drivers/staging/iio/ring_sw.c - kernel/mindspeed - Git at Google

 /* The industrial I/O simple minimally locked ring buffer.
  *
  * Copyright (c) 2008 Jonathan Cameron
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 as published by
  * the Free Software Foundation.
  */

 #include <linux/slab.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/device.h>
 #include <linux/workqueue.h>
 #include <linux/sched.h>
 #include <linux/poll.h>
 #include "ring_sw.h"
 #include "trigger.h"

 /**
  * struct iio_sw_ring_buffer - software ring buffer
  * @buf:		generic ring buffer elements
  * @data:		the ring buffer memory
  * @read_p:		read pointer (oldest available)
  * @write_p:		write pointer
  * @last_written_p:	read pointer (newest available)
  * @half_p:		half buffer length behind write_p (event generation)
  * @use_count:		reference count to prevent resizing when in use
  * @update_needed:	flag to indicated change in size requested
  * @use_lock:		lock to prevent change in size when in use
  *
  * Note that the first element of all ring buffers must be a
  * struct iio_buffer.
 **/
 struct iio_sw_ring_buffer {
 	struct iio_buffer  buf;
 	unsigned char		*data;
 	unsigned char		*read_p;
 	unsigned char		*write_p;
 	unsigned char		*last_written_p;
 	/* used to act as a point at which to signal an event */
 	unsigned char		*half_p;
 	int			use_count;
 	int			update_needed;
 	spinlock_t		use_lock;
 };

 #define iio_to_sw_ring(r) container_of(r, struct iio_sw_ring_buffer, buf)

 static inline int __iio_allocate_sw_ring_buffer(struct iio_sw_ring_buffer *ring,
 						int bytes_per_datum, int length)
 {
 	if ((length == 0) || (bytes_per_datum == 0))
 		return -EINVAL;
 	__iio_update_buffer(&ring->buf, bytes_per_datum, length);
 	ring->data = kmalloc(length*ring->buf.bytes_per_datum, GFP_ATOMIC);
 	ring->read_p = NULL;
 	ring->write_p = NULL;
 	ring->last_written_p = NULL;
 	ring->half_p = NULL;
 	return ring->data ? 0 : -ENOMEM;
 }

 static inline void __iio_init_sw_ring_buffer(struct iio_sw_ring_buffer *ring)
 {
 	spin_lock_init(&ring->use_lock);
 }

 static inline void __iio_free_sw_ring_buffer(struct iio_sw_ring_buffer *ring)
 {
 	kfree(ring->data);
 }

 static void iio_mark_sw_rb_in_use(struct iio_buffer *r)
 {
 	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
 	spin_lock(&ring->use_lock);
 	ring->use_count++;
 	spin_unlock(&ring->use_lock);
 }

 static void iio_unmark_sw_rb_in_use(struct iio_buffer *r)
 {
 	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
 	spin_lock(&ring->use_lock);
 	ring->use_count--;
 	spin_unlock(&ring->use_lock);
 }


 /* Ring buffer related functionality */
 /* Store to ring is typically called in the bh of a data ready interrupt handler
  * in the device driver */
 /* Lock always held if their is a chance this may be called */
 /* Only one of these per ring may run concurrently - enforced by drivers */
 static int iio_store_to_sw_ring(struct iio_sw_ring_buffer *ring,
 				unsigned char *data, s64 timestamp)
 {
 	int ret = 0;
 	unsigned char *temp_ptr, *change_test_ptr;

