[deliverable/lttng-modules.git] / lib / ringbuffer / ring_buffer_backend.c

/*
 * ring_buffer_backend.c
 *
 * Copyright (C) 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * Dual LGPL v2.1/GPL v2 license.
 */

#include <linux/stddef.h>
#include <linux/module.h>
#include <linux/string.h>
#include <linux/bitops.h>
#include <linux/delay.h>
#include <linux/errno.h>
#include <linux/slab.h>
#include <linux/cpu.h>
#include <linux/mm.h>

#include "../../wrapper/vmalloc.h"      /* for wrapper_vmalloc_sync_all() */
#include "../../wrapper/ringbuffer/config.h"
#include "../../wrapper/ringbuffer/backend.h"
#include "../../wrapper/ringbuffer/frontend.h"

/**
 * lib_ring_buffer_backend_allocate - allocate a channel buffer
 * @config: ring buffer instance configuration
 * @buf: the buffer struct
 * @size: total size of the buffer
 * @num_subbuf: number of subbuffers
 * @extra_reader_sb: need extra subbuffer for reader
 */
static
int lib_ring_buffer_backend_allocate(const struct lib_ring_buffer_config *config,
                                     struct lib_ring_buffer_backend *bufb,
                                     size_t size, size_t num_subbuf,
                                     int extra_reader_sb)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        unsigned long j, num_pages, num_pages_per_subbuf, page_idx = 0;
        unsigned long subbuf_size, mmap_offset = 0;
        unsigned long num_subbuf_alloc;
        struct page **pages;
        void **virt;
        unsigned long i;

        num_pages = size >> PAGE_SHIFT;
        num_pages_per_subbuf = num_pages >> get_count_order(num_subbuf);
        subbuf_size = chanb->subbuf_size;
        num_subbuf_alloc = num_subbuf;

        if (extra_reader_sb) {
                num_pages += num_pages_per_subbuf; /* Add pages for reader */
                num_subbuf_alloc++;
        }

        pages = kmalloc_node(ALIGN(sizeof(*pages) * num_pages,
                                   1 << INTERNODE_CACHE_SHIFT),
                        GFP_KERNEL, cpu_to_node(max(bufb->cpu, 0)));
        if (unlikely(!pages))
                goto pages_error;

        virt = kmalloc_node(ALIGN(sizeof(*virt) * num_pages,
                                  1 << INTERNODE_CACHE_SHIFT),
                        GFP_KERNEL, cpu_to_node(max(bufb->cpu, 0)));
        if (unlikely(!virt))
                goto virt_error;

        bufb->array = kmalloc_node(ALIGN(sizeof(*bufb->array)
                                         * num_subbuf_alloc,
                                  1 << INTERNODE_CACHE_SHIFT),
                        GFP_KERNEL, cpu_to_node(max(bufb->cpu, 0)));
        if (unlikely(!bufb->array))
                goto array_error;

        for (i = 0; i < num_pages; i++) {
                pages[i] = alloc_pages_node(cpu_to_node(max(bufb->cpu, 0)),
                                            GFP_KERNEL | __GFP_ZERO, 0);
                if (unlikely(!pages[i]))
                        goto depopulate;
                virt[i] = page_address(pages[i]);
        }
        bufb->num_pages_per_subbuf = num_pages_per_subbuf;

        /* Allocate backend pages array elements */
        for (i = 0; i < num_subbuf_alloc; i++) {
                bufb->array[i] =
                        kzalloc_node(ALIGN(
                                sizeof(struct lib_ring_buffer_backend_pages) +
                                sizeof(struct lib_ring_buffer_backend_page)
                                * num_pages_per_subbuf,
                                1 << INTERNODE_CACHE_SHIFT),
                                GFP_KERNEL, cpu_to_node(max(bufb->cpu, 0)));
                if (!bufb->array[i])
                        goto free_array;
        }

        /* Allocate write-side subbuffer table */
        bufb->buf_wsb = kzalloc_node(ALIGN(
                                sizeof(struct lib_ring_buffer_backend_subbuffer)
                                * num_subbuf,
                                1 << INTERNODE_CACHE_SHIFT),
                                GFP_KERNEL, cpu_to_node(max(bufb->cpu, 0)));
        if (unlikely(!bufb->buf_wsb))
                goto free_array;

        for (i = 0; i < num_subbuf; i++)
                bufb->buf_wsb[i].id = subbuffer_id(config, 0, 1, i);

