USB: serial: cp210x: honour device-type maximum line speed

Newer cp210x devices support higher line speeds than the older ones
which supported a discrete set of speeds up to 921.6 kbaud.

To support these higher speeds, we have for some time mapped speeds
lower than 1 Mbaud to the speeds supported by older devices, while
allowing the device to pick the closest possible rate for higher speeds
(without trying to guess and report back what rate was actually chosen).

As this implementation can lead to undefined behaviour for older devices
which do not support the higher rates, let's use the later-added
device-type detection to determine the maximum supported speed.

This will also be useful when adding support for cp2102n which can
handle rates up to 3 Mbaud.

As per the data sheets the following maximum speeds are used

	cp2101		921.6 kbaud
	cp2102/3	1 Mbaud
	cp2104/8	2 Mbaud
	cp2105
	 - ECI port	2 Mbaud
	 - SCI port	921.6 kbaud

while keeping the maximum 2 Mbaud for unknown device types in order to
avoid any regressions.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -229,6 +229,7 @@ struct cp210x_serial_private {
 	bool			gpio_registered;
 #endif
 	u8			partnum;
+	speed_t			max_speed;
 };
 
 struct cp210x_port_private {
@@ -1052,19 +1053,20 @@ static speed_t cp210x_get_an205_rate(speed_t baud)
 static void cp210x_change_speed(struct tty_struct *tty,
 		struct usb_serial_port *port, struct ktermios *old_termios)
 {
+	struct cp210x_serial_private *priv = usb_get_serial_data(port->serial);
 	u32 baud;
 
 	baud = tty->termios.c_ospeed;
 
 	/* This maps the requested rate to a rate valid on cp2102 or cp2103,
-	 * or to an arbitrary rate in [1M,2M].
+	 * or to an arbitrary rate in [1M, max_speed]
 	 *
 	 * NOTE: B0 is not implemented.
 	 */
 	if (baud < 1000000)
 		baud = cp210x_get_an205_rate(baud);
-	else if (baud > 2000000)
-		baud = 2000000;
+	else if (baud > priv->max_speed)
+		baud = priv->max_speed;
 
 	dev_dbg(&port->dev, "%s - setting baud rate to %u\n", __func__, baud);
 	if (cp210x_write_u32_reg(port, CP210X_SET_BAUDRATE, baud)) {
@@ -1495,6 +1497,37 @@ static int cp210x_port_remove(struct usb_serial_port *port)
 	return 0;
 }
 
+static void cp210x_init_max_speed(struct usb_serial *serial)
+{
+	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
+	speed_t max;
+
+	switch (priv->partnum) {
+	case CP210X_PARTNUM_CP2101:
+		max = 921600;
+		break;
+	case CP210X_PARTNUM_CP2102:
+	case CP210X_PARTNUM_CP2103:
+		max = 1000000;
+		break;
+	case CP210X_PARTNUM_CP2104:
+	case CP210X_PARTNUM_CP2108:
+		max = 2000000;
+		break;
+	case CP210X_PARTNUM_CP2105:
+		if (cp210x_interface_num(serial) == 0)
+			max = 2000000;	/* ECI */
+		else
+			max = 921600;	/* SCI */
+		break;
+	default:
+		max = 2000000;
+		break;
+	}
+
+	priv->max_speed = max;
+}
+
 static int cp210x_attach(struct usb_serial *serial)
 {
 	int result;
@@ -1515,6 +1548,8 @@ static int cp210x_attach(struct usb_serial *serial)
 
 	usb_set_serial_data(serial, priv);
 
+	cp210x_init_max_speed(serial);
+
 	if (priv->partnum == CP210X_PARTNUM_CP2105) {
 		result = cp2105_shared_gpio_init(serial);
 		if (result < 0) {