 	/* initial store */
 	if (unlikely(ring->write_p == NULL)) {
 		ring->write_p = ring->data;
 		/* Doesn't actually matter if this is out of the set
 		 * as long as the read pointer is valid before this
 		 * passes it - guaranteed as set later in this function.
 		 */
 		ring->half_p = ring->data - ring->buf.length*ring->buf.bytes_per_datum/2;
 	}
 	/* Copy data to where ever the current write pointer says */
 	memcpy(ring->write_p, data, ring->buf.bytes_per_datum);
 	barrier();
 	/* Update the pointer used to get most recent value.
 	 * Always valid as either points to latest or second latest value.
 	 * Before this runs it is null and read attempts fail with -EAGAIN.
 	 */
 	ring->last_written_p = ring->write_p;
 	barrier();
 	/* temp_ptr used to ensure we never have an invalid pointer
 	 * it may be slightly lagging, but never invalid
 	 */
 	temp_ptr = ring->write_p + ring->buf.bytes_per_datum;
 	/* End of ring, back to the beginning */
 	if (temp_ptr == ring->data + ring->buf.length*ring->buf.bytes_per_datum)
 		temp_ptr = ring->data;
 	/* Update the write pointer
 	 * always valid as long as this is the only function able to write.
 	 * Care needed with smp systems to ensure more than one ring fill
 	 * is never scheduled.
 	 */
 	ring->write_p = temp_ptr;

 	if (ring->read_p == NULL)
 		ring->read_p = ring->data;
 	/* Buffer full - move the read pointer and create / escalate
 	 * ring event */
 	/* Tricky case - if the read pointer moves before we adjust it.
 	 * Handle by not pushing if it has moved - may result in occasional
 	 * unnecessary buffer full events when it wasn't quite true.
 	 */
 	else if (ring->write_p == ring->read_p) {
 		change_test_ptr = ring->read_p;
 		temp_ptr = change_test_ptr + ring->buf.bytes_per_datum;
 		if (temp_ptr
 		    == ring->data + ring->buf.length*ring->buf.bytes_per_datum) {
 			temp_ptr = ring->data;
 		}
 		/* We are moving pointer on one because the ring is full.  Any
 		 * change to the read pointer will be this or greater.
 		 */
 		if (change_test_ptr == ring->read_p)
 			ring->read_p = temp_ptr;
 	}
 	/* investigate if our event barrier has been passed */
 	/* There are definite 'issues' with this and chances of
 	 * simultaneous read */
 	/* Also need to use loop count to ensure this only happens once */
 	ring->half_p += ring->buf.bytes_per_datum;
 	if (ring->half_p == ring->data + ring->buf.length*ring->buf.bytes_per_datum)
 		ring->half_p = ring->data;
 	if (ring->half_p == ring->read_p) {
 		ring->buf.stufftoread = true;
 		wake_up_interruptible(&ring->buf.pollq);
 	}
 	return ret;
 }

 static int iio_read_first_n_sw_rb(struct iio_buffer *r,
 				  size_t n, char __user *buf)
 {
 	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);

 	u8 *initial_read_p, *initial_write_p, *current_read_p, *end_read_p;
 	u8 *data;
 	int ret, max_copied, bytes_to_rip, dead_offset;

 	/* A userspace program has probably made an error if it tries to
 	 *  read something that is not a whole number of bpds.
 	 * Return an error.
 	 */
 	if (n % ring->buf.bytes_per_datum) {
 		ret = -EINVAL;
 		printk(KERN_INFO "Ring buffer read request not whole number of"
 		       "samples: Request bytes %zd, Current bytes per datum %d\n",
 		       n, ring->buf.bytes_per_datum);
 		goto error_ret;
 	}
 	/* Limit size to whole of ring buffer */
 	bytes_to_rip = min((size_t)(ring->buf.bytes_per_datum*ring->buf.length),
 			   n);

 	data = kmalloc(bytes_to_rip, GFP_KERNEL);
 	if (data == NULL) {
 		ret = -ENOMEM;
 		goto error_ret;
 	}

 	/* build local copy */
 	initial_read_p = ring->read_p;
 	if (unlikely(initial_read_p == NULL)) { /* No data here as yet */
 		ret = 0;
 		goto error_free_data_cpy;
 	}

 	initial_write_p = ring->write_p;

