drivers/tty/n_tracesink.c

   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  */
  57 static struct tty_struct *this_tty;
  58 static 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  */
  74 static 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  */
 100 static 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  */
 127 static 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  */
 151 static 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  */
 171 void 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 }
 180 EXPORT_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  */
 190 static 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  */
 208 static 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  */
 224 static 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
 232 module_init(n_tracesink_init);
 233 module_exit(n_tracesink_exit);
 234
 235 MODULE_LICENSE("GPL");
 236 MODULE_AUTHOR("Jay Freyensee");
 237 MODULE_ALIAS_LDISC(N_TRACESINK);
 238 MODULE_DESCRIPTION("Trace sink ldisc driver");