Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6

[linux-2.6] / drivers / char / hvcs.c

/*
 * 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 kobject construct.  This kobject is also used by
 * the vio bus to provide a vio device sysfs entry that this driver attaches
 * device specific attributes to, including partner information.  The vio bus
 * framework also provides a sysfs entry for each vio driver.  The hvcs driver
 * provides driver attributes in this entry.
 *
 * 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/kobject.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/spinlock.h>
#include <linux/stat.h>
#include <linux/tty.h>
#include <linux/tty_flip.h>
#include <asm/hvconsole.h>
#include <asm/hvcserver.h>
#include <asm/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
 * include/asm-powerpc/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 similarily 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);

char hvcs_driver_name[] = "hvcs";
char hvcs_device_node[] = "hvcs";
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 {
        spinlock_t lock;

        /*
         * This index identifies this hvcs device as the complement to a
         * specific tty index.
         */
        unsigned int index;

        struct tty_struct *tty;
        unsigned int open_count;

        /*
         * 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 the kobject is valid before a tty is connected and
         * stays valid after the tty is disconnected.  These shouldn't be
         * whacked until the koject refcount reaches zero though some entries
         * may be changed via sysfs initiatives.
         */
        struct kobject kobj; /* ref count & hvcs_struct lifetime */
        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;
};

/* Required to back map a kobject to its containing object */
#define from_kobj(kobj) container_of(kobj, struct hvcs_struct, kobj)

static struct list_head hvcs_structs = LIST_HEAD_INIT(hvcs_structs);
static DEFINE_SPINLOCK(hvcs_structs_lock);

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 void destroy_hvcs_struct(struct kobject *kobj);
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 __devinit hvcs_probe(struct vio_dev *dev,
                const struct vio_device_id *id);
static int __devexit hvcs_remove(struct vio_dev *dev);
static int __init hvcs_module_init(void);
static void __exit hvcs_module_exit(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 viod->dev.driver_data;
}
/* 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->open_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->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 commited 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->tty;

        hvcs_try_write(hvcsd);

        if (!tty || test_bit(TTY_THROTTLED, &tty->flags)) {
                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(tty, HVCS_BUFF_LEN) >= HVCS_BUFF_LEN) {
                got = hvc_get_chars(unit_address,
                                &buf[0],
                                HVCS_BUFF_LEN);
                tty_insert_flip_string(tty, 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(tty);

        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[] __devinitdata= {
        {"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;
}

/* callback when the kboject ref count reaches zero */
static void destroy_hvcs_struct(struct kobject *kobj)
{
        struct hvcs_struct *hvcsd = from_kobj(kobj);
        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 struct kobj_type hvcs_kobj_type = {
        .release = destroy_hvcs_struct,
};

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 __devinit hvcs_probe(
        struct vio_dev *dev,
        const struct vio_device_id *id)
{
        struct hvcs_struct *hvcsd;
        int index;
        int retval;

        if (!dev || !id) {
                printk(KERN_ERR "HVCS: probed with invalid parameter.\n");
                return -EPERM;
        }

        /* early to avoid cleanup on failure */
        index = hvcs_get_index();
        if (index < 0) {
                return -EFAULT;
        }

        hvcsd = kmalloc(sizeof(*hvcsd), GFP_KERNEL);
        if (!hvcsd)
                return -ENODEV;

        /* hvcsd->tty is zeroed out with the memset */
        memset(hvcsd, 0x00, sizeof(*hvcsd));

        spin_lock_init(&hvcsd->lock);
        /* Automatically incs the refcount the first time */
        kobject_init(&hvcsd->kobj);
        /* Set up the callback for terminating the hvcs_struct's life */
        hvcsd->kobj.ktype = &hvcs_kobj_type;

        hvcsd->vdev = dev;
        dev->dev.driver_data = 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 __devexit hvcs_remove(struct vio_dev *dev)
{
        struct hvcs_struct *hvcsd = dev->dev.driver_data;
        unsigned long flags;
        struct kobject *kobjp;
        struct tty_struct *tty;

        if (!hvcsd)
                return -ENODEV;

