diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
index fcb812bc832c..09b845d0da41 100644
--- a/drivers/usb/serial/cp210x.c
+++ b/drivers/usb/serial/cp210x.c
@@ -247,9 +247,9 @@ struct cp210x_serial_private {
 #ifdef CONFIG_GPIOLIB
 	struct gpio_chip	gc;
 	bool			gpio_registered;
-	u8			gpio_pushpull;
-	u8			gpio_altfunc;
-	u8			gpio_input;
+	u16			gpio_pushpull;
+	u16			gpio_altfunc;
+	u16			gpio_input;
 #endif
 	u8			partnum;
 	u32			fw_version;
@@ -534,6 +534,48 @@ struct cp210x_single_port_config {
 #define CP2104_GPIO1_RXLED_MODE		BIT(1)
 #define CP2104_GPIO2_RS485_MODE		BIT(2)
 
+struct cp210x_quad_port_state {
+	__le16 gpio_mode_pb0;
+	__le16 gpio_mode_pb1;
+	__le16 gpio_mode_pb2;
+	__le16 gpio_mode_pb3;
+	__le16 gpio_mode_pb4;
+
+	__le16 gpio_lowpower_pb0;
+	__le16 gpio_lowpower_pb1;
+	__le16 gpio_lowpower_pb2;
+	__le16 gpio_lowpower_pb3;
+	__le16 gpio_lowpower_pb4;
+
+	__le16 gpio_latch_pb0;
+	__le16 gpio_latch_pb1;
+	__le16 gpio_latch_pb2;
+	__le16 gpio_latch_pb3;
+	__le16 gpio_latch_pb4;
+};
+
+/*
+ * CP210X_VENDOR_SPECIFIC, CP210X_GET_PORTCONFIG call reads these 0x49 bytes
+ * on a CP2108 chip.
+ *
+ * See https://www.silabs.com/documents/public/application-notes/an978-cp210x-usb-to-uart-api-specification.pdf
+ */
+struct cp210x_quad_port_config {
+	struct cp210x_quad_port_state reset_state;
+	struct cp210x_quad_port_state suspend_state;
+	u8 ipdelay_ifc[4];
+	u8 enhancedfxn_ifc[4];
+	u8 enhancedfxn_device;
+	u8 extclkfreq[4];
+} __packed;
+
+#define CP2108_EF_IFC_GPIO_TXLED		0x01
+#define CP2108_EF_IFC_GPIO_RXLED		0x02
+#define CP2108_EF_IFC_GPIO_RS485		0x04
+#define CP2108_EF_IFC_GPIO_RS485_LOGIC		0x08
+#define CP2108_EF_IFC_GPIO_CLOCK		0x10
+#define CP2108_EF_IFC_DYNAMIC_SUSPEND		0x40
+
 /* CP2102N configuration array indices */
 #define CP210X_2NCONFIG_CONFIG_VERSION_IDX	2
 #define CP210X_2NCONFIG_GPIO_MODE_IDX		581
@@ -546,12 +588,24 @@ struct cp210x_single_port_config {
 #define CP2102N_QFN20_GPIO1_RS485_MODE		BIT(4)
 #define CP2102N_QFN20_GPIO0_CLK_MODE		BIT(6)
 
-/* CP210X_VENDOR_SPECIFIC, CP210X_WRITE_LATCH call writes these 0x2 bytes. */
+/*
+ * CP210X_VENDOR_SPECIFIC, CP210X_WRITE_LATCH call writes these 0x02 bytes
+ * for CP2102N, CP2103, CP2104 and CP2105.
+ */
 struct cp210x_gpio_write {
 	u8	mask;
 	u8	state;
 };
 
+/*
+ * CP210X_VENDOR_SPECIFIC, CP210X_WRITE_LATCH call writes these 0x04 bytes
+ * for CP2108.
+ */
+struct cp210x_gpio_write16 {
+	__le16	mask;
+	__le16	state;
+};
+
 /*
  * Helper to get interface number when we only have struct usb_serial.
  */
@@ -1434,52 +1488,84 @@ static int cp210x_gpio_get(struct gpio_chip *gc, unsigned int gpio)
 {
 	struct usb_serial *serial = gpiochip_get_data(gc);
 	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
-	u8 req_type = REQTYPE_DEVICE_TO_HOST;
+	u8 req_type;
+	u16 mask;
 	int result;
-	u8 buf;
-
-	if (priv->partnum == CP210X_PARTNUM_CP2105)
-		req_type = REQTYPE_INTERFACE_TO_HOST;
+	int len;
 
 	result = usb_autopm_get_interface(serial->interface);
 	if (result)
 		return result;
 
-	result = cp210x_read_vendor_block(serial, req_type,
-					  CP210X_READ_LATCH, &buf, sizeof(buf));
+	switch (priv->partnum) {
+	case CP210X_PARTNUM_CP2105:
+		req_type = REQTYPE_INTERFACE_TO_HOST;
+		len = 1;
+		break;
+	case CP210X_PARTNUM_CP2108:
+		req_type = REQTYPE_INTERFACE_TO_HOST;
+		len = 2;
+		break;
+	default:
+		req_type = REQTYPE_DEVICE_TO_HOST;
+		len = 1;
+		break;
+	}
+
+	mask = 0;
+	result = cp210x_read_vendor_block(serial, req_type, CP210X_READ_LATCH,
+					  &mask, len);
+
 	usb_autopm_put_interface(serial->interface);
+
 	if (result < 0)
 		return result;
 
-	return !!(buf & BIT(gpio));
+	le16_to_cpus(&mask);
+
+	return !!(mask & BIT(gpio));
 }
 
 static void cp210x_gpio_set(struct gpio_chip *gc, unsigned int gpio, int value)
 {
 	struct usb_serial *serial = gpiochip_get_data(gc);
 	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
+	struct cp210x_gpio_write16 buf16;
 	struct cp210x_gpio_write buf;
+	u16 mask, state;
+	u16 wIndex;
 	int result;
 
 	if (value == 1)
-		buf.state = BIT(gpio);
+		state = BIT(gpio);
 	else
-		buf.state = 0;
+		state = 0;
 
-	buf.mask = BIT(gpio);
+	mask = BIT(gpio);
 
 	result = usb_autopm_get_interface(serial->interface);
 	if (result)
 		goto out;
 
