drivers/base/sync.c

/*
 * drivers/base/sync.c
 *
 * Copyright (C) 2012 Google, Inc.
 *
 * This software is licensed under the terms of the GNU General Public
 * License version 2, as published by the Free Software Foundation, and
 * may be copied, distributed, and modified under those terms.
 *
 * 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.
 *
 */

#include <linux/debugfs.h>
#include <linux/export.h>
#include <linux/file.h>
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/poll.h>
#include <linux/sched.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/sync.h>
#include <linux/uaccess.h>

#include <linux/anon_inodes.h>

static void sync_fence_signal_pt(struct sync_pt *pt);
static int _sync_pt_has_signaled(struct sync_pt *pt);
static void sync_fence_free(struct kref *kref);
static void sync_dump(void);

static LIST_HEAD(sync_timeline_list_head);
static DEFINE_SPINLOCK(sync_timeline_list_lock);

static LIST_HEAD(sync_fence_list_head);
static DEFINE_SPINLOCK(sync_fence_list_lock);

struct sync_timeline *sync_timeline_create(const struct sync_timeline_ops *ops,
					   int size, const char *name)
{
	struct sync_timeline *obj;
	unsigned long flags;

	if (size < sizeof(struct sync_timeline))
		return NULL;

	obj = kzalloc(size, GFP_KERNEL);
	if (obj == NULL)
		return NULL;

	kref_init(&obj->kref);
	obj->ops = ops;
	strlcpy(obj->name, name, sizeof(obj->name));

	INIT_LIST_HEAD(&obj->child_list_head);
	spin_lock_init(&obj->child_list_lock);

	INIT_LIST_HEAD(&obj->active_list_head);
	spin_lock_init(&obj->active_list_lock);

	spin_lock_irqsave(&sync_timeline_list_lock, flags);
	list_add_tail(&obj->sync_timeline_list, &sync_timeline_list_head);
	spin_unlock_irqrestore(&sync_timeline_list_lock, flags);

	return obj;
}
EXPORT_SYMBOL(sync_timeline_create);

static void sync_timeline_free(struct kref *kref)
{
	struct sync_timeline *obj =
		container_of(kref, struct sync_timeline, kref);
	unsigned long flags;

	if (obj->ops->release_obj)
		obj->ops->release_obj(obj);

	spin_lock_irqsave(&sync_timeline_list_lock, flags);
	list_del(&obj->sync_timeline_list);
	spin_unlock_irqrestore(&sync_timeline_list_lock, flags);

	kfree(obj);
}

void sync_timeline_destroy(struct sync_timeline *obj)
{
	obj->destroyed = true;

	/*
	 * If this is not the last reference, signal any children
	 * that their parent is going away.
	 */

	if (!kref_put(&obj->kref, sync_timeline_free))
		sync_timeline_signal(obj);
}
EXPORT_SYMBOL(sync_timeline_destroy);

static void sync_timeline_add_pt(struct sync_timeline *obj, struct sync_pt *pt)
{
	unsigned long flags;

	pt->parent = obj;

	spin_lock_irqsave(&obj->child_list_lock, flags);
	list_add_tail(&pt->child_list, &obj->child_list_head);
	spin_unlock_irqrestore(&obj->child_list_lock, flags);
}

static void sync_timeline_remove_pt(struct sync_pt *pt)
{
	struct sync_timeline *obj = pt->parent;
	unsigned long flags;

	spin_lock_irqsave(&obj->active_list_lock, flags);
	if (!list_empty(&pt->active_list))
		list_del_init(&pt->active_list);
	spin_unlock_irqrestore(&obj->active_list_lock, flags);

	spin_lock_irqsave(&obj->child_list_lock, flags);
	if (!list_empty(&pt->child_list)) {
		list_del_init(&pt->child_list);
	}
	spin_unlock_irqrestore(&obj->child_list_lock, flags);
}

void sync_timeline_signal(struct sync_timeline *obj)
{
	unsigned long flags;
	LIST_HEAD(signaled_pts);
	struct list_head *pos, *n;

	spin_lock_irqsave(&obj->active_list_lock, flags);

	list_for_each_safe(pos, n, &obj->active_list_head) {
		struct sync_pt *pt =
			container_of(pos, struct sync_pt, active_list);

		if (_sync_pt_has_signaled(pt)) {
			list_del_init(pos);
			list_add(&pt->signaled_list, &signaled_pts);
			kref_get(&pt->fence->kref);
		}
	}

