text: /*
   * IBM eServer Hypervisor Virtual Console Server Device Driver
   * Copyright (C) 2003, 2004 IBM Corp.
   *  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
   *
   * Author(s) :  Ryan S. Arnold <rsa@us.ibm.com>
   *
   * This is the device driver for the IBM Hypervisor Virtual Console Server,
   * "hvcs".  The IBM hvcs provides a tty driver interface to allow Linux
   * user space applications access to the system consoles of logically
   * partitioned operating systems, e.g. Linux, running on the same partitioned
   * Power5 ppc64 system.  Physical hardware consoles per partition are not
   * practical on this hardware so system consoles are accessed by this driver
   * using inter-partition firmware interfaces to virtual terminal devices.
   *
   * A vty is known to the HMC as a "virtual serial server adapter".  It is a
   * virtual terminal device that is created by firmware upon partition creation
   * to act as a partitioned OS's console device.
   *
   * Firmware dynamically (via hotplug) exposes vty-servers to a running ppc64
   * Linux system upon their creation by the HMC or their exposure during boot.
   * The non-user interactive backend of this driver is implemented as a vio
   * device driver so that it can receive notification of vty-server lifetimes
   * after it registers with the vio bus to handle vty-server probe and remove
   * callbacks.
   *
   * Many vty-servers can be configured to connect to one vty, but a vty can
   * only be actively connected to by a single vty-server, in any manner, at one
   * time.  If the HMC is currently hosting the console for a target Linux
   * partition; attempts to open the tty device to the partition's console using
   * the hvcs on any partition will return -EBUSY with every open attempt until
   * the HMC frees the connection between its vty-server and the desired
   * partition's vty device.  Conversely, a vty-server may only be connected to
   * a single vty at one time even though it may have several configured vty
   * partner possibilities.
   *
   * Firmware does not provide notification of vty partner changes to this
   * driver.  This means that an HMC Super Admin may add or remove partner vtys
   * from a vty-server's partner list but the changes will not be signaled to
   * the vty-server.  Firmware only notifies the driver when a vty-server is
   * added or removed from the system.  To compensate for this deficiency, this
   * driver implements a sysfs update attribute which provides a method for
   * rescanning partner information upon a user's request.
   *
   * Each vty-server, prior to being exposed to this driver is reference counted
   * using the 2.6 Linux kernel kref construct.
   *
   * For direction on installation and usage of this driver please reference
   * Documentation/powerpc/hvcs.txt.
   */
  
  #include <linux/device.h>
  #include <linux/init.h>
  #include <linux/interrupt.h>
  #include <linux/kernel.h>
  #include <linux/kref.h>
  #include <linux/kthread.h>
  #include <linux/list.h>
  #include <linux/major.h>
  #include <linux/module.h>
  #include <linux/moduleparam.h>
  #include <linux/sched.h>
  #include <linux/slab.h>
  #include <linux/spinlock.h>
  #include <linux/stat.h>
  #include <linux/tty.h>
  #include <linux/tty_flip.h>
  #include <asm/hvconsole.h>
  #include <asm/hvcserver.h>
  #include <linux/uaccess.h>
  #include <asm/vio.h>
  
  /*
   * 1.3.0 -> 1.3.1 In hvcs_open memset(..,0x00,..) instead of memset(..,0x3F,00).
   * Removed braces around single statements following conditionals.  Removed '=
   * 0' after static int declarations since these default to zero.  Removed
   * list_for_each_safe() and replaced with list_for_each_entry() in
   * hvcs_get_by_index().  The 'safe' version is un-needed now that the driver is
   * using spinlocks.  Changed spin_lock_irqsave() to spin_lock() when locking
   * hvcs_structs_lock and hvcs_pi_lock since these are not touched in an int
   * handler.  Initialized hvcs_structs_lock and hvcs_pi_lock to
   * SPIN_LOCK_UNLOCKED at declaration time rather than in hvcs_module_init().
   * Added spin_lock around list_del() in destroy_hvcs_struct() to protect the
   * list traversals from a deletion.  Removed '= NULL' from pointer declaration
   * statements since they are initialized NULL by default.  Removed wmb()
   * instances from hvcs_try_write().  They probably aren't needed with locking in
   * place.  Added check and cleanup for hvcs_pi_buff = kmalloc() in
   * hvcs_module_init().  Exposed hvcs_struct.index via a sysfs attribute so that
   * the coupling between /dev/hvcs* and a vty-server can be automatically
   * determined.  Moved kobject_put() in hvcs_open outside of the
   * spin_unlock_irqrestore().
   *
   * 1.3.1 -> 1.3.2 Changed method for determining hvcs_struct->index and had it
   * align with how the tty layer always assigns the lowest index available.  This
   * change resulted in a list of ints that denotes which indexes are available.
   * Device additions and removals use the new hvcs_get_index() and
   * hvcs_return_index() helper functions.  The list is created with
   * hvsc_alloc_index_list() and it is destroyed with hvcs_free_index_list().
   * Without these fixes hotplug vty-server adapter support goes crazy with this
   * driver if the user removes a vty-server adapter.  Moved free_irq() outside of
   * the hvcs_final_close() function in order to get it out of the spinlock.
   * Rearranged hvcs_close().  Cleaned up some printks and did some housekeeping
   * on the changelog.  Removed local CLC_LENGTH and used HVCS_CLC_LENGTH from
   * arch/powerepc/include/asm/hvcserver.h
   *
   * 1.3.2 -> 1.3.3 Replaced yield() in hvcs_close() with tty_wait_until_sent() to
   * prevent possible lockup with realtime scheduling as similarly pointed out by
   * akpm in hvc_console.  Changed resulted in the removal of hvcs_final_close()
   * to reorder cleanup operations and prevent discarding of pending data during
   * an hvcs_close().  Removed spinlock protection of hvcs_struct data members in
   * hvcs_write_room() and hvcs_chars_in_buffer() because they aren't needed.
   */
  
  #define HVCS_DRIVER_VERSION "1.3.3"
  
  MODULE_AUTHOR("Ryan S. Arnold <rsa@us.ibm.com>");
  MODULE_DESCRIPTION("IBM hvcs (Hypervisor Virtual Console Server) Driver");
  MODULE_LICENSE("GPL");
  MODULE_VERSION(HVCS_DRIVER_VERSION);
  
  /*
   * Wait this long per iteration while trying to push buffered data to the
   * hypervisor before allowing the tty to complete a close operation.
   */
  #define HVCS_CLOSE_WAIT (HZ/100) /* 1/10 of a second */
  
  /*
   * Since the Linux TTY code does not currently (2-04-2004) support dynamic
   * addition of tty derived devices and we shouldn't allocate thousands of
   * tty_device pointers when the number of vty-server & vty partner connections
   * will most often be much lower than this, we'll arbitrarily allocate
   * HVCS_DEFAULT_SERVER_ADAPTERS tty_structs and cdev's by default when we
   * register the tty_driver. This can be overridden using an insmod parameter.
   */
  #define HVCS_DEFAULT_SERVER_ADAPTERS	64
  