-	if (priv->partnum == CP210X_PARTNUM_CP2105) {
+	switch (priv->partnum) {
+	case CP210X_PARTNUM_CP2105:
+		buf.mask = (u8)mask;
+		buf.state = (u8)state;
 		result = cp210x_write_vendor_block(serial,
 						   REQTYPE_HOST_TO_INTERFACE,
 						   CP210X_WRITE_LATCH, &buf,
 						   sizeof(buf));
-	} else {
-		u16 wIndex = buf.state << 8 | buf.mask;
-
+		break;
+	case CP210X_PARTNUM_CP2108:
+		buf16.mask = cpu_to_le16(mask);
+		buf16.state = cpu_to_le16(state);
+		result = cp210x_write_vendor_block(serial,
+						   REQTYPE_HOST_TO_INTERFACE,
+						   CP210X_WRITE_LATCH, &buf16,
+						   sizeof(buf16));
+		break;
+	default:
+		wIndex = state << 8 | mask;
 		result = usb_control_msg(serial->dev,
 					 usb_sndctrlpipe(serial->dev, 0),
 					 CP210X_VENDOR_SPECIFIC,
@@ -1487,6 +1573,7 @@ static void cp210x_gpio_set(struct gpio_chip *gc, unsigned int gpio, int value)
 					 CP210X_WRITE_LATCH,
 					 wIndex,
 					 NULL, 0, USB_CTRL_SET_TIMEOUT);
+		break;
 	}
 
 	usb_autopm_put_interface(serial->interface);
@@ -1696,6 +1783,61 @@ static int cp2104_gpioconf_init(struct usb_serial *serial)
 	return 0;
 }
 
+static int cp2108_gpio_init(struct usb_serial *serial)
+{
+	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
+	struct cp210x_quad_port_config config;
+	u16 gpio_latch;
+	int result;
+	u8 i;
+
+	result = cp210x_read_vendor_block(serial, REQTYPE_DEVICE_TO_HOST,
+					  CP210X_GET_PORTCONFIG, &config,
+					  sizeof(config));
+	if (result < 0)
+		return result;
+
+	priv->gc.ngpio = 16;
+	priv->gpio_pushpull = le16_to_cpu(config.reset_state.gpio_mode_pb1);
+	gpio_latch = le16_to_cpu(config.reset_state.gpio_latch_pb1);
+
+	/*
+	 * Mark all pins which are not in GPIO mode.
+	 *
+	 * Refer to table 9.1 "GPIO Mode alternate Functions" in the datasheet:
+	 * https://www.silabs.com/documents/public/data-sheets/cp2108-datasheet.pdf
+	 *
+	 * Alternate functions of GPIO0 to GPIO3 are determine by enhancedfxn_ifc[0]
+	 * and the similarly for the other pins; enhancedfxn_ifc[1]: GPIO4 to GPIO7,
+	 * enhancedfxn_ifc[2]: GPIO8 to GPIO11, enhancedfxn_ifc[3]: GPIO12 to GPIO15.
+	 */
+	for (i = 0; i < 4; i++) {
+		if (config.enhancedfxn_ifc[i] & CP2108_EF_IFC_GPIO_TXLED)
+			priv->gpio_altfunc |= BIT(i * 4);
+		if (config.enhancedfxn_ifc[i] & CP2108_EF_IFC_GPIO_RXLED)
+			priv->gpio_altfunc |= BIT((i * 4) + 1);
+		if (config.enhancedfxn_ifc[i] & CP2108_EF_IFC_GPIO_RS485)
+			priv->gpio_altfunc |= BIT((i * 4) + 2);
+		if (config.enhancedfxn_ifc[i] & CP2108_EF_IFC_GPIO_CLOCK)
+			priv->gpio_altfunc |= BIT((i * 4) + 3);
+	}
+
+	/*
+	 * Like CP2102N, CP2108 has also no strict input and output pin
+	 * modes. Do the same input mode emulation as CP2102N.
+	 */
+	for (i = 0; i < priv->gc.ngpio; ++i) {
+		/*
+		 * Set direction to "input" iff pin is open-drain and reset
+		 * value is 1.
+		 */
+		if (!(priv->gpio_pushpull & BIT(i)) && (gpio_latch & BIT(i)))
+			priv->gpio_input |= BIT(i);
+	}
+
+	return 0;
+}
+
 static int cp2102n_gpioconf_init(struct usb_serial *serial)
 {
 	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
@@ -1812,6 +1954,15 @@ static int cp210x_gpio_init(struct usb_serial *serial)
 	case CP210X_PARTNUM_CP2105:
 		result = cp2105_gpioconf_init(serial);
 		break;
+	case CP210X_PARTNUM_CP2108:
+		/*
+		 * The GPIOs are not tied to any specific port so only register
+		 * once for interface 0.
+		 */
+		if (cp210x_interface_num(serial) != 0)
+			return 0;
+		result = cp2108_gpio_init(serial);
+		break;
 	case CP210X_PARTNUM_CP2102N_QFN28:
 	case CP210X_PARTNUM_CP2102N_QFN24:
 	case CP210X_PARTNUM_CP2102N_QFN20:
diff --git a/drivers/usb/serial/cyberjack.c b/drivers/usb/serial/cyberjack.c
index cf389224d528..51e5aac3bf4c 100644
--- a/drivers/usb/serial/cyberjack.c
+++ b/drivers/usb/serial/cyberjack.c
@@ -53,7 +53,7 @@ static int  cyberjack_open(struct tty_struct *tty,
 static void cyberjack_close(struct usb_serial_port *port);
 static int cyberjack_write(struct tty_struct *tty,
 	struct usb_serial_port *port, const unsigned char *buf, int count);
-static int cyberjack_write_room(struct tty_struct *tty);
+static unsigned int cyberjack_write_room(struct tty_struct *tty);
 static void cyberjack_read_int_callback(struct urb *urb);
 static void cyberjack_read_bulk_callback(struct urb *urb);
 static void cyberjack_write_bulk_callback(struct urb *urb);
@@ -240,7 +240,7 @@ static int cyberjack_write(struct tty_struct *tty,
 	return count;
 }
 
