Root/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 */
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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