n_gsm: add a documentation

* this documentation gives some details on how to get the n_gsm
line discipline to work with modems supporting 07.10 basic option.

* it was tested on Telit and Simcom modems.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/Documentation/serial/n_gsm.txt b/Documentation/serial/n_gsm.txt
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+n_gsm.c GSM 0710 tty multiplexor HOWTO
+===================================================
+
+This line discipline implements the GSM 07.10 multiplexing protocol
+detailed in the following 3GPP document :
+http://www.3gpp.org/ftp/Specs/archive/07_series/07.10/0710-720.zip
+
+This document give some hints on how to use this driver with GPRS and 3G
+modems connected to a physical serial port.
+
+How to use it
+-------------
+1- initialize the modem in 0710 mux mode (usually AT+CMUX= command) through
+its serial port. Depending on the modem used, you can pass more or less
+parameters to this command,
+2- switch the serial line to using the n_gsm line discipline by using
+TIOCSETD ioctl,
+3- configure the mux using GSMIOC_GETCONF / GSMIOC_SETCONF ioctl,
+
+Major parts of the initialization program :
+(a good starting point is util-linux-ng/sys-utils/ldattach.c)
+#include <linux/gsmmux.h>
+#define N_GSM0710      21      /* GSM 0710 Mux */
+#define DEFAULT_SPEED  B115200
+#define SERIAL_PORT    /dev/ttyS0
+
+       int ldisc = N_GSM0710;
+       struct gsm_config c;
+       struct termios configuration;
+
+       /* open the serial port connected to the modem */
+       fd = open(SERIAL_PORT, O_RDWR | O_NOCTTY | O_NDELAY);
+
+       /* configure the serial port : speed, flow control ... */
+
+       /* send the AT commands to switch the modem to CMUX mode
+          and check that it's succesful (should return OK) */
+       write(fd, "AT+CMUX=0\r", 10);
+
+       /* experience showed that some modems need some time before
+          being able to answer to the first MUX packet so a delay
+          may be needed here in some case */
+       sleep(3);
+
+       /* use n_gsm line discipline */
+       ioctl(fd, TIOCSETD, &ldisc);
+
+       /* get n_gsm configuration */
+       ioctl(fd, GSMIOC_GETCONF, &c);
+       /* we are initiator and need encoding 0 (basic) */
+       c.initiator = 1;
+       c.encapsulation = 0;
+       /* our modem defaults to a maximum size of 127 bytes */
+       c.mru = 127;
+       c.mtu = 127;
+       /* set the new configuration */
+       ioctl(fd, GSMIOC_SETCONF, &c);
+
+       /* and wait for ever to keep the line discipline enabled */
+       daemon(0,0);
+       pause();
+
+4- create the devices corresponding to the "virtual" serial ports (take care,
+each modem has its configuration and some DLC have dedicated functions,
+for example GPS), starting with minor 1 (DLC0 is reserved for the management
+of the mux)
+
+MAJOR=`cat /proc/devices |grep gsmtty | awk '{print $1}`
+for i in `seq 1 4`; do
+       mknod /dev/ttygsm$i c $MAJOR $i
+done
+
+5- use these devices as plain serial ports.
+for example, it's possible :
+- and to use gnokii to send / receive SMS on ttygsm1
+- to use ppp to establish a datalink on ttygsm2
+
+6- first close all virtual ports before closing the physical port.
+
+Additional Documentation
+------------------------
+More practical details on the protocol and how it's supported by industrial
+modems can be found in the following documents :
+http://www.telit.com/module/infopool/download.php?id=616
+http://www.u-blox.com/images/downloads/Product_Docs/LEON-G100-G200-MuxImplementation_ApplicationNote_%28GSM%20G1-CS-10002%29.pdf
+http://www.sierrawireless.com/Support/Downloads/AirPrime/WMP_Series/~/media/Support_Downloads/AirPrime/Application_notes/CMUX_Feature_Application_Note-Rev004.ashx
+http://wm.sim.com/sim/News/photo/2010721161442.pdf
+
+11-03-08 - Eric Bénard - <eric@eukrea.com>