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>

Documentation/serial/n_gsm.txt

+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>