-static int cyberjack_write_room(struct tty_struct *tty)
+static unsigned int cyberjack_write_room(struct tty_struct *tty)
 {
 	/* FIXME: .... */
 	return CYBERJACK_LOCAL_BUF_SIZE;
diff --git a/drivers/usb/serial/cypress_m8.c b/drivers/usb/serial/cypress_m8.c
index 166ee2286fda..1dca04e1519d 100644
--- a/drivers/usb/serial/cypress_m8.c
+++ b/drivers/usb/serial/cypress_m8.c
@@ -122,14 +122,14 @@ static void cypress_dtr_rts(struct usb_serial_port *port, int on);
 static int  cypress_write(struct tty_struct *tty, struct usb_serial_port *port,
 			const unsigned char *buf, int count);
 static void cypress_send(struct usb_serial_port *port);
-static int  cypress_write_room(struct tty_struct *tty);
+static unsigned int cypress_write_room(struct tty_struct *tty);
 static void cypress_earthmate_init_termios(struct tty_struct *tty);
 static void cypress_set_termios(struct tty_struct *tty,
 			struct usb_serial_port *port, struct ktermios *old);
 static int  cypress_tiocmget(struct tty_struct *tty);
 static int  cypress_tiocmset(struct tty_struct *tty,
 			unsigned int set, unsigned int clear);
-static int  cypress_chars_in_buffer(struct tty_struct *tty);
+static unsigned int cypress_chars_in_buffer(struct tty_struct *tty);
 static void cypress_throttle(struct tty_struct *tty);
 static void cypress_unthrottle(struct tty_struct *tty);
 static void cypress_set_dead(struct usb_serial_port *port);
@@ -789,18 +789,18 @@ send:
 
 
 /* returns how much space is available in the soft buffer */
-static int cypress_write_room(struct tty_struct *tty)
+static unsigned int cypress_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct cypress_private *priv = usb_get_serial_port_data(port);
-	int room = 0;
+	unsigned int room;
 	unsigned long flags;
 
 	spin_lock_irqsave(&priv->lock, flags);
 	room = kfifo_avail(&priv->write_fifo);
 	spin_unlock_irqrestore(&priv->lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, room);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, room);
 	return room;
 }
 
@@ -970,18 +970,18 @@ static void cypress_set_termios(struct tty_struct *tty,
 
 
 /* returns amount of data still left in soft buffer */
-static int cypress_chars_in_buffer(struct tty_struct *tty)
+static unsigned int cypress_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct cypress_private *priv = usb_get_serial_port_data(port);
-	int chars = 0;
+	unsigned int chars;
 	unsigned long flags;
 
 	spin_lock_irqsave(&priv->lock, flags);
 	chars = kfifo_len(&priv->write_fifo);
 	spin_unlock_irqrestore(&priv->lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, chars);
 	return chars;
 }
 
diff --git a/drivers/usb/serial/digi_acceleport.c b/drivers/usb/serial/digi_acceleport.c
index 8b2f06539f2c..af65eb863d70 100644
--- a/drivers/usb/serial/digi_acceleport.c
+++ b/drivers/usb/serial/digi_acceleport.c
@@ -223,8 +223,8 @@ static int digi_tiocmset(struct tty_struct *tty, unsigned int set,
 static int digi_write(struct tty_struct *tty, struct usb_serial_port *port,
 		const unsigned char *buf, int count);
 static void digi_write_bulk_callback(struct urb *urb);
-static int digi_write_room(struct tty_struct *tty);
-static int digi_chars_in_buffer(struct tty_struct *tty);
+static unsigned int digi_write_room(struct tty_struct *tty);
+static unsigned int digi_chars_in_buffer(struct tty_struct *tty);
 static int digi_open(struct tty_struct *tty, struct usb_serial_port *port);
 static void digi_close(struct usb_serial_port *port);
 static void digi_dtr_rts(struct usb_serial_port *port, int on);
@@ -372,7 +372,7 @@ static int digi_write_oob_command(struct usb_serial_port *port,
 	int len;
 	struct usb_serial_port *oob_port = (struct usb_serial_port *)((struct digi_serial *)(usb_get_serial_data(port->serial)))->ds_oob_port;
 	struct digi_port *oob_priv = usb_get_serial_port_data(oob_port);
-	unsigned long flags = 0;
+	unsigned long flags;
 
 	dev_dbg(&port->dev,
 		"digi_write_oob_command: TOP: port=%d, count=%d\n",
@@ -430,7 +430,7 @@ static int digi_write_inb_command(struct usb_serial_port *port,
 	int len;
 	struct digi_port *priv = usb_get_serial_port_data(port);
 	unsigned char *data = port->write_urb->transfer_buffer;
-	unsigned long flags = 0;
+	unsigned long flags;
 
 	dev_dbg(&port->dev, "digi_write_inb_command: TOP: port=%d, count=%d\n",
 		priv->dp_port_num, count);
@@ -511,8 +511,7 @@ static int digi_set_modem_signals(struct usb_serial_port *port,
 	struct usb_serial_port *oob_port = (struct usb_serial_port *) ((struct digi_serial *)(usb_get_serial_data(port->serial)))->ds_oob_port;
 	struct digi_port *oob_priv = usb_get_serial_port_data(oob_port);
 	unsigned char *data = oob_port->write_urb->transfer_buffer;
-	unsigned long flags = 0;
-
+	unsigned long flags;
 
 	dev_dbg(&port->dev,
 		"digi_set_modem_signals: TOP: port=%d, modem_signals=0x%x\n",
@@ -577,7 +576,7 @@ static int digi_transmit_idle(struct usb_serial_port *port,
 	int ret;
 	unsigned char buf[2];
 	struct digi_port *priv = usb_get_serial_port_data(port);
-	unsigned long flags = 0;
+	unsigned long flags;
 
 	spin_lock_irqsave(&priv->dp_port_lock, flags);
 	priv->dp_transmit_idle = 0;
@@ -887,7 +886,7 @@ static int digi_write(struct tty_struct *tty, struct usb_serial_port *port,
 	int ret, data_len, new_len;
 	struct digi_port *priv = usb_get_serial_port_data(port);
 	unsigned char *data = port->write_urb->transfer_buffer;
-	unsigned long flags = 0;
+	unsigned long flags;
 
 	dev_dbg(&port->dev, "digi_write: TOP: port=%d, count=%d\n",
 		priv->dp_port_num, count);
@@ -1020,12 +1019,12 @@ static void digi_write_bulk_callback(struct urb *urb)
 		tty_port_tty_wakeup(&port->port);
 }
 
-static int digi_write_room(struct tty_struct *tty)
+static unsigned int digi_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct digi_port *priv = usb_get_serial_port_data(port);
-	int room;
-	unsigned long flags = 0;
+	unsigned long flags;
+	unsigned int room;
 
 	spin_lock_irqsave(&priv->dp_port_lock, flags);
 
