Staging: IIO: Ring buffer: Initial pass at rarely locked ring buffer

Please note this ring buffer implementation is very much a
work in progress (and hence RFC).  In it's current form
it is stable and reasonably efficient.  There are a couple
of unlikely cases that will lead to more data being lost
that is strictly necessary. The target was for the case
of requiring regular sampling even during user space reads.

All comments welcome.

The intention is to make this only one of several
implementations with run time selection.  For now there
is only one, so it is hard coded into the drivers using it.
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/Kconfig

@@ -17,6 +17,18 @@ config IIO_RING_BUFFER
 	  Provide core support for various ring buffer based data
 	  acquisition methods.
 
+if IIO_RING_BUFFER
+
+config IIO_SW_RING
+	tristate "Industrial I/O lock free software ring"
+	help
+	example software ring buffer implementation.  The design aim
+	of this particular realization was to minize write locking
+	with the intention that some devices would be able to write
+	in interrupt context.
+
+endif # IIO_RINGBUFFER
+
 config IIO_TRIGGER
 	boolean "Enable triggered sampling support"
 	help

drivers/staging/iio/Makefile

@@ -7,6 +7,8 @@ industrialio-y := industrialio-core.o
 industrialio-$(CONFIG_IIO_RING_BUFFER) += industrialio-ring.o
 industrialio-$(CONFIG_IIO_TRIGGER) += industrialio-trigger.o
 
+obj-$(CONFIG_IIO_SW_RING) += ring_sw.o
+
 obj-y += accel/
 obj-y += adc/
 obj-y += light/
\ No newline at end of file

