// SPDX-License-Identifier: GPL-2.0
/*
 * Public API and common code for kernel->userspace relay file support.
 *
 * See Documentation/filesystems/relay.rst for an overview.
 *
 * Copyright (C) 2002-2005 - Tom Zanussi (zanussi@us.ibm.com), IBM Corp
 * Copyright (C) 1999-2005 - Karim Yaghmour (karim@opersys.com)
 *
 * Moved to kernel/relay.c by Paul Mundt, 2006.
 * November 2006 - CPU hotplug support by Mathieu Desnoyers
 *      (mathieu.desnoyers@polymtl.ca)
 */
#include <linux/errno.h>
#include <linux/stddef.h>
#include <linux/slab.h>
#include <linux/export.h>
#include <linux/string.h>
#include <linux/relay.h>
#include <linux/vmalloc.h>
#include <linux/mm.h>
#include <linux/cpu.h>
#include <linux/splice.h>

/* list of open channels, for cpu hotplug */
static DEFINE_MUTEX(relay_channels_mutex);
static LIST_HEAD(relay_channels);

/*
 * fault() vm_op implementation for relay file mapping.
 */
static vm_fault_t relay_buf_fault(struct vm_fault *vmf)
{
        struct page *page;
        struct rchan_buf *buf = vmf->vma->vm_private_data;
        pgoff_t pgoff = vmf->pgoff;

        if (!buf)
                return VM_FAULT_OOM;

        page = vmalloc_to_page(buf->start + (pgoff << PAGE_SHIFT));
        if (!page)
                return VM_FAULT_SIGBUS;
        get_page(page);
        vmf->page = page;

        return 0;
}

/*
 * vm_ops for relay file mappings.
 */
static const struct vm_operations_struct relay_file_mmap_ops = {
        .fault = relay_buf_fault,
};

/*
 * allocate an array of pointers of struct page
 */
static struct page **relay_alloc_page_array(unsigned int n_pages)
{
        return kvzalloc_objs(struct page *, n_pages);
}

/*
 * free an array of pointers of struct page
 */
static void relay_free_page_array(struct page **array)
{
        kvfree(array);
}

/**
 *      relay_mmap_prepare_buf: - mmap channel buffer to process address space
 *      @buf: the relay channel buffer
 *      @desc: describing what to map
 *
 *      Returns 0 if ok, negative on error
 *
 *      Caller should already have grabbed mmap_lock.
 */
static int relay_mmap_prepare_buf(struct rchan_buf *buf,
                                  struct vm_area_desc *desc)
{
        unsigned long length = vma_desc_size(desc);

        if (!buf)
                return -EBADF;

        if (length != (unsigned long)buf->chan->alloc_size)
                return -EINVAL;

        desc->vm_ops = &relay_file_mmap_ops;
        vma_desc_set_flags(desc, VMA_DONTEXPAND_BIT);
        desc->private_data = buf;

        return 0;
}

/**
 *      relay_alloc_buf - allocate a channel buffer
 *      @buf: the buffer struct
 *      @size: total size of the buffer
 *
 *      Returns a pointer to the resulting buffer, %NULL if unsuccessful. The
 *      passed in size will get page aligned, if it isn't already.
 */
static void *relay_alloc_buf(struct rchan_buf *buf, size_t *size)
{
        void *mem;
        unsigned int i, j, n_pages;

        *size = PAGE_ALIGN(*size);
        n_pages = *size >> PAGE_SHIFT;

        buf->page_array = relay_alloc_page_array(n_pages);
        if (!buf->page_array)
                return NULL;

        for (i = 0; i < n_pages; i++) {
                buf->page_array[i] = alloc_page(GFP_KERNEL | __GFP_ZERO);
                if (unlikely(!buf->page_array[i]))
                        goto depopulate;
                set_page_private(buf->page_array[i], (unsigned long)buf);
        }
        mem = vmap(buf->page_array, n_pages, VM_MAP, PAGE_KERNEL);
        if (!mem)
                goto depopulate;

        buf->page_count = n_pages;
        return mem;

depopulate:
        for (j = 0; j < i; j++)
                __free_page(buf->page_array[j]);
        relay_free_page_array(buf->page_array);
        return NULL;
}