@@ -1035,27 +1034,28 @@ static int digi_write_room(struct tty_struct *tty)
 		room = port->bulk_out_size - 2 - priv->dp_out_buf_len;
 
 	spin_unlock_irqrestore(&priv->dp_port_lock, flags);
-	dev_dbg(&port->dev, "digi_write_room: port=%d, room=%d\n", priv->dp_port_num, room);
+	dev_dbg(&port->dev, "digi_write_room: port=%d, room=%u\n", priv->dp_port_num, room);
 	return room;
 
 }
 
-static int digi_chars_in_buffer(struct tty_struct *tty)
+static unsigned int digi_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct digi_port *priv = usb_get_serial_port_data(port);
+	unsigned long flags;
+	unsigned int chars;
 
-	if (priv->dp_write_urb_in_use) {
-		dev_dbg(&port->dev, "digi_chars_in_buffer: port=%d, chars=%d\n",
-			priv->dp_port_num, port->bulk_out_size - 2);
-		/* return(port->bulk_out_size - 2); */
-		return 256;
-	} else {
-		dev_dbg(&port->dev, "digi_chars_in_buffer: port=%d, chars=%d\n",
-			priv->dp_port_num, priv->dp_out_buf_len);
-		return priv->dp_out_buf_len;
-	}
+	spin_lock_irqsave(&priv->dp_port_lock, flags);
+	if (priv->dp_write_urb_in_use)
+		chars = port->bulk_out_size - 2;
+	else
+		chars = priv->dp_out_buf_len;
+	spin_unlock_irqrestore(&priv->dp_port_lock, flags);
 
+	dev_dbg(&port->dev, "%s: port=%d, chars=%d\n", __func__,
+			priv->dp_port_num, chars);
+	return chars;
 }
 
 static void digi_dtr_rts(struct usb_serial_port *port, int on)
diff --git a/drivers/usb/serial/garmin_gps.c b/drivers/usb/serial/garmin_gps.c
index 50e8bdc77e71..756d1ac7e96f 100644
--- a/drivers/usb/serial/garmin_gps.c
+++ b/drivers/usb/serial/garmin_gps.c
@@ -1113,7 +1113,7 @@ static int garmin_write(struct tty_struct *tty, struct usb_serial_port *port,
 }
 
 
-static int garmin_write_room(struct tty_struct *tty)
+static unsigned int garmin_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	/*
diff --git a/drivers/usb/serial/generic.c b/drivers/usb/serial/generic.c
index d10aa3d2ee49..15b6dee3a8e5 100644
--- a/drivers/usb/serial/generic.c
+++ b/drivers/usb/serial/generic.c
@@ -230,11 +230,11 @@ int usb_serial_generic_write(struct tty_struct *tty,
 }
 EXPORT_SYMBOL_GPL(usb_serial_generic_write);
 
-int usb_serial_generic_write_room(struct tty_struct *tty)
+unsigned int usb_serial_generic_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	unsigned long flags;
-	int room;
+	unsigned int room;
 
 	if (!port->bulk_out_size)
 		return 0;
@@ -243,15 +243,15 @@ int usb_serial_generic_write_room(struct tty_struct *tty)
 	room = kfifo_avail(&port->write_fifo);
 	spin_unlock_irqrestore(&port->lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, room);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, room);
 	return room;
 }
 
-int usb_serial_generic_chars_in_buffer(struct tty_struct *tty)
+unsigned int usb_serial_generic_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	unsigned long flags;
-	int chars;
+	unsigned int chars;
 
 	if (!port->bulk_out_size)
 		return 0;
@@ -260,7 +260,7 @@ int usb_serial_generic_chars_in_buffer(struct tty_struct *tty)
 	chars = kfifo_len(&port->write_fifo) + port->tx_bytes;
 	spin_unlock_irqrestore(&port->lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, chars);
 	return chars;
 }
 EXPORT_SYMBOL_GPL(usb_serial_generic_chars_in_buffer);
diff --git a/drivers/usb/serial/io_edgeport.c b/drivers/usb/serial/io_edgeport.c
index e6fe3882bf69..ea4edf5eed27 100644
--- a/drivers/usb/serial/io_edgeport.c
+++ b/drivers/usb/serial/io_edgeport.c
@@ -1351,33 +1351,21 @@ exit_send:
 /*****************************************************************************
  * edge_write_room
  *	this function is called by the tty driver when it wants to know how
- *	many bytes of data we can accept for a specific port. If successful,
- *	we return the amount of room that we have for this port	(the txCredits)
- *	otherwise we return a negative error number.
+ *	many bytes of data we can accept for a specific port.
  *****************************************************************************/
-static int edge_write_room(struct tty_struct *tty)
+static unsigned int edge_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct edgeport_port *edge_port = usb_get_serial_port_data(port);
-	int room;
+	unsigned int room;
 	unsigned long flags;
 
-	if (edge_port == NULL)
-		return 0;
-	if (edge_port->closePending)
-		return 0;
-
-	if (!edge_port->open) {
-		dev_dbg(&port->dev, "%s - port not opened\n", __func__);
-		return 0;
-	}
-
 	/* total of both buffers is still txCredit */
 	spin_lock_irqsave(&edge_port->ep_lock, flags);
 	room = edge_port->txCredits - edge_port->txfifo.count;
 	spin_unlock_irqrestore(&edge_port->ep_lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, room);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, room);
 	return room;
 }
 
@@ -1387,33 +1375,20 @@ static int edge_write_room(struct tty_struct *tty)
  *	this function is called by the tty driver when it wants to know how
  *	many bytes of data we currently have outstanding in the port (data that
  *	has been written, but hasn't made it out the port yet)
- *	If successful, we return the number of bytes left to be written in the
- *	system,
- *	Otherwise we return a negative error number.
  *****************************************************************************/
-static int edge_chars_in_buffer(struct tty_struct *tty)
+static unsigned int edge_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct edgeport_port *edge_port = usb_get_serial_port_data(port);
-	int num_chars;
+	unsigned int num_chars;
 	unsigned long flags;
 
