1// SPDX-License-Identifier: GPL-2.0
2/*
3 *  n_tracerouter.c - Trace data router through tty space
4 *
5 *  Copyright (C) Intel 2011
6 *
7 * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8 *
9 * This trace router uses the Linux line discipline framework to route
10 * trace data coming from a HW Modem to a PTI (Parallel Trace Module) port.
11 * The solution is not specific to a HW modem and this line disciple can
12 * be used to route any stream of data in kernel space.
13 * This is part of a solution for the P1149.7, compact JTAG, standard.
14 */
15
16#include <linux/init.h>
17#include <linux/kernel.h>
18#include <linux/module.h>
19#include <linux/types.h>
20#include <linux/ioctl.h>
21#include <linux/tty.h>
22#include <linux/tty_ldisc.h>
23#include <linux/errno.h>
24#include <linux/string.h>
25#include <linux/mutex.h>
26#include <linux/slab.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_tracerouter"
37
38/*
39 * struct to hold private configuration data for this ldisc.
40 * opencalled is used to hold if this ldisc has been opened.
41 * kref_tty holds the tty reference the ldisc sits on top of.
42 */
43struct tracerouter_data {
44	u8 opencalled;
45	struct tty_struct *kref_tty;
46};
47static struct tracerouter_data *tr_data;
48
49/* lock for when tty reference is being used */
50static DEFINE_MUTEX(routelock);
51
52/**
53 * n_tracerouter_open() - Called when a tty is opened by a SW entity.
54 * @tty: terminal device to the ldisc.
55 *
56 * Return:
57 *      0 for success.
58 *
59 * Caveats: This should only be opened one time per SW entity.
60 */
61static int n_tracerouter_open(struct tty_struct *tty)
62{
63	int retval = -EEXIST;
64
65	mutex_lock(&routelock);
66	if (tr_data->opencalled == 0) {
67
68		tr_data->kref_tty = tty_kref_get(tty);
69		if (tr_data->kref_tty == NULL) {
70			retval = -EFAULT;
71		} else {
72			tr_data->opencalled = 1;
73			tty->disc_data      = tr_data;
74			tty->receive_room   = RECEIVE_ROOM;
75			tty_driver_flush_buffer(tty);
76			retval = 0;
77		}
78	}
79	mutex_unlock(&routelock);
80	return retval;
81}
82
83/**
84 * n_tracerouter_close() - close connection
85 * @tty: terminal device to the ldisc.
86 *
87 * Called when a software entity wants to close a connection.
88 */
89static void n_tracerouter_close(struct tty_struct *tty)
90{
91	struct tracerouter_data *tptr = tty->disc_data;
92
93	mutex_lock(&routelock);
94	WARN_ON(tptr->kref_tty != tr_data->kref_tty);
95	tty_driver_flush_buffer(tty);
96	tty_kref_put(tr_data->kref_tty);
97	tr_data->kref_tty = NULL;
98	tr_data->opencalled = 0;
99	tty->disc_data = NULL;
100	mutex_unlock(&routelock);
101}
102
103/**
104 * n_tracerouter_read() - read request from user space
105 * @tty:  terminal device passed into the ldisc.
106 * @file: pointer to open file object.
107 * @buf:  pointer to the data buffer that gets eventually returned.
108 * @nr:   number of bytes of the data buffer that is returned.
109 *
110 * function that allows read() functionality in userspace. By default if this
111 * is not implemented it returns -EIO. This module is functioning like a
112 * router via n_tracerouter_receivebuf(), and there is no real requirement
113 * to implement this function. However, an error return value other than
114 * -EIO should be used just to show that there was an intent not to have
115 * this function implemented.  Return value based on read() man pages.
116 *
117 * Return:
118 *	 -EINVAL
119 */
120static ssize_t n_tracerouter_read(struct tty_struct *tty, struct file *file,
121				  unsigned char *buf, size_t nr,
122				  void **cookie, unsigned long offset)
123{
124	return -EINVAL;
125}
126
127/**
128 * n_tracerouter_write() - Function that allows write() in userspace.