/**
 *      relay_create_buf - allocate and initialize a channel buffer
 *      @chan: the relay channel
 *
 *      Returns channel buffer if successful, %NULL otherwise.
 */
static struct rchan_buf *relay_create_buf(struct rchan *chan)
{
        struct rchan_buf *buf;

        if (chan->n_subbufs > KMALLOC_MAX_SIZE / sizeof(size_t))
                return NULL;

        buf = kzalloc_obj(struct rchan_buf);
        if (!buf)
                return NULL;
        buf->padding = kmalloc_objs(size_t, chan->n_subbufs);
        if (!buf->padding)
                goto free_buf;

        buf->start = relay_alloc_buf(buf, &chan->alloc_size);
        if (!buf->start)
                goto free_buf;

        buf->chan = chan;
        kref_get(&buf->chan->kref);
        return buf;

free_buf:
        kfree(buf->padding);
        kfree(buf);
        return NULL;
}

/**
 *      relay_destroy_channel - free the channel struct
 *      @kref: target kernel reference that contains the relay channel
 *
 *      Should only be called from kref_put().
 */
static void relay_destroy_channel(struct kref *kref)
{
        struct rchan *chan = container_of(kref, struct rchan, kref);
        free_percpu(chan->buf);
        kfree(chan);
}

/**
 *      relay_destroy_buf - destroy an rchan_buf struct and associated buffer
 *      @buf: the buffer struct
 */
static void relay_destroy_buf(struct rchan_buf *buf)
{
        struct rchan *chan = buf->chan;
        unsigned int i;

        if (likely(buf->start)) {
                vunmap(buf->start);
                for (i = 0; i < buf->page_count; i++)
                        __free_page(buf->page_array[i]);
                relay_free_page_array(buf->page_array);
        }
        *per_cpu_ptr(chan->buf, buf->cpu) = NULL;
        kfree(buf->padding);
        kfree(buf);
        kref_put(&chan->kref, relay_destroy_channel);
}

/**
 *      relay_remove_buf - remove a channel buffer
 *      @kref: target kernel reference that contains the relay buffer
 *
 *      Removes the file from the filesystem, which also frees the
 *      rchan_buf_struct and the channel buffer.  Should only be called from
 *      kref_put().
 */
static void relay_remove_buf(struct kref *kref)
{
        struct rchan_buf *buf = container_of(kref, struct rchan_buf, kref);
        relay_destroy_buf(buf);
}

/**
 *      relay_buf_empty - boolean, is the channel buffer empty?
 *      @buf: channel buffer
 *
 *      Returns 1 if the buffer is empty, 0 otherwise.
 */
static int relay_buf_empty(struct rchan_buf *buf)
{
        return (buf->subbufs_produced - buf->subbufs_consumed) ? 0 : 1;
}

/**
 *      relay_buf_full - boolean, is the channel buffer full?
 *      @buf: channel buffer
 *
 *      Returns 1 if the buffer is full, 0 otherwise.
 */
int relay_buf_full(struct rchan_buf *buf)
{
        size_t ready = buf->subbufs_produced - buf->subbufs_consumed;
        return (ready >= buf->chan->n_subbufs) ? 1 : 0;
}
EXPORT_SYMBOL_GPL(relay_buf_full);

/*
 * High-level relay kernel API and associated functions.
 */

static int relay_subbuf_start(struct rchan_buf *buf, void *subbuf,
                              void *prev_subbuf)
{
        int full = relay_buf_full(buf);

        if (full)
                buf->stats.full_count++;

        if (!buf->chan->cb->subbuf_start)
                return !full;

        return buf->chan->cb->subbuf_start(buf, subbuf,
                                           prev_subbuf);
}

/**
 *      wakeup_readers - wake up readers waiting on a channel
 *      @work: contains the channel buffer
 *
 *      This is the function used to defer reader waking
 */
static void wakeup_readers(struct irq_work *work)
{
        struct rchan_buf *buf;

        buf = container_of(work, struct rchan_buf, wakeup_work);
        wake_up_interruptible(&buf->read_wait);
}

