsigned long flags;
  
  	spin_lock(&hvc_structs_lock);
  
  	list_for_each_entry(hp, &hvc_structs, next) {
  		spin_lock_irqsave(&hp->lock, flags);
  		if (hp->index == index) {
  			kref_get(&hp->kref);
  			spin_unlock_irqrestore(&hp->lock, flags);
  			spin_unlock(&hvc_structs_lock);
  			return hp;
  		}
  		spin_unlock_irqrestore(&hp->lock, flags);
  	}
  	hp = NULL;
  
  	spin_unlock(&hvc_structs_lock);
  	return hp;
  }
  
  
  /*
   * Initial console vtermnos for console API usage prior to full console
   * initialization.  Any vty adapter outside this range will not have usable
   * console interfaces but can still be used as a tty device.  This has to be
   * static because kmalloc will not work during early console init.
   */
  static const struct hv_ops *cons_ops[MAX_NR_HVC_CONSOLES];
  static uint32_t vtermnos[MAX_NR_HVC_CONSOLES] =
  	{[0 ... MAX_NR_HVC_CONSOLES - 1] = -1};
  
  /*
   * Console APIs, NOT TTY.  These APIs are available immediately when
   * hvc_console_setup() finds adapters.
   */
  
  static void hvc_console_print(struct console *co, const char *b,
  			      unsigned count)
  {
  	char c[N_OUTBUF] __ALIGNED__;
  	unsigned i = 0, n = 0;
  	int r, donecr = 0, index = co->index;
  
  	/* Console access attempt outside of acceptable console range. */
  	if (index >= MAX_NR_HVC_CONSOLES)
  		return;
  
  	/* This console adapter was removed so it is not usable. */
  	if (vtermnos[index] == -1)
  		return;
  
  	while (count > 0 || i > 0) {
  		if (count > 0 && i < sizeof(c)) {
  			if (b[n] == '\n' && !donecr) {
  				c[i++] = '\r';
  				donecr = 1;
  			} else {
  				c[i++] = b[n++];
  				donecr = 0;
  				--count;
  			}
  		} else {
  			r = cons_ops[index]->put_chars(vtermnos[index], c, i);
  			if (r <= 0) {
  				/* throw away chars on error */
  				i = 0;
  			} else if (r > 0) {
  				i -= r;
  				if (i > 0)
  					memmove(c, c+r, i);
  			}
  		}
  	}
  }
  
  static struct tty_driver *hvc_console_device(struct console *c, int *index)
  {
  	if (vtermnos[c->index] == -1)
  		return NULL;
  
  	*index = c->index;
  	return hvc_driver;
  }
  
  static int __init hvc_console_setup(struct console *co, char *options)
  {
  	if (co->index < 0 || co->index >= MAX_NR_HVC_CONSOLES)
  		return -ENODEV;
  
  	if (vtermnos[co->index] == -1)
  		return -ENODEV;
  
  	return 0;
  }
  
  static struct console hvc_con_driver = {
  	.name		= "hvc",
  	.write		= hvc_console_print,
  	.device		= hvc_console_device,
  	.setup		= hvc_console_setup,
  	.flags		= CON_PRINTBUFFER,
  	.index		= -1,
  };
  
  /*
   * Early console initialization.  Precedes driver initialization.
   *
   * (1) we are first, and the user specified another driver
   * -- index will remain -1
   * (2) we are first and the user specified no driver
   * -- index will be set to 0, then we will fail setup.
   * (3)  we are first and the user specified our driver
   * -- index will be set to user specified driver, and we will fail
   * (4) we are after driver, and this initcall will register us
   * -- if the user didn't specify a driver then the console will match
   *
   * Note that for cases 2 and 3, we will match later when the io driver
   * calls hvc_instantiate() and call register again.
   */
  static int __init hvc_console_init(void)
  {
  	register_console(&hvc_con_driver);
  	return 0;
  }
  console_initcall(hvc_console_init);
  