        /* Assign read-side subbuffer table */
        if (extra_reader_sb)
                bufb->buf_rsb.id = subbuffer_id(config, 0, 1,
                                                num_subbuf_alloc - 1);
        else
                bufb->buf_rsb.id = subbuffer_id(config, 0, 1, 0);

        /* Assign pages to page index */
        for (i = 0; i < num_subbuf_alloc; i++) {
                for (j = 0; j < num_pages_per_subbuf; j++) {
                        CHAN_WARN_ON(chanb, page_idx > num_pages);
                        bufb->array[i]->p[j].virt = virt[page_idx];
                        bufb->array[i]->p[j].page = pages[page_idx];
                        page_idx++;
                }
                if (config->output == RING_BUFFER_MMAP) {
                        bufb->array[i]->mmap_offset = mmap_offset;
                        mmap_offset += subbuf_size;
                }
        }

        /*
         * If kmalloc ever uses vmalloc underneath, make sure the buffer pages
         * will not fault.
         */
        wrapper_vmalloc_sync_all();
        kfree(virt);
        kfree(pages);
        return 0;

free_array:
        for (i = 0; (i < num_subbuf_alloc && bufb->array[i]); i++)
                kfree(bufb->array[i]);
depopulate:
        /* Free all allocated pages */
        for (i = 0; (i < num_pages && pages[i]); i++)
                __free_page(pages[i]);
        kfree(bufb->array);
array_error:
        kfree(virt);
virt_error:
        kfree(pages);
pages_error:
        return -ENOMEM;
}

int lib_ring_buffer_backend_create(struct lib_ring_buffer_backend *bufb,
                                   struct channel_backend *chanb, int cpu)
{
        const struct lib_ring_buffer_config *config = &chanb->config;

        bufb->chan = container_of(chanb, struct channel, backend);
        bufb->cpu = cpu;

        return lib_ring_buffer_backend_allocate(config, bufb, chanb->buf_size,
                                                chanb->num_subbuf,
                                                chanb->extra_reader_sb);
}

void lib_ring_buffer_backend_free(struct lib_ring_buffer_backend *bufb)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        unsigned long i, j, num_subbuf_alloc;

        num_subbuf_alloc = chanb->num_subbuf;
        if (chanb->extra_reader_sb)
                num_subbuf_alloc++;

        kfree(bufb->buf_wsb);
        for (i = 0; i < num_subbuf_alloc; i++) {
                for (j = 0; j < bufb->num_pages_per_subbuf; j++)
                        __free_page(bufb->array[i]->p[j].page);
                kfree(bufb->array[i]);
        }
        kfree(bufb->array);
        bufb->allocated = 0;
}

void lib_ring_buffer_backend_reset(struct lib_ring_buffer_backend *bufb)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        unsigned long num_subbuf_alloc;
        unsigned int i;

        num_subbuf_alloc = chanb->num_subbuf;
        if (chanb->extra_reader_sb)
                num_subbuf_alloc++;

        for (i = 0; i < chanb->num_subbuf; i++)
                bufb->buf_wsb[i].id = subbuffer_id(config, 0, 1, i);
        if (chanb->extra_reader_sb)
                bufb->buf_rsb.id = subbuffer_id(config, 0, 1,
                                                num_subbuf_alloc - 1);
        else
                bufb->buf_rsb.id = subbuffer_id(config, 0, 1, 0);

        for (i = 0; i < num_subbuf_alloc; i++) {
                /* Don't reset mmap_offset */
                v_set(config, &bufb->array[i]->records_commit, 0);
                v_set(config, &bufb->array[i]->records_unread, 0);
                bufb->array[i]->data_size = 0;
                /* Don't reset backend page and virt addresses */
        }
        /* Don't reset num_pages_per_subbuf, cpu, allocated */
        v_set(config, &bufb->records_read, 0);
}

/*
 * The frontend is responsible for also calling ring_buffer_backend_reset for
 * each buffer when calling channel_backend_reset.
 */
void channel_backend_reset(struct channel_backend *chanb)
{
        struct channel *chan = container_of(chanb, struct channel, backend);
        const struct lib_ring_buffer_config *config = &chanb->config;