/**
 *      __relay_reset - reset a channel buffer
 *      @buf: the channel buffer
 *      @init: 1 if this is a first-time initialization
 *
 *      See relay_reset() for description of effect.
 */
static void __relay_reset(struct rchan_buf *buf, unsigned int init)
{
        size_t i;

        if (init) {
                init_waitqueue_head(&buf->read_wait);
                kref_init(&buf->kref);
                init_irq_work(&buf->wakeup_work, wakeup_readers);
        } else {
                irq_work_sync(&buf->wakeup_work);
        }

        buf->subbufs_produced = 0;
        buf->subbufs_consumed = 0;
        buf->bytes_consumed = 0;
        buf->finalized = 0;
        buf->data = buf->start;
        buf->offset = 0;
        buf->stats.full_count = 0;
        buf->stats.big_count = 0;

        for (i = 0; i < buf->chan->n_subbufs; i++)
                buf->padding[i] = 0;

        relay_subbuf_start(buf, buf->data, NULL);
}

/**
 *      relay_reset - reset the channel
 *      @chan: the channel
 *
 *      This has the effect of erasing all data from all channel buffers
 *      and restarting the channel in its initial state.  The buffers
 *      are not freed, so any mappings are still in effect.
 *
 *      NOTE. Care should be taken that the channel isn't actually
 *      being used by anything when this call is made.
 */
void relay_reset(struct rchan *chan)
{
        struct rchan_buf *buf;
        unsigned int i;

        if (!chan)
                return;

        if (chan->is_global && (buf = *per_cpu_ptr(chan->buf, 0))) {
                __relay_reset(buf, 0);
                return;
        }

        mutex_lock(&relay_channels_mutex);
        for_each_possible_cpu(i)
                if ((buf = *per_cpu_ptr(chan->buf, i)))
                        __relay_reset(buf, 0);
        mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_reset);

static inline void relay_set_buf_dentry(struct rchan_buf *buf,
                                        struct dentry *dentry)
{
        buf->dentry = dentry;
        d_inode(buf->dentry)->i_size = buf->early_bytes;
}

static struct dentry *relay_create_buf_file(struct rchan *chan,
                                            struct rchan_buf *buf,
                                            unsigned int cpu)
{
        struct dentry *dentry;
        char *tmpname;

        tmpname = kasprintf(GFP_KERNEL, "%s%d", chan->base_filename, cpu);
        if (!tmpname)
                return NULL;

        /* Create file in fs */
        dentry = chan->cb->create_buf_file(tmpname, chan->parent,
                                           S_IRUSR, buf,
                                           &chan->is_global);
        if (IS_ERR(dentry))
                dentry = NULL;

        kfree(tmpname);

        return dentry;
}

/*
 *      relay_open_buf - create a new relay channel buffer
 *
 *      used by relay_open() and CPU hotplug.
 */
static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
{
        struct rchan_buf *buf;
        struct dentry *dentry;

        if (chan->is_global)
                return *per_cpu_ptr(chan->buf, 0);

        buf = relay_create_buf(chan);
        if (!buf)
                return NULL;

        if (chan->has_base_filename) {
                dentry = relay_create_buf_file(chan, buf, cpu);
                if (!dentry)
                        goto free_buf;
                relay_set_buf_dentry(buf, dentry);
        } else {
                /* Only retrieve global info, nothing more, nothing less */
                dentry = chan->cb->create_buf_file(NULL, NULL,
                                                   S_IRUSR, buf,
                                                   &chan->is_global);
                if (IS_ERR_OR_NULL(dentry))
                        goto free_buf;
        }

        buf->cpu = cpu;
        __relay_reset(buf, 1);

        if(chan->is_global) {
                *per_cpu_ptr(chan->buf, 0) = buf;
                buf->cpu = 0;
        }

        return buf;

free_buf:
        relay_destroy_buf(buf);
        return NULL;
}

/**
 *      relay_close_buf - close a channel buffer
 *      @buf: channel buffer
 *
 *      Marks the buffer finalized and restores the default callbacks.
 *      The channel buffer and channel buffer data structure are then freed
 *      automatically when the last reference is given up.
 */
static void relay_close_buf(struct rchan_buf *buf)
{
        buf->finalized = 1;
        irq_work_sync(&buf->wakeup_work);
        buf->chan->cb->remove_buf_file(buf->dentry);
        kref_put(&buf->kref, relay_remove_buf);
}

int relay_prepare_cpu(unsigned int cpu)
{
        struct rchan *chan;
        struct rchan_buf *buf;

        mutex_lock(&relay_channels_mutex);
        list_for_each_entry(chan, &relay_channels, list) {
                if (*per_cpu_ptr(chan->buf, cpu))
                        continue;
                buf = relay_open_buf(chan, cpu);
                if (!buf) {
                        pr_err("relay: cpu %d buffer creation failed\n", cpu);
                        mutex_unlock(&relay_channels_mutex);
                        return -ENOMEM;
                }
                *per_cpu_ptr(chan->buf, cpu) = buf;
        }
        mutex_unlock(&relay_channels_mutex);
        return 0;
}

