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");