Kernel documentation for the PTI feature.

This provides Kernel documentation for the PTI
feature and setting line discipline drivers
on top of tty's for Linux mobile solutions.
Signed-off-by: NJ Freyensee <james_p_freyensee@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

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;