/*
 * drivers/misc/logger.c
 *
 * A Logging Subsystem
 *
 * Copyright (C) 2007-2008 Google, Inc.
 *
 * Robert Love <rlove@google.com>
 *
 * This software is licensed under the terms of the GNU General Public
 * License version 2, as published by the Free Software Foundation, and
 * may be copied, distributed, and modified under those terms.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 */

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/miscdevice.h>
#include <linux/uaccess.h>
#include <linux/poll.h>
#include <linux/time.h>
#include "logger.h"

#include <asm/ioctls.h>

/*
 * struct logger_log - represents a specific log, such as 'main' or 'radio'
 *
 * This structure lives from module insertion until module removal, so it does
 * not need additional reference counting. The structure is protected by the
 * mutex 'mutex'.
 */
struct logger_log {
        unsigned char           *buffer;/* the ring buffer itself */
        struct miscdevice       misc;   /* misc device representing the log */
        wait_queue_head_t       wq;     /* wait queue for readers */
        struct list_head        readers; /* this log's readers */
        struct mutex            mutex;  /* mutex protecting buffer */
        size_t                  w_off;  /* current write head offset */
        size_t                  head;   /* new readers start here */
        size_t                  size;   /* size of the log */
};

/*
 * struct logger_reader - a logging device open for reading
 *
 * This object lives from open to release, so we don't need additional
 * reference counting. The structure is protected by log->mutex.
 */
struct logger_reader {
        struct logger_log       *log;   /* associated log */
        struct list_head        list;   /* entry in logger_log's list */
        size_t                  r_off;  /* current read head offset */
};

/* logger_offset - returns index 'n' into the log via (optimized) modulus */
#define logger_offset(n)        ((n) & (log->size - 1))

/*
 * file_get_log - Given a file structure, return the associated log
 *
 * This isn't aesthetic. We have several goals:
 *
 *      1) Need to quickly obtain the associated log during an I/O operation
 *      2) Readers need to maintain state (logger_reader)
 *      3) Writers need to be very fast (open() should be a near no-op)
 *
 * In the reader case, we can trivially go file->logger_reader->logger_log.
 * For a writer, we don't want to maintain a logger_reader, so we just go
 * file->logger_log. Thus what file->private_data points at depends on whether
 * or not the file was opened for reading. This function hides that dirtiness.
 */
static inline struct logger_log *file_get_log(struct file *file)
{
        if (file->f_mode & FMODE_READ) {
                struct logger_reader *reader = file->private_data;
                return reader->log;
        } else
                return file->private_data;
}

/*
 * get_entry_len - Grabs the length of the payload of the next entry starting
 * from 'off'.
 *
 * Caller needs to hold log->mutex.
 */
static __u32 get_entry_len(struct logger_log *log, size_t off)
{
        __u16 val;

        switch (log->size - off) {
        case 1:
                memcpy(&val, log->buffer + off, 1);
                memcpy(((char *) &val) + 1, log->buffer, 1);
                break;
        default:
                memcpy(&val, log->buffer + off, 2);
        }

        return sizeof(struct logger_entry) + val;
}

/*
 * do_read_log_to_user - reads exactly 'count' bytes from 'log' into the
 * user-space buffer 'buf'. Returns 'count' on success.
 *
 * Caller must hold log->mutex.
 */
static ssize_t do_read_log_to_user(struct logger_log *log,
                                   struct logger_reader *reader,
                                   char __user *buf,
                                   size_t count)
{
        size_t len;

        /*
         * We read from the log in two disjoint operations. First, we read from
         * the current read head offset up to 'count' bytes or to the end of
         * the log, whichever comes first.
         */
        len = min(count, log->size - reader->r_off);
        if (copy_to_user(buf, log->buffer + reader->r_off, len))
                return -EFAULT;