 	/* Need a consistent pair */
 	while ((initial_read_p != ring->read_p)
 	       || (initial_write_p != ring->write_p)) {
 		initial_read_p = ring->read_p;
 		initial_write_p = ring->write_p;
 	}
 	if (initial_write_p == initial_read_p) {
 		/* No new data available.*/
 		ret = 0;
 		goto error_free_data_cpy;
 	}

 	if (initial_write_p >= initial_read_p + bytes_to_rip) {
 		/* write_p is greater than necessary, all is easy */
 		max_copied = bytes_to_rip;
 		memcpy(data, initial_read_p, max_copied);
 		end_read_p = initial_read_p + max_copied;
 	} else if (initial_write_p > initial_read_p) {
 		/*not enough data to cpy */
 		max_copied = initial_write_p - initial_read_p;
 		memcpy(data, initial_read_p, max_copied);
 		end_read_p = initial_write_p;
 	} else {
 		/* going through 'end' of ring buffer */
 		max_copied = ring->data
 			+ ring->buf.length*ring->buf.bytes_per_datum - initial_read_p;
 		memcpy(data, initial_read_p, max_copied);
 		/* possible we are done if we align precisely with end */
 		if (max_copied == bytes_to_rip)
 			end_read_p = ring->data;
 		else if (initial_write_p
 			 > ring->data + bytes_to_rip - max_copied) {
 			/* enough data to finish */
 			memcpy(data + max_copied, ring->data,
 			       bytes_to_rip - max_copied);
 			max_copied = bytes_to_rip;
 			end_read_p = ring->data + (bytes_to_rip - max_copied);
 		} else {  /* not enough data */
 			memcpy(data + max_copied, ring->data,
 			       initial_write_p - ring->data);
 			max_copied += initial_write_p - ring->data;
 			end_read_p = initial_write_p;
 		}
 	}
 	/* Now to verify which section was cleanly copied - i.e. how far
 	 * read pointer has been pushed */
 	current_read_p = ring->read_p;

 	if (initial_read_p <= current_read_p)
 		dead_offset = current_read_p - initial_read_p;
 	else
 		dead_offset = ring->buf.length*ring->buf.bytes_per_datum
 			- (initial_read_p - current_read_p);

 	/* possible issue if the initial write has been lapped or indeed
 	 * the point we were reading to has been passed */
 	/* No valid data read.
 	 * In this case the read pointer is already correct having been
 	 * pushed further than we would look. */
 	if (max_copied - dead_offset < 0) {
 		ret = 0;
 		goto error_free_data_cpy;
 	}

 	/* setup the next read position */
 	/* Beware, this may fail due to concurrency fun and games.
 	 *  Possible that sufficient fill commands have run to push the read
 	 * pointer past where we would be after the rip. If this occurs, leave
 	 * it be.
 	 */
 	/* Tricky - deal with loops */

 	while (ring->read_p != end_read_p)
 		ring->read_p = end_read_p;

 	ret = max_copied - dead_offset;

 	if (copy_to_user(buf, data + dead_offset, ret))  {
 		ret =  -EFAULT;
 		goto error_free_data_cpy;
 	}

 	if (bytes_to_rip >= ring->buf.length*ring->buf.bytes_per_datum/2)
 		ring->buf.stufftoread = 0;

 error_free_data_cpy:
 	kfree(data);
 error_ret:

 	return ret;
 }

 static int iio_store_to_sw_rb(struct iio_buffer *r,
 			      u8 *data,
 			      s64 timestamp)
 {
 	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
 	return iio_store_to_sw_ring(ring, data, timestamp);
 }

 static int iio_read_last_from_sw_ring(struct iio_sw_ring_buffer *ring,
 				      unsigned char *data)
 {
 	unsigned char *last_written_p_copy;

 	iio_mark_sw_rb_in_use(&ring->buf);
 again:
 	barrier();
 	last_written_p_copy = ring->last_written_p;
 	barrier(); /*unnessecary? */
 	/* Check there is anything here */
 	if (last_written_p_copy == NULL)
 		return -EAGAIN;
 	memcpy(data, last_written_p_copy, ring->buf.bytes_per_datum);

 	if (unlikely(ring->last_written_p != last_written_p_copy))
 		goto again;