  /*
   * The user can't insmod with more than HVCS_MAX_SERVER_ADAPTERS hvcs device
   * nodes as a sanity check.  Theoretically there can be over 1 Billion
   * vty-server & vty partner connections.
   */
  #define HVCS_MAX_SERVER_ADAPTERS	1024
  
  /*
   * We let Linux assign us a major number and we start the minors at zero.  There
   * is no intuitive mapping between minor number and the target vty-server
   * adapter except that each new vty-server adapter is always assigned to the
   * smallest minor number available.
   */
  #define HVCS_MINOR_START	0
  
  /*
   * The hcall interface involves putting 8 chars into each of two registers.
   * We load up those 2 registers (in arch/powerpc/platforms/pseries/hvconsole.c)
   * by casting char[16] to long[2].  It would work without __ALIGNED__, but a 
   * little (tiny) bit slower because an unaligned load is slower than aligned 
   * load.
   */
  #define __ALIGNED__	__attribute__((__aligned__(8)))
  
  /*
   * How much data can firmware send with each hvc_put_chars()?  Maybe this
   * should be moved into an architecture specific area.
   */
  #define HVCS_BUFF_LEN	16
  
  /*
   * This is the maximum amount of data we'll let the user send us (hvcs_write) at
   * once in a chunk as a sanity check.
   */
  #define HVCS_MAX_FROM_USER	4096
  
  /*
   * Be careful when adding flags to this line discipline.  Don't add anything
   * that will cause echoing or we'll go into recursive loop echoing chars back
   * and forth with the console drivers.
   */
  static struct ktermios hvcs_tty_termios = {
  	.c_iflag = IGNBRK | IGNPAR,
  	.c_oflag = OPOST,
  	.c_cflag = B38400 | CS8 | CREAD | HUPCL,
  	.c_cc = INIT_C_CC,
  	.c_ispeed = 38400,
  	.c_ospeed = 38400
  };
  
  /*
   * This value is used to take the place of a command line parameter when the
   * module is inserted.  It starts as -1 and stays as such if the user doesn't
   * specify a module insmod parameter.  If they DO specify one then it is set to
   * the value of the integer passed in.
   */
  static int hvcs_parm_num_devs = -1;
  module_param(hvcs_parm_num_devs, int, 0);
  
  static const char hvcs_driver_name[] = "hvcs";
  static const char hvcs_device_node[] = "hvcs";
  static const char hvcs_driver_string[]
  	= "IBM hvcs (Hypervisor Virtual Console Server) Driver";
  
  /* Status of partner info rescan triggered via sysfs. */
  static int hvcs_rescan_status;
  
  static struct tty_driver *hvcs_tty_driver;
  
  /*
   * In order to be somewhat sane this driver always associates the hvcs_struct
   * index element with the numerically equal tty->index.  This means that a
   * hotplugged vty-server adapter will always map to the lowest index valued
   * device node.  If vty-servers were hotplug removed from the system and then
   * new ones added the new vty-server may have the largest slot number of all
   * the vty-server adapters in the partition but it may have the lowest dev node
   * index of all the adapters due to the hole left by the hotplug removed
   * adapter.  There are a set of functions provided to get the lowest index for
   * a new device as well as return the index to the list.  This list is allocated
   * with a number of elements equal to the number of device nodes requested when
   * the module was inserted.
   */
  static int *hvcs_index_list;
  
  /*
   * How large is the list?  This is kept for traversal since the list is
   * dynamically created.
   */
  static int hvcs_index_count;
  
  /*
   * Used by the khvcsd to pick up I/O operations when the kernel_thread is
   * already awake but potentially shifted to TASK_INTERRUPTIBLE state.
   */
  static int hvcs_kicked;
  
  /*
   * Use by the kthread construct for task operations like waking the sleeping
   * thread and stopping the kthread.
   */
  static struct task_struct *hvcs_task;
  
  /*
   * We allocate this for the use of all of the hvcs_structs when they fetch
   * partner info.
   */
  static unsigned long *hvcs_pi_buff;
  
  /* Only allow one hvcs_struct to use the hvcs_pi_buff at a time. */
  static DEFINE_SPINLOCK(hvcs_pi_lock);
  
  /* One vty-server per hvcs_struct */
  struct hvcs_struct {
  	struct tty_port port;
  	spinlock_t lock;
  
  	/*
  	 * This index identifies this hvcs device as the complement to a
  	 * specific tty index.
  	 */
  	unsigned int index;
  
  	/*
  	 * Used to tell the driver kernel_thread what operations need to take
  	 * place upon this hvcs_struct instance.
  	 */
  	int todo_mask;
  
  	/*
  	 * This buffer is required so that when hvcs_write_room() reports that
  	 * it can send HVCS_BUFF_LEN characters that it will buffer the full
  	 * HVCS_BUFF_LEN characters if need be.  This is essential for opost
  	 * writes since they do not do high level buffering and expect to be
  	 * able to send what the driver commits to sending buffering
  	 * [e.g. tab to space conversions in n_tty.c opost()].
  	 */
  	char buffer[HVCS_BUFF_LEN];
  	int chars_in_buffer;
  
  	/*
  	 * Any variable below is valid before a tty is connected and
  	 * stays valid after the tty is disconnected.  These shouldn't be
  	 * whacked until the kobject refcount reaches zero though some entries
  	 * may be changed via sysfs initiatives.
  	 */
  	int connected; /* is the vty-server currently connected to a vty? */
  	uint32_t p_unit_address; /* partner unit address */
  	uint32_t p_partition_ID; /* partner partition ID */
  	char p_location_code[HVCS_CLC_LENGTH + 1]; /* CLC + Null Term */
  	struct list_head next; /* list management */
  	struct vio_dev *vdev;
  };
  
  static LIST_HEAD(hvcs_structs);
  static DEFINE_SPINLOCK(hvcs_structs_lock);
  static DEFINE_MUTEX(hvcs_init_mutex);
  
  static void hvcs_unthrottle(struct tty_struct *tty);
  static void hvcs_throttle(struct tty_struct *tty);
  static irqreturn_t hvcs_handle_interrupt(int irq, void *dev_instance);
  
  static int hvcs_write(struct tty_struct *tty,
  		const unsigned char *buf, int count);
  static int hvcs_write_room(struct tty_struct *tty);
  static int hvcs_chars_in_buffer(struct tty_struct *tty);
  
  static int hvcs_has_pi(struct hvcs_struct *hvcsd);
  static void hvcs_set_pi(struct hvcs_partner_info *pi,
  		struct hvcs_struct *hvcsd);
  static int hvcs_get_pi(struct hvcs_struct *hvcsd);
  static int hvcs_rescan_devices_list(void);
  
  static int hvcs_partner_connect(struct hvcs_struct *hvcsd);
  static void hvcs_partner_free(struct hvcs_struct *hvcsd);
  
  static int hvcs_enable_device(struct hvcs_struct *hvcsd,
  		uint32_t unit_address, unsigned int irq, struct vio_dev *dev);
  
  static int hvcs_open(struct tty_struct *tty, struct file *filp);
  static void hvcs_close(struct tty_struct *tty, struct file *filp);
  static void hvcs_hangup(struct tty_struct * tty);
  
  static int hvcs_probe(struct vio_dev *dev,
  		const struct vio_device_id *id);
  static int hvcs_remove(struct vio_dev *dev);
  static int __init hvcs_module_init(void);
  static void __exit hvcs_module_exit(void);
  static int hvcs_initialize(void);
  