	spin_unlock_irqrestore(&obj->active_list_lock, flags);

	list_for_each_safe(pos, n, &signaled_pts) {
		struct sync_pt *pt =
			container_of(pos, struct sync_pt, signaled_list);

		list_del_init(pos);
		sync_fence_signal_pt(pt);
		kref_put(&pt->fence->kref, sync_fence_free);
	}
}
EXPORT_SYMBOL(sync_timeline_signal);

struct sync_pt *sync_pt_create(struct sync_timeline *parent, int size)
{
	struct sync_pt *pt;

	if (size < sizeof(struct sync_pt))
		return NULL;

	pt = kzalloc(size, GFP_KERNEL);
	if (pt == NULL)
		return NULL;

	INIT_LIST_HEAD(&pt->active_list);
	kref_get(&parent->kref);
	sync_timeline_add_pt(parent, pt);

	return pt;
}
EXPORT_SYMBOL(sync_pt_create);

void sync_pt_free(struct sync_pt *pt)
{
	if (pt->parent->ops->free_pt)
		pt->parent->ops->free_pt(pt);

	sync_timeline_remove_pt(pt);

	kref_put(&pt->parent->kref, sync_timeline_free);

	kfree(pt);
}
EXPORT_SYMBOL(sync_pt_free);

/* call with pt->parent->active_list_lock held */
static int _sync_pt_has_signaled(struct sync_pt *pt)
{
	int old_status = pt->status;

	if (!pt->status)
		pt->status = pt->parent->ops->has_signaled(pt);

	if (!pt->status && pt->parent->destroyed)
		pt->status = -ENOENT;

	if (pt->status != old_status)
		pt->timestamp = ktime_get();

	return pt->status;
}

static struct sync_pt *sync_pt_dup(struct sync_pt *pt)
{
	return pt->parent->ops->dup(pt);
}

/* Adds a sync pt to the active queue.  Called when added to a fence */
static void sync_pt_activate(struct sync_pt *pt)
{
	struct sync_timeline *obj = pt->parent;
	unsigned long flags;
	int err;

	spin_lock_irqsave(&obj->active_list_lock, flags);

	err = _sync_pt_has_signaled(pt);
	if (err != 0) {
		spin_unlock_irqrestore(&obj->active_list_lock, flags);
		sync_fence_signal_pt(pt);
		return;
	}

	list_add_tail(&pt->active_list, &obj->active_list_head);
	spin_unlock_irqrestore(&obj->active_list_lock, flags);
}

static int sync_fence_release(struct inode *inode, struct file *file);
static unsigned int sync_fence_poll(struct file *file, poll_table *wait);
static long sync_fence_ioctl(struct file *file, unsigned int cmd,
			     unsigned long arg);


static const struct file_operations sync_fence_fops = {
	.release = sync_fence_release,
	.poll = sync_fence_poll,
	.unlocked_ioctl = sync_fence_ioctl,
};

static struct sync_fence *sync_fence_alloc(const char *name)
{
	struct sync_fence *fence;
	unsigned long flags;

	fence = kzalloc(sizeof(struct sync_fence), GFP_KERNEL);
	if (fence == NULL)
		return NULL;

	fence->file = anon_inode_getfile("sync_fence", &sync_fence_fops,
					 fence, 0);
	if (fence->file == NULL)
		goto err;

	kref_init(&fence->kref);
	strlcpy(fence->name, name, sizeof(fence->name));

	INIT_LIST_HEAD(&fence->pt_list_head);
	INIT_LIST_HEAD(&fence->waiter_list_head);
	spin_lock_init(&fence->waiter_list_lock);

	init_waitqueue_head(&fence->wq);

	spin_lock_irqsave(&sync_fence_list_lock, flags);
	list_add_tail(&fence->sync_fence_list, &sync_fence_list_head);
	spin_unlock_irqrestore(&sync_fence_list_lock, flags);

	return fence;

err:
	kfree(fence);
	return NULL;
}

/* TODO: implement a create which takes more that one sync_pt */
struct sync_fence *sync_fence_create(const char *name, struct sync_pt *pt)
{
	struct sync_fence *fence;

	if (pt->fence)
		return NULL;

	fence = sync_fence_alloc(name);
	if (fence == NULL)
		return NULL;

	pt->fence = fence;
	list_add(&pt->pt_list, &fence->pt_list_head);
	sync_pt_activate(pt);

	return fence;
}
EXPORT_SYMBOL(sync_fence_create);

