USB: serial: cp210x: generalise CP2102N line-speed handling

The CP2102N equations for determining the actual baud rate can be used
also for other device types, so let's factor it out.

Note that this removes the now unused cp210x_is_cp2102n() helper.
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -230,6 +230,7 @@ struct cp210x_serial_private {
 #endif
 	u8			partnum;
 	speed_t			max_speed;
+	bool			use_actual_rate;
 };
 
 struct cp210x_port_private {
@@ -457,15 +458,6 @@ struct cp210x_gpio_write {
 	u8	state;
 } __packed;
 
-static bool cp210x_is_cp2102n(struct usb_serial *serial)
-{
-	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
-
-	return	(priv->partnum == CP210X_PARTNUM_CP2102N_QFN28) ||
-		(priv->partnum == CP210X_PARTNUM_CP2102N_QFN24) ||
-		(priv->partnum == CP210X_PARTNUM_CP2102N_QFN20);
-}
-
 /*
  * Helper to get interface number when we only have struct usb_serial.
  */
@@ -1036,6 +1028,23 @@ static speed_t cp210x_get_an205_rate(speed_t baud)
 	return cp210x_an205_table1[i].rate;
 }
 
+static speed_t cp210x_get_actual_rate(struct usb_serial *serial, speed_t baud)
+{
+	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
+	unsigned int prescale = 1;
+	unsigned int div;
+
+	baud = clamp(baud, 300u, priv->max_speed);
+
+	if (baud <= 365)
+		prescale = 4;
+
+	div = DIV_ROUND_CLOSEST(48000000, 2 * prescale * baud);
+	baud = 48000000 / (2 * prescale * div);
+
+	return baud;
+}
+
 /*
  * CP2101 supports the following baud rates:
  *
@@ -1072,25 +1081,17 @@ static void cp210x_change_speed(struct tty_struct *tty,
 	baud = tty->termios.c_ospeed;
 
 	/*
-	 * This maps the requested rate to the actual rate on cp2102n, a valid
-	 * rate on cp2102 or cp2103, or to an arbitrary rate in
-	 * [1M, max_speed].
+	 * This maps the requested rate to the actual rate, a valid rate on
+	 * cp2102 or cp2103, or to an arbitrary rate in [1M, max_speed].
 	 *
 	 * NOTE: B0 is not implemented.
 	 */
-	if (cp210x_is_cp2102n(serial)) {
-		int clk_div;
-		int prescaler;
-
-		baud = clamp(baud, 300u, priv->max_speed);
-		prescaler = (baud <= 365) ? 4 : 1;
-		clk_div = DIV_ROUND_CLOSEST(48000000, 2 * prescaler * baud);
-		baud = 48000000 / (2 * prescaler * clk_div);
-	} else if (baud < 1000000) {
+	if (priv->use_actual_rate)
+		baud = cp210x_get_actual_rate(serial, baud);
+	else if (baud < 1000000)
 		baud = cp210x_get_an205_rate(baud);
-	} else if (baud > priv->max_speed) {
+	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)) {
@@ -1524,6 +1525,7 @@ static int cp210x_port_remove(struct usb_serial_port *port)
 static void cp210x_init_max_speed(struct usb_serial *serial)
 {
 	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
+	bool use_actual_rate = false;
 	speed_t max;
 
 	switch (priv->partnum) {
@@ -1547,6 +1549,7 @@ static void cp210x_init_max_speed(struct usb_serial *serial)
 	case CP210X_PARTNUM_CP2102N_QFN28:
 	case CP210X_PARTNUM_CP2102N_QFN24:
 	case CP210X_PARTNUM_CP2102N_QFN20:
+		use_actual_rate = true;
 		max = 3000000;
 		break;
 	default:
@@ -1555,6 +1558,7 @@ static void cp210x_init_max_speed(struct usb_serial *serial)
 	}
 
 	priv->max_speed = max;
+	priv->use_actual_rate = use_actual_rate;
 }
 
 static int cp210x_attach(struct usb_serial *serial)