        /* By this time the vty-server won't be getting any more interrups */

        spin_lock_irqsave(&hvcsd->lock, flags);

        tty = hvcsd->tty;

        kobjp = &hvcsd->kobj;

        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.
         */
        kobject_put (kobjp);

        /*
         * 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,
        .driver         = {
                .name   = hvcs_driver_name,
                .owner  = THIS_MODULE,
        }
};

/* Only called from hvcs_get_pi please */
static void hvcs_set_pi(struct hvcs_partner_info *pi, struct hvcs_struct *hvcsd)
{
        int clclength;

        hvcsd->p_unit_address = pi->unit_address;
        hvcsd->p_partition_ID  = pi->partition_ID;
        clclength = strlen(&pi->location_code[0]);
        if (clclength > HVCS_CLC_LENGTH)
                clclength = HVCS_CLC_LENGTH;

        /* copy the null-term char too */
        strncpy(&hvcsd->p_location_code[0],
                        &pi->location_code[0], clclength + 1);
}

/*
 * 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.
         */
        if (!(rc = request_irq(irq, &hvcs_handle_interrupt,
                                IRQF_DISABLED, "ibmhvcs", hvcsd))) {
                /*
                 * 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 kobject 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.
 */
struct hvcs_struct *hvcs_get_by_index(int index)
{
        struct hvcs_struct *hvcsd = NULL;
        unsigned long flags;

        spin_lock(&hvcs_structs_lock);
        /* We can immediately discard OOB requests */
        if (index >= 0 && index < HVCS_MAX_SERVER_ADAPTERS) {
                list_for_each_entry(hvcsd, &hvcs_structs, next) {
                        spin_lock_irqsave(&hvcsd->lock, flags);
                        if (hvcsd->index == index) {
                                kobject_get(&hvcsd->kobj);
                                spin_unlock_irqrestore(&hvcsd->lock, flags);
                                spin_unlock(&hvcs_structs_lock);
                                return hvcsd;
                        }
                        spin_unlock_irqrestore(&hvcsd->lock, flags);
                }
                hvcsd = NULL;
        }

        spin_unlock(&hvcs_structs_lock);
        return hvcsd;
}

/*
 * 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;
        int rc, retval = 0;
        unsigned long flags;
        unsigned int irq;
        struct vio_dev *vdev;
        unsigned long unit_address;
        struct kobject *kobjp;

        if (tty->driver_data)
                goto fast_open;

        /*
         * Is there a vty-server that shares the same index?
         * This function increments the kobject index.
         */
        if (!(hvcsd = hvcs_get_by_index(tty->index))) {
                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)
                if ((retval = hvcs_partner_connect(hvcsd)))
                        goto error_release;

        hvcsd->open_count = 1;
        hvcsd->tty = tty;
        tty->driver_data = hvcsd;

        /*
         * Set this driver to low latency so that we actually have a chance at
         * catching a throttled TTY after we flip_buffer_push.  Otherwise the
         * flush_to_async may not execute until after the kernel_thread has
         * yielded and resumed the next flip_buffer_push resulting in data
         * loss.
         */
        tty->low_latency = 1;

        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.
         */
        if (((rc = hvcs_enable_device(hvcsd, unit_address, irq, vdev)))) {
                kobject_put(&hvcsd->kobj);
                printk(KERN_WARNING "HVCS: enable device failed.\n");
                return rc;
        }

        goto open_success;

fast_open:
        hvcsd = tty->driver_data;

        spin_lock_irqsave(&hvcsd->lock, flags);
        if (!kobject_get(&hvcsd->kobj)) {
                spin_unlock_irqrestore(&hvcsd->lock, flags);
                printk(KERN_ERR "HVCS: Kobject of open"
                        " hvcs doesn't exist.\n");
                return -EFAULT; /* Is this the right return value? */
        }

        hvcsd->open_count++;

        hvcsd->todo_mask |= HVCS_SCHED_READ;
        spin_unlock_irqrestore(&hvcsd->lock, flags);
open_success:
        hvcs_kick();

