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 *buf, size_t nr,
119				void **cookie, unsigned long offset)
120{
121	return -EINVAL;
122}
123
124/**
125 * n_tracesink_write() - Function that allows write() in userspace.
126 * @tty:  terminal device passed into the ldisc.
127 * @file: pointer to open file object.
128 * @buf:  pointer to the data buffer that gets eventually returned.
129 * @nr:   number of bytes of the data buffer that is returned.
130 *
131 * By default if this is not implemented, it returns -EIO.
132 * This should not be implemented, ever, because
133 * 1. this driver is functioning like a router via
134 *    n_tracesink_receivebuf()
135 * 2. No writes to HW will ever go through this line discpline driver.
136 * However, an error return value other than -EIO should be used
137 * just to show that there was an intent not to have this function
138 * implemented.  Return value based on write() man pages.
139 *
140 * Return:
141 *	-EINVAL
142 */
143static ssize_t n_tracesink_write(struct tty_struct *tty, struct file *file,
144				 const unsigned char *buf, size_t nr) {
145	return -EINVAL;
146}
147
148/**
149 * n_tracesink_datadrain() - Kernel API function used to route
150 *			     trace debugging data to user-defined
151 *			     port like USB.
152 *
153 * @buf:   Trace debuging data buffer to write to tty target
154 *         port. Null value will return with no write occurring.
155 * @count: Size of buf. Value of 0 or a negative number will
156 *         return with no write occuring.
157 *
158 * Caveat: If this line discipline does not set the tty it sits
159 * on top of via an open() call, this API function will not
160 * call the tty's write() call because it will have no pointer
161 * to call the write().
162 */
163void n_tracesink_datadrain(u8 *buf, int count)
164{
165	mutex_lock(&writelock);
166
167	if ((buf != NULL) && (count > 0) && (this_tty != NULL))
168		this_tty->ops->write(this_tty, buf, count);
169
170	mutex_unlock(&writelock);
171}
172EXPORT_SYMBOL_GPL(n_tracesink_datadrain);
173
174/*
175 * Flush buffer is not impelemented as the ldisc has no internal buffering
176 * so the tty_driver_flush_buffer() is sufficient for this driver's needs.
177 */
178
179/*
180 * tty_ldisc function operations for this driver.
181 */
182static struct tty_ldisc_ops tty_n_tracesink = {
183	.owner		= THIS_MODULE,
184	.magic		= TTY_LDISC_MAGIC,
185	.name		= DRIVERNAME,
186	.open		= n_tracesink_open,
187	.close		= n_tracesink_close,
188	.read		= n_tracesink_read,
189	.write		= n_tracesink_write
190};
191
192/**
193 * n_tracesink_init-	module initialisation
194 *
195 * Registers this module as a line discipline driver.
196 *
197 * Return:
198 *	0 for success, any other value error.
199 */
200static int __init n_tracesink_init(void)
201{
202	/* Note N_TRACESINK is defined in linux/tty.h */
203	int retval = tty_register_ldisc(N_TRACESINK, &tty_n_tracesink);
204
205	if (retval < 0)
206		pr_err("%s: Registration failed: %d\n", __func__, retval);
207
208	return retval;
209}
210
211/**
212 * n_tracesink_exit -	module unload
213 *
214 * Removes this module as a line discipline driver.
215 */
216static void __exit n_tracesink_exit(void)
217{
218	int retval = tty_unregister_ldisc(N_TRACESINK);
219
220	if (retval < 0)
221		pr_err("%s: Unregistration failed: %d\n", __func__,  retval);
222}
223
224module_init(n_tracesink_init);
225module_exit(n_tracesink_exit);
226
227MODULE_LICENSE("GPL");
228MODULE_AUTHOR("Jay Freyensee");
229MODULE_ALIAS_LDISC(N_TRACESINK);
230MODULE_DESCRIPTION("Trace sink ldisc driver");
231