  #define HVCS_SCHED_READ	0x00000001
  #define HVCS_QUICK_READ	0x00000002
  #define HVCS_TRY_WRITE	0x00000004
  #define HVCS_READ_MASK	(HVCS_SCHED_READ | HVCS_QUICK_READ)
  
  static inline struct hvcs_struct *from_vio_dev(struct vio_dev *viod)
  {
  	return dev_get_drvdata(&viod->dev);
  }
  /* The sysfs interface for the driver and devices */
  
  static ssize_t hvcs_partner_vtys_show(struct device *dev, struct device_attribute *attr, char *buf)
  {
  	struct vio_dev *viod = to_vio_dev(dev);
  	struct hvcs_struct *hvcsd = from_vio_dev(viod);
  	unsigned long flags;
  	int retval;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	retval = sprintf(buf, "%X\n", hvcsd->p_unit_address);
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	return retval;
  }
  static DEVICE_ATTR(partner_vtys, S_IRUGO, hvcs_partner_vtys_show, NULL);
  
  static ssize_t hvcs_partner_clcs_show(struct device *dev, struct device_attribute *attr, char *buf)
  {
  	struct vio_dev *viod = to_vio_dev(dev);
  	struct hvcs_struct *hvcsd = from_vio_dev(viod);
  	unsigned long flags;
  	int retval;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	retval = sprintf(buf, "%s\n", &hvcsd->p_location_code[0]);
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	return retval;
  }
  static DEVICE_ATTR(partner_clcs, S_IRUGO, hvcs_partner_clcs_show, NULL);
  
  static ssize_t hvcs_current_vty_store(struct device *dev, struct device_attribute *attr, const char * buf,
  		size_t count)
  {
  	/*
  	 * Don't need this feature at the present time because firmware doesn't
  	 * yet support multiple partners.
  	 */
  	printk(KERN_INFO "HVCS: Denied current_vty change: -EPERM.\n");
  	return -EPERM;
  }
  
  static ssize_t hvcs_current_vty_show(struct device *dev, struct device_attribute *attr, char *buf)
  {
  	struct vio_dev *viod = to_vio_dev(dev);
  	struct hvcs_struct *hvcsd = from_vio_dev(viod);
  	unsigned long flags;
  	int retval;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	retval = sprintf(buf, "%s\n", &hvcsd->p_location_code[0]);
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	return retval;
  }
  
  static DEVICE_ATTR(current_vty,
  	S_IRUGO | S_IWUSR, hvcs_current_vty_show, hvcs_current_vty_store);
  
  static ssize_t hvcs_vterm_state_store(struct device *dev, struct device_attribute *attr, const char *buf,
  		size_t count)
  {
  	struct vio_dev *viod = to_vio_dev(dev);
  	struct hvcs_struct *hvcsd = from_vio_dev(viod);
  	unsigned long flags;
  
  	/* writing a '0' to this sysfs entry will result in the disconnect. */
  	if (simple_strtol(buf, NULL, 0) != 0)
  		return -EINVAL;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  
  	if (hvcsd->port.count > 0) {
  		spin_unlock_irqrestore(&hvcsd->lock, flags);
  		printk(KERN_INFO "HVCS: vterm state unchanged.  "
  				"The hvcs device node is still in use.\n");
  		return -EPERM;
  	}
  
  	if (hvcsd->connected == 0) {
  		spin_unlock_irqrestore(&hvcsd->lock, flags);
  		printk(KERN_INFO "HVCS: vterm state unchanged. The"
  				" vty-server is not connected to a vty.\n");
  		return -EPERM;
  	}
  
  	hvcs_partner_free(hvcsd);
  	printk(KERN_INFO "HVCS: Closed vty-server@%X and"
  			" partner vty@%X:%d connection.\n",
  			hvcsd->vdev->unit_address,
  			hvcsd->p_unit_address,
  			(uint32_t)hvcsd->p_partition_ID);
  
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	return count;
  }
  
  static ssize_t hvcs_vterm_state_show(struct device *dev, struct device_attribute *attr, char *buf)
  {
  	struct vio_dev *viod = to_vio_dev(dev);
  	struct hvcs_struct *hvcsd = from_vio_dev(viod);
  	unsigned long flags;
  	int retval;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	retval = sprintf(buf, "%d\n", hvcsd->connected);
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	return retval;
  }
  static DEVICE_ATTR(vterm_state, S_IRUGO | S_IWUSR,
  		hvcs_vterm_state_show, hvcs_vterm_state_store);
  
  static ssize_t hvcs_index_show(struct device *dev, struct device_attribute *attr, char *buf)
  {
  	struct vio_dev *viod = to_vio_dev(dev);
  	struct hvcs_struct *hvcsd = from_vio_dev(viod);
  	unsigned long flags;
  	int retval;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	retval = sprintf(buf, "%d\n", hvcsd->index);
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	return retval;
  }
  
  static DEVICE_ATTR(index, S_IRUGO, hvcs_index_show, NULL);
  
  static struct attribute *hvcs_attrs[] = {
  	&dev_attr_partner_vtys.attr,
  	&dev_attr_partner_clcs.attr,
  	&dev_attr_current_vty.attr,
  	&dev_attr_vterm_state.attr,
  	&dev_attr_index.attr,
  	NULL,
  };
  
  static struct attribute_group hvcs_attr_group = {
  	.attrs = hvcs_attrs,
  };
  
  static ssize_t hvcs_rescan_show(struct device_driver *ddp, char *buf)
  {
  	/* A 1 means it is updating, a 0 means it is done updating */
  	return snprintf(buf, PAGE_SIZE, "%d\n", hvcs_rescan_status);
  }
  
  static ssize_t hvcs_rescan_store(struct device_driver *ddp, const char * buf,
  		size_t count)
  {
  	if ((simple_strtol(buf, NULL, 0) != 1)
  		&& (hvcs_rescan_status != 0))
  		return -EINVAL;
  
  	hvcs_rescan_status = 1;
  	printk(KERN_INFO "HVCS: rescanning partner info for all"
  		" vty-servers.\n");
  	hvcs_rescan_devices_list();
  	hvcs_rescan_status = 0;
  	return count;
  }
  
  static DRIVER_ATTR(rescan,
  	S_IRUGO | S_IWUSR, hvcs_rescan_show, hvcs_rescan_store);
  
  static void hvcs_kick(void)
  {
  	hvcs_kicked = 1;
  	wmb();
  	wake_up_process(hvcs_task);
  }
  
  static void hvcs_unthrottle(struct tty_struct *tty)
  {
  	struct hvcs_struct *hvcsd = tty->driver_data;
  	unsigned long flags;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	hvcsd->todo_mask |= HVCS_SCHED_READ;
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	hvcs_kick();
  }
  
  static void hvcs_throttle(struct tty_struct *tty)
  {
  	struct hvcs_struct *hvcsd = tty->driver_data;
  	unsigned long flags;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	vio_disable_interrupts(hvcsd->vdev);
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  }
  
