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+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+Intel MID PTI
+=============
+
+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;