-	if (edge_port == NULL)
-		return 0;
-	if (edge_port->closePending)
-		return 0;
-
-	if (!edge_port->open) {
-		dev_dbg(&port->dev, "%s - port not opened\n", __func__);
-		return 0;
-	}
-
 	spin_lock_irqsave(&edge_port->ep_lock, flags);
 	num_chars = edge_port->maxTxCredits - edge_port->txCredits +
 						edge_port->txfifo.count;
 	spin_unlock_irqrestore(&edge_port->ep_lock, flags);
 	if (num_chars) {
-		dev_dbg(&port->dev, "%s - returns %d\n", __func__, num_chars);
+		dev_dbg(&port->dev, "%s - returns %u\n", __func__, num_chars);
 	}
 
 	return num_chars;
diff --git a/drivers/usb/serial/io_ti.c b/drivers/usb/serial/io_ti.c
index 39503fdccebf..84b848d2794e 100644
--- a/drivers/usb/serial/io_ti.c
+++ b/drivers/usb/serial/io_ti.c
@@ -2067,11 +2067,11 @@ static void edge_send(struct usb_serial_port *port, struct tty_struct *tty)
 		tty_wakeup(tty);
 }
 
-static int edge_write_room(struct tty_struct *tty)
+static unsigned int edge_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct edgeport_port *edge_port = usb_get_serial_port_data(port);
-	int room = 0;
+	unsigned int room;
 	unsigned long flags;
 
 	if (edge_port == NULL)
@@ -2083,15 +2083,15 @@ static int edge_write_room(struct tty_struct *tty)
 	room = kfifo_avail(&port->write_fifo);
 	spin_unlock_irqrestore(&edge_port->ep_lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, room);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, room);
 	return room;
 }
 
-static int edge_chars_in_buffer(struct tty_struct *tty)
+static unsigned int edge_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct edgeport_port *edge_port = usb_get_serial_port_data(port);
-	int chars = 0;
+	unsigned int chars;
 	unsigned long flags;
 	if (edge_port == NULL)
 		return 0;
@@ -2100,7 +2100,7 @@ static int edge_chars_in_buffer(struct tty_struct *tty)
 	chars = kfifo_len(&port->write_fifo);
 	spin_unlock_irqrestore(&edge_port->ep_lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, chars);
 	return chars;
 }
 
diff --git a/drivers/usb/serial/ir-usb.c b/drivers/usb/serial/ir-usb.c
index 172261a908d8..7b44dbea95cd 100644
--- a/drivers/usb/serial/ir-usb.c
+++ b/drivers/usb/serial/ir-usb.c
@@ -47,7 +47,7 @@ static int xbof = -1;
 static int  ir_startup (struct usb_serial *serial);
 static int ir_write(struct tty_struct *tty, struct usb_serial_port *port,
 		const unsigned char *buf, int count);
-static int ir_write_room(struct tty_struct *tty);
+static unsigned int ir_write_room(struct tty_struct *tty);
 static void ir_write_bulk_callback(struct urb *urb);
 static void ir_process_read_urb(struct urb *urb);
 static void ir_set_termios(struct tty_struct *tty,
@@ -339,10 +339,10 @@ static void ir_write_bulk_callback(struct urb *urb)
 	usb_serial_port_softint(port);
 }
 
-static int ir_write_room(struct tty_struct *tty)
+static unsigned int ir_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
-	int count = 0;
+	unsigned int count = 0;
 
 	if (port->bulk_out_size == 0)
 		return 0;
diff --git a/drivers/usb/serial/keyspan.c b/drivers/usb/serial/keyspan.c
index b04a029e3657..87b89c99d517 100644
--- a/drivers/usb/serial/keyspan.c
+++ b/drivers/usb/serial/keyspan.c
@@ -1453,13 +1453,13 @@ static void usa67_glocont_callback(struct urb *urb)
 	}
 }
 
-static int keyspan_write_room(struct tty_struct *tty)
+static unsigned int keyspan_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct keyspan_port_private	*p_priv;
 	const struct keyspan_device_details	*d_details;
 	int				flip;
-	int				data_len;
+	unsigned int			data_len;
 	struct urb			*this_urb;
 
 	p_priv = usb_get_serial_port_data(port);
diff --git a/drivers/usb/serial/kobil_sct.c b/drivers/usb/serial/kobil_sct.c
index a9bc546626ab..4ed8b8b0a361 100644
--- a/drivers/usb/serial/kobil_sct.c
+++ b/drivers/usb/serial/kobil_sct.c
@@ -53,7 +53,7 @@ static int  kobil_open(struct tty_struct *tty, struct usb_serial_port *port);
 static void kobil_close(struct usb_serial_port *port);
 static int  kobil_write(struct tty_struct *tty, struct usb_serial_port *port,
 			 const unsigned char *buf, int count);
-static int  kobil_write_room(struct tty_struct *tty);
+static unsigned int kobil_write_room(struct tty_struct *tty);
 static int  kobil_ioctl(struct tty_struct *tty,
 			unsigned int cmd, unsigned long arg);
 static int  kobil_tiocmget(struct tty_struct *tty);
@@ -358,7 +358,7 @@ static int kobil_write(struct tty_struct *tty, struct usb_serial_port *port,
 }
 
 
-static int kobil_write_room(struct tty_struct *tty)
+static unsigned int kobil_write_room(struct tty_struct *tty)
 {
 	/* FIXME */
 	return 8;
diff --git a/drivers/usb/serial/metro-usb.c b/drivers/usb/serial/metro-usb.c
index f9ce9e7b9b80..30ab565e0738 100644
--- a/drivers/usb/serial/metro-usb.c
+++ b/drivers/usb/serial/metro-usb.c
@@ -109,9 +109,9 @@ static void metrousb_read_int_callback(struct urb *urb)
 	struct usb_serial_port *port = urb->context;
 	struct metrousb_private *metro_priv = usb_get_serial_port_data(port);
 	unsigned char *data = urb->transfer_buffer;
+	unsigned long flags;
 	int throttled = 0;
 	int result = 0;
-	unsigned long flags = 0;
 
 	dev_dbg(&port->dev, "%s\n", __func__);
 
@@ -171,7 +171,7 @@ static int metrousb_open(struct tty_struct *tty, struct usb_serial_port *port)
 {
 	struct usb_serial *serial = port->serial;
 	struct metrousb_private *metro_priv = usb_get_serial_port_data(port);
-	unsigned long flags = 0;
+	unsigned long flags;
 	int result = 0;
 