drivers/staging/iio/ring_sw.c

+/* 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/kernel.h>
+#include <linux/device.h>
+#include <linux/module.h>
+#include <linux/device.h>
+#include <linux/workqueue.h>
+#include "ring_sw.h"
+
+static inline int __iio_init_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_init_ring_buffer(&ring->buf, bytes_per_datum, length);
+	ring->use_lock = __SPIN_LOCK_UNLOCKED((ring)->use_lock);
+	ring->data = kmalloc(length*ring->buf.bpd, GFP_KERNEL);
+	ring->read_p = 0;
+	ring->write_p = 0;
+	ring->last_written_p = 0;
+	ring->half_p = 0;
+	return ring->data ? 0 : -ENOMEM;
+}
+
+static inline void __iio_free_sw_ring_buffer(struct iio_sw_ring_buffer *ring)
+{
+	kfree(ring->data);
+}
+
+void iio_mark_sw_rb_in_use(struct iio_ring_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);
+}
+EXPORT_SYMBOL(iio_mark_sw_rb_in_use);
+
+void iio_unmark_sw_rb_in_use(struct iio_ring_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);
+}
+EXPORT_SYMBOL(iio_unmark_sw_rb_in_use);
+
+
+/* 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 */
+int iio_store_to_sw_ring(struct iio_sw_ring_buffer *ring,
+			 unsigned char *data,
+			 s64 timestamp)
+{
+	int ret = 0;
+	int code;
+	unsigned char *temp_ptr, *change_test_ptr;
+
+	/* initial store */
+	if (unlikely(ring->write_p == 0)) {
+		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.bpd/2;
+	}
+	/* Copy data to where ever the current write pointer says */
+	memcpy(ring->write_p, data, ring->buf.bpd);
+	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.bpd;
+	/* End of ring, back to the beginning */
+	if (temp_ptr == ring->data + ring->buf.length*ring->buf.bpd)
+		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 == 0)
+		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.bpd;
+		if (temp_ptr
+		    == ring->data + ring->buf.length*ring->buf.bpd) {
+			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;
+
+		spin_lock(&ring->buf.shared_ev_pointer.lock);
+
+		ret = iio_push_or_escallate_ring_event(&ring->buf,
+						       IIO_EVENT_CODE_RING_100_FULL,
+						       timestamp);
+		spin_unlock(&ring->buf.shared_ev_pointer.lock);
+		if (ret)
+			goto error_ret;
+	}
+	/* 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.bpd;
+	if (ring->half_p == ring->data + ring->buf.length*ring->buf.bpd)
+		ring->half_p = ring->data;
+	if (ring->half_p == ring->read_p) {
+		spin_lock(&ring->buf.shared_ev_pointer.lock);
+		code = IIO_EVENT_CODE_RING_50_FULL;
+		ret = __iio_push_event(&ring->buf.ev_int,
+				       code,
+				       timestamp,
+				       &ring->buf.shared_ev_pointer);
+		spin_unlock(&ring->buf.shared_ev_pointer.lock);
+	}
+error_ret:
+	return ret;
+}
+
+int iio_rip_sw_rb(struct iio_ring_buffer *r,
+		  size_t count, u8 **data, int *dead_offset)
+{
+	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
+
+	u8 *initial_read_p, *initial_write_p, *current_read_p, *end_read_p;
+	int ret, max_copied;
+	int bytes_to_rip;
+
+	/* 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 (count % ring->buf.bpd) {
+		ret = -EINVAL;
+		printk(KERN_INFO "Ring buffer read request not whole number of"
+		       "samples: Request bytes %zd, Current bpd %d\n",
+		       count, ring->buf.bpd);
+		goto error_ret;
+	}
+	/* Limit size to whole of ring buffer */
+	bytes_to_rip = min((size_t)(ring->buf.bpd*ring->buf.length), count);
+
+	*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 == 0)) { /* 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.bpd - 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.bpd