 	iio_unmark_sw_rb_in_use(&ring->buf);
 	return 0;
 }

 static int iio_read_last_from_sw_rb(struct iio_buffer *r,
 			     unsigned char *data)
 {
 	return iio_read_last_from_sw_ring(iio_to_sw_ring(r), data);
 }

 static int iio_request_update_sw_rb(struct iio_buffer *r)
 {
 	int ret = 0;
 	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);

 	r->stufftoread = false;
 	spin_lock(&ring->use_lock);
 	if (!ring->update_needed)
 		goto error_ret;
 	if (ring->use_count) {
 		ret = -EAGAIN;
 		goto error_ret;
 	}
 	__iio_free_sw_ring_buffer(ring);
 	ret = __iio_allocate_sw_ring_buffer(ring, ring->buf.bytes_per_datum,
 					    ring->buf.length);
 error_ret:
 	spin_unlock(&ring->use_lock);
 	return ret;
 }

 static int iio_get_bytes_per_datum_sw_rb(struct iio_buffer *r)
 {
 	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
 	return ring->buf.bytes_per_datum;
 }

 static int iio_set_bytes_per_datum_sw_rb(struct iio_buffer *r, size_t bpd)
 {
 	if (r->bytes_per_datum != bpd) {
 		r->bytes_per_datum = bpd;
 		if (r->access->mark_param_change)
 			r->access->mark_param_change(r);
 	}
 	return 0;
 }

 static int iio_get_length_sw_rb(struct iio_buffer *r)
 {
 	return r->length;
 }

 static int iio_set_length_sw_rb(struct iio_buffer *r, int length)
 {
 	if (r->length != length) {
 		r->length = length;
 		if (r->access->mark_param_change)
 			r->access->mark_param_change(r);
 	}
 	return 0;
 }

 static int iio_mark_update_needed_sw_rb(struct iio_buffer *r)
 {
 	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
 	ring->update_needed = true;
 	return 0;
 }

 static IIO_BUFFER_ENABLE_ATTR;
 static IIO_BUFFER_BYTES_PER_DATUM_ATTR;
 static IIO_BUFFER_LENGTH_ATTR;

 /* Standard set of ring buffer attributes */
 static struct attribute *iio_ring_attributes[] = {
 	&dev_attr_length.attr,
 	&dev_attr_bytes_per_datum.attr,
 	&dev_attr_enable.attr,
 	NULL,
 };

 static struct attribute_group iio_ring_attribute_group = {
 	.attrs = iio_ring_attributes,
 	.name = "buffer",
 };

 struct iio_buffer *iio_sw_rb_allocate(struct iio_dev *indio_dev)
 {
 	struct iio_buffer *buf;
 	struct iio_sw_ring_buffer *ring;

 	ring = kzalloc(sizeof *ring, GFP_KERNEL);
 	if (!ring)
 		return NULL;
 	ring->update_needed = true;
 	buf = &ring->buf;
 	iio_buffer_init(buf, indio_dev);
 	__iio_init_sw_ring_buffer(ring);
 	buf->attrs = &iio_ring_attribute_group;

 	return buf;
 }
 EXPORT_SYMBOL(iio_sw_rb_allocate);

 void iio_sw_rb_free(struct iio_buffer *r)
 {
 	kfree(iio_to_sw_ring(r));
 }
 EXPORT_SYMBOL(iio_sw_rb_free);

 const struct iio_buffer_access_funcs ring_sw_access_funcs = {
 	.mark_in_use = &iio_mark_sw_rb_in_use,
 	.unmark_in_use = &iio_unmark_sw_rb_in_use,
 	.store_to = &iio_store_to_sw_rb,
 	.read_last = &iio_read_last_from_sw_rb,
 	.read_first_n = &iio_read_first_n_sw_rb,
 	.mark_param_change = &iio_mark_update_needed_sw_rb,
 	.request_update = &iio_request_update_sw_rb,
 	.get_bytes_per_datum = &iio_get_bytes_per_datum_sw_rb,
 	.set_bytes_per_datum = &iio_set_bytes_per_datum_sw_rb,
 	.get_length = &iio_get_length_sw_rb,
 	.set_length = &iio_set_length_sw_rb,
 };
 EXPORT_SYMBOL(ring_sw_access_funcs);