/**
 *      relay_open - create a new relay channel
 *      @base_filename: base name of files to create
 *      @parent: dentry of parent directory, %NULL for root directory or buffer
 *      @subbuf_size: size of sub-buffers
 *      @n_subbufs: number of sub-buffers
 *      @cb: client callback functions
 *      @private_data: user-defined data
 *
 *      Returns channel pointer if successful, %NULL otherwise.
 *
 *      Creates a channel buffer for each cpu using the sizes and
 *      attributes specified.  The created channel buffer files
 *      will be named base_filename0...base_filenameN-1.  File
 *      permissions will be %S_IRUSR.
 */
struct rchan *relay_open(const char *base_filename,
                         struct dentry *parent,
                         size_t subbuf_size,
                         size_t n_subbufs,
                         const struct rchan_callbacks *cb,
                         void *private_data)
{
        unsigned int i;
        struct rchan *chan;
        struct rchan_buf *buf;

        if (!(subbuf_size && n_subbufs))
                return NULL;
        if (subbuf_size > UINT_MAX / n_subbufs)
                return NULL;
        if (!cb || !cb->create_buf_file || !cb->remove_buf_file)
                return NULL;

        chan = kzalloc_obj(struct rchan);
        if (!chan)
                return NULL;

        chan->buf = alloc_percpu(struct rchan_buf *);
        if (!chan->buf) {
                kfree(chan);
                return NULL;
        }

        chan->version = RELAYFS_CHANNEL_VERSION;
        chan->n_subbufs = n_subbufs;
        chan->subbuf_size = subbuf_size;
        chan->alloc_size = PAGE_ALIGN(subbuf_size * n_subbufs);
        chan->parent = parent;
        chan->private_data = private_data;
        if (base_filename) {
                chan->has_base_filename = 1;
                strscpy(chan->base_filename, base_filename, NAME_MAX);
        }
        chan->cb = cb;
        kref_init(&chan->kref);

        mutex_lock(&relay_channels_mutex);
        for_each_online_cpu(i) {
                buf = relay_open_buf(chan, i);
                if (!buf)
                        goto free_bufs;
                *per_cpu_ptr(chan->buf, i) = buf;
        }
        list_add(&chan->list, &relay_channels);
        mutex_unlock(&relay_channels_mutex);

        return chan;

free_bufs:
        for_each_possible_cpu(i) {
                if ((buf = *per_cpu_ptr(chan->buf, i)))
                        relay_close_buf(buf);
        }

        kref_put(&chan->kref, relay_destroy_channel);
        mutex_unlock(&relay_channels_mutex);
        return NULL;
}
EXPORT_SYMBOL_GPL(relay_open);

struct rchan_percpu_buf_dispatcher {
        struct rchan_buf *buf;
        struct dentry *dentry;
};

/**
 *      relay_switch_subbuf - switch to a new sub-buffer
 *      @buf: channel buffer
 *      @length: size of current event
 *
 *      Returns either the length passed in or 0 if full.
 *
 *      Performs sub-buffer-switch tasks such as invoking callbacks,
 *      updating padding counts, waking up readers, etc.
 */
size_t relay_switch_subbuf(struct rchan_buf *buf, size_t length)
{
        void *old, *new;
        size_t old_subbuf, new_subbuf;

        if (unlikely(length > buf->chan->subbuf_size))
                goto toobig;

        if (buf->offset != buf->chan->subbuf_size + 1) {
                size_t prev_padding;

                prev_padding = buf->chan->subbuf_size - buf->offset;
                old_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
                buf->padding[old_subbuf] = prev_padding;
                buf->subbufs_produced++;
                if (buf->dentry)
                        d_inode(buf->dentry)->i_size +=
                                buf->chan->subbuf_size -
                                buf->padding[old_subbuf];
                else
                        buf->early_bytes += buf->chan->subbuf_size -
                                            buf->padding[old_subbuf];
                smp_mb();
                if (waitqueue_active(&buf->read_wait)) {
                        /*
                         * Calling wake_up_interruptible() from here
                         * will deadlock if we happen to be logging
                         * from the scheduler (trying to re-grab
                         * rq->lock), so defer it.
                         */
                        irq_work_queue(&buf->wakeup_work);
                }
        }