+			- (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;
+
+	return max_copied - *dead_offset;
+
+error_free_data_cpy:
+	kfree(*data);
+error_ret:
+	return ret;
+}
+EXPORT_SYMBOL(iio_rip_sw_rb);
+
+int iio_store_to_sw_rb(struct iio_ring_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);
+}
+EXPORT_SYMBOL(iio_store_to_sw_rb);
+
+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 == 0)
+		return -EAGAIN;
+	memcpy(data, last_written_p_copy, ring->buf.bpd);
+
+	if (unlikely(ring->last_written_p >= last_written_p_copy))
+		goto again;
+
+	iio_unmark_sw_rb_in_use(&ring->buf);
+	return 0;
+}
+
+int iio_read_last_from_sw_rb(struct iio_ring_buffer *r,
+			     unsigned char *data)
+{
+	return iio_read_last_from_sw_ring(iio_to_sw_ring(r), data);
+}
+EXPORT_SYMBOL(iio_read_last_from_sw_rb);
+
+int iio_request_update_sw_rb(struct iio_ring_buffer *r)
+{
+	int ret = 0;
+	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
+
+	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_init_sw_ring_buffer(ring, ring->buf.bpd, ring->buf.length);
+error_ret:
+	spin_unlock(&ring->use_lock);
+	return ret;
+}
+EXPORT_SYMBOL(iio_request_update_sw_rb);
+
+int iio_get_bpd_sw_rb(struct iio_ring_buffer *r)
+{
+	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
+	return ring->buf.bpd;
+}
+EXPORT_SYMBOL(iio_get_bpd_sw_rb);
+
+int iio_set_bpd_sw_rb(struct iio_ring_buffer *r, size_t bpd)
+{
+	if (r->bpd != bpd) {
+		r->bpd = bpd;
+		if (r->access.mark_param_change)
+			r->access.mark_param_change(r);
+	}
+	return 0;
+}
+EXPORT_SYMBOL(iio_set_bpd_sw_rb);
+
+int iio_get_length_sw_rb(struct iio_ring_buffer *r)
+{
+	return r->length;
+}
+EXPORT_SYMBOL(iio_get_length_sw_rb);
+
+int iio_set_length_sw_rb(struct iio_ring_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;
+}
+EXPORT_SYMBOL(iio_set_length_sw_rb);
+
+int iio_mark_update_needed_sw_rb(struct iio_ring_buffer *r)
+{
+	struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
+	ring->update_needed = true;
+	return 0;
+}
+EXPORT_SYMBOL(iio_mark_update_needed_sw_rb);
+
+static void iio_sw_rb_release(struct device *dev)
+{
+	struct iio_ring_buffer *r = to_iio_ring_buffer(dev);
+	kfree(iio_to_sw_ring(r));
+}
+
+static IIO_RING_ENABLE_ATTR;
+static IIO_RING_BPS_ATTR;
+static IIO_RING_LENGTH_ATTR;
+
+/* Standard set of ring buffer attributes */
+static struct attribute *iio_ring_attributes[] = {
+	&dev_attr_length.attr,
+	&dev_attr_bps.attr,
+	&dev_attr_ring_enable.attr,
+	NULL,
+};
+
+static struct attribute_group iio_ring_attribute_group = {
+	.attrs = iio_ring_attributes,
+};
+
+static struct attribute_group *iio_ring_attribute_groups[] = {
+	&iio_ring_attribute_group,
+	NULL
+};
+
+static struct device_type iio_sw_ring_type = {
+	.release = iio_sw_rb_release,
+	.groups = iio_ring_attribute_groups,
+};
+
+struct iio_ring_buffer *iio_sw_rb_allocate(struct iio_dev *indio_dev)
+{
+	struct iio_ring_buffer *buf;
+	struct iio_sw_ring_buffer *ring;
+
+	ring = kzalloc(sizeof *ring, GFP_KERNEL);
+	if (!ring)
+		return 0;
+	buf = &ring->buf;
+
+	iio_ring_buffer_init(buf, indio_dev);
+	buf->dev.type = &iio_sw_ring_type;
+	device_initialize(&buf->dev);
+	buf->dev.parent = &indio_dev->dev;
+	buf->dev.class = &iio_class;
+	dev_set_drvdata(&buf->dev, (void *)buf);
+
+	return buf;
+}
+EXPORT_SYMBOL(iio_sw_rb_allocate);
+
+void iio_sw_rb_free(struct iio_ring_buffer *r)
+{
+	if (r)
+		iio_put_ring_buffer(r);
+}
+EXPORT_SYMBOL(iio_sw_rb_free);
+MODULE_DESCRIPTION("Industrialio I/O software ring buffer");
+MODULE_LICENSE("GPL");