  /* callback when the kboject ref count reaches zero. */
  static void destroy_hvc_struct(struct kref *kref)
  {
  	struct hvc_struct *hp = container_of(kref, struct hvc_struct, kref);
  	unsigned long flags;
  
  	spin_lock(&hvc_structs_lock);
  
  	spin_lock_irqsave(&hp->lock, flags);
  	list_del(&(hp->next));
  	spin_unlock_irqrestore(&hp->lock, flags);
  
  	spin_unlock(&hvc_structs_lock);
  
  	kfree(hp);
  }
  
  /*
   * hvc_instantiate() is an early console discovery method which locates
   * consoles * prior to the vio subsystem discovering them.  Hotplugged
   * vty adapters do NOT get an hvc_instantiate() callback since they
   * appear after early console init.
   */
  int hvc_instantiate(uint32_t vtermno, int index, const struct hv_ops *ops)
  {
  	struct hvc_struct *hp;
  
  	if (index < 0 || index >= MAX_NR_HVC_CONSOLES)
  		return -1;
  
  	if (vtermnos[index] != -1)
  		return -1;
  
  	/* make sure no no tty has been registered in this index */
  	hp = hvc_get_by_index(index);
  	if (hp) {
  		kref_put(&hp->kref, destroy_hvc_struct);
  		return -1;
  	}
  
  	vtermnos[index] = vtermno;
  	cons_ops[index] = ops;
  
  	/* reserve all indices up to and including this index */
  	if (last_hvc < index)
  		last_hvc = index;
  
  	/* if this index is what the user requested, then register
  	 * now (setup won't fail at this point).  It's ok to just
  	 * call register again if previously .setup failed.
  	 */
  	if (index == hvc_con_driver.index)
  		register_console(&hvc_con_driver);
  
  	return 0;
  }
  EXPORT_SYMBOL_GPL(hvc_instantiate);
  
  /* Wake the sleeping khvcd */
  void hvc_kick(void)
  {
  	hvc_kicked = 1;
  	wake_up_process(hvc_task);
  }
  EXPORT_SYMBOL_GPL(hvc_kick);
  
  static void hvc_unthrottle(struct tty_struct *tty)
  {
  	hvc_kick();
  }
  
  /*
   * The TTY interface won't be used until after the vio layer has exposed the vty
   * adapter to the kernel.
   */
  static int hvc_open(struct tty_struct *tty, struct file * filp)
  {
  	struct hvc_struct *hp;
  	unsigned long flags;
  	int rc = 0;
  
  	/* Auto increments kref reference if found. */
  	if (!(hp = hvc_get_by_index(tty->index)))
  		return -ENODEV;
  
  	spin_lock_irqsave(&hp->lock, flags);
  	/* Check and then increment for fast path open. */
  	if (hp->count++ > 0) {
  		tty_kref_get(tty);
  		spin_unlock_irqrestore(&hp->lock, flags);
  		hvc_kick();
  		return 0;
  	} /* else count == 0 */
  
  	tty->driver_data = hp;
  
  	hp->tty = tty_kref_get(tty);
  
  	spin_unlock_irqrestore(&hp->lock, flags);
  
  	if (hp->ops->notifier_add)
  		rc = hp->ops->notifier_add(hp, hp->data);
  
  	/*
  	 * If the notifier fails we return an error.  The tty layer
  	 * will call hvc_close() after a failed open but we don't want to clean
  	 * up there so we'll clean up here and clear out the previously set
  	 * tty fields and return the kref reference.
  	 */
  	if (rc) {
  		spin_lock_irqsave(&hp->lock, flags);
  		hp->tty = NULL;
  		spin_unlock_irqrestore(&hp->lock, flags);
  		tty_kref_put(tty);
  		tty->driver_data = NULL;
  		kref_put(&hp->kref, destroy_hvc_struct);
  		printk(KERN_ERR "hvc_open: request_irq failed with rc %d.\n", rc);
  	}
  	/* Force wakeup of the polling thread */
  	hvc_kick();
  
  	return rc;
  }
  
  static void hvc_close(struct tty_struct *tty, struct file * filp)
  {
  	struct hvc_struct *hp;
  	unsigned long flags;
  
  	if (tty_hung_up_p(filp))
  		return;
  
  	/*
  	 * No driver_data means that this close was issued after a failed
  	 * hvc_open by the tty layer's release_dev() function and we can just
  	 * exit cleanly because the kref reference wasn't made.
  	 */
  	if (!tty->driver_data)
  		return;
  
  	hp = tty->driver_data;
  
  	spin_lock_irqsave(&hp->lock, flags);
  
  	if (--hp->count == 0) {
  		/* We are done with the tty pointer now. */
  		hp->tty = NULL;
  		spin_unlock_irqrestore(&hp->lock, flags);
  
  		if (hp->ops->notifier_del)
  			hp->ops->notifier_del(hp, hp->data);
  