        old = buf->data;
        new_subbuf = buf->subbufs_produced % buf->chan->n_subbufs;
        new = buf->start + new_subbuf * buf->chan->subbuf_size;
        buf->offset = 0;
        if (!relay_subbuf_start(buf, new, old)) {
                buf->offset = buf->chan->subbuf_size + 1;
                return 0;
        }
        buf->data = new;
        buf->padding[new_subbuf] = 0;

        if (unlikely(length + buf->offset > buf->chan->subbuf_size))
                goto toobig;

        return length;

toobig:
        buf->stats.big_count++;
        return 0;
}
EXPORT_SYMBOL_GPL(relay_switch_subbuf);

/**
 *      relay_subbufs_consumed - update the buffer's sub-buffers-consumed count
 *      @chan: the channel
 *      @cpu: the cpu associated with the channel buffer to update
 *      @subbufs_consumed: number of sub-buffers to add to current buf's count
 *
 *      Adds to the channel buffer's consumed sub-buffer count.
 *      subbufs_consumed should be the number of sub-buffers newly consumed,
 *      not the total consumed.
 *
 *      NOTE. Kernel clients don't need to call this function if the channel
 *      mode is 'overwrite'.
 */
void relay_subbufs_consumed(struct rchan *chan,
                            unsigned int cpu,
                            size_t subbufs_consumed)
{
        struct rchan_buf *buf;

        if (!chan || cpu >= NR_CPUS)
                return;

        buf = *per_cpu_ptr(chan->buf, cpu);
        if (!buf || subbufs_consumed > chan->n_subbufs)
                return;

        if (subbufs_consumed > buf->subbufs_produced - buf->subbufs_consumed)
                buf->subbufs_consumed = buf->subbufs_produced;
        else
                buf->subbufs_consumed += subbufs_consumed;
}
EXPORT_SYMBOL_GPL(relay_subbufs_consumed);

/**
 *      relay_close - close the channel
 *      @chan: the channel
 *
 *      Closes all channel buffers and frees the channel.
 */
void relay_close(struct rchan *chan)
{
        struct rchan_buf *buf;
        unsigned int i;

        if (!chan)
                return;

        mutex_lock(&relay_channels_mutex);
        if (chan->is_global && (buf = *per_cpu_ptr(chan->buf, 0)))
                relay_close_buf(buf);
        else
                for_each_possible_cpu(i)
                        if ((buf = *per_cpu_ptr(chan->buf, i)))
                                relay_close_buf(buf);

        list_del(&chan->list);
        kref_put(&chan->kref, relay_destroy_channel);
        mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_close);

/**
 *      relay_flush - close the channel
 *      @chan: the channel
 *
 *      Flushes all channel buffers, i.e. forces buffer switch.
 */
void relay_flush(struct rchan *chan)
{
        struct rchan_buf *buf;
        unsigned int i;

        if (!chan)
                return;

        if (chan->is_global && (buf = *per_cpu_ptr(chan->buf, 0))) {
                relay_switch_subbuf(buf, 0);
                return;
        }

        mutex_lock(&relay_channels_mutex);
        for_each_possible_cpu(i)
                if ((buf = *per_cpu_ptr(chan->buf, i)))
                        relay_switch_subbuf(buf, 0);
        mutex_unlock(&relay_channels_mutex);
}
EXPORT_SYMBOL_GPL(relay_flush);

/**
 *      relay_stats - get channel buffer statistics
 *      @chan: the channel
 *      @flags: select particular information to get
 *
 *      Returns the count of certain field that caller specifies.
 */
size_t relay_stats(struct rchan *chan, int flags)
{
        unsigned int i, count = 0;
        struct rchan_buf *rbuf;

        if (!chan || flags > RELAY_STATS_LAST)
                return 0;

        if (chan->is_global) {
                rbuf = *per_cpu_ptr(chan->buf, 0);
                if (flags & RELAY_STATS_BUF_FULL)
                        count = rbuf->stats.full_count;
                else if (flags & RELAY_STATS_WRT_BIG)
                        count = rbuf->stats.big_count;
        } else {
                for_each_online_cpu(i) {
                        rbuf = *per_cpu_ptr(chan->buf, i);
                        if (rbuf) {
                                if (flags & RELAY_STATS_BUF_FULL)
                                        count += rbuf->stats.full_count;
                                else if (flags & RELAY_STATS_WRT_BIG)
                                        count += rbuf->stats.big_count;
                        }
                }
        }