 MODULE_DESCRIPTION("Industrialio I/O software ring buffer");
 MODULE_LICENSE("GPL");
	/* The industrial I/O simple minimally locked ring buffer.
	*
	* Copyright (c) 2008 Jonathan Cameron
	*
	* This program is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 as published by
	* the Free Software Foundation.
	*/

	#include <linux/slab.h>
	#include <linux/kernel.h>
	#include <linux/module.h>
	#include <linux/device.h>
	#include <linux/workqueue.h>
	#include <linux/sched.h>
	#include <linux/poll.h>
	#include "ring_sw.h"
	#include "trigger.h"

	/**
	* struct iio_sw_ring_buffer - software ring buffer
	* @buf: generic ring buffer elements
	* @data: the ring buffer memory
	* @read_p: read pointer (oldest available)
	* @write_p: write pointer
	* @last_written_p: read pointer (newest available)
	* @half_p: half buffer length behind write_p (event generation)
	* @use_count: reference count to prevent resizing when in use
	* @update_needed: flag to indicated change in size requested
	* @use_lock: lock to prevent change in size when in use
	*
	* Note that the first element of all ring buffers must be a
	* struct iio_buffer.
	**/
	struct iio_sw_ring_buffer {
	struct iio_buffer buf;
	unsigned char *data;
	unsigned char *read_p;
	unsigned char *write_p;
	unsigned char *last_written_p;
	/* used to act as a point at which to signal an event */
	unsigned char *half_p;
	int use_count;
	int update_needed;
	spinlock_t use_lock;
	};

	#define iio_to_sw_ring(r) container_of(r, struct iio_sw_ring_buffer, buf)

	static inline int __iio_allocate_sw_ring_buffer(struct iio_sw_ring_buffer *ring,
	int bytes_per_datum, int length)
	{
	if ((length == 0) \|\| (bytes_per_datum == 0))
	return -EINVAL;
	__iio_update_buffer(&ring->buf, bytes_per_datum, length);
	ring->data = kmalloc(length*ring->buf.bytes_per_datum, GFP_ATOMIC);
	ring->read_p = NULL;
	ring->write_p = NULL;
	ring->last_written_p = NULL;
	ring->half_p = NULL;
	return ring->data ? 0 : -ENOMEM;
	}

	static inline void __iio_init_sw_ring_buffer(struct iio_sw_ring_buffer *ring)
	{
	spin_lock_init(&ring->use_lock);
	}

	static inline void __iio_free_sw_ring_buffer(struct iio_sw_ring_buffer *ring)
	{
	kfree(ring->data);
	}

	static void iio_mark_sw_rb_in_use(struct iio_buffer *r)
	{
	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
	spin_lock(&ring->use_lock);
	ring->use_count++;
	spin_unlock(&ring->use_lock);
	}

	static void iio_unmark_sw_rb_in_use(struct iio_buffer *r)
	{
	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
	spin_lock(&ring->use_lock);
	ring->use_count--;
	spin_unlock(&ring->use_lock);
	}


	/* Ring buffer related functionality */
	/* Store to ring is typically called in the bh of a data ready interrupt handler
	* in the device driver */
	/* Lock always held if their is a chance this may be called */
	/* Only one of these per ring may run concurrently - enforced by drivers */
	static int iio_store_to_sw_ring(struct iio_sw_ring_buffer *ring,
	unsigned char *data, s64 timestamp)
	{
	int ret = 0;
	unsigned char temp_ptr, change_test_ptr;