  		/* cancel pending tty resize work */
  		cancel_work_sync(&hp->tty_resize);
  
  		/*
  		 * Chain calls chars_in_buffer() and returns immediately if
  		 * there is no buffered data otherwise sleeps on a wait queue
  		 * waking periodically to check chars_in_buffer().
  		 */
  		tty_wait_until_sent(tty, HVC_CLOSE_WAIT);
  	} else {
  		if (hp->count < 0)
  			printk(KERN_ERR "hvc_close %X: oops, count is %d\n",
  				hp->vtermno, hp->count);
  		spin_unlock_irqrestore(&hp->lock, flags);
  	}
  
  	tty_kref_put(tty);
  	kref_put(&hp->kref, destroy_hvc_struct);
  }
  
  static void hvc_hangup(struct tty_struct *tty)
  {
  	struct hvc_struct *hp = tty->driver_data;
  	unsigned long flags;
  	int temp_open_count;
  
  	if (!hp)
  		return;
  
  	/* cancel pending tty resize work */
  	cancel_work_sync(&hp->tty_resize);
  
  	spin_lock_irqsave(&hp->lock, flags);
  
  	/*
  	 * The N_TTY line discipline has problems such that in a close vs
  	 * open->hangup case this can be called after the final close so prevent
  	 * that from happening for now.
  	 */
  	if (hp->count <= 0) {
  		spin_unlock_irqrestore(&hp->lock, flags);
  		return;
  	}
  
  	temp_open_count = hp->count;
  	hp->count = 0;
  	hp->n_outbuf = 0;
  	hp->tty = NULL;
  
  	spin_unlock_irqrestore(&hp->lock, flags);
  
  	if (hp->ops->notifier_hangup)
  		hp->ops->notifier_hangup(hp, hp->data);
  
  	while(temp_open_count) {
  		--temp_open_count;
  		tty_kref_put(tty);
  		kref_put(&hp->kref, destroy_hvc_struct);
  	}
  }
  
  /*
   * Push buffered characters whether they were just recently buffered or waiting
   * on a blocked hypervisor.  Call this function with hp->lock held.
   */
  static int hvc_push(struct hvc_struct *hp)
  {
  	int n;
  
  	n = hp->ops->put_chars(hp->vtermno, hp->outbuf, hp->n_outbuf);
  	if (n <= 0) {
  		if (n == 0) {
  			hp->do_wakeup = 1;
  			return 0;
  		}
  		/* throw away output on error; this happens when
  		   there is no session connected to the vterm. */
  		hp->n_outbuf = 0;
  	} else
  		hp->n_outbuf -= n;
  	if (hp->n_outbuf > 0)
  		memmove(hp->outbuf, hp->outbuf + n, hp->n_outbuf);
  	else
  		hp->do_wakeup = 1;
  
  	return n;
  }
  
  static int hvc_write(struct tty_struct *tty, const unsigned char *buf, int count)
  {
  	struct hvc_struct *hp = tty->driver_data;
  	unsigned long flags;
  	int rsize, written = 0;
  
  	/* This write was probably executed during a tty close. */
  	if (!hp)
  		return -EPIPE;
  
  	if (hp->count <= 0)
  		return -EIO;
  
  	spin_lock_irqsave(&hp->lock, flags);
  
  	/* Push pending writes */
  	if (hp->n_outbuf > 0)
  		hvc_push(hp);
  
  	while (count > 0 && (rsize = hp->outbuf_size - hp->n_outbuf) > 0) {
  		if (rsize > count)
  			rsize = count;
  		memcpy(hp->outbuf + hp->n_outbuf, buf, rsize);
  		count -= rsize;
  		buf += rsize;
  		hp->n_outbuf += rsize;
  		written += rsize;
  		hvc_push(hp);
  	}
  	spin_unlock_irqrestore(&hp->lock, flags);
  
