ring_sw.c

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/* 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 <linux/iio/trigger.h>

struct iio_sw_ring_buffer {
    struct iio_buffer  buf;
    unsigned char       *data;
    unsigned char       *read_p;
    unsigned char       *write_p;
    /* used to act as a point at which to signal an event */
    unsigned char       *half_p;
    int         update_needed;
};

#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->half_p = NULL;
    return ring->data ? 0 : -ENOMEM;
}

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

/* 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)
{
    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.
     */
    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;
    size_t data_available, buffer_size;

    /* 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;
    }

    buffer_size = ring->buf.bytes_per_datum*ring->buf.length;

    /* Limit size to whole of ring buffer */
    bytes_to_rip = min_t(size_t, buffer_size, 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)
        data_available = initial_write_p - initial_read_p;
    else
        data_available = buffer_size - (initial_read_p - initial_write_p);

    if (data_available < bytes_to_rip)
        bytes_to_rip = data_available;

    if (initial_read_p + bytes_to_rip >= ring->data + buffer_size) {
        max_copied = ring->data + buffer_size - initial_read_p;
        memcpy(data, initial_read_p, max_copied);
        memcpy(data + max_copied, ring->data, bytes_to_rip - max_copied);
        end_read_p = ring->data + bytes_to_rip - max_copied;
    } else {
        memcpy(data, initial_read_p, bytes_to_rip);
        end_read_p = initial_read_p + bytes_to_rip;
    }

    /* 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 = buffer_size - (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 (bytes_to_rip - 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 = bytes_to_rip - 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)
{
    struct iio_sw_ring_buffer *ring = iio_to_sw_ring(r);
    return iio_store_to_sw_ring(ring, 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;
    if (!ring->update_needed)
        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:
    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_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 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;
        iio_mark_update_needed_sw_rb(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;
        iio_mark_update_needed_sw_rb(r);
    }
    return 0;
}

static IIO_BUFFER_ENABLE_ATTR;
static IIO_BUFFER_LENGTH_ATTR;

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

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

static const struct iio_buffer_access_funcs ring_sw_access_funcs = {
    .store_to = &iio_store_to_sw_rb,
    .read_first_n = &iio_read_first_n_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,
};

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);
    buf->attrs = &iio_ring_attribute_group;
    buf->access = &ring_sw_access_funcs;

    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);

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