 	/* Set the private data information for the port. */
@@ -268,7 +268,7 @@ static void metrousb_throttle(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct metrousb_private *metro_priv = usb_get_serial_port_data(port);
-	unsigned long flags = 0;
+	unsigned long flags;
 
 	/* Set the private information for the port to stop reading data. */
 	spin_lock_irqsave(&metro_priv->lock, flags);
@@ -281,7 +281,7 @@ static int metrousb_tiocmget(struct tty_struct *tty)
 	unsigned long control_state = 0;
 	struct usb_serial_port *port = tty->driver_data;
 	struct metrousb_private *metro_priv = usb_get_serial_port_data(port);
-	unsigned long flags = 0;
+	unsigned long flags;
 
 	spin_lock_irqsave(&metro_priv->lock, flags);
 	control_state = metro_priv->control_state;
@@ -296,7 +296,7 @@ static int metrousb_tiocmset(struct tty_struct *tty,
 	struct usb_serial_port *port = tty->driver_data;
 	struct usb_serial *serial = port->serial;
 	struct metrousb_private *metro_priv = usb_get_serial_port_data(port);
-	unsigned long flags = 0;
+	unsigned long flags;
 	unsigned long control_state = 0;
 
 	dev_dbg(&port->dev, "%s - set=%d, clear=%d\n", __func__, set, clear);
@@ -323,7 +323,7 @@ static void metrousb_unthrottle(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct metrousb_private *metro_priv = usb_get_serial_port_data(port);
-	unsigned long flags = 0;
+	unsigned long flags;
 	int result = 0;
 
 	/* Set the private information for the port to resume reading data. */
diff --git a/drivers/usb/serial/mos7720.c b/drivers/usb/serial/mos7720.c
index 6ee83886e2c9..227f43d2bd56 100644
--- a/drivers/usb/serial/mos7720.c
+++ b/drivers/usb/serial/mos7720.c
@@ -945,27 +945,20 @@ static int mos7720_open(struct tty_struct *tty, struct usb_serial_port *port)
  *	this function is called by the tty driver when it wants to know how many
  *	bytes of data we currently have outstanding in the port (data that has
  *	been written, but hasn't made it out the port yet)
- *	If successful, we return the number of bytes left to be written in the
- *	system,
- *	Otherwise we return a negative error number.
  */
-static int mos7720_chars_in_buffer(struct tty_struct *tty)
+static unsigned int mos7720_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
+	struct moschip_port *mos7720_port = usb_get_serial_port_data(port);
 	int i;
-	int chars = 0;
-	struct moschip_port *mos7720_port;
-
-	mos7720_port = usb_get_serial_port_data(port);
-	if (mos7720_port == NULL)
-		return 0;
+	unsigned int chars = 0;
 
 	for (i = 0; i < NUM_URBS; ++i) {
 		if (mos7720_port->write_urb_pool[i] &&
 		    mos7720_port->write_urb_pool[i]->status == -EINPROGRESS)
 			chars += URB_TRANSFER_BUFFER_SIZE;
 	}
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, chars);
 	return chars;
 }
 
@@ -1030,20 +1023,14 @@ static void mos7720_break(struct tty_struct *tty, int break_state)
  * mos7720_write_room
  *	this function is called by the tty driver when it wants to know how many
  *	bytes of data we can accept for a specific port.
- *	If successful, we return the amount of room that we have for this port
- *	Otherwise we return a negative error number.
  */
-static int mos7720_write_room(struct tty_struct *tty)
+static unsigned int mos7720_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
-	struct moschip_port *mos7720_port;
-	int room = 0;
+	struct moschip_port *mos7720_port = usb_get_serial_port_data(port);
+	unsigned int room = 0;
 	int i;
 
-	mos7720_port = usb_get_serial_port_data(port);
-	if (mos7720_port == NULL)
-		return 0;
-
 	/* FIXME: Locking */
 	for (i = 0; i < NUM_URBS; ++i) {
 		if (mos7720_port->write_urb_pool[i] &&
@@ -1051,7 +1038,7 @@ static int mos7720_write_room(struct tty_struct *tty)
 			room += URB_TRANSFER_BUFFER_SIZE;
 	}
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, room);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, room);
 	return room;
 }
 
diff --git a/drivers/usb/serial/mos7840.c b/drivers/usb/serial/mos7840.c
index 28e4093794e0..d7fe33ca73e4 100644
--- a/drivers/usb/serial/mos7840.c
+++ b/drivers/usb/serial/mos7840.c
@@ -730,17 +730,14 @@ err:
  *	this function is called by the tty driver when it wants to know how many
  *	bytes of data we currently have outstanding in the port (data that has
  *	been written, but hasn't made it out the port yet)
- *	If successful, we return the number of bytes left to be written in the
- *	system,
- *	Otherwise we return zero.
  *****************************************************************************/
 
-static int mos7840_chars_in_buffer(struct tty_struct *tty)
+static unsigned int mos7840_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct moschip_port *mos7840_port = usb_get_serial_port_data(port);
 	int i;
-	int chars = 0;
+	unsigned int chars = 0;
 	unsigned long flags;
 
 	spin_lock_irqsave(&mos7840_port->pool_lock, flags);
@@ -751,7 +748,7 @@ static int mos7840_chars_in_buffer(struct tty_struct *tty)
 		}
 	}
 	spin_unlock_irqrestore(&mos7840_port->pool_lock, flags);
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, chars);
 	return chars;
 
 }
@@ -814,16 +811,14 @@ static void mos7840_break(struct tty_struct *tty, int break_state)
  * mos7840_write_room
  *	this function is called by the tty driver when it wants to know how many
  *	bytes of data we can accept for a specific port.
- *	If successful, we return the amount of room that we have for this port
- *	Otherwise we return a negative error number.
  *****************************************************************************/
 
-static int mos7840_write_room(struct tty_struct *tty)
+static unsigned int mos7840_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct moschip_port *mos7840_port = usb_get_serial_port_data(port);
 	int i;
-	int room = 0;
+	unsigned int room = 0;
 	unsigned long flags;
 
 	spin_lock_irqsave(&mos7840_port->pool_lock, flags);
@@ -834,7 +829,7 @@ static int mos7840_write_room(struct tty_struct *tty)
 	spin_unlock_irqrestore(&mos7840_port->pool_lock, flags);
 
 	room = (room == 0) ? 0 : room - URB_TRANSFER_BUFFER_SIZE + 1;
-	dev_dbg(&mos7840_port->port->dev, "%s - returns %d\n", __func__, room);
+	dev_dbg(&mos7840_port->port->dev, "%s - returns %u\n", __func__, room);
 	return room;
 