        return count;
}

/**
 *      relay_file_open - open file op for relay files
 *      @inode: the inode
 *      @filp: the file
 *
 *      Increments the channel buffer refcount.
 */
static int relay_file_open(struct inode *inode, struct file *filp)
{
        struct rchan_buf *buf = inode->i_private;
        kref_get(&buf->kref);
        filp->private_data = buf;

        return nonseekable_open(inode, filp);
}

/**
 *      relay_file_mmap_prepare - mmap file op for relay files
 *      @desc: describing what to map
 *
 *      Calls upon relay_mmap_prepare_buf() to map the file into user space.
 */
static int relay_file_mmap_prepare(struct vm_area_desc *desc)
{
        struct rchan_buf *buf = desc->file->private_data;

        return relay_mmap_prepare_buf(buf, desc);
}

/**
 *      relay_file_poll - poll file op for relay files
 *      @filp: the file
 *      @wait: poll table
 *
 *      Poll implemention.
 */
static __poll_t relay_file_poll(struct file *filp, poll_table *wait)
{
        __poll_t mask = 0;
        struct rchan_buf *buf = filp->private_data;

        if (buf->finalized)
                return EPOLLERR;

        if (filp->f_mode & FMODE_READ) {
                poll_wait(filp, &buf->read_wait, wait);
                if (!relay_buf_empty(buf))
                        mask |= EPOLLIN | EPOLLRDNORM;
        }

        return mask;
}

/**
 *      relay_file_release - release file op for relay files
 *      @inode: the inode
 *      @filp: the file
 *
 *      Decrements the channel refcount, as the filesystem is
 *      no longer using it.
 */
static int relay_file_release(struct inode *inode, struct file *filp)
{
        struct rchan_buf *buf = filp->private_data;
        kref_put(&buf->kref, relay_remove_buf);

        return 0;
}

/*
 *      relay_file_read_consume - update the consumed count for the buffer
 */
static void relay_file_read_consume(struct rchan_buf *buf,
                                    size_t read_pos,
                                    size_t bytes_consumed)
{
        size_t subbuf_size = buf->chan->subbuf_size;
        size_t n_subbufs = buf->chan->n_subbufs;
        size_t read_subbuf;

        if (buf->subbufs_produced == buf->subbufs_consumed &&
            buf->offset == buf->bytes_consumed)
                return;

        if (buf->bytes_consumed + bytes_consumed > subbuf_size) {
                relay_subbufs_consumed(buf->chan, buf->cpu, 1);
                buf->bytes_consumed = 0;
        }

        buf->bytes_consumed += bytes_consumed;
        if (!read_pos)
                read_subbuf = buf->subbufs_consumed % n_subbufs;
        else
                read_subbuf = read_pos / buf->chan->subbuf_size;
        if (buf->bytes_consumed + buf->padding[read_subbuf] == subbuf_size) {
                if ((read_subbuf == buf->subbufs_produced % n_subbufs) &&
                    (buf->offset == subbuf_size))
                        return;
                relay_subbufs_consumed(buf->chan, buf->cpu, 1);
                buf->bytes_consumed = 0;
        }
}

/*
 *      relay_file_read_avail - boolean, are there unconsumed bytes available?
 */
static int relay_file_read_avail(struct rchan_buf *buf)
{
        size_t subbuf_size = buf->chan->subbuf_size;
        size_t n_subbufs = buf->chan->n_subbufs;
        size_t produced = buf->subbufs_produced;
        size_t consumed;

        relay_file_read_consume(buf, 0, 0);

        consumed = buf->subbufs_consumed;

        if (unlikely(buf->offset > subbuf_size)) {
                if (produced == consumed)
                        return 0;
                return 1;
        }

        if (unlikely(produced - consumed >= n_subbufs)) {
                consumed = produced - n_subbufs + 1;
                buf->subbufs_consumed = consumed;
                buf->bytes_consumed = 0;
        }

        produced = (produced % n_subbufs) * subbuf_size + buf->offset;
        consumed = (consumed % n_subbufs) * subbuf_size + buf->bytes_consumed;

        if (consumed > produced)
                produced += n_subbufs * subbuf_size;

        if (consumed == produced) {
                if (buf->offset == subbuf_size &&
                    buf->subbufs_produced > buf->subbufs_consumed)
                        return 1;
                return 0;
        }