  /*
   * If the device is being removed we don't have to worry about this interrupt
   * handler taking any further interrupts because they are disabled which means
   * the hvcs_struct will always be valid in this handler.
   */
  static irqreturn_t hvcs_handle_interrupt(int irq, void *dev_instance)
  {
  	struct hvcs_struct *hvcsd = dev_instance;
  
  	spin_lock(&hvcsd->lock);
  	vio_disable_interrupts(hvcsd->vdev);
  	hvcsd->todo_mask |= HVCS_SCHED_READ;
  	spin_unlock(&hvcsd->lock);
  	hvcs_kick();
  
  	return IRQ_HANDLED;
  }
  
  /* This function must be called with the hvcsd->lock held */
  static void hvcs_try_write(struct hvcs_struct *hvcsd)
  {
  	uint32_t unit_address = hvcsd->vdev->unit_address;
  	struct tty_struct *tty = hvcsd->port.tty;
  	int sent;
  
  	if (hvcsd->todo_mask & HVCS_TRY_WRITE) {
  		/* won't send partial writes */
  		sent = hvc_put_chars(unit_address,
  				&hvcsd->buffer[0],
  				hvcsd->chars_in_buffer );
  		if (sent > 0) {
  			hvcsd->chars_in_buffer = 0;
  			/* wmb(); */
  			hvcsd->todo_mask &= ~(HVCS_TRY_WRITE);
  			/* wmb(); */
  
  			/*
  			 * We are still obligated to deliver the data to the
  			 * hypervisor even if the tty has been closed because
  			 * we committed to delivering it.  But don't try to wake
  			 * a non-existent tty.
  			 */
  			if (tty) {
  				tty_wakeup(tty);
  			}
  		}
  	}
  }
  
  static int hvcs_io(struct hvcs_struct *hvcsd)
  {
  	uint32_t unit_address;
  	struct tty_struct *tty;
  	char buf[HVCS_BUFF_LEN] __ALIGNED__;
  	unsigned long flags;
  	int got = 0;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  
  	unit_address = hvcsd->vdev->unit_address;
  	tty = hvcsd->port.tty;
  
  	hvcs_try_write(hvcsd);
  
  	if (!tty || tty_throttled(tty)) {
  		hvcsd->todo_mask &= ~(HVCS_READ_MASK);
  		goto bail;
  	} else if (!(hvcsd->todo_mask & (HVCS_READ_MASK)))
  		goto bail;
  
  	/* remove the read masks */
  	hvcsd->todo_mask &= ~(HVCS_READ_MASK);
  
  	if (tty_buffer_request_room(&hvcsd->port, HVCS_BUFF_LEN) >= HVCS_BUFF_LEN) {
  		got = hvc_get_chars(unit_address,
  				&buf[0],
  				HVCS_BUFF_LEN);
  		tty_insert_flip_string(&hvcsd->port, buf, got);
  	}
  
  	/* Give the TTY time to process the data we just sent. */
  	if (got)
  		hvcsd->todo_mask |= HVCS_QUICK_READ;
  
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	/* This is synch because tty->low_latency == 1 */
  	if(got)
  		tty_flip_buffer_push(&hvcsd->port);
  
  	if (!got) {
  		/* Do this _after_ the flip_buffer_push */
  		spin_lock_irqsave(&hvcsd->lock, flags);
  		vio_enable_interrupts(hvcsd->vdev);
  		spin_unlock_irqrestore(&hvcsd->lock, flags);
  	}
  
  	return hvcsd->todo_mask;
  
   bail:
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	return hvcsd->todo_mask;
  }
  
  static int khvcsd(void *unused)
  {
  	struct hvcs_struct *hvcsd;
  	int hvcs_todo_mask;
  
  	__set_current_state(TASK_RUNNING);
  
  	do {
  		hvcs_todo_mask = 0;
  		hvcs_kicked = 0;
  		wmb();
  
  		spin_lock(&hvcs_structs_lock);
  		list_for_each_entry(hvcsd, &hvcs_structs, next) {
  			hvcs_todo_mask |= hvcs_io(hvcsd);
  		}
  		spin_unlock(&hvcs_structs_lock);
  
  		/*
  		 * If any of the hvcs adapters want to try a write or quick read
  		 * don't schedule(), yield a smidgen then execute the hvcs_io
  		 * thread again for those that want the write.
  		 */
  		 if (hvcs_todo_mask & (HVCS_TRY_WRITE | HVCS_QUICK_READ)) {
  			yield();
  			continue;
  		}
  
  		set_current_state(TASK_INTERRUPTIBLE);
  		if (!hvcs_kicked)
  			schedule();
  		__set_current_state(TASK_RUNNING);
  	} while (!kthread_should_stop());
  
  	return 0;
  }
  
  static struct vio_device_id hvcs_driver_table[] = {
  	{"serial-server", "hvterm2"},
  	{ "", "" }
  };
  MODULE_DEVICE_TABLE(vio, hvcs_driver_table);
  
  static void hvcs_return_index(int index)
  {
  	/* Paranoia check */
  	if (!hvcs_index_list)
  		return;
  	if (index < 0 || index >= hvcs_index_count)
  		return;
  	if (hvcs_index_list[index] == -1)
  		return;
  	else
  		hvcs_index_list[index] = -1;
  }
  
  static void hvcs_destruct_port(struct tty_port *p)
  {
  	struct hvcs_struct *hvcsd = container_of(p, struct hvcs_struct, port);
  	struct vio_dev *vdev;
  	unsigned long flags;
  
  	spin_lock(&hvcs_structs_lock);
  	spin_lock_irqsave(&hvcsd->lock, flags);
  
  	/* the list_del poisons the pointers */
  	list_del(&(hvcsd->next));
  
  	if (hvcsd->connected == 1) {
  		hvcs_partner_free(hvcsd);
  		printk(KERN_INFO "HVCS: Closed vty-server@%X and"
  				" partner vty@%X:%d connection.\n",
  				hvcsd->vdev->unit_address,
  				hvcsd->p_unit_address,
  				(uint32_t)hvcsd->p_partition_ID);
  	}
  	printk(KERN_INFO "HVCS: Destroyed hvcs_struct for vty-server@%X.\n",
  			hvcsd->vdev->unit_address);
  
  	vdev = hvcsd->vdev;
  	hvcsd->vdev = NULL;
  
  	hvcsd->p_unit_address = 0;
  	hvcsd->p_partition_ID = 0;
  	hvcs_return_index(hvcsd->index);
  	memset(&hvcsd->p_location_code[0], 0x00, HVCS_CLC_LENGTH + 1);
  
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	spin_unlock(&hvcs_structs_lock);
  
  	sysfs_remove_group(&vdev->dev.kobj, &hvcs_attr_group);
  
  	kfree(hvcsd);
  }
  
  static const struct tty_port_operations hvcs_port_ops = {
  	.destruct = hvcs_destruct_port,
  };
  
  static int hvcs_get_index(void)
  {
  	int i;
  	/* Paranoia check */
  	if (!hvcs_index_list) {
  		printk(KERN_ERR "HVCS: hvcs_index_list NOT valid!.\n");
  		return -EFAULT;
  	}
  	/* Find the numerically lowest first free index. */
  	for(i = 0; i < hvcs_index_count; i++) {
  		if (hvcs_index_list[i] == -1) {
  			hvcs_index_list[i] = 0;
  			return i;
  		}
  	}
  	return -1;
  }
  
  static int hvcs_probe(
  	struct vio_dev *dev,
  	const struct vio_device_id *id)
  {
  	struct hvcs_struct *hvcsd;
  	int index, rc;
  	int retval;
  
  	if (!dev || !id) {
  		printk(KERN_ERR "HVCS: probed with invalid parameter.\n");
  		return -EPERM;
  	}
  