static int sync_fence_copy_pts(struct sync_fence *dst, struct sync_fence *src)
{
	struct list_head *pos;

	list_for_each(pos, &src->pt_list_head) {
		struct sync_pt *orig_pt =
			container_of(pos, struct sync_pt, pt_list);
		struct sync_pt *new_pt = sync_pt_dup(orig_pt);

		if (new_pt == NULL)
			return -ENOMEM;

		new_pt->fence = dst;
		list_add(&new_pt->pt_list, &dst->pt_list_head);
		sync_pt_activate(new_pt);
	}

	return 0;
}

static int sync_fence_merge_pts(struct sync_fence *dst, struct sync_fence *src)
{
	struct list_head *src_pos, *dst_pos, *n;

	list_for_each(src_pos, &src->pt_list_head) {
		struct sync_pt *src_pt =
			container_of(src_pos, struct sync_pt, pt_list);
		bool collapsed = false;

		list_for_each_safe(dst_pos, n, &dst->pt_list_head) {
			struct sync_pt *dst_pt =
				container_of(dst_pos, struct sync_pt, pt_list);
			/* collapse two sync_pts on the same timeline
			 * to a single sync_pt that will signal at
			 * the later of the two
			 */
			if (dst_pt->parent == src_pt->parent) {
				if (dst_pt->parent->ops->compare(dst_pt, src_pt) == -1) {
					struct sync_pt *new_pt =
						sync_pt_dup(src_pt);
					if (new_pt == NULL)
						return -ENOMEM;

					new_pt->fence = dst;
					list_replace(&dst_pt->pt_list,
						     &new_pt->pt_list);
					sync_pt_activate(new_pt);
					sync_pt_free(dst_pt);
				}
				collapsed = true;
				break;
			}
		}

		if (!collapsed) {
			struct sync_pt *new_pt = sync_pt_dup(src_pt);

			if (new_pt == NULL)
				return -ENOMEM;

			new_pt->fence = dst;
			list_add(&new_pt->pt_list, &dst->pt_list_head);
			sync_pt_activate(new_pt);
		}
	}

	return 0;
}

static void sync_fence_detach_pts(struct sync_fence *fence)
{
	struct list_head *pos, *n;

	list_for_each_safe(pos, n, &fence->pt_list_head) {
		struct sync_pt *pt = container_of(pos, struct sync_pt, pt_list);
		sync_timeline_remove_pt(pt);
	}
}

static void sync_fence_free_pts(struct sync_fence *fence)
{
	struct list_head *pos, *n;

	list_for_each_safe(pos, n, &fence->pt_list_head) {
		struct sync_pt *pt = container_of(pos, struct sync_pt, pt_list);
		sync_pt_free(pt);
	}
}

struct sync_fence *sync_fence_fdget(int fd)
{
	struct file *file = fget(fd);

	if (file == NULL)
		return NULL;

	if (file->f_op != &sync_fence_fops)
		goto err;

	return file->private_data;

err:
	fput(file);
	return NULL;
}
EXPORT_SYMBOL(sync_fence_fdget);

void sync_fence_put(struct sync_fence *fence)
{
	fput(fence->file);
}
EXPORT_SYMBOL(sync_fence_put);

void sync_fence_install(struct sync_fence *fence, int fd)
{
	fd_install(fd, fence->file);
}
EXPORT_SYMBOL(sync_fence_install);

static int sync_fence_get_status(struct sync_fence *fence)
{
	struct list_head *pos;
	int status = 1;

	list_for_each(pos, &fence->pt_list_head) {
		struct sync_pt *pt = container_of(pos, struct sync_pt, pt_list);
		int pt_status = pt->status;

		if (pt_status < 0) {
			status = pt_status;
			break;
		} else if (status == 1) {
			status = pt_status;
		}
	}

	return status;
}

struct sync_fence *sync_fence_merge(const char *name,
				    struct sync_fence *a, struct sync_fence *b)
{
	struct sync_fence *fence;
	int err;

	fence = sync_fence_alloc(name);
	if (fence == NULL)
		return NULL;

	err = sync_fence_copy_pts(fence, a);
	if (err < 0)
		goto err;

	err = sync_fence_merge_pts(fence, b);
	if (err < 0)
		goto err;

	fence->status = sync_fence_get_status(fence);

	return fence;
err:
	sync_fence_free_pts(fence);
	kfree(fence);
	return NULL;
}
EXPORT_SYMBOL(sync_fence_merge);