        /*
         * Don't reset buf_size, subbuf_size, subbuf_size_order,
         * num_subbuf_order, buf_size_order, extra_reader_sb, num_subbuf,
         * priv, notifiers, config, cpumask and name.
         */
        chanb->start_tsc = config->cb.ring_buffer_clock_read(chan);
}

#ifdef CONFIG_HOTPLUG_CPU
/**
 *      lib_ring_buffer_cpu_hp_callback - CPU hotplug callback
 *      @nb: notifier block
 *      @action: hotplug action to take
 *      @hcpu: CPU number
 *
 *      Returns the success/failure of the operation. (%NOTIFY_OK, %NOTIFY_BAD)
 */
static
int __cpuinit lib_ring_buffer_cpu_hp_callback(struct notifier_block *nb,
                                              unsigned long action,
                                              void *hcpu)
{
        unsigned int cpu = (unsigned long)hcpu;
        struct channel_backend *chanb = container_of(nb, struct channel_backend,
                                                     cpu_hp_notifier);
        const struct lib_ring_buffer_config *config = &chanb->config;
        struct lib_ring_buffer *buf;
        int ret;

        CHAN_WARN_ON(chanb, config->alloc == RING_BUFFER_ALLOC_GLOBAL);

        switch (action) {
        case CPU_UP_PREPARE:
        case CPU_UP_PREPARE_FROZEN:
                buf = per_cpu_ptr(chanb->buf, cpu);
                ret = lib_ring_buffer_create(buf, chanb, cpu);
                if (ret) {
                        printk(KERN_ERR
                          "ring_buffer_cpu_hp_callback: cpu %d "
                          "buffer creation failed\n", cpu);
                        return NOTIFY_BAD;
                }
                break;
        case CPU_DEAD:
        case CPU_DEAD_FROZEN:
                /* No need to do a buffer switch here, because it will happen
                 * when tracing is stopped, or will be done by switch timer CPU
                 * DEAD callback. */
                break;
        }
        return NOTIFY_OK;
}
#endif

/**
 * channel_backend_init - initialize a channel backend
 * @chanb: channel backend
 * @name: channel name
 * @config: client ring buffer configuration
 * @priv: client private data
 * @parent: dentry of parent directory, %NULL for root directory
 * @subbuf_size: size of sub-buffers (> PAGE_SIZE, power of 2)
 * @num_subbuf: number of sub-buffers (power of 2)
 *
 * Returns channel pointer if successful, %NULL otherwise.
 *
 * Creates per-cpu channel buffers using the sizes and attributes
 * specified.  The created channel buffer files will be named
 * name_0...name_N-1.  File permissions will be %S_IRUSR.
 *
 * Called with CPU hotplug disabled.
 */
int channel_backend_init(struct channel_backend *chanb,
                         const char *name,
                         const struct lib_ring_buffer_config *config,
                         void *priv, size_t subbuf_size, size_t num_subbuf)
{
        struct channel *chan = container_of(chanb, struct channel, backend);
        unsigned int i;
        int ret;

        if (!name)
                return -EPERM;

        if (!(subbuf_size && num_subbuf))
                return -EPERM;

        /* Check that the subbuffer size is larger than a page. */
        if (subbuf_size < PAGE_SIZE)
                return -EINVAL;

        /*
         * Make sure the number of subbuffers and subbuffer size are power of 2.
         */
        CHAN_WARN_ON(chanb, hweight32(subbuf_size) != 1);
        CHAN_WARN_ON(chanb, hweight32(num_subbuf) != 1);

        ret = subbuffer_id_check_index(config, num_subbuf);
        if (ret)
                return ret;

        chanb->priv = priv;
        chanb->buf_size = num_subbuf * subbuf_size;
        chanb->subbuf_size = subbuf_size;
        chanb->buf_size_order = get_count_order(chanb->buf_size);
        chanb->subbuf_size_order = get_count_order(subbuf_size);
        chanb->num_subbuf_order = get_count_order(num_subbuf);
        chanb->extra_reader_sb =
                        (config->mode == RING_BUFFER_OVERWRITE) ? 1 : 0;
        chanb->num_subbuf = num_subbuf;
        strlcpy(chanb->name, name, NAME_MAX);
        memcpy(&chanb->config, config, sizeof(chanb->config));