	/* initial store */
	if (unlikely(ring->write_p == NULL)) {
	ring->write_p = ring->data;
	/* Doesn't actually matter if this is out of the set
	* as long as the read pointer is valid before this
	* passes it - guaranteed as set later in this function.
	*/
	ring->half_p = ring->data - ring->buf.length*ring->buf.bytes_per_datum/2;
	}
	/* Copy data to where ever the current write pointer says */
	memcpy(ring->write_p, data, ring->buf.bytes_per_datum);
	barrier();
	/* Update the pointer used to get most recent value.
	* Always valid as either points to latest or second latest value.
	* Before this runs it is null and read attempts fail with -EAGAIN.
	*/
	ring->last_written_p = ring->write_p;
	barrier();
	/* temp_ptr used to ensure we never have an invalid pointer
	* it may be slightly lagging, but never invalid
	*/
	temp_ptr = ring->write_p + ring->buf.bytes_per_datum;
	/* End of ring, back to the beginning */
	if (temp_ptr == ring->data + ring->buf.length*ring->buf.bytes_per_datum)
	temp_ptr = ring->data;
	/* Update the write pointer
	* always valid as long as this is the only function able to write.
	* Care needed with smp systems to ensure more than one ring fill
	* is never scheduled.
	*/
	ring->write_p = temp_ptr;

	if (ring->read_p == NULL)
	ring->read_p = ring->data;
	/* Buffer full - move the read pointer and create / escalate
	* ring event */
	/* Tricky case - if the read pointer moves before we adjust it.
	* Handle by not pushing if it has moved - may result in occasional
	* unnecessary buffer full events when it wasn't quite true.
	*/
	else if (ring->write_p == ring->read_p) {
	change_test_ptr = ring->read_p;
	temp_ptr = change_test_ptr + ring->buf.bytes_per_datum;
	if (temp_ptr
	== ring->data + ring->buf.length*ring->buf.bytes_per_datum) {
	temp_ptr = ring->data;
	}
	/* We are moving pointer on one because the ring is full. Any
	* change to the read pointer will be this or greater.
	*/
	if (change_test_ptr == ring->read_p)
	ring->read_p = temp_ptr;
	}
	/* investigate if our event barrier has been passed */
	/* There are definite 'issues' with this and chances of
	* simultaneous read */
	/* Also need to use loop count to ensure this only happens once */
	ring->half_p += ring->buf.bytes_per_datum;
	if (ring->half_p == ring->data + ring->buf.length*ring->buf.bytes_per_datum)
	ring->half_p = ring->data;
	if (ring->half_p == ring->read_p) {
	ring->buf.stufftoread = true;
	wake_up_interruptible(&ring->buf.pollq);
	}
	return ret;
	}

	static int iio_read_first_n_sw_rb(struct iio_buffer *r,
	size_t n, char __user *buf)
	{
	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);

	u8 initial_read_p, initial_write_p, current_read_p, end_read_p;
	u8 *data;
	int ret, max_copied, bytes_to_rip, dead_offset;

	/* A userspace program has probably made an error if it tries to
	* read something that is not a whole number of bpds.
	* Return an error.
	*/
	if (n % ring->buf.bytes_per_datum) {
	ret = -EINVAL;
	printk(KERN_INFO "Ring buffer read request not whole number of"
	"samples: Request bytes %zd, Current bytes per datum %d\n",
	n, ring->buf.bytes_per_datum);
	goto error_ret;
	}
	/* Limit size to whole of ring buffer */
	bytes_to_rip = min((size_t)(ring->buf.bytes_per_datum*ring->buf.length),
	n);

	data = kmalloc(bytes_to_rip, GFP_KERNEL);
	if (data == NULL) {
	ret = -ENOMEM;
	goto error_ret;
	}

	/* build local copy */
	initial_read_p = ring->read_p;
	if (unlikely(initial_read_p == NULL)) { /* No data here as yet */
	ret = 0;
	goto error_free_data_cpy;
	}

	initial_write_p = ring->write_p;