static void sync_fence_signal_pt(struct sync_pt *pt)
{
	LIST_HEAD(signaled_waiters);
	struct sync_fence *fence = pt->fence;
	struct list_head *pos;
	struct list_head *n;
	unsigned long flags;
	int status;

	status = sync_fence_get_status(fence);

	spin_lock_irqsave(&fence->waiter_list_lock, flags);
	/*
	 * this should protect against two threads racing on the signaled
	 * false -> true transition
	 */
	if (status && !fence->status) {
		list_for_each_safe(pos, n, &fence->waiter_list_head)
			list_move(pos, &signaled_waiters);

		fence->status = status;
	} else {
		status = 0;
	}
	spin_unlock_irqrestore(&fence->waiter_list_lock, flags);

	if (status) {
		list_for_each_safe(pos, n, &signaled_waiters) {
			struct sync_fence_waiter *waiter =
				container_of(pos, struct sync_fence_waiter,
					     waiter_list);

			list_del(pos);
			waiter->callback(fence, waiter);
		}
		wake_up(&fence->wq);
	}
}

int sync_fence_wait_async(struct sync_fence *fence,
			  struct sync_fence_waiter *waiter)
{
	unsigned long flags;
	int err = 0;

	spin_lock_irqsave(&fence->waiter_list_lock, flags);

	if (fence->status) {
		err = fence->status;
		goto out;
	}

	list_add_tail(&waiter->waiter_list, &fence->waiter_list_head);
out:
	spin_unlock_irqrestore(&fence->waiter_list_lock, flags);

	return err;
}
EXPORT_SYMBOL(sync_fence_wait_async);

int sync_fence_cancel_async(struct sync_fence *fence,
			     struct sync_fence_waiter *waiter)
{
	struct list_head *pos;
	struct list_head *n;
	unsigned long flags;
	int ret = -ENOENT;

	spin_lock_irqsave(&fence->waiter_list_lock, flags);
	/*
	 * Make sure waiter is still in waiter_list because it is possible for
	 * the waiter to be removed from the list while the callback is still
	 * pending.
	 */
	list_for_each_safe(pos, n, &fence->waiter_list_head) {
		struct sync_fence_waiter *list_waiter =
			container_of(pos, struct sync_fence_waiter,
				     waiter_list);
		if (list_waiter == waiter) {
			list_del(pos);
			ret = 0;
			break;
		}
	}
	spin_unlock_irqrestore(&fence->waiter_list_lock, flags);
	return ret;
}
EXPORT_SYMBOL(sync_fence_cancel_async);

int sync_fence_wait(struct sync_fence *fence, long timeout)
{
	int err = 0;

	if (timeout > 0) {
		timeout = msecs_to_jiffies(timeout);
		err = wait_event_interruptible_timeout(fence->wq,
						       fence->status != 0,
						       timeout);
	} else if (timeout < 0) {
		err = wait_event_interruptible(fence->wq, fence->status != 0);
	}

	if (err < 0)
		return err;

	if (fence->status < 0)
		return fence->status;

	if (fence->status == 0) {
		pr_info("fence timeout on [%p] after %dms\n", fence,
			jiffies_to_msecs(timeout));
		sync_dump();
		return -ETIME;
	}

	return 0;
}
EXPORT_SYMBOL(sync_fence_wait);

static void sync_fence_free(struct kref *kref)
{
	struct sync_fence *fence = container_of(kref, struct sync_fence, kref);

	sync_fence_free_pts(fence);

	kfree(fence);
}

static int sync_fence_release(struct inode *inode, struct file *file)
{
	struct sync_fence *fence = file->private_data;
	unsigned long flags;

	/*
	 * We need to remove all ways to access this fence before droping
	 * our ref.
	 *
	 * start with its membership in the global fence list
	 */
	spin_lock_irqsave(&sync_fence_list_lock, flags);
	list_del(&fence->sync_fence_list);
	spin_unlock_irqrestore(&sync_fence_list_lock, flags);