        printk(KERN_INFO "HVCS: vty-server@%X connection opened.\n",
                hvcsd->vdev->unit_address );

        return 0;

error_release:
        kobjp = &hvcsd->kobj;
        spin_unlock_irqrestore(&hvcsd->lock, flags);
        kobject_put(&hvcsd->kobj);

        printk(KERN_WARNING "HVCS: partner connect failed.\n");
        return retval;
}

static void hvcs_close(struct tty_struct *tty, struct file *filp)
{
        struct hvcs_struct *hvcsd;
        unsigned long flags;
        struct kobject *kobjp;
        int irq = NO_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);
        kobjp = &hvcsd->kobj;
        if (--hvcsd->open_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->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);
                kobject_put(kobjp);
                return;
        } else if (hvcsd->open_count < 0) {
                printk(KERN_ERR "HVCS: vty-server@%X open_count: %d"
                                " is missmanaged.\n",
                hvcsd->vdev->unit_address, hvcsd->open_count);
        }

        spin_unlock_irqrestore(&hvcsd->lock, flags);
        kobject_put(kobjp);
}

static void hvcs_hangup(struct tty_struct * tty)
{
        struct hvcs_struct *hvcsd = tty->driver_data;
        unsigned long flags;
        int temp_open_count;
        struct kobject *kobjp;
        int irq = NO_IRQ;

        spin_lock_irqsave(&hvcsd->lock, flags);
        /* Preserve this so that we know how many kobject refs to put */
        temp_open_count = hvcsd->open_count;

        /*
         * Don't kobject put inside the spinlock because the destruction
         * callback may use the spinlock and it may get called before the
         * spinlock has been released.  Get a pointer to the kobject and
         * kobject_put on that after releasing the spinlock.
         */
        kobjp = &hvcsd->kobj;

        vio_disable_interrupts(hvcsd->vdev);

        hvcsd->todo_mask = 0;

        /* I don't think the tty needs the hvcs_struct pointer after a hangup */
        hvcsd->tty->driver_data = NULL;
        hvcsd->tty = NULL;

        hvcsd->open_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 kobject_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.
                 */
                kobject_put(kobjp);
        }
}

/*
 * 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 succedded 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->open_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 guarentee 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->open_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 = {
        .open = hvcs_open,
        .close = hvcs_close,
        .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 __init hvcs_module_init(void)
{
        int rc;
        int num_ttys_to_alloc;

        printk(KERN_INFO "Initializing %s\n", hvcs_driver_string);

        /* 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)
                return -ENOMEM;

        if (hvcs_alloc_index_list(num_ttys_to_alloc)) {
                rc = -ENOMEM;
                goto index_fail;
        }

        hvcs_tty_driver->owner = THIS_MODULE;

        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 = kmalloc(PAGE_SIZE, 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.  Driver not loaded.\n");
                rc = -EIO;
                goto kthread_fail;
        }

        rc = vio_register_driver(&hvcs_vio_driver);
        if (rc) {
                printk(KERN_ERR "HVCS: can't register vio driver\n");
                goto vio_fail;
        }

        /*
         * 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) {
                printk(KERN_ERR "HVCS: sysfs attr create failed\n");
                goto attr_fail;
        }

        printk(KERN_INFO "HVCS: driver module inserted.\n");

        return 0;

attr_fail:
        vio_unregister_driver(&hvcs_vio_driver);
vio_fail:
        kthread_stop(hvcs_task);
kthread_fail:
        kfree(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;
        return rc;
}

static void __exit hvcs_module_exit(void)
{
        /*
         * This driver receives hvcs_remove callbacks for each device upon
         * module removal.
         */

        /*
         * 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);
        kfree(hvcs_pi_buff);
        hvcs_pi_buff = NULL;
        spin_unlock(&hvcs_pi_lock);

        driver_remove_file(&hvcs_vio_driver.driver, &driver_attr_rescan);

        vio_unregister_driver(&hvcs_vio_driver);

        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);