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.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_ */