	/*
	 * remove its pts from their parents so that sync_timeline_signal()
	 * can't reference the fence.
	 */
	sync_fence_detach_pts(fence);

	kref_put(&fence->kref, sync_fence_free);

	return 0;
}

static unsigned int sync_fence_poll(struct file *file, poll_table *wait)
{
	struct sync_fence *fence = file->private_data;

	poll_wait(file, &fence->wq, wait);

	if (fence->status == 1)
		return POLLIN;
	else if (fence->status < 0)
		return POLLERR;
	else
		return 0;
}

static long sync_fence_ioctl_wait(struct sync_fence *fence, unsigned long arg)
{
	__s32 value;

	if (copy_from_user(&value, (void __user *)arg, sizeof(value)))
		return -EFAULT;

	return sync_fence_wait(fence, value);
}

static long sync_fence_ioctl_merge(struct sync_fence *fence, unsigned long arg)
{
	int fd = get_unused_fd();
	int err;
	struct sync_fence *fence2, *fence3;
	struct sync_merge_data data;

	if (copy_from_user(&data, (void __user *)arg, sizeof(data)))
		return -EFAULT;

	fence2 = sync_fence_fdget(data.fd2);
	if (fence2 == NULL) {
		err = -ENOENT;
		goto err_put_fd;
	}

	data.name[sizeof(data.name) - 1] = '\0';
	fence3 = sync_fence_merge(data.name, fence, fence2);
	if (fence3 == NULL) {
		err = -ENOMEM;
		goto err_put_fence2;
	}

	data.fence = fd;
	if (copy_to_user((void __user *)arg, &data, sizeof(data))) {
		err = -EFAULT;
		goto err_put_fence3;
	}

	sync_fence_install(fence3, fd);
	sync_fence_put(fence2);
	return 0;

err_put_fence3:
	sync_fence_put(fence3);

err_put_fence2:
	sync_fence_put(fence2);

err_put_fd:
	put_unused_fd(fd);
	return err;
}

static int sync_fill_pt_info(struct sync_pt *pt, void *data, int size)
{
	struct sync_pt_info *info = data;
	int ret;

	if (size < sizeof(struct sync_pt_info))
		return -ENOMEM;

	info->len = sizeof(struct sync_pt_info);

	if (pt->parent->ops->fill_driver_data) {
		ret = pt->parent->ops->fill_driver_data(pt, info->driver_data,
							size - sizeof(*info));
		if (ret < 0)
			return ret;

		info->len += ret;
	}

	strlcpy(info->obj_name, pt->parent->name, sizeof(info->obj_name));
	strlcpy(info->driver_name, pt->parent->ops->driver_name,
		sizeof(info->driver_name));
	info->status = pt->status;
	info->timestamp_ns = ktime_to_ns(pt->timestamp);

	return info->len;
}

static long sync_fence_ioctl_fence_info(struct sync_fence *fence,
					unsigned long arg)
{
	struct sync_fence_info_data *data;
	struct list_head *pos;
	__u32 size;
	__u32 len = 0;
	int ret;

	if (copy_from_user(&size, (void __user *)arg, sizeof(size)))
		return -EFAULT;

	if (size < sizeof(struct sync_fence_info_data))
		return -EINVAL;

	if (size > 4096)
		size = 4096;

	data = kzalloc(size, GFP_KERNEL);
	if (data == NULL)
		return -ENOMEM;

	strlcpy(data->name, fence->name, sizeof(data->name));
	data->status = fence->status;
	len = sizeof(struct sync_fence_info_data);

	list_for_each(pos, &fence->pt_list_head) {
		struct sync_pt *pt =
			container_of(pos, struct sync_pt, pt_list);

		ret = sync_fill_pt_info(pt, (u8 *)data + len, size - len);

		if (ret < 0)
			goto out;

		len += ret;
	}

	data->len = len;

	if (copy_to_user((void __user *)arg, data, len))
		ret = -EFAULT;
	else
		ret = 0;

out:
	kfree(data);

	return ret;
}

static long sync_fence_ioctl(struct file *file, unsigned int cmd,
			     unsigned long arg)
{
	struct sync_fence *fence = file->private_data;
	switch (cmd) {
	case SYNC_IOC_WAIT:
		return sync_fence_ioctl_wait(fence, arg);

	case SYNC_IOC_MERGE:
		return sync_fence_ioctl_merge(fence, arg);

	case SYNC_IOC_FENCE_INFO:
		return sync_fence_ioctl_fence_info(fence, arg);

	default:
		return -ENOTTY;
	}
}

#ifdef CONFIG_DEBUG_FS
static const char *sync_status_str(int status)
{
	if (status > 0)
		return "signaled";
	else if (status == 0)
		return "active";
	else
		return "error";
}

