tty: make everyone's write_room return >= 0

The tty line disciplines don't expect tty_operations::write_room to
return negative values. Fix the five drivers which violate this.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-44-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/gdm724x/gdm_tty.c

@@ -188,7 +188,7 @@ static int gdm_tty_write_room(struct tty_struct *tty)
 	struct gdm *gdm = tty->driver_data;
 
 	if (!GDM_TTY_READY(gdm))
-		return -ENODEV;
+		return 0;
 
 	return WRITE_SIZE;
 }

drivers/tty/ipwireless/tty.c

@@ -235,10 +235,10 @@ static int ipw_write_room(struct tty_struct *linux_tty)
 
 	/* FIXME: Exactly how is the tty object locked here .. */
 	if (!tty)
-		return -ENODEV;
+		return 0;
 
 	if (!tty->port.count)
-		return -EINVAL;
+		return 0;
 
 	room = IPWIRELESS_TX_QUEUE_SIZE - tty->tx_bytes_queued;
 	if (room < 0)

drivers/tty/n_gsm.c

@@ -3048,7 +3048,7 @@ static int gsmtty_write_room(struct tty_struct *tty)
 {
 	struct gsm_dlci *dlci = tty->driver_data;
 	if (dlci->state == DLCI_CLOSED)
-		return -EINVAL;
+		return 0;
 	return TX_SIZE - kfifo_len(&dlci->fifo);
 }
 

drivers/tty/vcc.c

@@ -878,7 +878,7 @@ static int vcc_write_room(struct tty_struct *tty)
 	port = vcc_get_ne(tty->index);
 	if (unlikely(!port)) {
 		pr_err("VCC: write_room: Failed to find VCC port\n");
-		return -ENODEV;
+		return 0;
 	}
 
 	num = VCC_BUFF_LEN - port->chars_in_buffer;

drivers/usb/serial/mos7720.c

@@ -1042,7 +1042,7 @@ static int mos7720_write_room(struct tty_struct *tty)
 
 	mos7720_port = usb_get_serial_port_data(port);
 	if (mos7720_port == NULL)
-		return -ENODEV;
+		return 0;
 
 	/* FIXME: Locking */
 	for (i = 0; i < NUM_URBS; ++i) {