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>

diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
index 72314734dfd0ea516a3174da1a9c860e2bef6267..2b9b5119ff1302a4abcc443b5e13b9a0051d0d9e 100644 (file)
--- a/drivers/usb/serial/ftdi_sio.c
+++ b/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;