/*
 *  n_tracerouter.c - Trace data router through tty space
 *
 *  Copyright (C) Intel 2011
 *
 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License version 2
 *  as published by the Free Software Foundation.
 *
 *  This 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.
 *
 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 *
 * This trace router uses the Linux line discipline framework to route
 * trace data coming from a HW Modem to a PTI (Parallel Trace Module) port.
 * The solution is not specific to a HW modem and this line disciple can
 * be used to route any stream of data in kernel space.
 * This is part of a solution for the P1149.7, compact JTAG, standard.
 */

#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/ioctl.h>
#include <linux/tty.h>
#include <linux/tty_ldisc.h>
#include <linux/errno.h>
#include <linux/string.h>
#include <linux/mutex.h>
#include <linux/slab.h>
#include <asm-generic/bug.h>
#include "n_tracesink.h"

/*
 * Other ldisc drivers use 65536 which basically means,
 * 'I can always accept 64k' and flow control is off.
 * This number is deemed appropriate for this driver.
 */
#define RECEIVE_ROOM    65536
#define DRIVERNAME      "n_tracerouter"

/*
 * struct to hold private configuration data for this ldisc.
 * opencalled is used to hold if this ldisc has been opened.
 * kref_tty holds the tty reference the ldisc sits on top of.
 */
struct tracerouter_data {
        u8 opencalled;
        struct tty_struct *kref_tty;
};
static struct tracerouter_data *tr_data;

/* lock for when tty reference is being used */
static DEFINE_MUTEX(routelock);

/**
 * n_tracerouter_open() - Called when a tty is opened by a SW entity.
 * @tty: terminal device to the ldisc.
 *
 * Return:
 *      0 for success.
 *
 * Caveats: This should only be opened one time per SW entity.
 */
static int n_tracerouter_open(struct tty_struct *tty)
{
        int retval = -EEXIST;

        mutex_lock(&routelock);
        if (tr_data->opencalled == 0) {

                tr_data->kref_tty = tty_kref_get(tty);
                if (tr_data->kref_tty == NULL) {
                        retval = -EFAULT;
                } else {
                        tr_data->opencalled = 1;
                        tty->disc_data      = tr_data;
                        tty->receive_room   = RECEIVE_ROOM;
                        tty_driver_flush_buffer(tty);
                        retval = 0;
                }
        }
        mutex_unlock(&routelock);
        return retval;
}

/**
 * n_tracerouter_close() - close connection
 * @tty: terminal device to the ldisc.
 *
 * Called when a software entity wants to close a connection.
 */
static void n_tracerouter_close(struct tty_struct *tty)
{
        struct tracerouter_data *tptr = tty->disc_data;

        mutex_lock(&routelock);
        WARN_ON(tptr->kref_tty != tr_data->kref_tty);
        tty_driver_flush_buffer(tty);
        tty_kref_put(tr_data->kref_tty);
        tr_data->kref_tty = NULL;
        tr_data->opencalled = 0;
        tty->disc_data = NULL;
        mutex_unlock(&routelock);
}

/**
 * n_tracerouter_read() - read request from user space
 * @tty:  terminal device passed into the ldisc.
 * @file: pointer to open file object.
 * @buf:  pointer to the data buffer that gets eventually returned.
 * @nr:   number of bytes of the data buffer that is returned.
 *
 * function that allows read() functionality in userspace. By default if this
 * is not implemented it returns -EIO. This module is functioning like a
 * router via n_tracerouter_receivebuf(), and there is no real requirement
 * to implement this function. However, an error return value other than
 * -EIO should be used just to show that there was an intent not to have
 * this function implemented.  Return value based on read() man pages.
 *
 * Return:
 *       -EINVAL
 */
static ssize_t n_tracerouter_read(struct tty_struct *tty, struct file *file,
                                  unsigned char __user *buf, size_t nr) {
        return -EINVAL;
}

/**
 * n_tracerouter_write() - Function that allows write() in userspace.
 * @tty:  terminal device passed into the ldisc.
 * @file: pointer to open file object.
 * @buf:  pointer to the data buffer that gets eventually returned.
 * @nr:   number of bytes of the data buffer that is returned.
 *
 * By default if this is not implemented, it returns -EIO.
 * This should not be implemented, ever, because
 * 1. this driver is functioning like a router via
 *    n_tracerouter_receivebuf()
 * 2. No writes to HW will ever go through this line discpline driver.
 * However, an error return value other than -EIO should be used
 * just to show that there was an intent not to have this function
 * implemented.  Return value based on write() man pages.
 *
 * Return:
 *      -EINVAL
 */
static ssize_t n_tracerouter_write(struct tty_struct *tty, struct file *file,
                                   const unsigned char *buf, size_t nr) {
        return -EINVAL;
}

/**
 * n_tracerouter_receivebuf() - Routing function for driver.
 * @tty: terminal device passed into the ldisc.  It's assumed
 *       tty will never be NULL.
 * @cp:  buffer, block of characters to be eventually read by
 *       someone, somewhere (user read() call or some kernel function).
 * @fp:  flag buffer.
 * @count: number of characters (aka, bytes) in cp.
 *
 * This function takes the input buffer, cp, and passes it to
 * an external API function for processing.
 */
static void n_tracerouter_receivebuf(struct tty_struct *tty,
                                        const unsigned char *cp,
                                        char *fp, int count)
{
        mutex_lock(&routelock);
        n_tracesink_datadrain((u8 *) cp, count);
        mutex_unlock(&routelock);
}

/*
 * Flush buffer is not impelemented as the ldisc has no internal buffering
 * so the tty_driver_flush_buffer() is sufficient for this driver's needs.
 */

static struct tty_ldisc_ops tty_ptirouter_ldisc = {
        .owner          = THIS_MODULE,
        .magic          = TTY_LDISC_MAGIC,
        .name           = DRIVERNAME,
        .open           = n_tracerouter_open,
        .close          = n_tracerouter_close,
        .read           = n_tracerouter_read,
        .write          = n_tracerouter_write,
        .receive_buf    = n_tracerouter_receivebuf
};

/**
 * n_tracerouter_init - module initialisation
 *
 * Registers this module as a line discipline driver.
 *
 * Return:
 *      0 for success, any other value error.
 */
static int __init n_tracerouter_init(void)
{
        int retval;

        tr_data = kzalloc(sizeof(struct tracerouter_data), GFP_KERNEL);
        if (tr_data == NULL)
                return -ENOMEM;


        /* Note N_TRACEROUTER is defined in linux/tty.h */
        retval = tty_register_ldisc(N_TRACEROUTER, &tty_ptirouter_ldisc);
        if (retval < 0) {
                pr_err("%s: Registration failed: %d\n", __func__, retval);
                kfree(tr_data);
        }
        return retval;
}

/**
 * n_tracerouter_exit - module unload
 *
 * Removes this module as a line discipline driver.
 */
static void __exit n_tracerouter_exit(void)
{
        int retval = tty_unregister_ldisc(N_TRACEROUTER);

        if (retval < 0)
                pr_err("%s: Unregistration failed: %d\n", __func__,  retval);
        else
                kfree(tr_data);
}

module_init(n_tracerouter_init);
module_exit(n_tracerouter_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Jay Freyensee");
MODULE_ALIAS_LDISC(N_TRACEROUTER);
MODULE_DESCRIPTION("Trace router ldisc driver");