tty: n_gsm: Add some instructions and code for requester

The gsm driver can configure initiator or requester by parameter
initiator,but the config code and using are different ,the doc has
initiator instructions only,it should be add instructions for requester.
Signed-off-by: NZhenguo Zhao <Zhenguo.Zhao1@unisoc.com>
Link: https://lore.kernel.org/r/1629461872-26965-1-git-send-email-zhenguo6858@gmail.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/serial/n_gsm.rst

@@ -12,13 +12,16 @@ 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,
-4. obtain base gsmtty number for the used serial port,
+1. config initiator
+^^^^^^^^^^^^^^^^^^^^^
+
+1.1 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.
+1.2 switch the serial line to using the n_gsm line discipline by using
+    TIOCSETD ioctl.
+1.3 configure the mux using GSMIOC_GETCONF / GSMIOC_SETCONF ioctl.
+1.4 obtain base gsmtty number for the used serial port.
 
 Major parts of the initialization program :
 (a good starting point is util-linux-ng/sys-utils/ldattach.c)::
@@ -70,14 +73,14 @@ Major parts of the initialization program :
 	daemon(0,0);
 	pause();
 
-5. use these devices as plain serial ports.
+1.5 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.
+1.6 first close all virtual ports before closing the physical port.
 
    Note that after closing the physical port the modem is still in multiplexing
    mode. This may prevent a successful re-opening of the port later. To avoid
@@ -87,6 +90,56 @@ Major parts of the initialization program :
 
       0xf9, 0x03, 0xef, 0x03, 0xc3, 0x16, 0xf9.
 
+2. config requester
+^^^^^^^^^^^^^^^^^^^^^
+
+2.1 receive string "AT+CMUX= command" through its serial port,initialize
+    mux mode config
+2.2 switch the serial line to using the n_gsm line discipline by using
+    TIOCSETD ioctl.
+2.3 configure the mux using GSMIOC_GETCONF / GSMIOC_SETCONF ioctl.
+2.4 obtain base gsmtty number for the used serial port,
+
+  #include <stdio.h>
+  #include <stdint.h>
+  #include <linux/gsmmux.h>
+  #include <linux/tty.h>
+  #define DEFAULT_SPEED	B115200
+  #define SERIAL_PORT	/dev/ttyS0
+
+	int ldisc = N_GSM0710;
+	struct gsm_config c;
+	struct termios configuration;
+	uint32_t first;
+
+	/* open the serial port */
+	fd = open(SERIAL_PORT, O_RDWR | O_NOCTTY | O_NDELAY);
+
+	/* configure the serial port : speed, flow control ... */
+
+	/* get serial data and check "AT+CMUX=command" parameter ... */
+
+	/* use n_gsm line discipline */
+	ioctl(fd, TIOCSETD, &ldisc);
+
+	/* get n_gsm configuration */
+	ioctl(fd, GSMIOC_GETCONF, &c);
+	/* we are requester and need encoding 0 (basic) */
+	c.initiator = 0;
+	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);
+	/* get first gsmtty device node */
+	ioctl(fd, GSMIOC_GETFIRST, &first);
+	printf("first muxed line: /dev/gsmtty%i\n", first);
+
+	/* and wait for ever to keep the line discipline enabled */
+	daemon(0,0);
+	pause();
+
 Additional Documentation
 ------------------------
 More practical details on the protocol and how it's supported by industrial