        /*
         * Second, we read any remaining bytes, starting back at the head of
         * the log.
         */
        if (count != len)
                if (copy_to_user(buf + len, log->buffer, count - len))
                        return -EFAULT;

        reader->r_off = logger_offset(reader->r_off + count);

        return count;
}

/*
 * logger_read - our log's read() method
 *
 * Behavior:
 *
 *      - O_NONBLOCK works
 *      - If there are no log entries to read, blocks until log is written to
 *      - Atomically reads exactly one log entry
 *
 * Optimal read size is LOGGER_ENTRY_MAX_LEN. Will set errno to EINVAL if read
 * buffer is insufficient to hold next entry.
 */
static ssize_t logger_read(struct file *file, char __user *buf,
                           size_t count, loff_t *pos)
{
        struct logger_reader *reader = file->private_data;
        struct logger_log *log = reader->log;
        ssize_t ret;
        DEFINE_WAIT(wait);

start:
        while (1) {
                prepare_to_wait(&log->wq, &wait, TASK_INTERRUPTIBLE);

                mutex_lock(&log->mutex);
                ret = (log->w_off == reader->r_off);
                mutex_unlock(&log->mutex);
                if (!ret)
                        break;

                if (file->f_flags & O_NONBLOCK) {
                        ret = -EAGAIN;
                        break;
                }

                if (signal_pending(current)) {
                        ret = -EINTR;
                        break;
                }

                schedule();
        }

        finish_wait(&log->wq, &wait);
        if (ret)
                return ret;

        mutex_lock(&log->mutex);

        /* is there still something to read or did we race? */
        if (unlikely(log->w_off == reader->r_off)) {
                mutex_unlock(&log->mutex);
                goto start;
        }

        /* get the size of the next entry */
        ret = get_entry_len(log, reader->r_off);
        if (count < ret) {
                ret = -EINVAL;
                goto out;
        }

        /* get exactly one entry from the log */
        ret = do_read_log_to_user(log, reader, buf, ret);

out:
        mutex_unlock(&log->mutex);

        return ret;
}

/*
 * get_next_entry - return the offset of the first valid entry at least 'len'
 * bytes after 'off'.
 *
 * Caller must hold log->mutex.
 */
static size_t get_next_entry(struct logger_log *log, size_t off, size_t len)
{
        size_t count = 0;

        do {
                size_t nr = get_entry_len(log, off);
                off = logger_offset(off + nr);
                count += nr;
        } while (count < len);

        return off;
}

/*
 * clock_interval - is a < c < b in mod-space? Put another way, does the line
 * from a to b cross c?
 */
static inline int clock_interval(size_t a, size_t b, size_t c)
{
        if (b < a) {
                if (a < c || b >= c)
                        return 1;
        } else {
                if (a < c && b >= c)
                        return 1;
        }

        return 0;
}

/*
 * fix_up_readers - walk the list of all readers and "fix up" any who were
 * lapped by the writer; also do the same for the default "start head".
 * We do this by "pulling forward" the readers and start head to the first
 * entry after the new write head.
 *
 * The caller needs to hold log->mutex.
 */
static void fix_up_readers(struct logger_log *log, size_t len)
{
        size_t old = log->w_off;
        size_t new = logger_offset(old + len);
        struct logger_reader *reader;

        if (clock_interval(old, new, log->head))
                log->head = get_next_entry(log, log->head, len);

        list_for_each_entry(reader, &log->readers, list)
                if (clock_interval(old, new, reader->r_off))
                        reader->r_off = get_next_entry(log, reader->r_off, len);
}

/*
 * do_write_log - writes 'len' bytes from 'buf' to 'log'
 *
 * The caller needs to hold log->mutex.
 */
static void do_write_log(struct logger_log *log, const void *buf, size_t count)
{
        size_t len;

        len = min(count, log->size - log->w_off);
        memcpy(log->buffer + log->w_off, buf, len);

        if (count != len)
                memcpy(log->buffer, buf + len, count - len);

        log->w_off = logger_offset(log->w_off + count);