	/* Need a consistent pair */
	while ((initial_read_p != ring->read_p)
	\|\| (initial_write_p != ring->write_p)) {
	initial_read_p = ring->read_p;
	initial_write_p = ring->write_p;
	}
	if (initial_write_p == initial_read_p) {
	/* No new data available.*/
	ret = 0;
	goto error_free_data_cpy;
	}

	if (initial_write_p >= initial_read_p + bytes_to_rip) {
	/* write_p is greater than necessary, all is easy */
	max_copied = bytes_to_rip;
	memcpy(data, initial_read_p, max_copied);
	end_read_p = initial_read_p + max_copied;
	} else if (initial_write_p > initial_read_p) {
	/not enough data to cpy /
	max_copied = initial_write_p - initial_read_p;
	memcpy(data, initial_read_p, max_copied);
	end_read_p = initial_write_p;
	} else {
	/* going through 'end' of ring buffer */
	max_copied = ring->data
	+ ring->buf.length*ring->buf.bytes_per_datum - initial_read_p;
	memcpy(data, initial_read_p, max_copied);
	/* possible we are done if we align precisely with end */
	if (max_copied == bytes_to_rip)
	end_read_p = ring->data;
	else if (initial_write_p
	> ring->data + bytes_to_rip - max_copied) {
	/* enough data to finish */
	memcpy(data + max_copied, ring->data,
	bytes_to_rip - max_copied);
	max_copied = bytes_to_rip;
	end_read_p = ring->data + (bytes_to_rip - max_copied);
	} else { /* not enough data */
	memcpy(data + max_copied, ring->data,
	initial_write_p - ring->data);
	max_copied += initial_write_p - ring->data;
	end_read_p = initial_write_p;
	}
	}
	/* Now to verify which section was cleanly copied - i.e. how far
	* read pointer has been pushed */
	current_read_p = ring->read_p;

	if (initial_read_p <= current_read_p)
	dead_offset = current_read_p - initial_read_p;
	else
	dead_offset = ring->buf.length*ring->buf.bytes_per_datum
	- (initial_read_p - current_read_p);

	/* possible issue if the initial write has been lapped or indeed
	* the point we were reading to has been passed */
	/* No valid data read.
	* In this case the read pointer is already correct having been
	* pushed further than we would look. */
	if (max_copied - dead_offset < 0) {
	ret = 0;
	goto error_free_data_cpy;
	}

	/* setup the next read position */
	/* Beware, this may fail due to concurrency fun and games.
	* Possible that sufficient fill commands have run to push the read
	* pointer past where we would be after the rip. If this occurs, leave
	* it be.
	*/
	/* Tricky - deal with loops */

	while (ring->read_p != end_read_p)
	ring->read_p = end_read_p;

	ret = max_copied - dead_offset;

	if (copy_to_user(buf, data + dead_offset, ret)) {
	ret = -EFAULT;
	goto error_free_data_cpy;
	}

	if (bytes_to_rip >= ring->buf.length*ring->buf.bytes_per_datum/2)
	ring->buf.stufftoread = 0;

	error_free_data_cpy:
	kfree(data);
	error_ret:

	return ret;
	}

	static int iio_store_to_sw_rb(struct iio_buffer *r,
	u8 *data,
	s64 timestamp)
	{
	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
	return iio_store_to_sw_ring(ring, data, timestamp);
	}

	static int iio_read_last_from_sw_ring(struct iio_sw_ring_buffer *ring,
	unsigned char *data)
	{
	unsigned char *last_written_p_copy;

	iio_mark_sw_rb_in_use(&ring->buf);
	again:
	barrier();
	last_written_p_copy = ring->last_written_p;
	barrier(); /unnessecary? /
	/* Check there is anything here */
	if (last_written_p_copy == NULL)
	return -EAGAIN;
	memcpy(data, last_written_p_copy, ring->buf.bytes_per_datum);

	if (unlikely(ring->last_written_p != last_written_p_copy))
	goto again;

	iio_unmark_sw_rb_in_use(&ring->buf);
	return 0;
	}

	static int iio_read_last_from_sw_rb(struct iio_buffer *r,
	unsigned char *data)
	{
	return iio_read_last_from_sw_ring(iio_to_sw_ring(r), data);
	}

