[PATCH 4/4] tty: n_gsm: add ioctl to map serial device to mux'ed tty

["Martin Hundebøll" <martin@geanix.com>]

Guessing the base tty for a gsm0710 multiplexed serial device is not
currently possible, which makes it racy to use with multiple modems.

Add a way to map the physical serial tty to its related mux devices
using a ioctl.

Signed-off-by: Martin Hundebøll <martin@geanix.com>
---
 Documentation/serial/n_gsm.rst | 8 ++++++++
 drivers/tty/n_gsm.c            | 6 ++++++
 include/uapi/linux/gsmmux.h    | 2 ++
 3 files changed, 16 insertions(+)

diff --git a/Documentation/serial/n_gsm.rst b/Documentation/serial/n_gsm.rst
index a3ce1b269018..bf5d7f82b5af 100644
--- a/Documentation/serial/n_gsm.rst
+++ b/Documentation/serial/n_gsm.rst
@@ -18,10 +18,12 @@ How to use it
 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,
 
 Major parts of the initialization program :
 (a good starting point is util-linux-ng/sys-utils/ldattach.c)::
 
+  #include <stdio.h>
   #include <linux/gsmmux.h>
   #include <linux/tty.h>
   #define DEFAULT_SPEED	B115200
@@ -30,6 +32,7 @@ Major parts of the initialization program :
 	int ldisc = N_GSM0710;
 	struct gsm_config c;
 	struct termios configuration;
+	int base;
 
 	/* open the serial port connected to the modem */
 	fd = open(SERIAL_PORT, O_RDWR | O_NOCTTY | O_NDELAY);
@@ -58,6 +61,11 @@ Major parts of the initialization program :
 	c.mtu = 127;
 	/* set the new configuration */
 	ioctl(fd, GSMIOC_SETCONF, &c);
+	/* get and print base gsmtty device node */
+	ioctl(fd, GSMIOC_GETBASE, &base);
+	/* the base node is used for mux control by the driver */
+	printf(first muxed line: /dev/gsmtty%i\n", base + 1);
+
 
 	/* and wait for ever to keep the line discipline enabled */
 	daemon(0,0);
diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index cba06063c44a..93c24d9b582b 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2612,6 +2612,7 @@ static int gsmld_ioctl(struct tty_struct *tty, struct file *file,
 {
 	struct gsm_config c;
 	struct gsm_mux *gsm = tty->disc_data;
+	int base;
 
 	switch (cmd) {
 	case GSMIOC_GETCONF:
@@ -2623,6 +2624,11 @@ static int gsmld_ioctl(struct tty_struct *tty, struct file *file,
 		if (copy_from_user(&c, (void *)arg, sizeof(c)))
 			return -EFAULT;
 		return gsm_config(gsm, &c);
+	case GSMIOC_GETBASE:
+		base = mux_num_to_base(gsm);
+		if (copy_to_user((void *)arg, &base, sizeof(base)))
+			return -EFAULT;
+		return 0;
 	default:
 		return n_tty_ioctl_helper(tty, file, cmd, arg);
 	}
diff --git a/include/uapi/linux/gsmmux.h b/include/uapi/linux/gsmmux.h
index 101d3c469acb..6eb63be3ed9e 100644
--- a/include/uapi/linux/gsmmux.h
+++ b/include/uapi/linux/gsmmux.h
@@ -37,5 +37,7 @@ struct gsm_netconfig {
 #define GSMIOC_ENABLE_NET      _IOW('G', 2, struct gsm_netconfig)
 #define GSMIOC_DISABLE_NET     _IO('G', 3)
 
+/* get the base tty number for a configured gsmmux tty */
+#define GSMIOC_GETBASE		_IOR('G', 4, int)
 
 #endif
-- 
2.22.0