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                if (!zalloc_cpumask_var(&chanb->cpumask, GFP_KERNEL))
                        return -ENOMEM;
        }

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                /* Allocating the buffer per-cpu structures */
                chanb->buf = alloc_percpu(struct lib_ring_buffer);
                if (!chanb->buf)
                        goto free_cpumask;

                /*
                 * In case of non-hotplug cpu, if the ring-buffer is allocated
                 * in early initcall, it will not be notified of secondary cpus.
                 * In that off case, we need to allocate for all possible cpus.
                 */
#ifdef CONFIG_HOTPLUG_CPU
                /*
                 * buf->backend.allocated test takes care of concurrent CPU
                 * hotplug.
                 * Priority higher than frontend, so we create the ring buffer
                 * before we start the timer.
                 */
                chanb->cpu_hp_notifier.notifier_call =
                                lib_ring_buffer_cpu_hp_callback;
                chanb->cpu_hp_notifier.priority = 5;
                register_hotcpu_notifier(&chanb->cpu_hp_notifier);

                get_online_cpus();
                for_each_online_cpu(i) {
                        ret = lib_ring_buffer_create(per_cpu_ptr(chanb->buf, i),
                                                 chanb, i);
                        if (ret)
                                goto free_bufs; /* cpu hotplug locked */
                }
                put_online_cpus();
#else
                for_each_possible_cpu(i) {
                        ret = lib_ring_buffer_create(per_cpu_ptr(chanb->buf, i),
                                                 chanb, i);
                        if (ret)
                                goto free_bufs; /* cpu hotplug locked */
                }
#endif
        } else {
                chanb->buf = kzalloc(sizeof(struct lib_ring_buffer), GFP_KERNEL);
                if (!chanb->buf)
                        goto free_cpumask;
                ret = lib_ring_buffer_create(chanb->buf, chanb, -1);
                if (ret)
                        goto free_bufs;
        }
        chanb->start_tsc = config->cb.ring_buffer_clock_read(chan);

        return 0;

free_bufs:
        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                for_each_possible_cpu(i) {
                        struct lib_ring_buffer *buf = per_cpu_ptr(chanb->buf, i);

                        if (!buf->backend.allocated)
                                continue;
                        lib_ring_buffer_free(buf);
                }
#ifdef CONFIG_HOTPLUG_CPU
                put_online_cpus();
#endif
                free_percpu(chanb->buf);
        } else
                kfree(chanb->buf);
free_cpumask:
        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
                free_cpumask_var(chanb->cpumask);
        return -ENOMEM;
}

/**
 * channel_backend_unregister_notifiers - unregister notifiers
 * @chan: the channel
 *
 * Holds CPU hotplug.
 */
void channel_backend_unregister_notifiers(struct channel_backend *chanb)
{
        const struct lib_ring_buffer_config *config = &chanb->config;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
                unregister_hotcpu_notifier(&chanb->cpu_hp_notifier);
}

/**
 * channel_backend_free - destroy the channel
 * @chan: the channel
 *
 * Destroy all channel buffers and frees the channel.
 */
void channel_backend_free(struct channel_backend *chanb)
{
        const struct lib_ring_buffer_config *config = &chanb->config;
        unsigned int i;

        if (config->alloc == RING_BUFFER_ALLOC_PER_CPU) {
                for_each_possible_cpu(i) {
                        struct lib_ring_buffer *buf = per_cpu_ptr(chanb->buf, i);

                        if (!buf->backend.allocated)
                                continue;
                        lib_ring_buffer_free(buf);
                }
                free_cpumask_var(chanb->cpumask);
                free_percpu(chanb->buf);
        } else {
                struct lib_ring_buffer *buf = chanb->buf;

                CHAN_WARN_ON(chanb, !buf->backend.allocated);
                lib_ring_buffer_free(buf);
                kfree(buf);
        }
}

/**
 * lib_ring_buffer_write - write data to a ring_buffer buffer.
 * @bufb : buffer backend
 * @offset : offset within the buffer
 * @src : source address
 * @len : length to write
 * @pagecpy : page size copied so far
 */
void _lib_ring_buffer_write(struct lib_ring_buffer_backend *bufb, size_t offset,
                            const void *src, size_t len, ssize_t pagecpy)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        size_t sbidx, index;
        struct lib_ring_buffer_backend_pages *rpages;
        unsigned long sb_bindex, id;

        do {
                len -= pagecpy;
                src += pagecpy;
                offset += pagecpy;
                sbidx = offset >> chanb->subbuf_size_order;
                index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;

