linux/drivers/tty/n_tracesink.c
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   1/*
   2 *  n_tracesink.c - Trace data router and sink path through tty space.
   3 *
   4 *  Copyright (C) Intel 2011
   5 *
   6 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   7 *
   8 *  This program is free software; you can redistribute it and/or modify
   9 *  it under the terms of the GNU General Public License version 2
  10 *  as published by the Free Software Foundation.
  11 *
  12 *  This program is distributed in the hope that it will be useful,
  13 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 *  GNU General Public License for more details.
  16 *
  17 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  18 *
  19 * The trace sink uses the Linux line discipline framework to receive
  20 * trace data coming from the PTI source line discipline driver
  21 * to a user-desired tty port, like USB.
  22 * This is to provide a way to extract modem trace data on
  23 * devices that do not have a PTI HW module, or just need modem
  24 * trace data to come out of a different HW output port.
  25 * This is part of a solution for the P1149.7, compact JTAG, standard.
  26 */
  27
  28#include <linux/init.h>
  29#include <linux/kernel.h>
  30#include <linux/module.h>
  31#include <linux/types.h>
  32#include <linux/ioctl.h>
  33#include <linux/tty.h>
  34#include <linux/tty_ldisc.h>
  35#include <linux/errno.h>
  36#include <linux/string.h>
  37#include <asm-generic/bug.h>
  38#include "n_tracesink.h"
  39
  40/*
  41 * Other ldisc drivers use 65536 which basically means,
  42 * 'I can always accept 64k' and flow control is off.
  43 * This number is deemed appropriate for this driver.
  44 */
  45#define RECEIVE_ROOM    65536
  46#define DRIVERNAME      "n_tracesink"
  47
  48/*
  49 * there is a quirk with this ldisc is he can write data
  50 * to a tty from anyone calling his kernel API, which
  51 * meets customer requirements in the drivers/misc/pti.c
  52 * project.  So he needs to know when he can and cannot write when
  53 * the API is called. In theory, the API can be called
  54 * after an init() but before a successful open() which
  55 * would crash the system if tty is not checked.
  56 */
  57static struct tty_struct *this_tty;
  58static DEFINE_MUTEX(writelock);
  59
  60/**
  61 * n_tracesink_open() - Called when a tty is opened by a SW entity.
  62 * @tty: terminal device to the ldisc.
  63 *
  64 * Return:
  65 *      0 for success,
  66 *      -EFAULT = couldn't get a tty kref n_tracesink will sit
  67 *       on top of
  68 *      -EEXIST = open() called successfully once and it cannot
  69 *      be called again.
  70 *
  71 * Caveats: open() should only be successful the first time a
  72 * SW entity calls it.
  73 */
  74static int n_tracesink_open(struct tty_struct *tty)
  75{
  76        int retval = -EEXIST;
  77
  78        mutex_lock(&writelock);
  79        if (this_tty == NULL) {
  80                this_tty = tty_kref_get(tty);
  81                if (this_tty == NULL) {
  82                        retval = -EFAULT;
  83                } else {
  84                        tty->disc_data = this_tty;
  85                        tty_driver_flush_buffer(tty);
  86                        retval = 0;
  87                }
  88        }
  89        mutex_unlock(&writelock);
  90
  91        return retval;
  92}
  93
  94/**
  95 * n_tracesink_close() - close connection
  96 * @tty: terminal device to the ldisc.
  97 *
  98 * Called when a software entity wants to close a connection.
  99 */
 100static void n_tracesink_close(struct tty_struct *tty)
 101{
 102        mutex_lock(&writelock);
 103        tty_driver_flush_buffer(tty);
 104        tty_kref_put(this_tty);
 105        this_tty = NULL;
 106        tty->disc_data = NULL;
 107        mutex_unlock(&writelock);
 108}
 109
 110/**
 111 * n_tracesink_read() - read request from user space
 112 * @tty:  terminal device passed into the ldisc.
 113 * @file: pointer to open file object.
 114 * @buf:  pointer to the data buffer that gets eventually returned.
 115 * @nr:   number of bytes of the data buffer that is returned.
 116 *
 117 * function that allows read() functionality in userspace. By default if this
 118 * is not implemented it returns -EIO. This module is functioning like a
 119 * router via n_tracesink_receivebuf(), and there is no real requirement
 120 * to implement this function. However, an error return value other than
 121 * -EIO should be used just to show that there was an intent not to have
 122 * this function implemented.  Return value based on read() man pages.