  	/*
  	 * Racy, but harmless, kick thread if there is still pending data.
  	 */
  	if (hp->n_outbuf)
  		hvc_kick();
  
  	return written;
  }
  
  /**
   * hvc_set_winsz() - Resize the hvc tty terminal window.
   * @work:	work structure.
   *
   * The routine shall not be called within an atomic context because it
   * might sleep.
   *
   * Locking:	hp->lock
   */
  static void hvc_set_winsz(struct work_struct *work)
  {
  	struct hvc_struct *hp;
  	unsigned long hvc_flags;
  	struct tty_struct *tty;
  	struct winsize ws;
  
  	hp = container_of(work, struct hvc_struct, tty_resize);
  
  	spin_lock_irqsave(&hp->lock, hvc_flags);
  	if (!hp->tty) {
  		spin_unlock_irqrestore(&hp->lock, hvc_flags);
  		return;
  	}
  	ws  = hp->ws;
  	tty = tty_kref_get(hp->tty);
  	spin_unlock_irqrestore(&hp->lock, hvc_flags);
  
  	tty_do_resize(tty, &ws);
  	tty_kref_put(tty);
  }
  
  /*
   * This is actually a contract between the driver and the tty layer outlining
   * how much write room the driver can guarantee will be sent OR BUFFERED.  This
   * driver MUST honor the return value.
   */
  static int hvc_write_room(struct tty_struct *tty)
  {
  	struct hvc_struct *hp = tty->driver_data;
  
  	if (!hp)
  		return -1;
  
  	return hp->outbuf_size - hp->n_outbuf;
  }
  
  static int hvc_chars_in_buffer(struct tty_struct *tty)
  {
  	struct hvc_struct *hp = tty->driver_data;
  
  	if (!hp)
  		return 0;
  	return hp->n_outbuf;
  }
  
  /*
   * timeout will vary between the MIN and MAX values defined here.  By default
   * and during console activity we will use a default MIN_TIMEOUT of 10.  When
   * the console is idle, we increase the timeout value on each pass through
   * msleep until we reach the max.  This may be noticeable as a brief (average
   * one second) delay on the console before the console responds to input when
   * there has been no input for some time.
   */
  #define MIN_TIMEOUT		(10)
  #define MAX_TIMEOUT		(2000)
  static u32 timeout = MIN_TIMEOUT;
  
  #define HVC_POLL_READ	0x00000001
  #define HVC_POLL_WRITE	0x00000002
  
  int hvc_poll(struct hvc_struct *hp)
  {
  	struct tty_struct *tty;
  	int i, n, poll_mask = 0;
  	char buf[N_INBUF] __ALIGNED__;
  	unsigned long flags;
  	int read_total = 0;
  	int written_total = 0;
  
  	spin_lock_irqsave(&hp->lock, flags);
  
  	/* Push pending writes */
  	if (hp->n_outbuf > 0)
  		written_total = hvc_push(hp);
  
  	/* Reschedule us if still some write pending */
  	if (hp->n_outbuf > 0) {
  		poll_mask |= HVC_POLL_WRITE;
  		/* If hvc_push() was not able to write, sleep a few msecs */
  		timeout = (written_total) ? 0 : MIN_TIMEOUT;
  	}
  
  	/* No tty attached, just skip */
  	tty = tty_kref_get(hp->tty);
  	if (tty == NULL)
  		goto bail;
  
  	/* Now check if we can get data (are we throttled ?) */
  	if (test_bit(TTY_THROTTLED, &tty->flags))
  		goto throttled;
  
  	/* If we aren't notifier driven and aren't throttled, we always
  	 * request a reschedule
  	 */
  	if (!hp->irq_requested)
  		poll_mask |= HVC_POLL_READ;
  
  	/* Read data if any */
  	for (;;) {
  		int count = tty_buffer_request_room(tty, N_INBUF);
  
  		/* If flip is full, just reschedule a later read */
  		if (count == 0) {
  			poll_mask |= HVC_POLL_READ;
  			break;
  		}
  
  		n = hp->ops->get_chars(hp->vtermno, buf, count);
  		if (n <= 0) {
  			/* Hangup the tty when disconnected from host */
  			if (n == -EPIPE) {
  				spin_unlock_irqrestore(&hp->lock, flags);
  				tty_hangup(tty);
  				spin_lock_irqsave(&hp->lock, flags);
  			} else if ( n == -EAGAIN ) {
  				/*
  				 * Some back-ends can only ensure a certain min
  				 * num of bytes read, which may be > 'count'.
  				 * Let the tty clear the flip buff to make room.
  				 */
  				poll_mask |= HVC_POLL_READ;
  			}
  			break;
  		}
  		for (i = 0; i < n; ++i) {
  #ifdef CONFIG_MAGIC_SYSRQ
  			if (hp->index == hvc_con_driver.index) {
  				/* Handle the SysRq Hack */
  				/* XXX should support a sequence */
  				if (buf[i] == '\x0f') {	/* ^O */
  					/* if ^O is pressed again, reset
  					 * sysrq_pressed and flip ^O char */
  					sysrq_pressed = !sysrq_pressed;
  					if (sysrq_pressed)
  						continue;
  				} else if (sysrq_pressed) {
  					handle_sysrq(buf[i], tty);
  					sysrq_pressed = 0;
  					continue;
  				}
  			}
  #endif /* CONFIG_MAGIC_SYSRQ */
  			tty_insert_flip_char(tty, buf[i], 0);
  		}
  