 }
diff --git a/drivers/usb/serial/opticon.c b/drivers/usb/serial/opticon.c
index 40c713fae0c3..aed28c35caff 100644
--- a/drivers/usb/serial/opticon.c
+++ b/drivers/usb/serial/opticon.c
@@ -267,7 +267,7 @@ error_no_buffer:
 	return ret;
 }
 
-static int opticon_write_room(struct tty_struct *tty)
+static unsigned int opticon_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct opticon_private *priv = usb_get_serial_port_data(port);
@@ -289,12 +289,12 @@ static int opticon_write_room(struct tty_struct *tty)
 	return 2048;
 }
 
-static int opticon_chars_in_buffer(struct tty_struct *tty)
+static unsigned int opticon_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct opticon_private *priv = usb_get_serial_port_data(port);
 	unsigned long flags;
-	int count;
+	unsigned int count;
 
 	spin_lock_irqsave(&priv->lock, flags);
 	count = priv->outstanding_bytes;
diff --git a/drivers/usb/serial/oti6858.c b/drivers/usb/serial/oti6858.c
index 65cd0341fa78..a5caedbe72e2 100644
--- a/drivers/usb/serial/oti6858.c
+++ b/drivers/usb/serial/oti6858.c
@@ -126,8 +126,8 @@ static void oti6858_read_bulk_callback(struct urb *urb);
 static void oti6858_write_bulk_callback(struct urb *urb);
 static int oti6858_write(struct tty_struct *tty, struct usb_serial_port *port,
 			const unsigned char *buf, int count);
-static int oti6858_write_room(struct tty_struct *tty);
-static int oti6858_chars_in_buffer(struct tty_struct *tty);
+static unsigned int oti6858_write_room(struct tty_struct *tty);
+static unsigned int oti6858_chars_in_buffer(struct tty_struct *tty);
 static int oti6858_tiocmget(struct tty_struct *tty);
 static int oti6858_tiocmset(struct tty_struct *tty,
 				unsigned int set, unsigned int clear);
@@ -363,10 +363,10 @@ static int oti6858_write(struct tty_struct *tty, struct usb_serial_port *port,
 	return count;
 }
 
-static int oti6858_write_room(struct tty_struct *tty)
+static unsigned int oti6858_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
-	int room = 0;
+	unsigned int room;
 	unsigned long flags;
 
 	spin_lock_irqsave(&port->lock, flags);
@@ -376,10 +376,10 @@ static int oti6858_write_room(struct tty_struct *tty)
 	return room;
 }
 
-static int oti6858_chars_in_buffer(struct tty_struct *tty)
+static unsigned int oti6858_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
-	int chars = 0;
+	unsigned int chars;
 	unsigned long flags;
 
 	spin_lock_irqsave(&port->lock, flags);
diff --git a/drivers/usb/serial/quatech2.c b/drivers/usb/serial/quatech2.c
index 067690dac24c..971907f083a3 100644
--- a/drivers/usb/serial/quatech2.c
+++ b/drivers/usb/serial/quatech2.c
@@ -870,12 +870,12 @@ static void qt2_update_lsr(struct usb_serial_port *port, unsigned char *ch)
 
 }
 
-static int qt2_write_room(struct tty_struct *tty)
+static unsigned int qt2_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct qt2_port_private *port_priv;
-	unsigned long flags = 0;
-	int r;
+	unsigned long flags;
+	unsigned int r;
 
 	port_priv = usb_get_serial_port_data(port);
 
diff --git a/drivers/usb/serial/sierra.c b/drivers/usb/serial/sierra.c
index 54e16ffc30a0..4b49b7c33364 100644
--- a/drivers/usb/serial/sierra.c
+++ b/drivers/usb/serial/sierra.c
@@ -613,7 +613,7 @@ static void sierra_instat_callback(struct urb *urb)
 	}
 }
 
-static int sierra_write_room(struct tty_struct *tty)
+static unsigned int sierra_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct sierra_port_private *portdata = usb_get_serial_port_data(port);
@@ -632,19 +632,19 @@ static int sierra_write_room(struct tty_struct *tty)
 	return 2048;
 }
 
-static int sierra_chars_in_buffer(struct tty_struct *tty)
+static unsigned int sierra_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct sierra_port_private *portdata = usb_get_serial_port_data(port);
 	unsigned long flags;
-	int chars;
+	unsigned int chars;
 
 	/* NOTE: This overcounts somewhat. */
 	spin_lock_irqsave(&portdata->lock, flags);
 	chars = portdata->outstanding_urbs * MAX_TRANSFER;
 	spin_unlock_irqrestore(&portdata->lock, flags);
 
-	dev_dbg(&port->dev, "%s - %d\n", __func__, chars);
+	dev_dbg(&port->dev, "%s - %u\n", __func__, chars);
 
 	return chars;
 }
diff --git a/drivers/usb/serial/ti_usb_3410_5052.c b/drivers/usb/serial/ti_usb_3410_5052.c
index 310db5abea9d..18c0bd853392 100644
--- a/drivers/usb/serial/ti_usb_3410_5052.c
+++ b/drivers/usb/serial/ti_usb_3410_5052.c
@@ -308,8 +308,8 @@ static int ti_open(struct tty_struct *tty, struct usb_serial_port *port);
 static void ti_close(struct usb_serial_port *port);
 static int ti_write(struct tty_struct *tty, struct usb_serial_port *port,
 		const unsigned char *data, int count);
-static int ti_write_room(struct tty_struct *tty);
-static int ti_chars_in_buffer(struct tty_struct *tty);
+static unsigned int ti_write_room(struct tty_struct *tty);
+static unsigned int ti_chars_in_buffer(struct tty_struct *tty);
 static bool ti_tx_empty(struct usb_serial_port *port);
 static void ti_throttle(struct tty_struct *tty);
 static void ti_unthrottle(struct tty_struct *tty);
@@ -813,34 +813,34 @@ static int ti_write(struct tty_struct *tty, struct usb_serial_port *port,
 }
 
 
-static int ti_write_room(struct tty_struct *tty)
+static unsigned int ti_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct ti_port *tport = usb_get_serial_port_data(port);
-	int room = 0;
+	unsigned int room;
 	unsigned long flags;
 
 	spin_lock_irqsave(&tport->tp_lock, flags);
 	room = kfifo_avail(&port->write_fifo);
 	spin_unlock_irqrestore(&tport->tp_lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, room);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, room);
 	return room;
 }
 
