Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1602 insertions, 0 deletions

diff --git a/drivers/tty/hvc/hvcs.c b/drivers/tty/hvc/hvcs.c
new file mode 100644
index 000000000..7853c6375
--- /dev/null
+++ b/drivers/tty/hvc/hvcs.c
@@ -0,0 +1,1602 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * IBM eServer Hypervisor Virtual Console Server Device Driver
+ * Copyright (C) 2003, 2004 IBM Corp.
+ *  Ryan S. Arnold (rsa@us.ibm.com)
+ *
+ * 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 const 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 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 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_RW(rescan);
+
+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 const 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);
+
+		free_irq(irq, hvcsd);
+		return;
+	} else if (hvcsd->port.count < 0) {
+		printk(KERN_ERR "HVCS: vty-server@%X open_count: %d is mismanaged.\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;
+
+	/*
+	 * 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;
+
+	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_array(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_warn("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);