diff --git a/drivers/usb/serial/belkin_sa.h b/drivers/usb/serial/belkin_sa.h
index a13a98d284f2..89ec30c63cc6 100644
--- a/drivers/usb/serial/belkin_sa.h
+++ b/drivers/usb/serial/belkin_sa.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Definitions for Belkin USB Serial Adapter Driver
  *
diff --git a/drivers/usb/serial/ch341.c b/drivers/usb/serial/ch341.c
index c5ecdcd51ffc..89675ee29645 100644
--- a/drivers/usb/serial/ch341.c
+++ b/drivers/usb/serial/ch341.c
@@ -73,6 +73,8 @@
 #define CH341_LCR_CS6          0x01
 #define CH341_LCR_CS5          0x00
 
+#define CH341_QUIRK_LIMITED_PRESCALER	BIT(0)
+
 static const struct usb_device_id id_table[] = {
 	{ USB_DEVICE(0x4348, 0x5523) },
 	{ USB_DEVICE(0x1a86, 0x7523) },
@@ -87,6 +89,7 @@ struct ch341_private {
 	u8 mcr;
 	u8 msr;
 	u8 lcr;
+	unsigned long quirks;
 };
 
 static void ch341_set_termios(struct tty_struct *tty,
@@ -159,9 +162,11 @@ static const speed_t ch341_min_rates[] = {
  *		2 <= div <= 256 if fact = 0, or
  *		9 <= div <= 256 if fact = 1
  */
-static int ch341_get_divisor(speed_t speed)
+static int ch341_get_divisor(struct ch341_private *priv)
 {
 	unsigned int fact, div, clk_div;
+	speed_t speed = priv->baud_rate;
+	bool force_fact0 = false;
 	int ps;
 
 	/*
@@ -187,8 +192,12 @@ static int ch341_get_divisor(speed_t speed)
 	clk_div = CH341_CLK_DIV(ps, fact);
 	div = CH341_CLKRATE / (clk_div * speed);
 
+	/* Some devices require a lower base clock if ps < 3. */
+	if (ps < 3 && (priv->quirks & CH341_QUIRK_LIMITED_PRESCALER))
+		force_fact0 = true;
+
 	/* Halve base clock (fact = 0) if required. */
-	if (div < 9 || div > 255) {
+	if (div < 9 || div > 255 || force_fact0) {
 		div /= 2;
 		clk_div *= 2;
 		fact = 0;
@@ -227,7 +236,7 @@ static int ch341_set_baudrate_lcr(struct usb_device *dev,
 	if (!priv->baud_rate)
 		return -EINVAL;
 
-	val = ch341_get_divisor(priv->baud_rate);
+	val = ch341_get_divisor(priv);
 	if (val < 0)
 		return -EINVAL;
 
@@ -308,6 +317,54 @@ out:	kfree(buffer);
 	return r;
 }
 
+static int ch341_detect_quirks(struct usb_serial_port *port)
+{
+	struct ch341_private *priv = usb_get_serial_port_data(port);
+	struct usb_device *udev = port->serial->dev;
+	const unsigned int size = 2;
+	unsigned long quirks = 0;
+	char *buffer;
+	int r;
+
+	buffer = kmalloc(size, GFP_KERNEL);
+	if (!buffer)
+		return -ENOMEM;
+
+	/*
+	 * A subset of CH34x devices does not support all features. The
+	 * prescaler is limited and there is no support for sending a RS232
+	 * break condition. A read failure when trying to set up the latter is
+	 * used to detect these devices.
+	 */
+	r = usb_control_msg(udev, usb_rcvctrlpipe(udev, 0), CH341_REQ_READ_REG,
+			    USB_TYPE_VENDOR | USB_RECIP_DEVICE | USB_DIR_IN,
+			    CH341_REG_BREAK, 0, buffer, size, DEFAULT_TIMEOUT);
+	if (r == -EPIPE) {
+		dev_dbg(&port->dev, "break control not supported\n");
+		quirks = CH341_QUIRK_LIMITED_PRESCALER;
+		r = 0;
+		goto out;
+	}
+
+	if (r != size) {
+		if (r >= 0)
+			r = -EIO;
+		dev_err(&port->dev, "failed to read break control: %d\n", r);
+		goto out;
+	}
+
+	r = 0;
+out:
+	kfree(buffer);
+
+	if (quirks) {
+		dev_dbg(&port->dev, "enabling quirk flags: 0x%02lx\n", quirks);
+		priv->quirks |= quirks;
+	}
+
+	return r;
+}
+
 static int ch341_port_probe(struct usb_serial_port *port)
 {
 	struct ch341_private *priv;
@@ -330,6 +387,11 @@ static int ch341_port_probe(struct usb_serial_port *port)
 		goto error;
 
