Kernel documentation for the PTI feature.

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;