 
-static int ti_chars_in_buffer(struct tty_struct *tty)
+static unsigned int ti_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct ti_port *tport = usb_get_serial_port_data(port);
-	int chars = 0;
+	unsigned int chars;
 	unsigned long flags;
 
 	spin_lock_irqsave(&tport->tp_lock, flags);
 	chars = kfifo_len(&port->write_fifo);
 	spin_unlock_irqrestore(&tport->tp_lock, flags);
 
-	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
+	dev_dbg(&port->dev, "%s - returns %u\n", __func__, chars);
 	return chars;
 }
 
diff --git a/drivers/usb/serial/usb-wwan.h b/drivers/usb/serial/usb-wwan.h
index b5331d03092f..519101945769 100644
--- a/drivers/usb/serial/usb-wwan.h
+++ b/drivers/usb/serial/usb-wwan.h
@@ -11,13 +11,13 @@ extern int usb_wwan_open(struct tty_struct *tty, struct usb_serial_port *port);
 extern void usb_wwan_close(struct usb_serial_port *port);
 extern int usb_wwan_port_probe(struct usb_serial_port *port);
 extern void usb_wwan_port_remove(struct usb_serial_port *port);
-extern int usb_wwan_write_room(struct tty_struct *tty);
+extern unsigned int usb_wwan_write_room(struct tty_struct *tty);
 extern int usb_wwan_tiocmget(struct tty_struct *tty);
 extern int usb_wwan_tiocmset(struct tty_struct *tty,
 			     unsigned int set, unsigned int clear);
 extern int usb_wwan_write(struct tty_struct *tty, struct usb_serial_port *port,
 			  const unsigned char *buf, int count);
-extern int usb_wwan_chars_in_buffer(struct tty_struct *tty);
+extern unsigned int usb_wwan_chars_in_buffer(struct tty_struct *tty);
 #ifdef CONFIG_PM
 extern int usb_wwan_suspend(struct usb_serial *serial, pm_message_t message);
 extern int usb_wwan_resume(struct usb_serial *serial);
diff --git a/drivers/usb/serial/usb_wwan.c b/drivers/usb/serial/usb_wwan.c
index 3eb72c59ede6..cb01283d4d15 100644
--- a/drivers/usb/serial/usb_wwan.c
+++ b/drivers/usb/serial/usb_wwan.c
@@ -278,12 +278,12 @@ static void usb_wwan_outdat_callback(struct urb *urb)
 	}
 }
 
-int usb_wwan_write_room(struct tty_struct *tty)
+unsigned int usb_wwan_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct usb_wwan_port_private *portdata;
 	int i;
-	int data_len = 0;
+	unsigned int data_len = 0;
 	struct urb *this_urb;
 
 	portdata = usb_get_serial_port_data(port);
@@ -294,17 +294,17 @@ int usb_wwan_write_room(struct tty_struct *tty)
 			data_len += OUT_BUFLEN;
 	}
 
-	dev_dbg(&port->dev, "%s: %d\n", __func__, data_len);
+	dev_dbg(&port->dev, "%s: %u\n", __func__, data_len);
 	return data_len;
 }
 EXPORT_SYMBOL(usb_wwan_write_room);
 
-int usb_wwan_chars_in_buffer(struct tty_struct *tty)
+unsigned int usb_wwan_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct usb_wwan_port_private *portdata;
 	int i;
-	int data_len = 0;
+	unsigned int data_len = 0;
 	struct urb *this_urb;
 
 	portdata = usb_get_serial_port_data(port);
@@ -316,7 +316,7 @@ int usb_wwan_chars_in_buffer(struct tty_struct *tty)
 		if (this_urb && test_bit(i, &portdata->out_busy))
 			data_len += this_urb->transfer_buffer_length;
 	}
-	dev_dbg(&port->dev, "%s: %d\n", __func__, data_len);
+	dev_dbg(&port->dev, "%s: %u\n", __func__, data_len);
 	return data_len;
 }
 EXPORT_SYMBOL(usb_wwan_chars_in_buffer);
diff --git a/include/linux/usb/serial.h b/include/linux/usb/serial.h
index b81eb604e092..16ea5a4cc586 100644
--- a/include/linux/usb/serial.h
+++ b/include/linux/usb/serial.h
@@ -276,7 +276,7 @@ struct usb_serial_driver {
 	int  (*write)(struct tty_struct *tty, struct usb_serial_port *port,
 			const unsigned char *buf, int count);
 	/* Called only by the tty layer */
-	int  (*write_room)(struct tty_struct *tty);
+	unsigned int (*write_room)(struct tty_struct *tty);
 	int  (*ioctl)(struct tty_struct *tty,
 		      unsigned int cmd, unsigned long arg);
 	void (*get_serial)(struct tty_struct *tty, struct serial_struct *ss);
@@ -284,7 +284,7 @@ struct usb_serial_driver {
 	void (*set_termios)(struct tty_struct *tty,
 			struct usb_serial_port *port, struct ktermios *old);
 	void (*break_ctl)(struct tty_struct *tty, int break_state);
-	int  (*chars_in_buffer)(struct tty_struct *tty);
+	unsigned int (*chars_in_buffer)(struct tty_struct *tty);
 	void (*wait_until_sent)(struct tty_struct *tty, long timeout);
 	bool (*tx_empty)(struct usb_serial_port *port);
 	void (*throttle)(struct tty_struct *tty);
@@ -347,8 +347,8 @@ int usb_serial_generic_write(struct tty_struct *tty, struct usb_serial_port *por
 		const unsigned char *buf, int count);
 void usb_serial_generic_close(struct usb_serial_port *port);
 int usb_serial_generic_resume(struct usb_serial *serial);
-int usb_serial_generic_write_room(struct tty_struct *tty);
-int usb_serial_generic_chars_in_buffer(struct tty_struct *tty);
+unsigned int usb_serial_generic_write_room(struct tty_struct *tty);
+unsigned int usb_serial_generic_chars_in_buffer(struct tty_struct *tty);
 void usb_serial_generic_wait_until_sent(struct tty_struct *tty, long timeout);
 void usb_serial_generic_read_bulk_callback(struct urb *urb);
 void usb_serial_generic_write_bulk_callback(struct urb *urb);