USB: serial: ftdi_sio: detect BM chip with iSerialNumber bug

If a BM type chip has iSerialNumber set to 0 in its EEPROM, an incorrect
value is read from the bcdDevice field of the USB descriptor, making it
look like an AM type chip.  Attempt to correct this in
ftdi_determine_type() by attempting to read the latency timer for an AM
type chip if it has iSerialNumber set to 0.  If that succeeds, assume it
is a BM type chip.

Currently, read_latency_timer() bails out without reading the latency
timer for an AM type chip, so factor out the guts of
read_latency_timer() into a new function _read_latency_timer() that
attempts to read the latency timer regardless of chip type, and returns
either the latency timer value or a negative error number.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ftdi_sio.c

@@ -1425,16 +1425,13 @@ static int write_latency_timer(struct usb_serial_port *port)
 	return rv;
 }
 
-static int read_latency_timer(struct usb_serial_port *port)
+static int _read_latency_timer(struct usb_serial_port *port)
 {
 	struct ftdi_private *priv = usb_get_serial_port_data(port);
 	struct usb_device *udev = port->serial->dev;
 	unsigned char *buf;
 	int rv;
 
-	if (priv->chip_type == SIO || priv->chip_type == FT8U232AM)
-		return -EINVAL;
-
 	buf = kmalloc(1, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
@@ -1446,11 +1443,10 @@ static int read_latency_timer(struct usb_serial_port *port)
 			     0, priv->interface,
 			     buf, 1, WDR_TIMEOUT);
 	if (rv < 1) {
-		dev_err(&port->dev, "Unable to read latency timer: %i\n", rv);
 		if (rv >= 0)
 			rv = -EIO;
 	} else {
-		priv->latency = buf[0];
+		rv = buf[0];
 	}
 
 	kfree(buf);
@@ -1458,6 +1454,25 @@ static int read_latency_timer(struct usb_serial_port *port)
 	return rv;
 }
 
+static int read_latency_timer(struct usb_serial_port *port)
+{
+	struct ftdi_private *priv = usb_get_serial_port_data(port);
+	int rv;
+
+	if (priv->chip_type == SIO || priv->chip_type == FT8U232AM)
+		return -EINVAL;
+
+	rv = _read_latency_timer(port);
+	if (rv < 0) {
+		dev_err(&port->dev, "Unable to read latency timer: %i\n", rv);
+		return rv;
+	}
+
+	priv->latency = rv;
+
+	return 0;
+}
+
 static int get_serial_info(struct usb_serial_port *port,
 				struct serial_struct __user *retinfo)
 {
@@ -1609,9 +1624,19 @@ static void ftdi_determine_type(struct usb_serial_port *port)
 		priv->baud_base = 12000000 / 16;
 	} else if (version < 0x400) {
 		/* Assume it's an FT8U232AM (or FT8U245AM) */
-		/* (It might be a BM because of the iSerialNumber bug,
-		 * but it will still work as an AM device.) */
 		priv->chip_type = FT8U232AM;
+		/*
+		 * It might be a BM type because of the iSerialNumber bug.
+		 * If iSerialNumber==0 and the latency timer is readable,
+		 * assume it is BM type.
+		 */
+		if (udev->descriptor.iSerialNumber == 0 &&
+				_read_latency_timer(port) >= 0) {
+			dev_dbg(&port->dev,
+				"%s: has latency timer so not an AM type\n",
+				__func__);
+			priv->chip_type = FT232BM;
+		}
 	} else if (version < 0x600) {
 		/* Assume it's an FT232BM (or FT245BM) */
 		priv->chip_type = FT232BM;