n_tracesink.c

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/*
 *  n_tracesink.c - Trace data router and sink path 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.
 *
 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 *
 * The trace sink uses the Linux line discipline framework to receive
 * trace data coming from the PTI source line discipline driver
 * to a user-desired tty port, like USB.
 * This is to provide a way to extract modem trace data on
 * devices that do not have a PTI HW module, or just need modem
 * trace data to come out of a different HW output port.
 * 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 <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_tracesink"

/*
 * there is a quirk with this ldisc is he can write data
 * to a tty from anyone calling his kernel API, which
 * meets customer requirements in the drivers/misc/pti.c
 * project.  So he needs to know when he can and cannot write when
 * the API is called. In theory, the API can be called
 * after an init() but before a successful open() which
 * would crash the system if tty is not checked.
 */
static struct tty_struct *this_tty;
static DEFINE_MUTEX(writelock);

static int n_tracesink_open(struct tty_struct *tty)
{
    int retval = -EEXIST;

    mutex_lock(&writelock);
    if (this_tty == NULL) {
        this_tty = tty_kref_get(tty);
        if (this_tty == NULL) {
            retval = -EFAULT;
        } else {
            tty->disc_data = this_tty;
            tty_driver_flush_buffer(tty);
            retval = 0;
        }
    }
    mutex_unlock(&writelock);

    return retval;
}

static void n_tracesink_close(struct tty_struct *tty)
{
    mutex_lock(&writelock);
    tty_driver_flush_buffer(tty);
    tty_kref_put(this_tty);
    this_tty = NULL;
    tty->disc_data = NULL;
    mutex_unlock(&writelock);
}

static ssize_t n_tracesink_read(struct tty_struct *tty, struct file *file,
                unsigned char __user *buf, size_t nr) {
    return -EINVAL;
}

static ssize_t n_tracesink_write(struct tty_struct *tty, struct file *file,
                 const unsigned char *buf, size_t nr) {
    return -EINVAL;
}

void n_tracesink_datadrain(u8 *buf, int count)
{
    mutex_lock(&writelock);

    if ((buf != NULL) && (count > 0) && (this_tty != NULL))
        this_tty->ops->write(this_tty, buf, count);

    mutex_unlock(&writelock);
}
EXPORT_SYMBOL_GPL(n_tracesink_datadrain);

/*
 * 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.
 */

/*
 * tty_ldisc function operations for this driver.
 */
static struct tty_ldisc_ops tty_n_tracesink = {
    .owner      = THIS_MODULE,
    .magic      = TTY_LDISC_MAGIC,
    .name       = DRIVERNAME,
    .open       = n_tracesink_open,
    .close      = n_tracesink_close,
    .read       = n_tracesink_read,
    .write      = n_tracesink_write
};

static int __init n_tracesink_init(void)
{
    /* Note N_TRACESINK is defined in linux/tty.h */
    int retval = tty_register_ldisc(N_TRACESINK, &tty_n_tracesink);

    if (retval < 0)
        pr_err("%s: Registration failed: %d\n", __func__, retval);

    return retval;
}

static void __exit n_tracesink_exit(void)
{
    int retval = tty_unregister_ldisc(N_TRACESINK);

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

module_init(n_tracesink_init);
module_exit(n_tracesink_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Jay Freyensee");
MODULE_ALIAS_LDISC(N_TRACESINK);
MODULE_DESCRIPTION("Trace sink ldisc driver");