static void sync_print_pt(struct seq_file *s, struct sync_pt *pt, bool fence)
{
	int status = pt->status;
	seq_printf(s, "  %s%spt %s",
		   fence ? pt->parent->name : "",
		   fence ? "_" : "",
		   sync_status_str(status));
	if (pt->status) {
		struct timeval tv = ktime_to_timeval(pt->timestamp);
		seq_printf(s, "@%ld.%06ld", tv.tv_sec, tv.tv_usec);
	}

	if (pt->parent->ops->print_pt) {
		seq_printf(s, ": ");
		pt->parent->ops->print_pt(s, pt);
	}

	seq_printf(s, "\n");
}

static void sync_print_obj(struct seq_file *s, struct sync_timeline *obj)
{
	struct list_head *pos;
	unsigned long flags;

	seq_printf(s, "%s %s", obj->name, obj->ops->driver_name);

	if (obj->ops->print_obj) {
		seq_printf(s, ": ");
		obj->ops->print_obj(s, obj);
	}

	seq_printf(s, "\n");

	spin_lock_irqsave(&obj->child_list_lock, flags);
	list_for_each(pos, &obj->child_list_head) {
		struct sync_pt *pt =
			container_of(pos, struct sync_pt, child_list);
		sync_print_pt(s, pt, false);
	}
	spin_unlock_irqrestore(&obj->child_list_lock, flags);
}

static void sync_print_fence(struct seq_file *s, struct sync_fence *fence)
{
	struct list_head *pos;
	unsigned long flags;

	seq_printf(s, "[%p] %s: %s\n", fence, fence->name,
		   sync_status_str(fence->status));

	list_for_each(pos, &fence->pt_list_head) {
		struct sync_pt *pt =
			container_of(pos, struct sync_pt, pt_list);
		sync_print_pt(s, pt, true);
	}

	spin_lock_irqsave(&fence->waiter_list_lock, flags);
	list_for_each(pos, &fence->waiter_list_head) {
		struct sync_fence_waiter *waiter =
			container_of(pos, struct sync_fence_waiter,
				     waiter_list);

		seq_printf(s, "waiter %pF\n", waiter->callback);
	}
	spin_unlock_irqrestore(&fence->waiter_list_lock, flags);
}

static int sync_debugfs_show(struct seq_file *s, void *unused)
{
	unsigned long flags;
	struct list_head *pos;

	seq_printf(s, "objs:\n--------------\n");

	spin_lock_irqsave(&sync_timeline_list_lock, flags);
	list_for_each(pos, &sync_timeline_list_head) {
		struct sync_timeline *obj =
			container_of(pos, struct sync_timeline,
				     sync_timeline_list);

		sync_print_obj(s, obj);
		seq_printf(s, "\n");
	}
	spin_unlock_irqrestore(&sync_timeline_list_lock, flags);

	seq_printf(s, "fences:\n--------------\n");

	spin_lock_irqsave(&sync_fence_list_lock, flags);
	list_for_each(pos, &sync_fence_list_head) {
		struct sync_fence *fence =
			container_of(pos, struct sync_fence, sync_fence_list);

		sync_print_fence(s, fence);
		seq_printf(s, "\n");
	}
	spin_unlock_irqrestore(&sync_fence_list_lock, flags);
	return 0;
}

static int sync_debugfs_open(struct inode *inode, struct file *file)
{
	return single_open(file, sync_debugfs_show, inode->i_private);
}

static const struct file_operations sync_debugfs_fops = {
	.open           = sync_debugfs_open,
	.read           = seq_read,
	.llseek         = seq_lseek,
	.release        = single_release,
};

static __init int sync_debugfs_init(void)
{
	debugfs_create_file("sync", S_IRUGO, NULL, NULL, &sync_debugfs_fops);
	return 0;
}
late_initcall(sync_debugfs_init);

#define DUMP_CHUNK 256
static char sync_dump_buf[64 * 1024];
void sync_dump(void)
{
       struct seq_file s = {
               .buf = sync_dump_buf,
               .size = sizeof(sync_dump_buf) - 1,
       };
       int i;

       sync_debugfs_show(&s, NULL);

       for (i = 0; i < s.count; i += DUMP_CHUNK) {
               if ((s.count - i) > DUMP_CHUNK) {
                       char c = s.buf[i + DUMP_CHUNK];
                       s.buf[i + DUMP_CHUNK] = 0;
                       pr_cont("%s", s.buf + i);
                       s.buf[i + DUMP_CHUNK] = c;
               } else {
                       s.buf[s.count] = 0;
                       pr_cont("%s", s.buf + i);
               }
       }
}
#else
static void sync_dump(void)
{
}
#endif