tty: n_gsm: add ioctl for DLC config via ldisc handle

The application which initializes the n_gsm ldisc may like to set
individual default parameters for each DLCI or take over of the application
specific DLCI configuration completely. This is currently not possible.
It is either possible to set common default parameters for all DLCIs or let
the user application set its DLCI specific configuration parameters.

Add support of GSMIOC_GETCONF_DLCI and GSMIOC_SETCONF_DLCI for the n_gsm
ldisc handle to support DLCI specific parameter configuration upfront.

Add a code example for this use case to the n_gsm documentation.

Signed-off-by: Daniel Starke <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/driver-api/tty/n_gsm.rst

···
       int ldisc = N_GSM0710;
       struct gsm_config c;
       struct gsm_config_ext ce;
+      struct gsm_dlci_config dc;
       struct termios configuration;
       uint32_t first;
 
···
       c.mtu = 127;
       /* set the new configuration */
       ioctl(fd, GSMIOC_SETCONF, &c);
+      /* get DLC 1 configuration */
+      dc.channel = 1;
+      ioctl(fd, GSMIOC_GETCONF_DLCI, &dc);
+      /* the first user channel gets a higher priority */
+      dc.priority = 1;
+      /* set the new DLC 1 specific configuration */
+      ioctl(fd, GSMIOC_SETCONF_DLCI, &dc);
       /* get first gsmtty device node */
       ioctl(fd, GSMIOC_GETFIRST, &first);
       printf("first muxed line: /dev/gsmtty%i\n", first);
···
 	int ldisc = N_GSM0710;
 	struct gsm_config c;
 	struct gsm_config_ext ce;
+	struct gsm_dlci_config dc;
 	struct termios configuration;
 	uint32_t first;
 
···
 	c.mtu = 127;
 	/* set the new configuration */
 	ioctl(fd, GSMIOC_SETCONF, &c);
+	/* get DLC 1 configuration */
+	dc.channel = 1;
+	ioctl(fd, GSMIOC_GETCONF_DLCI, &dc);
+	/* the first user channel gets a higher priority */
+	dc.priority = 1;
+	/* set the new DLC 1 specific configuration */
+	ioctl(fd, GSMIOC_SETCONF_DLCI, &dc);
 	/* get first gsmtty device node */
 	ioctl(fd, GSMIOC_GETFIRST, &first);
 	printf("first muxed line: /dev/gsmtty%i\n", first);

drivers/tty/n_gsm.c

···
 {
 	struct gsm_config c;
 	struct gsm_config_ext ce;
+	struct gsm_dlci_config dc;
 	struct gsm_mux *gsm = tty->disc_data;
+	struct gsm_dlci *dlci;
 	unsigned int base;
 
 	switch (cmd) {
···
 		if (copy_from_user(&ce, (void __user *)arg, sizeof(ce)))
 			return -EFAULT;
 		return gsm_config_ext(gsm, &ce);
+	case GSMIOC_GETCONF_DLCI:
+		if (copy_from_user(&dc, (void __user *)arg, sizeof(dc)))
+			return -EFAULT;
+		if (dc.channel == 0 || dc.channel >= NUM_DLCI)
+			return -EINVAL;
+		dlci = gsm->dlci[dc.channel];
+		if (!dlci) {
+			dlci = gsm_dlci_alloc(gsm, dc.channel);
+			if (!dlci)
+				return -ENOMEM;
+		}
+		gsm_dlci_copy_config_values(dlci, &dc);
+		if (copy_to_user((void __user *)arg, &dc, sizeof(dc)))
+			return -EFAULT;
+		return 0;
+	case GSMIOC_SETCONF_DLCI:
+		if (copy_from_user(&dc, (void __user *)arg, sizeof(dc)))
+			return -EFAULT;
+		if (dc.channel == 0 || dc.channel >= NUM_DLCI)
+			return -EINVAL;
+		dlci = gsm->dlci[dc.channel];
+		if (!dlci) {
+			dlci = gsm_dlci_alloc(gsm, dc.channel);
+			if (!dlci)
+				return -ENOMEM;
+		}
+		return gsm_dlci_config(dlci, &dc, 0);
 	default:
 		return n_tty_ioctl_helper(tty, cmd, arg);
 	}