}

/*
 * do_write_log_user - writes 'len' bytes from the user-space buffer 'buf' to
 * the log 'log'
 *
 * The caller needs to hold log->mutex.
 *
 * Returns 'count' on success, negative error code on failure.
 */
static ssize_t do_write_log_from_user(struct logger_log *log,
                                      const void __user *buf, size_t count)
{
        size_t len;

        len = min(count, log->size - log->w_off);
        if (len && copy_from_user(log->buffer + log->w_off, buf, len))
                return -EFAULT;

        if (count != len)
                if (copy_from_user(log->buffer, buf + len, count - len))
                        return -EFAULT;

        log->w_off = logger_offset(log->w_off + count);

        return count;
}

/*
 * logger_aio_write - our write method, implementing support for write(),
 * writev(), and aio_write(). Writes are our fast path, and we try to optimize
 * them above all else.
 */
ssize_t logger_aio_write(struct kiocb *iocb, const struct iovec *iov,
                         unsigned long nr_segs, loff_t ppos)
{
        struct logger_log *log = file_get_log(iocb->ki_filp);
        size_t orig = log->w_off;
        struct logger_entry header;
        struct timespec now;
        ssize_t ret = 0;

        now = current_kernel_time();

        header.pid = current->tgid;
        header.tid = current->pid;
        header.sec = now.tv_sec;
        header.nsec = now.tv_nsec;
        header.len = min_t(size_t, iocb->ki_left, LOGGER_ENTRY_MAX_PAYLOAD);

        /* null writes succeed, return zero */
        if (unlikely(!header.len))
                return 0;

        mutex_lock(&log->mutex);

        /*
         * Fix up any readers, pulling them forward to the first readable
         * entry after (what will be) the new write offset. We do this now
         * because if we partially fail, we can end up with clobbered log
         * entries that encroach on readable buffer.
         */
        fix_up_readers(log, sizeof(struct logger_entry) + header.len);

        do_write_log(log, &header, sizeof(struct logger_entry));

        while (nr_segs-- > 0) {
                size_t len;
                ssize_t nr;

                /* figure out how much of this vector we can keep */
                len = min_t(size_t, iov->iov_len, header.len - ret);

                /* write out this segment's payload */
                nr = do_write_log_from_user(log, iov->iov_base, len);
                if (unlikely(nr < 0)) {
                        log->w_off = orig;
                        mutex_unlock(&log->mutex);
                        return nr;
                }

                iov++;
                ret += nr;
        }

        mutex_unlock(&log->mutex);

        /* wake up any blocked readers */
        wake_up_interruptible(&log->wq);

        return ret;
}

static struct logger_log *get_log_from_minor(int);

/*
 * logger_open - the log's open() file operation
 *
 * Note how near a no-op this is in the write-only case. Keep it that way!
 */
static int logger_open(struct inode *inode, struct file *file)
{
        struct logger_log *log;
        int ret;

        ret = nonseekable_open(inode, file);
        if (ret)
                return ret;

        log = get_log_from_minor(MINOR(inode->i_rdev));
        if (!log)
                return -ENODEV;

        if (file->f_mode & FMODE_READ) {
                struct logger_reader *reader;

                reader = kmalloc(sizeof(struct logger_reader), GFP_KERNEL);
                if (!reader)
                        return -ENOMEM;

                reader->log = log;
                INIT_LIST_HEAD(&reader->list);

                mutex_lock(&log->mutex);
                reader->r_off = log->head;
                list_add_tail(&reader->list, &log->readers);
                mutex_unlock(&log->mutex);

                file->private_data = reader;
        } else
                file->private_data = log;

        return 0;
}

/*
 * logger_release - the log's release file operation
 *
 * Note this is a total no-op in the write-only case. Keep it that way!
 */
static int logger_release(struct inode *ignored, struct file *file)
{
        if (file->f_mode & FMODE_READ) {
                struct logger_reader *reader = file->private_data;
                list_del(&reader->list);
                kfree(reader);
        }

        return 0;
}

/*
 * logger_poll - the log's poll file operation, for poll/select/epoll
 *
 * Note we always return POLLOUT, because you can always write() to the log.
 * Note also that, strictly speaking, a return value of POLLIN does not
 * guarantee that the log is readable without blocking, as there is a small
 * chance that the writer can lap the reader in the interim between poll()
 * returning and the read() request.
 */
static unsigned int logger_poll(struct file *file, poll_table *wait)
{
        struct logger_reader *reader;
        struct logger_log *log;
        unsigned int ret = POLLOUT | POLLWRNORM;

        if (!(file->f_mode & FMODE_READ))
                return ret;

        reader = file->private_data;
        log = reader->log;

        poll_wait(file, &log->wq, wait);

        mutex_lock(&log->mutex);
        if (log->w_off != reader->r_off)
                ret |= POLLIN | POLLRDNORM;
        mutex_unlock(&log->mutex);