  		read_total += n;
  	}
   throttled:
  	/* Wakeup write queue if necessary */
  	if (hp->do_wakeup) {
  		hp->do_wakeup = 0;
  		tty_wakeup(tty);
  	}
   bail:
  	spin_unlock_irqrestore(&hp->lock, flags);
  
  	if (read_total) {
  		/* Activity is occurring, so reset the polling backoff value to
  		   a minimum for performance. */
  		timeout = MIN_TIMEOUT;
  
  		tty_flip_buffer_push(tty);
  	}
  	if (tty)
  		tty_kref_put(tty);
  
  	return poll_mask;
  }
  EXPORT_SYMBOL_GPL(hvc_poll);
  
  /**
   * __hvc_resize() - Update terminal window size information.
   * @hp:		HVC console pointer
   * @ws:		Terminal window size structure
   *
   * Stores the specified window size information in the hvc structure of @hp.
   * The function schedule the tty resize update.
   *
   * Locking:	Locking free; the function MUST be called holding hp->lock
   */
  void __hvc_resize(struct hvc_struct *hp, struct winsize ws)
  {
  	hp->ws = ws;
  	schedule_work(&hp->tty_resize);
  }
  EXPORT_SYMBOL_GPL(__hvc_resize);
  
  /*
   * This kthread is either polling or interrupt driven.  This is determined by
   * calling hvc_poll() who determines whether a console adapter support
   * interrupts.
   */
  static int khvcd(void *unused)
  {
  	int poll_mask;
  	struct hvc_struct *hp;
  
  	set_freezable();
  	__set_current_state(TASK_RUNNING);
  	do {
  		poll_mask = 0;
  		hvc_kicked = 0;
  		try_to_freeze();
  		wmb();
  		if (!cpus_are_in_xmon()) {
  			spin_lock(&hvc_structs_lock);
  			list_for_each_entry(hp, &hvc_structs, next) {
  				poll_mask |= hvc_poll(hp);
  			}
  			spin_unlock(&hvc_structs_lock);
  		} else
  			poll_mask |= HVC_POLL_READ;
  		if (hvc_kicked)
  			continue;
  		set_current_state(TASK_INTERRUPTIBLE);
  		if (!hvc_kicked) {
  			if (poll_mask == 0)
  				schedule();
  			else {
  				if (timeout < MAX_TIMEOUT)
  					timeout += (timeout >> 6) + 1;
  
  				msleep_interruptible(timeout);
  			}
  		}
  		__set_current_state(TASK_RUNNING);
  	} while (!kthread_should_stop());
  
  	return 0;
  }
  
  static const struct tty_operations hvc_ops = {
  	.open = hvc_open,
  	.close = hvc_close,
  	.write = hvc_write,
  	.hangup = hvc_hangup,
  	.unthrottle = hvc_unthrottle,
  	.write_room = hvc_write_room,
  	.chars_in_buffer = hvc_chars_in_buffer,
  };
  
  struct hvc_struct *hvc_alloc(uint32_t vtermno, int data,
  			     const struct hv_ops *ops,
  			     int outbuf_size)
  {
  	struct hvc_struct *hp;
  	int i;
  
  	/* We wait until a driver actually comes along */
  	if (!hvc_driver) {
  		int err = hvc_init();
  		if (err)
  			return ERR_PTR(err);
  	}
  
  	hp = kzalloc(ALIGN(sizeof(*hp), sizeof(long)) + outbuf_size,
  			GFP_KERNEL);
  	if (!hp)
  		return ERR_PTR(-ENOMEM);
  
  	hp->vtermno = vtermno;
  	hp->data = data;
  	hp->ops = ops;
  	hp->outbuf_size = outbuf_size;
  	hp->outbuf = &((char *)hp)[ALIGN(sizeof(*hp), sizeof(long))];
  
  	kref_init(&hp->kref);
  