        return 1;
}

/**
 *      relay_file_read_subbuf_avail - return bytes available in sub-buffer
 *      @read_pos: file read position
 *      @buf: relay channel buffer
 */
static size_t relay_file_read_subbuf_avail(size_t read_pos,
                                           struct rchan_buf *buf)
{
        size_t padding, avail = 0;
        size_t read_subbuf, read_offset, write_subbuf, write_offset;
        size_t subbuf_size = buf->chan->subbuf_size;

        write_subbuf = (buf->data - buf->start) / subbuf_size;
        write_offset = buf->offset > subbuf_size ? subbuf_size : buf->offset;
        read_subbuf = read_pos / subbuf_size;
        read_offset = read_pos % subbuf_size;
        padding = buf->padding[read_subbuf];

        if (read_subbuf == write_subbuf) {
                if (read_offset + padding < write_offset)
                        avail = write_offset - (read_offset + padding);
        } else
                avail = (subbuf_size - padding) - read_offset;

        return avail;
}

/**
 *      relay_file_read_start_pos - find the first available byte to read
 *      @buf: relay channel buffer
 *
 *      If the read_pos is in the middle of padding, return the
 *      position of the first actually available byte, otherwise
 *      return the original value.
 */
static size_t relay_file_read_start_pos(struct rchan_buf *buf)
{
        size_t read_subbuf, padding, padding_start, padding_end;
        size_t subbuf_size = buf->chan->subbuf_size;
        size_t n_subbufs = buf->chan->n_subbufs;
        size_t consumed = buf->subbufs_consumed % n_subbufs;
        size_t read_pos = (consumed * subbuf_size + buf->bytes_consumed)
                        % (n_subbufs * subbuf_size);

        read_subbuf = read_pos / subbuf_size;
        padding = buf->padding[read_subbuf];
        padding_start = (read_subbuf + 1) * subbuf_size - padding;
        padding_end = (read_subbuf + 1) * subbuf_size;
        if (read_pos >= padding_start && read_pos < padding_end) {
                read_subbuf = (read_subbuf + 1) % n_subbufs;
                read_pos = read_subbuf * subbuf_size;
        }

        return read_pos;
}

/**
 *      relay_file_read_end_pos - return the new read position
 *      @read_pos: file read position
 *      @buf: relay channel buffer
 *      @count: number of bytes to be read
 */
static size_t relay_file_read_end_pos(struct rchan_buf *buf,
                                      size_t read_pos,
                                      size_t count)
{
        size_t read_subbuf, padding, end_pos;
        size_t subbuf_size = buf->chan->subbuf_size;
        size_t n_subbufs = buf->chan->n_subbufs;

        read_subbuf = read_pos / subbuf_size;
        padding = buf->padding[read_subbuf];
        if (read_pos % subbuf_size + count + padding == subbuf_size)
                end_pos = (read_subbuf + 1) * subbuf_size;
        else
                end_pos = read_pos + count;
        if (end_pos >= subbuf_size * n_subbufs)
                end_pos = 0;

        return end_pos;
}

static ssize_t relay_file_read(struct file *filp,
                               char __user *buffer,
                               size_t count,
                               loff_t *ppos)
{
        struct rchan_buf *buf = filp->private_data;
        size_t read_start, avail;
        size_t written = 0;
        int ret;

        if (!count)
                return 0;

        inode_lock(file_inode(filp));
        do {
                void *from;

                if (!relay_file_read_avail(buf))
                        break;

                read_start = relay_file_read_start_pos(buf);
                avail = relay_file_read_subbuf_avail(read_start, buf);
                if (!avail)
                        break;

                avail = min(count, avail);
                from = buf->start + read_start;
                ret = avail;
                if (copy_to_user(buffer, from, avail))
                        break;

                buffer += ret;
                written += ret;
                count -= ret;

                relay_file_read_consume(buf, read_start, ret);
                *ppos = relay_file_read_end_pos(buf, read_start, ret);
        } while (count);
        inode_unlock(file_inode(filp));

        return written;
}


const struct file_operations relay_file_operations = {
        .open           = relay_file_open,
        .poll           = relay_file_poll,
        .mmap_prepare   = relay_file_mmap_prepare,
        .read           = relay_file_read,
        .release        = relay_file_release,
};
EXPORT_SYMBOL_GPL(relay_file_operations);