	static int iio_request_update_sw_rb(struct iio_buffer *r)
	{
	int ret = 0;
	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);

	r->stufftoread = false;
	spin_lock(&ring->use_lock);
	if (!ring->update_needed)
	goto error_ret;
	if (ring->use_count) {
	ret = -EAGAIN;
	goto error_ret;
	}
	__iio_free_sw_ring_buffer(ring);
	ret = __iio_allocate_sw_ring_buffer(ring, ring->buf.bytes_per_datum,
	ring->buf.length);
	error_ret:
	spin_unlock(&ring->use_lock);
	return ret;
	}

	static int iio_get_bytes_per_datum_sw_rb(struct iio_buffer *r)
	{
	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
	return ring->buf.bytes_per_datum;
	}

	static int iio_set_bytes_per_datum_sw_rb(struct iio_buffer *r, size_t bpd)
	{
	if (r->bytes_per_datum != bpd) {
	r->bytes_per_datum = bpd;
	if (r->access->mark_param_change)
	r->access->mark_param_change(r);
	}
	return 0;
	}

	static int iio_get_length_sw_rb(struct iio_buffer *r)
	{
	return r->length;
	}

	static int iio_set_length_sw_rb(struct iio_buffer *r, int length)
	{
	if (r->length != length) {
	r->length = length;
	if (r->access->mark_param_change)
	r->access->mark_param_change(r);
	}
	return 0;
	}

	static int iio_mark_update_needed_sw_rb(struct iio_buffer *r)
	{
	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
	ring->update_needed = true;
	return 0;
	}

	static IIO_BUFFER_ENABLE_ATTR;
	static IIO_BUFFER_BYTES_PER_DATUM_ATTR;
	static IIO_BUFFER_LENGTH_ATTR;

	/* Standard set of ring buffer attributes */
	static struct attribute *iio_ring_attributes[] = {
	&dev_attr_length.attr,
	&dev_attr_bytes_per_datum.attr,
	&dev_attr_enable.attr,
	NULL,
	};

	static struct attribute_group iio_ring_attribute_group = {
	.attrs = iio_ring_attributes,
	.name = "buffer",
	};

	struct iio_buffer iio_sw_rb_allocate(struct iio_dev indio_dev)
	{
	struct iio_buffer *buf;
	struct iio_sw_ring_buffer *ring;

	ring = kzalloc(sizeof *ring, GFP_KERNEL);
	if (!ring)
	return NULL;
	ring->update_needed = true;
	buf = &ring->buf;
	iio_buffer_init(buf, indio_dev);
	__iio_init_sw_ring_buffer(ring);
	buf->attrs = &iio_ring_attribute_group;

	return buf;
	}
	EXPORT_SYMBOL(iio_sw_rb_allocate);

	void iio_sw_rb_free(struct iio_buffer *r)
	{
	kfree(iio_to_sw_ring(r));
	}
	EXPORT_SYMBOL(iio_sw_rb_free);

	const struct iio_buffer_access_funcs ring_sw_access_funcs = {
	.mark_in_use = &iio_mark_sw_rb_in_use,
	.unmark_in_use = &iio_unmark_sw_rb_in_use,
	.store_to = &iio_store_to_sw_rb,
	.read_last = &iio_read_last_from_sw_rb,
	.read_first_n = &iio_read_first_n_sw_rb,
	.mark_param_change = &iio_mark_update_needed_sw_rb,
	.request_update = &iio_request_update_sw_rb,
	.get_bytes_per_datum = &iio_get_bytes_per_datum_sw_rb,
	.set_bytes_per_datum = &iio_set_bytes_per_datum_sw_rb,
	.get_length = &iio_get_length_sw_rb,
	.set_length = &iio_set_length_sw_rb,
	};
	EXPORT_SYMBOL(ring_sw_access_funcs);

	MODULE_DESCRIPTION("Industrialio I/O software ring buffer");
	MODULE_LICENSE("GPL");