  	/* Make sure we are properly initialized */
  	rc = hvcs_initialize();
  	if (rc) {
  		pr_err("HVCS: Failed to initialize core driver.\n");
  		return rc;
  	}
  
  	/* early to avoid cleanup on failure */
  	index = hvcs_get_index();
  	if (index < 0) {
  		return -EFAULT;
  	}
  
  	hvcsd = kzalloc(sizeof(*hvcsd), GFP_KERNEL);
  	if (!hvcsd)
  		return -ENODEV;
  
  	tty_port_init(&hvcsd->port);
  	hvcsd->port.ops = &hvcs_port_ops;
  	spin_lock_init(&hvcsd->lock);
  
  	hvcsd->vdev = dev;
  	dev_set_drvdata(&dev->dev, hvcsd);
  
  	hvcsd->index = index;
  
  	/* hvcsd->index = ++hvcs_struct_count; */
  	hvcsd->chars_in_buffer = 0;
  	hvcsd->todo_mask = 0;
  	hvcsd->connected = 0;
  
  	/*
  	 * This will populate the hvcs_struct's partner info fields for the
  	 * first time.
  	 */
  	if (hvcs_get_pi(hvcsd)) {
  		printk(KERN_ERR "HVCS: Failed to fetch partner"
  			" info for vty-server@%X on device probe.\n",
  			hvcsd->vdev->unit_address);
  	}
  
  	/*
  	 * If a user app opens a tty that corresponds to this vty-server before
  	 * the hvcs_struct has been added to the devices list then the user app
  	 * will get -ENODEV.
  	 */
  	spin_lock(&hvcs_structs_lock);
  	list_add_tail(&(hvcsd->next), &hvcs_structs);
  	spin_unlock(&hvcs_structs_lock);
  
  	retval = sysfs_create_group(&dev->dev.kobj, &hvcs_attr_group);
  	if (retval) {
  		printk(KERN_ERR "HVCS: Can't create sysfs attrs for vty-server@%X\n",
  		       hvcsd->vdev->unit_address);
  		return retval;
  	}
  
  	printk(KERN_INFO "HVCS: vty-server@%X added to the vio bus.\n", dev->unit_address);
  
  	/*
  	 * DON'T enable interrupts here because there is no user to receive the
  	 * data.
  	 */
  	return 0;
  }
  
  static int hvcs_remove(struct vio_dev *dev)
  {
  	struct hvcs_struct *hvcsd = dev_get_drvdata(&dev->dev);
  	unsigned long flags;
  	struct tty_struct *tty;
  
  	if (!hvcsd)
  		return -ENODEV;
  
  	/* By this time the vty-server won't be getting any more interrupts */
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  
  	tty = hvcsd->port.tty;
  
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  
  	/*
  	 * Let the last holder of this object cause it to be removed, which
  	 * would probably be tty_hangup below.
  	 */
  	tty_port_put(&hvcsd->port);
  
  	/*
  	 * The hangup is a scheduled function which will auto chain call
  	 * hvcs_hangup.  The tty should always be valid at this time unless a
  	 * simultaneous tty close already cleaned up the hvcs_struct.
  	 */
  	if (tty)
  		tty_hangup(tty);
  
  	printk(KERN_INFO "HVCS: vty-server@%X removed from the"
  			" vio bus.\n", dev->unit_address);
  	return 0;
  };
  
  static struct vio_driver hvcs_vio_driver = {
  	.id_table	= hvcs_driver_table,
  	.probe		= hvcs_probe,
  	.remove		= hvcs_remove,
  	.name		= hvcs_driver_name,
  };
  
  /* Only called from hvcs_get_pi please */
  static void hvcs_set_pi(struct hvcs_partner_info *pi, struct hvcs_struct *hvcsd)
  {
  	hvcsd->p_unit_address = pi->unit_address;
  	hvcsd->p_partition_ID  = pi->partition_ID;
  
  	/* copy the null-term char too */
  	strlcpy(&hvcsd->p_location_code[0],
  			&pi->location_code[0], sizeof(hvcsd->p_location_code));
  }
  
  /*
   * Traverse the list and add the partner info that is found to the hvcs_struct
   * struct entry. NOTE: At this time I know that partner info will return a
   * single entry but in the future there may be multiple partner info entries per
   * vty-server and you'll want to zero out that list and reset it.  If for some
   * reason you have an old version of this driver but there IS more than one
   * partner info then hvcsd->p_* will hold the last partner info data from the
   * firmware query.  A good way to update this code would be to replace the three
   * partner info fields in hvcs_struct with a list of hvcs_partner_info
   * instances.
   *
   * This function must be called with the hvcsd->lock held.
   */
  static int hvcs_get_pi(struct hvcs_struct *hvcsd)
  {
  	struct hvcs_partner_info *pi;
  	uint32_t unit_address = hvcsd->vdev->unit_address;
  	struct list_head head;
  	int retval;
  
  	spin_lock(&hvcs_pi_lock);
  	if (!hvcs_pi_buff) {
  		spin_unlock(&hvcs_pi_lock);
  		return -EFAULT;
  	}
  	retval = hvcs_get_partner_info(unit_address, &head, hvcs_pi_buff);
  	spin_unlock(&hvcs_pi_lock);
  	if (retval) {
  		printk(KERN_ERR "HVCS: Failed to fetch partner"
  			" info for vty-server@%x.\n", unit_address);
  		return retval;
  	}
  
  	/* nixes the values if the partner vty went away */
  	hvcsd->p_unit_address = 0;
  	hvcsd->p_partition_ID = 0;
  
  	list_for_each_entry(pi, &head, node)
  		hvcs_set_pi(pi, hvcsd);
  
  	hvcs_free_partner_info(&head);
  	return 0;
  }
  
  /*
   * This function is executed by the driver "rescan" sysfs entry.  It shouldn't
   * be executed elsewhere, in order to prevent deadlock issues.
   */
  static int hvcs_rescan_devices_list(void)
  {
  	struct hvcs_struct *hvcsd;
  	unsigned long flags;
  
  	spin_lock(&hvcs_structs_lock);
  
  	list_for_each_entry(hvcsd, &hvcs_structs, next) {
  		spin_lock_irqsave(&hvcsd->lock, flags);
  		hvcs_get_pi(hvcsd);
  		spin_unlock_irqrestore(&hvcsd->lock, flags);
  	}
  
  	spin_unlock(&hvcs_structs_lock);
  
  	return 0;
  }
  
  /*
   * Farm this off into its own function because it could be more complex once
   * multiple partners support is added. This function should be called with
   * the hvcsd->lock held.
   */
  static int hvcs_has_pi(struct hvcs_struct *hvcsd)
  {
  	if ((!hvcsd->p_unit_address) || (!hvcsd->p_partition_ID))
  		return 0;
  	return 1;
  }
  