                /*
                 * Underlying layer should never ask for writes across
                 * subbuffers.
                 */
                CHAN_WARN_ON(chanb, offset >= chanb->buf_size);

                pagecpy = min_t(size_t, len, PAGE_SIZE - (offset & ~PAGE_MASK));
                id = bufb->buf_wsb[sbidx].id;
                sb_bindex = subbuffer_id_get_index(config, id);
                rpages = bufb->array[sb_bindex];
                CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                             && subbuffer_id_is_noref(config, id));
                lib_ring_buffer_do_copy(config,
                                        rpages->p[index].virt
                                                + (offset & ~PAGE_MASK),
                                        src, pagecpy);
        } while (unlikely(len != pagecpy));
}
EXPORT_SYMBOL_GPL(_lib_ring_buffer_write);


/**
 * lib_ring_buffer_memset - write len bytes of c to a ring_buffer buffer.
 * @bufb : buffer backend
 * @offset : offset within the buffer
 * @c : the byte to write
 * @len : length to write
 * @pagecpy : page size copied so far
 */
void _lib_ring_buffer_memset(struct lib_ring_buffer_backend *bufb,
                             size_t offset,
                             int c, size_t len, ssize_t pagecpy)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        size_t sbidx, index;
        struct lib_ring_buffer_backend_pages *rpages;
        unsigned long sb_bindex, id;

        do {
                len -= pagecpy;
                offset += pagecpy;
                sbidx = offset >> chanb->subbuf_size_order;
                index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;

                /*
                 * Underlying layer should never ask for writes across
                 * subbuffers.
                 */
                CHAN_WARN_ON(chanb, offset >= chanb->buf_size);

                pagecpy = min_t(size_t, len, PAGE_SIZE - (offset & ~PAGE_MASK));
                id = bufb->buf_wsb[sbidx].id;
                sb_bindex = subbuffer_id_get_index(config, id);
                rpages = bufb->array[sb_bindex];
                CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                             && subbuffer_id_is_noref(config, id));
                lib_ring_buffer_do_memset(rpages->p[index].virt
                                          + (offset & ~PAGE_MASK),
                                          c, pagecpy);
        } while (unlikely(len != pagecpy));
}
EXPORT_SYMBOL_GPL(_lib_ring_buffer_memset);


/**
 * lib_ring_buffer_copy_from_user - write user data to a ring_buffer buffer.
 * @bufb : buffer backend
 * @offset : offset within the buffer
 * @src : source address
 * @len : length to write
 * @pagecpy : page size copied so far
 *
 * This function deals with userspace pointers, it should never be called
 * directly without having the src pointer checked with access_ok()
 * previously.
 */
void _lib_ring_buffer_copy_from_user(struct lib_ring_buffer_backend *bufb,
                                      size_t offset,
                                      const void __user *src, size_t len,
                                      ssize_t pagecpy)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        size_t sbidx, index;
        struct lib_ring_buffer_backend_pages *rpages;
        unsigned long sb_bindex, id;
        int ret;

        do {
                len -= pagecpy;
                src += pagecpy;
                offset += pagecpy;
                sbidx = offset >> chanb->subbuf_size_order;
                index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;

                /*
                 * Underlying layer should never ask for writes across
                 * subbuffers.
                 */
                CHAN_WARN_ON(chanb, offset >= chanb->buf_size);

                pagecpy = min_t(size_t, len, PAGE_SIZE - (offset & ~PAGE_MASK));
                id = bufb->buf_wsb[sbidx].id;
                sb_bindex = subbuffer_id_get_index(config, id);
                rpages = bufb->array[sb_bindex];
                CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                                && subbuffer_id_is_noref(config, id));
                ret = lib_ring_buffer_do_copy_from_user(rpages->p[index].virt
                                                        + (offset & ~PAGE_MASK),
                                                        src, pagecpy) != 0;
                if (ret > 0) {
                        offset += (pagecpy - ret);
                        len -= (pagecpy - ret);
                        _lib_ring_buffer_memset(bufb, offset, 0, len, 0);
                        break; /* stop copy */
                }
        } while (unlikely(len != pagecpy));
}
EXPORT_SYMBOL_GPL(_lib_ring_buffer_copy_from_user);

