tty: n_gsm: add TIOCMIWAIT support

Add support for the TIOCMIWAIT ioctl on the virtual ttys. This enables the
user to wait for virtual modem signals like RING.

More work is needed to support also TIOCGICOUNT.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230206114606.2133-4-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dstarke-siemens

drivers/tty/n_gsm.c

@@ -1536,6 +1536,7 @@ static void gsm_process_modem(struct tty_struct *tty, struct gsm_dlci *dlci,
 	if (brk & 0x01)
 		tty_insert_flip_char(&dlci->port, 0, TTY_BREAK);
 	dlci->modem_rx = mlines;
+	wake_up_interruptible(&dlci->gsm->event);
 }
 
 /**
@@ -2071,7 +2072,7 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
 	/* A DLCI 0 close is a MUX termination so we need to kick that
 	   back to userspace somehow */
 	gsm_dlci_data_kick(dlci);
-	wake_up(&dlci->gsm->event);
+	wake_up_all(&dlci->gsm->event);
 }
 
 /**
@@ -2272,6 +2273,7 @@ static void gsm_dlci_begin_close(struct gsm_dlci *dlci)
 	dlci->state = DLCI_CLOSING;
 	gsm_command(dlci->gsm, dlci->addr, DISC|PF);
 	mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
+	wake_up_interruptible(&gsm->event);
 }
 
 /**
@@ -3775,6 +3777,33 @@ static int gsm_modem_update(struct gsm_dlci *dlci, u8 brk)
 	return -EPROTONOSUPPORT;
 }
 
+/**
+ * gsm_wait_modem_change - wait for modem status line change
+ * @dlci: channel
+ * @mask: modem status line bits
+ *
+ * The function returns if:
+ * - any given modem status line bit changed
+ * - the wait event function got interrupted (e.g. by a signal)
+ * - the underlying DLCI was closed
+ * - the underlying ldisc device was removed
+ */
+static int gsm_wait_modem_change(struct gsm_dlci *dlci, u32 mask)
+{
+	struct gsm_mux *gsm = dlci->gsm;
+	u32 old = dlci->modem_rx;
+	int ret;
+
+	ret = wait_event_interruptible(gsm->event, gsm->dead ||
+				       dlci->state != DLCI_OPEN ||
+				       (old ^ dlci->modem_rx) & mask);
+	if (gsm->dead)
+		return -ENODEV;
+	if (dlci->state != DLCI_OPEN)
+		return -EL2NSYNC;
+	return ret;
+}
+
 static bool gsm_carrier_raised(struct tty_port *port)
 {
 	struct gsm_dlci *dlci = container_of(port, struct gsm_dlci, port);
@@ -4034,6 +4063,8 @@ static int gsmtty_ioctl(struct tty_struct *tty,
 		gsm_destroy_network(dlci);
 		mutex_unlock(&dlci->mutex);
 		return 0;
+	case TIOCMIWAIT:
+		return gsm_wait_modem_change(dlci, (u32)arg);
 	default:
 		return -ENOIOCTLCMD;
 	}