129 * @tty:  terminal device passed into the ldisc.
130 * @file: pointer to open file object.
131 * @buf:  pointer to the data buffer that gets eventually returned.
132 * @nr:   number of bytes of the data buffer that is returned.
133 *
134 * By default if this is not implemented, it returns -EIO.
135 * This should not be implemented, ever, because
136 * 1. this driver is functioning like a router via
137 *    n_tracerouter_receivebuf()
138 * 2. No writes to HW will ever go through this line discpline driver.
139 * However, an error return value other than -EIO should be used
140 * just to show that there was an intent not to have this function
141 * implemented.  Return value based on write() man pages.
142 *
143 * Return:
144 *	-EINVAL
145 */
146static ssize_t n_tracerouter_write(struct tty_struct *tty, struct file *file,
147				   const unsigned char *buf, size_t nr) {
148	return -EINVAL;
149}
150
151/**
152 * n_tracerouter_receivebuf() - Routing function for driver.
153 * @tty: terminal device passed into the ldisc.  It's assumed
154 *       tty will never be NULL.
155 * @cp:  buffer, block of characters to be eventually read by
156 *       someone, somewhere (user read() call or some kernel function).
157 * @fp:  flag buffer.
158 * @count: number of characters (aka, bytes) in cp.
159 *
160 * This function takes the input buffer, cp, and passes it to
161 * an external API function for processing.
162 */
163static void n_tracerouter_receivebuf(struct tty_struct *tty,
164					const unsigned char *cp,
165					char *fp, int count)
166{
167	mutex_lock(&routelock);
168	n_tracesink_datadrain((u8 *) cp, count);
169	mutex_unlock(&routelock);
170}
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
177static struct tty_ldisc_ops tty_ptirouter_ldisc = {
178	.owner		= THIS_MODULE,
179	.magic		= TTY_LDISC_MAGIC,
180	.name		= DRIVERNAME,
181	.open		= n_tracerouter_open,
182	.close		= n_tracerouter_close,
183	.read		= n_tracerouter_read,
184	.write		= n_tracerouter_write,
185	.receive_buf	= n_tracerouter_receivebuf
186};
187
188/**
189 * n_tracerouter_init -	module initialisation
190 *
191 * Registers this module as a line discipline driver.
192 *
193 * Return:
194 *	0 for success, any other value error.
195 */
196static int __init n_tracerouter_init(void)
197{
198	int retval;
199
200	tr_data = kzalloc(sizeof(struct tracerouter_data), GFP_KERNEL);
201	if (tr_data == NULL)
202		return -ENOMEM;
203
204
205	/* Note N_TRACEROUTER is defined in linux/tty.h */
206	retval = tty_register_ldisc(N_TRACEROUTER, &tty_ptirouter_ldisc);
207	if (retval < 0) {
208		pr_err("%s: Registration failed: %d\n", __func__, retval);
209		kfree(tr_data);
210	}
211	return retval;
212}
213
214/**
215 * n_tracerouter_exit -	module unload
216 *
217 * Removes this module as a line discipline driver.
218 */
219static void __exit n_tracerouter_exit(void)
220{
221	int retval = tty_unregister_ldisc(N_TRACEROUTER);
222
223	if (retval < 0)
224		pr_err("%s: Unregistration failed: %d\n", __func__,  retval);
225	else
226		kfree(tr_data);
227}
228
229module_init(n_tracerouter_init);
230module_exit(n_tracerouter_exit);
231
232MODULE_LICENSE("GPL");
233MODULE_AUTHOR("Jay Freyensee");
234MODULE_ALIAS_LDISC(N_TRACEROUTER);
235MODULE_DESCRIPTION("Trace router ldisc driver");
236