/**
 * lib_ring_buffer_read - read data from ring_buffer_buffer.
 * @bufb : buffer backend
 * @offset : offset within the buffer
 * @dest : destination address
 * @len : length to copy to destination
 *
 * Should be protected by get_subbuf/put_subbuf.
 * Returns the length copied.
 */
size_t lib_ring_buffer_read(struct lib_ring_buffer_backend *bufb, size_t offset,
                            void *dest, size_t len)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        size_t index;
        ssize_t pagecpy, orig_len;
        struct lib_ring_buffer_backend_pages *rpages;
        unsigned long sb_bindex, id;

        orig_len = len;
        offset &= chanb->buf_size - 1;
        index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
        if (unlikely(!len))
                return 0;
        for (;;) {
                pagecpy = min_t(size_t, len, PAGE_SIZE - (offset & ~PAGE_MASK));
                id = bufb->buf_rsb.id;
                sb_bindex = subbuffer_id_get_index(config, id);
                rpages = bufb->array[sb_bindex];
                CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                             && subbuffer_id_is_noref(config, id));
                memcpy(dest, rpages->p[index].virt + (offset & ~PAGE_MASK),
                       pagecpy);
                len -= pagecpy;
                if (likely(!len))
                        break;
                dest += pagecpy;
                offset += pagecpy;
                index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
                /*
                 * Underlying layer should never ask for reads across
                 * subbuffers.
                 */
                CHAN_WARN_ON(chanb, offset >= chanb->buf_size);
        }
        return orig_len;
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_read);

/**
 * __lib_ring_buffer_copy_to_user - read data from ring_buffer to userspace
 * @bufb : buffer backend
 * @offset : offset within the buffer
 * @dest : destination userspace address
 * @len : length to copy to destination
 *
 * Should be protected by get_subbuf/put_subbuf.
 * access_ok() must have been performed on dest addresses prior to call this
 * function.
 * Returns -EFAULT on error, 0 if ok.
 */
int __lib_ring_buffer_copy_to_user(struct lib_ring_buffer_backend *bufb,
                                   size_t offset, void __user *dest, size_t len)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        size_t index;
        ssize_t pagecpy;
        struct lib_ring_buffer_backend_pages *rpages;
        unsigned long sb_bindex, id;

        offset &= chanb->buf_size - 1;
        index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
        if (unlikely(!len))
                return 0;
        for (;;) {
                pagecpy = min_t(size_t, len, PAGE_SIZE - (offset & ~PAGE_MASK));
                id = bufb->buf_rsb.id;
                sb_bindex = subbuffer_id_get_index(config, id);
                rpages = bufb->array[sb_bindex];
                CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                             && subbuffer_id_is_noref(config, id));
                if (__copy_to_user(dest,
                               rpages->p[index].virt + (offset & ~PAGE_MASK),
                               pagecpy))
                        return -EFAULT;
                len -= pagecpy;
                if (likely(!len))
                        break;
                dest += pagecpy;
                offset += pagecpy;
                index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
                /*
                 * Underlying layer should never ask for reads across
                 * subbuffers.
                 */
                CHAN_WARN_ON(chanb, offset >= chanb->buf_size);
        }
        return 0;
}
EXPORT_SYMBOL_GPL(__lib_ring_buffer_copy_to_user);

/**
 * lib_ring_buffer_read_cstr - read a C-style string from ring_buffer.
 * @bufb : buffer backend
 * @offset : offset within the buffer
 * @dest : destination address
 * @len : destination's length
 *
 * return string's length
 * Should be protected by get_subbuf/put_subbuf.
 */
int lib_ring_buffer_read_cstr(struct lib_ring_buffer_backend *bufb, size_t offset,
                              void *dest, size_t len)
{
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        size_t index;
        ssize_t pagecpy, pagelen, strpagelen, orig_offset;
        char *str;
        struct lib_ring_buffer_backend_pages *rpages;
        unsigned long sb_bindex, id;

