[PATCH] tty: n_gsm: add some instructions and code for slaver

[Zhenguo Zhao]

From: Zhenguo Zhao <Zhenguo.Zhao1@xxxxxxxxxx>

The gsm driver config to master or slaver by initiator,the config code
is different ,the doc has master instructions only,it should be add
instructions for responder.

Signed-off-by: Zhenguo Zhao <Zhenguo.Zhao1@xxxxxxxxxx>
---
 Documentation/driver-api/serial/n_gsm.rst | 67 ++++++++++++++++++++++++++-----
 1 file changed, 58 insertions(+), 9 deletions(-)

diff --git a/Documentation/driver-api/serial/n_gsm.rst b/Documentation/driver-api/serial/n_gsm.rst
index 87dfcd5..8cf08c4 100644
--- a/Documentation/driver-api/serial/n_gsm.rst
+++ b/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 +90,52 @@ Major parts of the initialization program :
 
       0xf9, 0x03, 0xef, 0x03, 0xc3, 0x16, 0xf9.
 
+2. config responter
+^^^^^^^^^^^^^^^^^^^^^
+
+2.1 switch the serial line to using the n_gsm line discipline by using
+    TIOCSETD ioctl.
+2.2 configure the mux using GSMIOC_GETCONF / GSMIOC_SETCONF ioctl.
+2.3 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 ... */
+
+	/* use n_gsm line discipline */
+	ioctl(fd, TIOCSETD, &ldisc);
+
+	/* get n_gsm configuration */
+	ioctl(fd, GSMIOC_GETCONF, &c);
+	/* we are responter 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
-- 
1.9.1