  /*
   * NOTE: It is possible that the super admin removed a partner vty and then
   * added a different vty as the new partner.
   *
   * This function must be called with the hvcsd->lock held.
   */
  static int hvcs_partner_connect(struct hvcs_struct *hvcsd)
  {
  	int retval;
  	unsigned int unit_address = hvcsd->vdev->unit_address;
  
  	/*
  	 * If there wasn't any pi when the device was added it doesn't meant
  	 * there isn't any now.  This driver isn't notified when a new partner
  	 * vty is added to a vty-server so we discover changes on our own.
  	 * Please see comments in hvcs_register_connection() for justification
  	 * of this bizarre code.
  	 */
  	retval = hvcs_register_connection(unit_address,
  			hvcsd->p_partition_ID,
  			hvcsd->p_unit_address);
  	if (!retval) {
  		hvcsd->connected = 1;
  		return 0;
  	} else if (retval != -EINVAL)
  		return retval;
  
  	/*
  	 * As per the spec re-get the pi and try again if -EINVAL after the
  	 * first connection attempt.
  	 */
  	if (hvcs_get_pi(hvcsd))
  		return -ENOMEM;
  
  	if (!hvcs_has_pi(hvcsd))
  		return -ENODEV;
  
  	retval = hvcs_register_connection(unit_address,
  			hvcsd->p_partition_ID,
  			hvcsd->p_unit_address);
  	if (retval != -EINVAL) {
  		hvcsd->connected = 1;
  		return retval;
  	}
  
  	/*
  	 * EBUSY is the most likely scenario though the vty could have been
  	 * removed or there really could be an hcall error due to the parameter
  	 * data but thanks to ambiguous firmware return codes we can't really
  	 * tell.
  	 */
  	printk(KERN_INFO "HVCS: vty-server or partner"
  			" vty is busy.  Try again later.\n");
  	return -EBUSY;
  }
  
  /* This function must be called with the hvcsd->lock held */
  static void hvcs_partner_free(struct hvcs_struct *hvcsd)
  {
  	int retval;
  	do {
  		retval = hvcs_free_connection(hvcsd->vdev->unit_address);
  	} while (retval == -EBUSY);
  	hvcsd->connected = 0;
  }
  
  /* This helper function must be called WITHOUT the hvcsd->lock held */
  static int hvcs_enable_device(struct hvcs_struct *hvcsd, uint32_t unit_address,
  		unsigned int irq, struct vio_dev *vdev)
  {
  	unsigned long flags;
  	int rc;
  
  	/*
  	 * It is possible that the vty-server was removed between the time that
  	 * the conn was registered and now.
  	 */
  	rc = request_irq(irq, &hvcs_handle_interrupt, 0, "ibmhvcs", hvcsd);
  	if (!rc) {
  		/*
  		 * It is possible the vty-server was removed after the irq was
  		 * requested but before we have time to enable interrupts.
  		 */
  		if (vio_enable_interrupts(vdev) == H_SUCCESS)
  			return 0;
  		else {
  			printk(KERN_ERR "HVCS: int enable failed for"
  					" vty-server@%X.\n", unit_address);
  			free_irq(irq, hvcsd);
  		}
  	} else
  		printk(KERN_ERR "HVCS: irq req failed for"
  				" vty-server@%X.\n", unit_address);
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	hvcs_partner_free(hvcsd);
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  
  	return rc;
  
  }
  
  /*
   * This always increments the kref ref count if the call is successful.
   * Please remember to dec when you are done with the instance.
   *
   * NOTICE: Do NOT hold either the hvcs_struct.lock or hvcs_structs_lock when
   * calling this function or you will get deadlock.
   */
  static struct hvcs_struct *hvcs_get_by_index(int index)
  {
  	struct hvcs_struct *hvcsd;
  	unsigned long flags;
  
  	spin_lock(&hvcs_structs_lock);
  	list_for_each_entry(hvcsd, &hvcs_structs, next) {
  		spin_lock_irqsave(&hvcsd->lock, flags);
  		if (hvcsd->index == index) {
  			tty_port_get(&hvcsd->port);
  			spin_unlock_irqrestore(&hvcsd->lock, flags);
  			spin_unlock(&hvcs_structs_lock);
  			return hvcsd;
  		}
  		spin_unlock_irqrestore(&hvcsd->lock, flags);
  	}
  	spin_unlock(&hvcs_structs_lock);
  
  	return NULL;
  }
  
  static int hvcs_install(struct tty_driver *driver, struct tty_struct *tty)
  {
  	struct hvcs_struct *hvcsd;
  	struct vio_dev *vdev;
  	unsigned long unit_address, flags;
  	unsigned int irq;
  	int retval;
  
  	/*
  	 * Is there a vty-server that shares the same index?
  	 * This function increments the kref index.
  	 */
  	hvcsd = hvcs_get_by_index(tty->index);
  	if (!hvcsd) {
  		printk(KERN_WARNING "HVCS: open failed, no device associated"
  				" with tty->index %d.\n", tty->index);
  		return -ENODEV;
  	}
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  
  	if (hvcsd->connected == 0) {
  		retval = hvcs_partner_connect(hvcsd);
  		if (retval) {
  			spin_unlock_irqrestore(&hvcsd->lock, flags);
  			printk(KERN_WARNING "HVCS: partner connect failed.\n");
  			goto err_put;
  		}
  	}
  
  	hvcsd->port.count = 0;
  	hvcsd->port.tty = tty;
  	tty->driver_data = hvcsd;
  
  	memset(&hvcsd->buffer[0], 0x00, HVCS_BUFF_LEN);
  
  	/*
  	 * Save these in the spinlock for the enable operations that need them
  	 * outside of the spinlock.
  	 */
  	irq = hvcsd->vdev->irq;
  	vdev = hvcsd->vdev;
  	unit_address = hvcsd->vdev->unit_address;
  
  	hvcsd->todo_mask |= HVCS_SCHED_READ;
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  
  	/*
  	 * This must be done outside of the spinlock because it requests irqs
  	 * and will grab the spinlock and free the connection if it fails.
  	 */
  	retval = hvcs_enable_device(hvcsd, unit_address, irq, vdev);
  	if (retval) {
  		printk(KERN_WARNING "HVCS: enable device failed.\n");
  		goto err_put;
  	}
  
  	retval = tty_port_install(&hvcsd->port, driver, tty);
  	if (retval)
  		goto err_irq;
  
  	return 0;
  err_irq:
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	vio_disable_interrupts(hvcsd->vdev);
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  	free_irq(irq, hvcsd);
  err_put:
  	tty_port_put(&hvcsd->port);
  
  	return retval;
  }
  
  /*
   * This is invoked via the tty_open interface when a user app connects to the
   * /dev node.
   */
  static int hvcs_open(struct tty_struct *tty, struct file *filp)
  {
  	struct hvcs_struct *hvcsd = tty->driver_data;
  	unsigned long flags;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	hvcsd->port.count++;
  	hvcsd->todo_mask |= HVCS_SCHED_READ;
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  
  	hvcs_kick();
  
  	printk(KERN_INFO "HVCS: vty-server@%X connection opened.\n",
  		hvcsd->vdev->unit_address );
  
  	return 0;
  }
  
  static void hvcs_close(struct tty_struct *tty, struct file *filp)
  {
  	struct hvcs_struct *hvcsd;
  	unsigned long flags;
  	int irq;
  