drivers/staging/iio/ring_sw.h

+/* 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.
+ *
+ * This code is deliberately kept separate from the main industrialio I/O core
+ * as it is intended that in the future a number of different software ring
+ * buffer implementations will exist with different characteristics to suit
+ * different applications.
+ *
+ * This particular one was designed for a data capture application where it was
+ * particularly important that no userspace reads would interrupt the capture
+ * process. To this end the ring is not locked during a read.
+ *
+ * Comments on this buffer design welcomed. It's far from efficient and some of
+ * my understanding of the effects of scheduling on this are somewhat limited.
+ * Frankly, to my mind, this is the current weak point in the industrial I/O
+ * patch set.
+ */
+
+#ifndef _IIO_RING_SW_H_
+#define _IIO_RING_SW_H_
+/* NEEDS COMMENTS */
+/* The intention is that this should be a separate module from the iio core.
+ * This is a bit like supporting algorithms dependent on what the device
+ * driver requests - some may support multiple options */
+
+
+#include <linux/autoconf.h>
+#include "iio.h"
+#include "ring_generic.h"
+
+#if defined CONFIG_IIO_SW_RING || defined CONFIG_IIO_SW_RING_MODULE
+
+/**
+ * iio_create_sw_rb() software ring buffer allocation
+ * @r:		pointer to ring buffer pointer
+ **/
+int iio_create_sw_rb(struct iio_ring_buffer **r);
+
+/**
+ * iio_init_sw_rb() initialize the software ring buffer
+ * @r:		pointer to a software ring buffer created by an
+ *		iio_create_sw_rb call.
+ **/
+int iio_init_sw_rb(struct iio_ring_buffer *r, struct iio_dev *indio_dev);
+/**
+ * iio_exit_sw_rb() reverse what was done in iio_init_sw_rb
+ **/
+void iio_exit_sw_rb(struct iio_ring_buffer *r);
+
+/**
+ * iio_free_sw_rb() free memory occupied by the core ring buffer struct
+ **/
+void iio_free_sw_rb(struct iio_ring_buffer *r);
+
+/**
+ * iio_mark_sw_rb_in_use() reference counting to prevent incorrect chances
+ **/
+void iio_mark_sw_rb_in_use(struct iio_ring_buffer *r);
+
+/**
+ *  iio_unmark_sw_rb_in_use() notify the ring buffer that we don't care anymore
+ **/
+void iio_unmark_sw_rb_in_use(struct iio_ring_buffer *r);
+
+/**
+ * iio_read_last_from_sw_rb() attempt to read the last stored datum from the rb
+ **/
+int iio_read_last_from_sw_rb(struct iio_ring_buffer *r, u8 *data);
+
+/**
+ * iio_store_to_sw_rb() store a new datum to the ring buffer
+ * @rb:	pointer to ring buffer instance
+ * @data:	the datum to be stored including timestamp if relevant.
+ * @timestamp:	timestamp which will be attached to buffer events if relevant.
+ **/
+int iio_store_to_sw_rb(struct iio_ring_buffer *r, u8 *data, s64 timestamp);
+
+/**
+ * iio_rip_sw_rb() attempt to read data from the ring buffer
+ * @r:			ring buffer instance
+ * @count:		number of datum's to try and read
+ * @data:		where the data will be stored.
+ * @dead_offset:	how much of the stored data was possibly invalidated by
+ *			the end of the copy.
+ **/
+int iio_rip_sw_rb(struct iio_ring_buffer *r,
+		  size_t count,
+		  u8 **data,
+		  int *dead_offset);
+
+/**
+ * iio_request_update_sw_rb() update params if update needed
+ **/
+int iio_request_update_sw_rb(struct iio_ring_buffer *r);
+
+/**
+ * iio_mark_update_needed_sw_rb() tell the ring buffer it needs a param update
+ **/
+int iio_mark_update_needed_sw_rb(struct iio_ring_buffer *r);
+
+
+/**
+ * iio_get_bpd_sw_rb() get the datum size in bytes
+ **/
+int iio_get_bpd_sw_rb(struct iio_ring_buffer *r);
+
+/**
+ * iio_set_bpd_sw_rb() set the datum size in bytes
+ **/
+int iio_set_bpd_sw_rb(struct iio_ring_buffer *r, size_t bpd);
+
+/**
+ * iio_get_length_sw_rb() get how many datums the rb may contain
+ **/
+int iio_get_length_sw_rb(struct iio_ring_buffer *r);
+
+/**
+ * iio_set_length_sw_rb() set how many datums the rb may contain
+ **/
+int iio_set_length_sw_rb(struct iio_ring_buffer *r, int length);
+
+/**
+ * iio_ring_sw_register_funcs() helper function to set up rb access
+ **/
+static inline void iio_ring_sw_register_funcs(struct iio_ring_access_funcs *ra)
+{
+	ra->mark_in_use = &iio_mark_sw_rb_in_use;
+	ra->unmark_in_use = &iio_unmark_sw_rb_in_use;
+
+	ra->store_to = &iio_store_to_sw_rb;
+	ra->read_last = &iio_read_last_from_sw_rb;
+	ra->rip_lots = &iio_rip_sw_rb;
+
+	ra->mark_param_change = &iio_mark_update_needed_sw_rb;
+	ra->request_update = &iio_request_update_sw_rb;
+
+	ra->get_bpd = &iio_get_bpd_sw_rb;
+	ra->set_bpd = &iio_set_bpd_sw_rb;
+
+	ra->get_length = &iio_get_length_sw_rb;
+	ra->set_length = &iio_set_length_sw_rb;
+};
+
+/**
+ * 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_ring_buffer.
+**/
+
+struct iio_sw_ring_buffer {
+	struct iio_ring_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)
+
+struct iio_ring_buffer *iio_sw_rb_allocate(struct iio_dev *indio_dev);
+void iio_sw_rb_free(struct iio_ring_buffer *ring);
+
+
+
+#else /* CONFIG_IIO_RING_BUFFER*/
+static inline void iio_ring_sw_register_funcs(struct iio_ring_access_funcs *ra)
+{};
+#endif /* !CONFIG_IIO_RING_BUFFER */
+#endif /* _IIO_RING_SW_H_ */