1// SPDX-License-Identifier: GPL-2.0
2/*
3 * n_tracesink.c - Trace data router and sink path through tty space.
4 *
5 * Copyright (C) Intel 2011
6 *
7 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8 *
9 * The trace sink uses the Linux line discipline framework to receive
10 * trace data coming from the PTI source line discipline driver
11 * to a user-desired tty port, like USB.
12 * This is to provide a way to extract modem trace data on
13 * devices that do not have a PTI HW module, or just need modem
14 * trace data to come out of a different HW output port.
15 * This is part of a solution for the P1149.7, compact JTAG, standard.
16 */
17
18#include <linux/init.h>
19#include <linux/kernel.h>
20#include <linux/module.h>
21#include <linux/types.h>
22#include <linux/ioctl.h>
23#include <linux/tty.h>
24#include <linux/tty_ldisc.h>
25#include <linux/errno.h>
26#include <linux/string.h>
27#include <linux/bug.h>
28#include "n_tracesink.h"
29
30/*
31 * Other ldisc drivers use 65536 which basically means,
32 * 'I can always accept 64k' and flow control is off.
33 * This number is deemed appropriate for this driver.
34 */
35#define RECEIVE_ROOM 65536
36#define DRIVERNAME "n_tracesink"
37
38/*
39 * there is a quirk with this ldisc is he can write data
40 * to a tty from anyone calling his kernel API, which
41 * meets customer requirements in the drivers/misc/pti.c
42 * project. So he needs to know when he can and cannot write when
43 * the API is called. In theory, the API can be called
44 * after an init() but before a successful open() which
45 * would crash the system if tty is not checked.
46 */
47static struct tty_struct *this_tty;
48static DEFINE_MUTEX(writelock);
49
50/**
51 * n_tracesink_open() - Called when a tty is opened by a SW entity.
52 * @tty: terminal device to the ldisc.
53 *
54 * Return:
55 * 0 for success,
56 * -EFAULT = couldn't get a tty kref n_tracesink will sit
57 * on top of
58 * -EEXIST = open() called successfully once and it cannot
59 * be called again.
60 *
61 * Caveats: open() should only be successful the first time a
62 * SW entity calls it.
63 */
64static int n_tracesink_open(struct tty_struct *tty)
65{
66 int retval = -EEXIST;
67
68 mutex_lock(&writelock);
69 if (this_tty == NULL) {
70 this_tty = tty_kref_get(tty);
71 if (this_tty == NULL) {
72 retval = -EFAULT;
73 } else {
74 tty->disc_data = this_tty;
75 tty_driver_flush_buffer(tty);
76 retval = 0;
77 }
78 }
79 mutex_unlock(&writelock);
80
81 return retval;
82}
83
84/**
85 * n_tracesink_close() - close connection
86 * @tty: terminal device to the ldisc.
87 *
88 * Called when a software entity wants to close a connection.
89 */
90static void n_tracesink_close(struct tty_struct *tty)
91{
92 mutex_lock(&writelock);
93 tty_driver_flush_buffer(tty);
94 tty_kref_put(this_tty);
95 this_tty = NULL;
96 tty->disc_data = NULL;
97 mutex_unlock(&writelock);
98}
99
100/**
101 * n_tracesink_read() - read request from user space
102 * @tty: terminal device passed into the ldisc.
103 * @file: pointer to open file object.
104 * @buf: pointer to the data buffer that gets eventually returned.
105 * @nr: number of bytes of the data buffer that is returned.
106 *
107 * function that allows read() functionality in userspace. By default if this
108 * is not implemented it returns -EIO. This module is functioning like a
109 * router via n_tracesink_receivebuf(), and there is no real requirement
110 * to implement this function. However, an error return value other than
111 * -EIO should be used just to show that there was an intent not to have
112 * this function implemented. Return value based on read() man pages.