  	/*
  	 * Is someone trying to close the file associated with this device after
  	 * we have hung up?  If so tty->driver_data wouldn't be valid.
  	 */
  	if (tty_hung_up_p(filp))
  		return;
  
  	/*
  	 * No driver_data means that this close was probably issued after a
  	 * failed hvcs_open by the tty layer's release_dev() api and we can just
  	 * exit cleanly.
  	 */
  	if (!tty->driver_data)
  		return;
  
  	hvcsd = tty->driver_data;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	if (--hvcsd->port.count == 0) {
  
  		vio_disable_interrupts(hvcsd->vdev);
  
  		/*
  		 * NULL this early so that the kernel_thread doesn't try to
  		 * execute any operations on the TTY even though it is obligated
  		 * to deliver any pending I/O to the hypervisor.
  		 */
  		hvcsd->port.tty = NULL;
  
  		irq = hvcsd->vdev->irq;
  		spin_unlock_irqrestore(&hvcsd->lock, flags);
  
  		tty_wait_until_sent(tty, HVCS_CLOSE_WAIT);
  
  		/*
  		 * This line is important because it tells hvcs_open that this
  		 * device needs to be re-configured the next time hvcs_open is
  		 * called.
  		 */
  		tty->driver_data = NULL;
  
  		free_irq(irq, hvcsd);
  		return;
  	} else if (hvcsd->port.count < 0) {
  		printk(KERN_ERR "HVCS: vty-server@%X open_count: %d"
  				" is missmanaged.\n",
  		hvcsd->vdev->unit_address, hvcsd->port.count);
  	}
  
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  }
  
  static void hvcs_cleanup(struct tty_struct * tty)
  {
  	struct hvcs_struct *hvcsd = tty->driver_data;
  
  	tty_port_put(&hvcsd->port);
  }
  
  static void hvcs_hangup(struct tty_struct * tty)
  {
  	struct hvcs_struct *hvcsd = tty->driver_data;
  	unsigned long flags;
  	int temp_open_count;
  	int irq;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  	/* Preserve this so that we know how many kref refs to put */
  	temp_open_count = hvcsd->port.count;
  
  	/*
  	 * Don't kref put inside the spinlock because the destruction
  	 * callback may use the spinlock and it may get called before the
  	 * spinlock has been released.
  	 */
  	vio_disable_interrupts(hvcsd->vdev);
  
  	hvcsd->todo_mask = 0;
  
  	/* I don't think the tty needs the hvcs_struct pointer after a hangup */
  	tty->driver_data = NULL;
  	hvcsd->port.tty = NULL;
  
  	hvcsd->port.count = 0;
  
  	/* This will drop any buffered data on the floor which is OK in a hangup
  	 * scenario. */
  	memset(&hvcsd->buffer[0], 0x00, HVCS_BUFF_LEN);
  	hvcsd->chars_in_buffer = 0;
  
  	irq = hvcsd->vdev->irq;
  
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  
  	free_irq(irq, hvcsd);
  
  	/*
  	 * We need to kref_put() for every open_count we have since the
  	 * tty_hangup() function doesn't invoke a close per open connection on a
  	 * non-console device.
  	 */
  	while(temp_open_count) {
  		--temp_open_count;
  		/*
  		 * The final put will trigger destruction of the hvcs_struct.
  		 * NOTE:  If this hangup was signaled from user space then the
  		 * final put will never happen.
  		 */
  		tty_port_put(&hvcsd->port);
  	}
  }
  
  /*
   * NOTE: This is almost always from_user since user level apps interact with the
   * /dev nodes. I'm trusting that if hvcs_write gets called and interrupted by
   * hvcs_remove (which removes the target device and executes tty_hangup()) that
   * tty_hangup will allow hvcs_write time to complete execution before it
   * terminates our device.
   */
  static int hvcs_write(struct tty_struct *tty,
  		const unsigned char *buf, int count)
  {
  	struct hvcs_struct *hvcsd = tty->driver_data;
  	unsigned int unit_address;
  	const unsigned char *charbuf;
  	unsigned long flags;
  	int total_sent = 0;
  	int tosend = 0;
  	int result = 0;
  
  	/*
  	 * If they don't check the return code off of their open they may
  	 * attempt this even if there is no connected device.
  	 */
  	if (!hvcsd)
  		return -ENODEV;
  
  	/* Reasonable size to prevent user level flooding */
  	if (count > HVCS_MAX_FROM_USER) {
  		printk(KERN_WARNING "HVCS write: count being truncated to"
  				" HVCS_MAX_FROM_USER.\n");
  		count = HVCS_MAX_FROM_USER;
  	}
  
  	charbuf = buf;
  
  	spin_lock_irqsave(&hvcsd->lock, flags);
  
  	/*
  	 * Somehow an open succeeded but the device was removed or the
  	 * connection terminated between the vty-server and partner vty during
  	 * the middle of a write operation?  This is a crummy place to do this
  	 * but we want to keep it all in the spinlock.
  	 */
  	if (hvcsd->port.count <= 0) {
  		spin_unlock_irqrestore(&hvcsd->lock, flags);
  		return -ENODEV;
  	}
  
  	unit_address = hvcsd->vdev->unit_address;
  
  	while (count > 0) {
  		tosend = min(count, (HVCS_BUFF_LEN - hvcsd->chars_in_buffer));
  		/*
  		 * No more space, this probably means that the last call to
  		 * hvcs_write() didn't succeed and the buffer was filled up.
  		 */
  		if (!tosend)
  			break;
  
  		memcpy(&hvcsd->buffer[hvcsd->chars_in_buffer],
  				&charbuf[total_sent],
  				tosend);
  
  		hvcsd->chars_in_buffer += tosend;
  
  		result = 0;
  
  		/*
  		 * If this is true then we don't want to try writing to the
  		 * hypervisor because that is the kernel_threads job now.  We'll
  		 * just add to the buffer.
  		 */
  		if (!(hvcsd->todo_mask & HVCS_TRY_WRITE))
  			/* won't send partial writes */
  			result = hvc_put_chars(unit_address,
  					&hvcsd->buffer[0],
  					hvcsd->chars_in_buffer);
  
