linux-next/Documentation/pti/pti_intel_mid.txt

The Intel MID PTI project is HW implemented in Intel Atom
system-on-a-chip designs based on the Parallel Trace
Interface for MIPI P1149.7 cJTAG standard.  The kernel solution
for this platform involves the following files:

./include/linux/pti.h
./drivers/.../n_tracesink.h
./drivers/.../n_tracerouter.c
./drivers/.../n_tracesink.c
./drivers/.../pti.c

pti.c is the driver that enables various debugging features
popular on platforms from certain mobile manufacturers.
n_tracerouter.c and n_tracesink.c allow extra system information to
be collected and routed to the pti driver, such as trace
debugging data from a modem.  Although n_tracerouter
and n_tracesink are a part of the complete PTI solution,
these two line disciplines can work separately from
pti.c and route any data stream from one /dev/tty node
to another /dev/tty node via kernel-space.  This provides
a stable, reliable connection that will not break unless
the user-space application shuts down (plus avoids
kernel->user->kernel context switch overheads of routing
data).

An example debugging usage for this driver system:
   *Hook /dev/ttyPTI0 to syslogd.  Opening this port will also start
    a console device to further capture debugging messages to PTI.
   *Hook /dev/ttyPTI1 to modem debugging data to write to PTI HW.
    This is where n_tracerouter and n_tracesink are used.
   *Hook /dev/pti to a user-level debugging application for writing
    to PTI HW.
   *Use mipi_* Kernel Driver API in other device drivers for
    debugging to PTI by first requesting a PTI write address via
    mipi_request_masterchannel(1).

Below is example pseudo-code on how a 'privileged' application
can hook up n_tracerouter and n_tracesink to any tty on
a system.  'Privileged' means the application has enough
privileges to successfully manipulate the ldisc drivers
but is not just blindly executing as 'root'. Keep in mind
the use of ioctl(,TIOCSETD,) is not specific to the n_tracerouter
and n_tracesink line discpline drivers but is a generic
operation for a program to use a line discpline driver
on a tty port other than the default n_tty.

/////////// To hook up n_tracerouter and n_tracesink /////////

// Note that n_tracerouter depends on n_tracesink.
#include <errno.h>
#define ONE_TTY "/dev/ttyOne"
#define TWO_TTY "/dev/ttyTwo"

// needed global to hand onto ldisc connection
static int g_fd_source = -1;
static int g_fd_sink  = -1;

// these two vars used to grab LDISC values from loaded ldisc drivers
// in OS.  Look at /proc/tty/ldiscs to get the right numbers from
// the ldiscs loaded in the system.
int source_ldisc_num, sink_ldisc_num = -1;
int retval;

g_fd_source = open(ONE_TTY, O_RDWR); // must be R/W
g_fd_sink   = open(TWO_TTY, O_RDWR); // must be R/W

if (g_fd_source <= 0) || (g_fd_sink <= 0) {
   // doubt you'll want to use these exact error lines of code
   printf("Error on open(). errno: %d\n",errno);
   return errno;
}

retval = ioctl(g_fd_sink, TIOCSETD, &sink_ldisc_num);
if (retval < 0) {
   printf("Error on ioctl().  errno: %d\n", errno);
   return errno;
}

retval = ioctl(g_fd_source, TIOCSETD, &source_ldisc_num);
if (retval < 0) {
   printf("Error on ioctl().  errno: %d\n", errno);
   return errno;
}

/////////// To disconnect n_tracerouter and n_tracesink ////////

// First make sure data through the ldiscs has stopped.

// Second, disconnect ldiscs.  This provides a
// little cleaner shutdown on tty stack.
sink_ldisc_num = 0;
source_ldisc_num = 0;
ioctl(g_fd_uart, TIOCSETD, &sink_ldisc_num);
ioctl(g_fd_gadget, TIOCSETD, &source_ldisc_num);

// Three, program closes connection, and cleanup:
close(g_fd_uart);
close(g_fd_gadget);
g_fd_uart = g_fd_gadget = NULL;