113 *
114 * Return:
115 * -EINVAL
116 */
117static ssize_t n_tracesink_read(struct tty_struct *tty, struct file *file,
118 unsigned char __user *buf, size_t nr) {
119 return -EINVAL;
120}
121
122/**
123 * n_tracesink_write() - Function that allows write() in userspace.
124 * @tty: terminal device passed into the ldisc.
125 * @file: pointer to open file object.
126 * @buf: pointer to the data buffer that gets eventually returned.
127 * @nr: number of bytes of the data buffer that is returned.
128 *
129 * By default if this is not implemented, it returns -EIO.
130 * This should not be implemented, ever, because
131 * 1. this driver is functioning like a router via
132 * n_tracesink_receivebuf()
133 * 2. No writes to HW will ever go through this line discpline driver.
134 * However, an error return value other than -EIO should be used
135 * just to show that there was an intent not to have this function
136 * implemented. Return value based on write() man pages.
137 *
138 * Return:
139 * -EINVAL
140 */
141static ssize_t n_tracesink_write(struct tty_struct *tty, struct file *file,
142 const unsigned char *buf, size_t nr) {
143 return -EINVAL;
144}
145
146/**
147 * n_tracesink_datadrain() - Kernel API function used to route
148 * trace debugging data to user-defined
149 * port like USB.
150 *
151 * @buf: Trace debuging data buffer to write to tty target
152 * port. Null value will return with no write occurring.
153 * @count: Size of buf. Value of 0 or a negative number will
154 * return with no write occuring.
155 *
156 * Caveat: If this line discipline does not set the tty it sits
157 * on top of via an open() call, this API function will not
158 * call the tty's write() call because it will have no pointer
159 * to call the write().
160 */
161void n_tracesink_datadrain(u8 *buf, int count)
162{
163 mutex_lock(&writelock);
164
165 if ((buf != NULL) && (count > 0) && (this_tty != NULL))
166 this_tty->ops->write(this_tty, buf, count);
167
168 mutex_unlock(&writelock);
169}
170EXPORT_SYMBOL_GPL(n_tracesink_datadrain);
171
172/*
173 * Flush buffer is not impelemented as the ldisc has no internal buffering
174 * so the tty_driver_flush_buffer() is sufficient for this driver's needs.
175 */
176
177/*
178 * tty_ldisc function operations for this driver.
179 */
180static struct tty_ldisc_ops tty_n_tracesink = {
181 .owner = THIS_MODULE,
182 .magic = TTY_LDISC_MAGIC,
183 .name = DRIVERNAME,
184 .open = n_tracesink_open,
185 .close = n_tracesink_close,
186 .read = n_tracesink_read,
187 .write = n_tracesink_write
188};
189
190/**
191 * n_tracesink_init- module initialisation
192 *
193 * Registers this module as a line discipline driver.
194 *
195 * Return:
196 * 0 for success, any other value error.
197 */
198static int __init n_tracesink_init(void)
199{
200 /* Note N_TRACESINK is defined in linux/tty.h */
201 int retval = tty_register_ldisc(N_TRACESINK, &tty_n_tracesink);
202
203 if (retval < 0)
204 pr_err("%s: Registration failed: %d\n", __func__, retval);
205
206 return retval;
207}
208
209/**
210 * n_tracesink_exit - module unload
211 *
212 * Removes this module as a line discipline driver.
213 */
214static void __exit n_tracesink_exit(void)
215{
216 int retval = tty_unregister_ldisc(N_TRACESINK);
217
218 if (retval < 0)
219 pr_err("%s: Unregistration failed: %d\n", __func__, retval);
220}
221
222module_init(n_tracesink_init);
223module_exit(n_tracesink_exit);
224
225MODULE_LICENSE("GPL");
226MODULE_AUTHOR("Jay Freyensee");
227MODULE_ALIAS_LDISC(N_TRACESINK);
228MODULE_DESCRIPTION("Trace sink ldisc driver");
229