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// SPDX-License-Identifier: GPL-2.0
/*
* n_tracesink.c - Trace data router and sink path through tty space.
*
* Copyright (C) Intel 2011
*
* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
*
* The trace sink uses the Linux line discipline framework to receive
* trace data coming from the PTI source line discipline driver
* to a user-desired tty port, like USB.
* This is to provide a way to extract modem trace data on
* devices that do not have a PTI HW module, or just need modem
* trace data to come out of a different HW output port.
* This is part of a solution for the P1149.7, compact JTAG, standard.
*/
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/ioctl.h>
#include <linux/tty.h>
#include <linux/tty_ldisc.h>
#include <linux/errno.h>
#include <linux/string.h>
#include <linux/bug.h>
#include "n_tracesink.h"
/*
* Other ldisc drivers use 65536 which basically means,
* 'I can always accept 64k' and flow control is off.
* This number is deemed appropriate for this driver.
*/
#define RECEIVE_ROOM 65536
#define DRIVERNAME "n_tracesink"
/*
* there is a quirk with this ldisc is he can write data
* to a tty from anyone calling his kernel API, which
* meets customer requirements in the drivers/misc/pti.c
* project. So he needs to know when he can and cannot write when
* the API is called. In theory, the API can be called
* after an init() but before a successful open() which
* would crash the system if tty is not checked.
*/
static struct tty_struct *this_tty;
static DEFINE_MUTEX(writelock);
/**
* n_tracesink_open() - Called when a tty is opened by a SW entity.
* @tty: terminal device to the ldisc.
*
* Return:
* 0 for success,
* -EFAULT = couldn't get a tty kref n_tracesink will sit
* on top of
* -EEXIST = open() called successfully once and it cannot
* be called again.
*
* Caveats: open() should only be successful the first time a
* SW entity calls it.
*/
static int n_tracesink_open(struct tty_struct *tty)
{
int retval = -EEXIST;
mutex_lock(&writelock);
if (this_tty == NULL) {
this_tty = tty_kref_get(tty);
if (this_tty == NULL) {
retval = -EFAULT;
} else {
tty->disc_data = this_tty;
tty_driver_flush_buffer(tty);
retval = 0;
}
}
mutex_unlock(&writelock);
return retval;
}
/**
* n_tracesink_close() - close connection
* @tty: terminal device to the ldisc.
*
* Called when a software entity wants to close a connection.
*/
static void n_tracesink_close(struct tty_struct *tty)
{
mutex_lock(&writelock);
tty_driver_flush_buffer(tty);
tty_kref_put(this_tty);
this_tty = NULL;
tty->disc_data = NULL;
mutex_unlock(&writelock);
}
/**
* n_tracesink_read() - read request from user space
* @tty: terminal device passed into the ldisc.
* @file: pointer to open file object.
* @buf: pointer to the data buffer that gets eventually returned.
* @nr: number of bytes of the data buffer that is returned.
*
* function that allows read() functionality in userspace. By default if this
* is not implemented it returns -EIO. This module is functioning like a
* router via n_tracesink_receivebuf(), and there is no real requirement
* to implement this function. However, an error return value other than
* -EIO should be used just to show that there was an intent not to have
* this function implemented. Return value based on read() man pages.
*
* Return:
* -EINVAL
*/
static ssize_t n_tracesink_read(struct tty_struct *tty, struct file *file,
unsigned char __user *buf, size_t nr) {
return -EINVAL;
}
/**
* n_tracesink_write() - Function that allows write() in userspace.
* @tty: terminal device passed into the ldisc.
* @file: pointer to open file object.
* @buf: pointer to the data buffer that gets eventually returned.
* @nr: number of bytes of the data buffer that is returned.
*
* By default if this is not implemented, it returns -EIO.
* This should not be implemented, ever, because
* 1. this driver is functioning like a router via
* n_tracesink_receivebuf()
* 2. No writes to HW will ever go through this line discpline driver.
* However, an error return value other than -EIO should be used
* just to show that there was an intent not to have this function
* implemented. Return value based on write() man pages.
*
* Return:
* -EINVAL
*/
static ssize_t n_tracesink_write(struct tty_struct *tty, struct file *file,
const unsigned char *buf, size_t nr) {
return -EINVAL;
}
/**
* n_tracesink_datadrain() - Kernel API function used to route
* trace debugging data to user-defined
* port like USB.
*
* @buf: Trace debuging data buffer to write to tty target
* port. Null value will return with no write occurring.
* @count: Size of buf. Value of 0 or a negative number will
* return with no write occuring.
*
* Caveat: If this line discipline does not set the tty it sits
* on top of via an open() call, this API function will not
* call the tty's write() call because it will have no pointer
* to call the write().
*/
void n_tracesink_datadrain(u8 *buf, int count)
{
mutex_lock(&writelock);
if ((buf != NULL) && (count > 0) && (this_tty != NULL))
this_tty->ops->write(this_tty, buf, count);
mutex_unlock(&writelock);
}
EXPORT_SYMBOL_GPL(n_tracesink_datadrain);
/*
* Flush buffer is not impelemented as the ldisc has no internal buffering
* so the tty_driver_flush_buffer() is sufficient for this driver's needs.
*/
/*
* tty_ldisc function operations for this driver.
*/
static struct tty_ldisc_ops tty_n_tracesink = {
.owner = THIS_MODULE,
.magic = TTY_LDISC_MAGIC,
.name = DRIVERNAME,
.open = n_tracesink_open,
.close = n_tracesink_close,
.read = n_tracesink_read,
.write = n_tracesink_write
};
/**
* n_tracesink_init- module initialisation
*
* Registers this module as a line discipline driver.
*
* Return:
* 0 for success, any other value error.
*/
static int __init n_tracesink_init(void)
{
/* Note N_TRACESINK is defined in linux/tty.h */
int retval = tty_register_ldisc(N_TRACESINK, &tty_n_tracesink);
if (retval < 0)
pr_err("%s: Registration failed: %d\n", __func__, retval);
return retval;
}
/**
* n_tracesink_exit - module unload
*
* Removes this module as a line discipline driver.
*/
static void __exit n_tracesink_exit(void)
{
int retval = tty_unregister_ldisc(N_TRACESINK);
if (retval < 0)
pr_err("%s: Unregistration failed: %d\n", __func__, retval);
}
module_init(n_tracesink_init);
module_exit(n_tracesink_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Jay Freyensee");
MODULE_ALIAS_LDISC(N_TRACESINK);
MODULE_DESCRIPTION("Trace sink ldisc driver");
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