  	INIT_WORK(&hp->tty_resize, hvc_set_winsz);
  	spin_lock_init(&hp->lock);
  	spin_lock(&hvc_structs_lock);
  
  	/*
  	 * find index to use:
  	 * see if this vterm id matches one registered for console.
  	 */
  	for (i=0; i < MAX_NR_HVC_CONSOLES; i++)
  		if (vtermnos[i] == hp->vtermno &&
  		    cons_ops[i] == hp->ops)
  			break;
  
  	/* no matching slot, just use a counter */
  	if (i >= MAX_NR_HVC_CONSOLES)
  		i = ++last_hvc;
  
  	hp->index = i;
  
  	list_add_tail(&(hp->next), &hvc_structs);
  	spin_unlock(&hvc_structs_lock);
  
  	return hp;
  }
  EXPORT_SYMBOL_GPL(hvc_alloc);
  
  int hvc_remove(struct hvc_struct *hp)
  {
  	unsigned long flags;
  	struct tty_struct *tty;
  
  	spin_lock_irqsave(&hp->lock, flags);
  	tty = tty_kref_get(hp->tty);
  
  	if (hp->index < MAX_NR_HVC_CONSOLES)
  		vtermnos[hp->index] = -1;
  
  	/* Don't whack hp->irq because tty_hangup() will need to free the irq. */
  
  	spin_unlock_irqrestore(&hp->lock, flags);
  
  	/*
  	 * We 'put' the instance that was grabbed when the kref instance
  	 * was initialized using kref_init().  Let the last holder of this
  	 * kref cause it to be removed, which will probably be the tty_vhangup
  	 * below.
  	 */
  	kref_put(&hp->kref, destroy_hvc_struct);
  
  	/*
  	 * This function call will auto chain call hvc_hangup.
  	 */
  	if (tty) {
  		tty_vhangup(tty);
  		tty_kref_put(tty);
  	}
  	return 0;
  }
  EXPORT_SYMBOL_GPL(hvc_remove);
  
  /* Driver initialization: called as soon as someone uses hvc_alloc(). */
  static int hvc_init(void)
  {
  	struct tty_driver *drv;
  	int err;
  
  	/* We need more than hvc_count adapters due to hotplug additions. */
  	drv = alloc_tty_driver(HVC_ALLOC_TTY_ADAPTERS);
  	if (!drv) {
  		err = -ENOMEM;
  		goto out;
  	}
  
  	drv->owner = THIS_MODULE;
  	drv->driver_name = "hvc";
  	drv->name = "hvc";
  	drv->major = HVC_MAJOR;
  	drv->minor_start = HVC_MINOR;
  	drv->type = TTY_DRIVER_TYPE_SYSTEM;
  	drv->init_termios = tty_std_termios;
  	drv->flags = TTY_DRIVER_REAL_RAW | TTY_DRIVER_RESET_TERMIOS;
  	tty_set_operations(drv, &hvc_ops);
  
  	/* Always start the kthread because there can be hotplug vty adapters
  	 * added later. */
  	hvc_task = kthread_run(khvcd, NULL, "khvcd");
  	if (IS_ERR(hvc_task)) {
  		printk(KERN_ERR "Couldn't create kthread for console.\n");
  		err = PTR_ERR(hvc_task);
  		goto put_tty;
  	}
  
  	err = tty_register_driver(drv);
  	if (err) {
  		printk(KERN_ERR "Couldn't register hvc console driver\n");
  		goto stop_thread;
  	}
  
  	/*
  	 * Make sure tty is fully registered before allowing it to be
  	 * found by hvc_console_device.
  	 */
  	smp_mb();
  	hvc_driver = drv;
  	return 0;
  
  stop_thread:
  	kthread_stop(hvc_task);
  	hvc_task = NULL;
  put_tty:
  	put_tty_driver(drv);
  out:
  	return err;
  }
  
  /* This isn't particularly necessary due to this being a console driver
   * but it is nice to be thorough.
   */
  static void __exit hvc_exit(void)
  {
  	if (hvc_driver) {
  		kthread_stop(hvc_task);
  
  		tty_unregister_driver(hvc_driver);
  		/* return tty_struct instances allocated in hvc_init(). */
  		put_tty_driver(hvc_driver);
  		unregister_console(&hvc_con_driver);
  	}
  }
  module_exit(hvc_exit);
  