 123 *
 124 * Return:
 125 *       -EINVAL
 126 */
 127static ssize_t n_tracesink_read(struct tty_struct *tty, struct file *file,
 128                                unsigned char __user *buf, size_t nr) {
 129        return -EINVAL;
 130}
 131
 132/**
 133 * n_tracesink_write() - Function that allows write() in userspace.
 134 * @tty:  terminal device passed into the ldisc.
 135 * @file: pointer to open file object.
 136 * @buf:  pointer to the data buffer that gets eventually returned.
 137 * @nr:   number of bytes of the data buffer that is returned.
 138 *
 139 * By default if this is not implemented, it returns -EIO.
 140 * This should not be implemented, ever, because
 141 * 1. this driver is functioning like a router via
 142 *    n_tracesink_receivebuf()
 143 * 2. No writes to HW will ever go through this line discpline driver.
 144 * However, an error return value other than -EIO should be used
 145 * just to show that there was an intent not to have this function
 146 * implemented.  Return value based on write() man pages.
 147 *
 148 * Return:
 149 *      -EINVAL
 150 */
 151static ssize_t n_tracesink_write(struct tty_struct *tty, struct file *file,
 152                                 const unsigned char *buf, size_t nr) {
 153        return -EINVAL;
 154}
 155
 156/**
 157 * n_tracesink_datadrain() - Kernel API function used to route
 158 *                           trace debugging data to user-defined
 159 *                           port like USB.
 160 *
 161 * @buf:   Trace debuging data buffer to write to tty target
 162 *         port. Null value will return with no write occurring.
 163 * @count: Size of buf. Value of 0 or a negative number will
 164 *         return with no write occuring.
 165 *
 166 * Caveat: If this line discipline does not set the tty it sits
 167 * on top of via an open() call, this API function will not
 168 * call the tty's write() call because it will have no pointer
 169 * to call the write().
 170 */
 171void n_tracesink_datadrain(u8 *buf, int count)
 172{
 173        mutex_lock(&writelock);
 174
 175        if ((buf != NULL) && (count > 0) && (this_tty != NULL))
 176                this_tty->ops->write(this_tty, buf, count);
 177
 178        mutex_unlock(&writelock);
 179}
 180EXPORT_SYMBOL_GPL(n_tracesink_datadrain);
 181
 182/*
 183 * Flush buffer is not impelemented as the ldisc has no internal buffering
 184 * so the tty_driver_flush_buffer() is sufficient for this driver's needs.
 185 */
 186
 187/*
 188 * tty_ldisc function operations for this driver.
 189 */
 190static struct tty_ldisc_ops tty_n_tracesink = {
 191        .owner          = THIS_MODULE,
 192        .magic          = TTY_LDISC_MAGIC,
 193        .name           = DRIVERNAME,
 194        .open           = n_tracesink_open,
 195        .close          = n_tracesink_close,
 196        .read           = n_tracesink_read,
 197        .write          = n_tracesink_write
 198};
 199
 200/**
 201 * n_tracesink_init-    module initialisation
 202 *
 203 * Registers this module as a line discipline driver.
 204 *
 205 * Return:
 206 *      0 for success, any other value error.
 207 */
 208static int __init n_tracesink_init(void)
 209{
 210        /* Note N_TRACESINK is defined in linux/tty.h */
 211        int retval = tty_register_ldisc(N_TRACESINK, &tty_n_tracesink);
 212
 213        if (retval < 0)
 214                pr_err("%s: Registration failed: %d\n", __func__, retval);
 215
 216        return retval;
 217}
 218
 219/**
 220 * n_tracesink_exit -   module unload
 221 *
 222 * Removes this module as a line discipline driver.
 223 */
 224static void __exit n_tracesink_exit(void)
 225{
 226        int retval = tty_unregister_ldisc(N_TRACESINK);
 227
 228        if (retval < 0)
 229                pr_err("%s: Unregistration failed: %d\n", __func__,  retval);
 230}
 231
 232module_init(n_tracesink_init);
 233module_exit(n_tracesink_exit);
 234
 235MODULE_LICENSE("GPL");
 236MODULE_AUTHOR("Jay Freyensee");
 237MODULE_ALIAS_LDISC(N_TRACESINK);
 238MODULE_DESCRIPTION("Trace sink ldisc driver");
 239
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