  		/*
  		 * Since we know we have enough room in hvcsd->buffer for
  		 * tosend we record that it was sent regardless of whether the
  		 * hypervisor actually took it because we have it buffered.
  		 */
  		total_sent+=tosend;
  		count-=tosend;
  		if (result == 0) {
  			hvcsd->todo_mask |= HVCS_TRY_WRITE;
  			hvcs_kick();
  			break;
  		}
  
  		hvcsd->chars_in_buffer = 0;
  		/*
  		 * Test after the chars_in_buffer reset otherwise this could
  		 * deadlock our writes if hvc_put_chars fails.
  		 */
  		if (result < 0)
  			break;
  	}
  
  	spin_unlock_irqrestore(&hvcsd->lock, flags);
  
  	if (result == -1)
  		return -EIO;
  	else
  		return total_sent;
  }
  
  /*
   * This is really asking how much can we guarantee that we can send or that we
   * absolutely WILL BUFFER if we can't send it.  This driver MUST honor the
   * return value, hence the reason for hvcs_struct buffering.
   */
  static int hvcs_write_room(struct tty_struct *tty)
  {
  	struct hvcs_struct *hvcsd = tty->driver_data;
  
  	if (!hvcsd || hvcsd->port.count <= 0)
  		return 0;
  
  	return HVCS_BUFF_LEN - hvcsd->chars_in_buffer;
  }
  
  static int hvcs_chars_in_buffer(struct tty_struct *tty)
  {
  	struct hvcs_struct *hvcsd = tty->driver_data;
  
  	return hvcsd->chars_in_buffer;
  }
  
  static const struct tty_operations hvcs_ops = {
  	.install = hvcs_install,
  	.open = hvcs_open,
  	.close = hvcs_close,
  	.cleanup = hvcs_cleanup,
  	.hangup = hvcs_hangup,
  	.write = hvcs_write,
  	.write_room = hvcs_write_room,
  	.chars_in_buffer = hvcs_chars_in_buffer,
  	.unthrottle = hvcs_unthrottle,
  	.throttle = hvcs_throttle,
  };
  
  static int hvcs_alloc_index_list(int n)
  {
  	int i;
  
  	hvcs_index_list = kmalloc(n * sizeof(hvcs_index_count),GFP_KERNEL);
  	if (!hvcs_index_list)
  		return -ENOMEM;
  	hvcs_index_count = n;
  	for (i = 0; i < hvcs_index_count; i++)
  		hvcs_index_list[i] = -1;
  	return 0;
  }
  
  static void hvcs_free_index_list(void)
  {
  	/* Paranoia check to be thorough. */
  	kfree(hvcs_index_list);
  	hvcs_index_list = NULL;
  	hvcs_index_count = 0;
  }
  
  static int hvcs_initialize(void)
  {
  	int rc, num_ttys_to_alloc;
  
  	mutex_lock(&hvcs_init_mutex);
  	if (hvcs_task) {
  		mutex_unlock(&hvcs_init_mutex);
  		return 0;
  	}
  
  	/* Has the user specified an overload with an insmod param? */
  	if (hvcs_parm_num_devs <= 0 ||
  		(hvcs_parm_num_devs > HVCS_MAX_SERVER_ADAPTERS)) {
  		num_ttys_to_alloc = HVCS_DEFAULT_SERVER_ADAPTERS;
  	} else
  		num_ttys_to_alloc = hvcs_parm_num_devs;
  
  	hvcs_tty_driver = alloc_tty_driver(num_ttys_to_alloc);
  	if (!hvcs_tty_driver) {
  		mutex_unlock(&hvcs_init_mutex);
  		return -ENOMEM;
  	}
  
  	if (hvcs_alloc_index_list(num_ttys_to_alloc)) {
  		rc = -ENOMEM;
  		goto index_fail;
  	}
  
  	hvcs_tty_driver->driver_name = hvcs_driver_name;
  	hvcs_tty_driver->name = hvcs_device_node;
  
  	/*
  	 * We'll let the system assign us a major number, indicated by leaving
  	 * it blank.
  	 */
  
  	hvcs_tty_driver->minor_start = HVCS_MINOR_START;
  	hvcs_tty_driver->type = TTY_DRIVER_TYPE_SYSTEM;
  
  	/*
  	 * We role our own so that we DONT ECHO.  We can't echo because the
  	 * device we are connecting to already echoes by default and this would
  	 * throw us into a horrible recursive echo-echo-echo loop.
  	 */
  	hvcs_tty_driver->init_termios = hvcs_tty_termios;
  	hvcs_tty_driver->flags = TTY_DRIVER_REAL_RAW;
  
  	tty_set_operations(hvcs_tty_driver, &hvcs_ops);
  
  	/*
  	 * The following call will result in sysfs entries that denote the
  	 * dynamically assigned major and minor numbers for our devices.
  	 */
  	if (tty_register_driver(hvcs_tty_driver)) {
  		printk(KERN_ERR "HVCS: registration as a tty driver failed.\n");
  		rc = -EIO;
  		goto register_fail;
  	}
  
  	hvcs_pi_buff = (unsigned long *) __get_free_page(GFP_KERNEL);
  	if (!hvcs_pi_buff) {
  		rc = -ENOMEM;
  		goto buff_alloc_fail;
  	}
  
  	hvcs_task = kthread_run(khvcsd, NULL, "khvcsd");
  	if (IS_ERR(hvcs_task)) {
  		printk(KERN_ERR "HVCS: khvcsd creation failed.\n");
  		rc = -EIO;
  		goto kthread_fail;
  	}
  	mutex_unlock(&hvcs_init_mutex);
  	return 0;
  
  kthread_fail:
  	free_page((unsigned long)hvcs_pi_buff);
  buff_alloc_fail:
  	tty_unregister_driver(hvcs_tty_driver);
  register_fail:
  	hvcs_free_index_list();
  index_fail:
  	put_tty_driver(hvcs_tty_driver);
  	hvcs_tty_driver = NULL;
  	mutex_unlock(&hvcs_init_mutex);
  	return rc;
  }
  
  static int __init hvcs_module_init(void)
  {
  	int rc = vio_register_driver(&hvcs_vio_driver);
  	if (rc) {
  		printk(KERN_ERR "HVCS: can't register vio driver\n");
  		return rc;
  	}
  
  	pr_info("HVCS: Driver registered.\n");
  
  	/* This needs to be done AFTER the vio_register_driver() call or else
  	 * the kobjects won't be initialized properly.
  	 */
  	rc = driver_create_file(&(hvcs_vio_driver.driver), &driver_attr_rescan);
  	if (rc)
  		pr_warning("HVCS: Failed to create rescan file (err %d)\n", rc);
  
  	return 0;
  }
  
  static void __exit hvcs_module_exit(void)
  {
  	/*
  	 * This driver receives hvcs_remove callbacks for each device upon
  	 * module removal.
  	 */
  	vio_unregister_driver(&hvcs_vio_driver);
  	if (!hvcs_task)
  		return;
  
  	/*
  	 * This synchronous operation  will wake the khvcsd kthread if it is
  	 * asleep and will return when khvcsd has terminated.
  	 */
  	kthread_stop(hvcs_task);
  
  	spin_lock(&hvcs_pi_lock);
  	free_page((unsigned long)hvcs_pi_buff);
  	hvcs_pi_buff = NULL;
  	spin_unlock(&hvcs_pi_lock);
  
  	driver_remove_file(&hvcs_vio_driver.driver, &driver_attr_rescan);
  
  	tty_unregister_driver(hvcs_tty_driver);
  
  	hvcs_free_index_list();
  
  	put_tty_driver(hvcs_tty_driver);
  
  	printk(KERN_INFO "HVCS: driver module removed.\n");
  }
  
  module_init(hvcs_module_init);
  module_exit(hvcs_module_exit);
  