        offset &= chanb->buf_size - 1;
        index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
        orig_offset = offset;
        for (;;) {
                id = bufb->buf_rsb.id;
                sb_bindex = subbuffer_id_get_index(config, id);
                rpages = bufb->array[sb_bindex];
                CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                             && subbuffer_id_is_noref(config, id));
                str = (char *)rpages->p[index].virt + (offset & ~PAGE_MASK);
                pagelen = PAGE_SIZE - (offset & ~PAGE_MASK);
                strpagelen = strnlen(str, pagelen);
                if (len) {
                        pagecpy = min_t(size_t, len, strpagelen);
                        if (dest) {
                                memcpy(dest, str, pagecpy);
                                dest += pagecpy;
                        }
                        len -= pagecpy;
                }
                offset += strpagelen;
                index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
                if (strpagelen < pagelen)
                        break;
                /*
                 * Underlying layer should never ask for reads across
                 * subbuffers.
                 */
                CHAN_WARN_ON(chanb, offset >= chanb->buf_size);
        }
        if (dest && len)
                ((char *)dest)[0] = 0;
        return offset - orig_offset;
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_read_cstr);

/**
 * lib_ring_buffer_read_get_page - Get a whole page to read from
 * @bufb : buffer backend
 * @offset : offset within the buffer
 * @virt : pointer to page address (output)
 *
 * Should be protected by get_subbuf/put_subbuf.
 * Returns the pointer to the page struct pointer.
 */
struct page **lib_ring_buffer_read_get_page(struct lib_ring_buffer_backend *bufb,
                                            size_t offset, void ***virt)
{
        size_t index;
        struct lib_ring_buffer_backend_pages *rpages;
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        unsigned long sb_bindex, id;

        offset &= chanb->buf_size - 1;
        index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
        id = bufb->buf_rsb.id;
        sb_bindex = subbuffer_id_get_index(config, id);
        rpages = bufb->array[sb_bindex];
        CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                     && subbuffer_id_is_noref(config, id));
        *virt = &rpages->p[index].virt;
        return &rpages->p[index].page;
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_read_get_page);

/**
 * lib_ring_buffer_read_offset_address - get address of a buffer location
 * @bufb : buffer backend
 * @offset : offset within the buffer.
 *
 * Return the address where a given offset is located (for read).
 * Should be used to get the current subbuffer header pointer. Given we know
 * it's never on a page boundary, it's safe to write directly to this address,
 * as long as the write is never bigger than a page size.
 */
void *lib_ring_buffer_read_offset_address(struct lib_ring_buffer_backend *bufb,
                                          size_t offset)
{
        size_t index;
        struct lib_ring_buffer_backend_pages *rpages;
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        unsigned long sb_bindex, id;

        offset &= chanb->buf_size - 1;
        index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
        id = bufb->buf_rsb.id;
        sb_bindex = subbuffer_id_get_index(config, id);
        rpages = bufb->array[sb_bindex];
        CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                     && subbuffer_id_is_noref(config, id));
        return rpages->p[index].virt + (offset & ~PAGE_MASK);
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_read_offset_address);

/**
 * lib_ring_buffer_offset_address - get address of a location within the buffer
 * @bufb : buffer backend
 * @offset : offset within the buffer.
 *
 * Return the address where a given offset is located.
 * Should be used to get the current subbuffer header pointer. Given we know
 * it's always at the beginning of a page, it's safe to write directly to this
 * address, as long as the write is never bigger than a page size.
 */
void *lib_ring_buffer_offset_address(struct lib_ring_buffer_backend *bufb,
                                     size_t offset)
{
        size_t sbidx, index;
        struct lib_ring_buffer_backend_pages *rpages;
        struct channel_backend *chanb = &bufb->chan->backend;
        const struct lib_ring_buffer_config *config = &chanb->config;
        unsigned long sb_bindex, id;

        offset &= chanb->buf_size - 1;
        sbidx = offset >> chanb->subbuf_size_order;
        index = (offset & (chanb->subbuf_size - 1)) >> PAGE_SHIFT;
        id = bufb->buf_wsb[sbidx].id;
        sb_bindex = subbuffer_id_get_index(config, id);
        rpages = bufb->array[sb_bindex];
        CHAN_WARN_ON(chanb, config->mode == RING_BUFFER_OVERWRITE
                     && subbuffer_id_is_noref(config, id));
        return rpages->p[index].virt + (offset & ~PAGE_MASK);
}
EXPORT_SYMBOL_GPL(lib_ring_buffer_offset_address);