 	usb_set_serial_port_data(port, priv);
+
+	r = ch341_detect_quirks(port);
+	if (r < 0)
+		goto error;
+
 	return 0;
 
 error:	kfree(priv);
diff --git a/drivers/usb/serial/io_16654.h b/drivers/usb/serial/io_16654.h
index 4980f72dc56f..f18501f056cf 100644
--- a/drivers/usb/serial/io_16654.h
+++ b/drivers/usb/serial/io_16654.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /************************************************************************
  *
  *	16654.H		Definitions for 16C654 UART used on EdgePorts
diff --git a/drivers/usb/serial/io_edgeport.h b/drivers/usb/serial/io_edgeport.h
index 2e7fedbaf2ff..43ba53a3a6fa 100644
--- a/drivers/usb/serial/io_edgeport.h
+++ b/drivers/usb/serial/io_edgeport.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /************************************************************************
  *
  *	io_edgeport.h	Edgeport Linux Interface definitions
diff --git a/drivers/usb/serial/io_ionsp.h b/drivers/usb/serial/io_ionsp.h
index 4b8e4823bd45..db4fce815c97 100644
--- a/drivers/usb/serial/io_ionsp.h
+++ b/drivers/usb/serial/io_ionsp.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /************************************************************************
  *
  *	IONSP.H		Definitions for I/O Networks Serial Protocol
diff --git a/drivers/usb/serial/io_ti.h b/drivers/usb/serial/io_ti.h
index 9bbcee37524e..50b899d55ed0 100644
--- a/drivers/usb/serial/io_ti.h
+++ b/drivers/usb/serial/io_ti.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*****************************************************************************
  *
  *	Copyright (C) 1997-2002 Inside Out Networks, Inc.
diff --git a/drivers/usb/serial/io_usbvend.h b/drivers/usb/serial/io_usbvend.h
index 0d1a5bb4636e..52cbc353051f 100644
--- a/drivers/usb/serial/io_usbvend.h
+++ b/drivers/usb/serial/io_usbvend.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /************************************************************************
  *
  *	USBVEND.H		Vendor-specific USB definitions
diff --git a/drivers/usb/serial/iuu_phoenix.h b/drivers/usb/serial/iuu_phoenix.h
index b400b262f72e..87992b24d904 100644
--- a/drivers/usb/serial/iuu_phoenix.h
+++ b/drivers/usb/serial/iuu_phoenix.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Infinity Unlimited USB Phoenix driver
  *
diff --git a/drivers/usb/serial/mct_u232.h b/drivers/usb/serial/mct_u232.h
index 0084edf518e8..e3d09a83cab1 100644
--- a/drivers/usb/serial/mct_u232.h
+++ b/drivers/usb/serial/mct_u232.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Definitions for MCT (Magic Control Technology) USB-RS232 Converter Driver
  *
diff --git a/drivers/usb/serial/oti6858.h b/drivers/usb/serial/oti6858.h
index 1226bf2347eb..5c25836fdcd9 100644
--- a/drivers/usb/serial/oti6858.h
+++ b/drivers/usb/serial/oti6858.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Ours Technology Inc. OTi-6858 USB to serial adapter driver.
  */
diff --git a/drivers/usb/serial/pl2303.h b/drivers/usb/serial/pl2303.h
index 52db5519aaf0..7d3090ee7e0c 100644
--- a/drivers/usb/serial/pl2303.h
+++ b/drivers/usb/serial/pl2303.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Prolific PL2303 USB to serial adaptor driver header file
  */
diff --git a/drivers/usb/serial/usb_wwan.c b/drivers/usb/serial/usb_wwan.c
index 13be21aad2f4..4b9845807bee 100644
--- a/drivers/usb/serial/usb_wwan.c
+++ b/drivers/usb/serial/usb_wwan.c
@@ -270,6 +270,10 @@ static void usb_wwan_indat_callback(struct urb *urb)
 	if (status) {
 		dev_dbg(dev, "%s: nonzero status: %d on endpoint %02x.\n",
 			__func__, status, endpoint);
+
+		/* don't resubmit on fatal errors */
+		if (status == -ESHUTDOWN || status == -ENOENT)
+			return;
 	} else {
 		if (urb->actual_length) {
 			tty_insert_flip_string(&port->port, data,
diff --git a/drivers/usb/serial/visor.h b/drivers/usb/serial/visor.h
index 4bd69d047036..622d639ce74e 100644
--- a/drivers/usb/serial/visor.h
+++ b/drivers/usb/serial/visor.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * USB HandSpring Visor driver
  *
diff --git a/drivers/usb/serial/whiteheat.h b/drivers/usb/serial/whiteheat.h
index 269e727a92f9..7e63074c9128 100644
--- a/drivers/usb/serial/whiteheat.h
+++ b/drivers/usb/serial/whiteheat.h
@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * USB ConnectTech WhiteHEAT driver
  *