        return ret;
}

static long logger_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
        struct logger_log *log = file_get_log(file);
        struct logger_reader *reader;
        long ret = -ENOTTY;

        mutex_lock(&log->mutex);

        switch (cmd) {
        case LOGGER_GET_LOG_BUF_SIZE:
                ret = log->size;
                break;
        case LOGGER_GET_LOG_LEN:
                if (!(file->f_mode & FMODE_READ)) {
                        ret = -EBADF;
                        break;
                }
                reader = file->private_data;
                if (log->w_off >= reader->r_off)
                        ret = log->w_off - reader->r_off;
                else
                        ret = (log->size - reader->r_off) + log->w_off;
                break;
        case LOGGER_GET_NEXT_ENTRY_LEN:
                if (!(file->f_mode & FMODE_READ)) {
                        ret = -EBADF;
                        break;
                }
                reader = file->private_data;
                if (log->w_off != reader->r_off)
                        ret = get_entry_len(log, reader->r_off);
                else
                        ret = 0;
                break;
        case LOGGER_FLUSH_LOG:
                if (!(file->f_mode & FMODE_WRITE)) {
                        ret = -EBADF;
                        break;
                }
                list_for_each_entry(reader, &log->readers, list)
                        reader->r_off = log->w_off;
                log->head = log->w_off;
                ret = 0;
                break;
        }

        mutex_unlock(&log->mutex);

        return ret;
}

static const struct file_operations logger_fops = {
        .owner = THIS_MODULE,
        .read = logger_read,
        .aio_write = logger_aio_write,
        .poll = logger_poll,
        .unlocked_ioctl = logger_ioctl,
        .compat_ioctl = logger_ioctl,
        .open = logger_open,
        .release = logger_release,
};

/*
 * Defines a log structure with name 'NAME' and a size of 'SIZE' bytes, which
 * must be a power of two, greater than LOGGER_ENTRY_MAX_LEN, and less than
 * LONG_MAX minus LOGGER_ENTRY_MAX_LEN.
 */
#define DEFINE_LOGGER_DEVICE(VAR, NAME, SIZE) \
static unsigned char _buf_ ## VAR[SIZE]; \
static struct logger_log VAR = { \
        .buffer = _buf_ ## VAR, \
        .misc = { \
                .minor = MISC_DYNAMIC_MINOR, \
                .name = NAME, \
                .fops = &logger_fops, \
                .parent = NULL, \
        }, \
        .wq = __WAIT_QUEUE_HEAD_INITIALIZER(VAR .wq), \
        .readers = LIST_HEAD_INIT(VAR .readers), \
        .mutex = __MUTEX_INITIALIZER(VAR .mutex), \
        .w_off = 0, \
        .head = 0, \
        .size = SIZE, \
};

DEFINE_LOGGER_DEVICE(log_main, LOGGER_LOG_MAIN, 64*1024)
DEFINE_LOGGER_DEVICE(log_events, LOGGER_LOG_EVENTS, 256*1024)
DEFINE_LOGGER_DEVICE(log_radio, LOGGER_LOG_RADIO, 64*1024)

static struct logger_log *get_log_from_minor(int minor)
{
        if (log_main.misc.minor == minor)
                return &log_main;
        if (log_events.misc.minor == minor)
                return &log_events;
        if (log_radio.misc.minor == minor)
                return &log_radio;
        return NULL;
}

static int __init init_log(struct logger_log *log)
{
        int ret;

        ret = misc_register(&log->misc);
        if (unlikely(ret)) {
                printk(KERN_ERR "logger: failed to register misc "
                       "device for log '%s'!\n", log->misc.name);
                return ret;
        }

        printk(KERN_INFO "logger: created %luK log '%s'\n",
               (unsigned long) log->size >> 10, log->misc.name);

        return 0;
}

static int __init logger_init(void)
{
        int ret;

        ret = init_log(&log_main);
        if (unlikely(ret))
                goto out;

        ret = init_log(&log_events);
        if (unlikely(ret))
                goto out;

        ret = init_log(&log_radio);
        if (unlikely(ret))
                goto out;

out:
        return ret;
}
device_initcall(logger_init);