USB: serial: cp210x: add minimum baud rate for CP2105 SCI

Add minimum baud rate to the cp210x driver.

According to the datasheet for CP2105, the SCI supports 2400 as the
lowest baud rate. As this is not heeded in the current code, an error
message 'failed set req 0x1e size 4 status: -32' when trying to set a
lower baud rate such as 300.
The other cp210x models to date supports a minimum baud rate of 300.
Signed-off-by: Johanna Abrahamsson <johanna.abrahamsson@afconsult.com>
[ johan: simplify min_speed init, move clamp after comment, and drop
         unused serial-data pointer from cp210x_get_actual_rate() ]
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -245,6 +245,7 @@ struct cp210x_serial_private {
 	u8			gpio_input;
 #endif
 	u8			partnum;
+	speed_t			min_speed;
 	speed_t			max_speed;
 	bool			use_actual_rate;
 };
@@ -1072,14 +1073,11 @@ 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)
+static speed_t cp210x_get_actual_rate(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;
 
@@ -1122,20 +1120,18 @@ static void cp210x_change_speed(struct tty_struct *tty,
 	struct cp210x_serial_private *priv = usb_get_serial_data(serial);
 	u32 baud;
 
-	baud = tty->termios.c_ospeed;
-
 	/*
 	 * 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.
 	 */
+	baud = clamp(tty->termios.c_ospeed, priv->min_speed, priv->max_speed);
+
 	if (priv->use_actual_rate)
-		baud = cp210x_get_actual_rate(serial, baud);
+		baud = cp210x_get_actual_rate(baud);
 	else if (baud < 1000000)
 		baud = cp210x_get_an205_rate(baud);
-	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)) {
@@ -1797,6 +1793,7 @@ 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 min = 300;
 	speed_t max;
 
 	switch (priv->partnum) {
@@ -1819,6 +1816,7 @@ static void cp210x_init_max_speed(struct usb_serial *serial)
 			use_actual_rate = true;
 			max = 2000000;	/* ECI */
 		} else {
+			min = 2400;
 			max = 921600;	/* SCI */
 		}
 		break;
@@ -1833,6 +1831,7 @@ static void cp210x_init_max_speed(struct usb_serial *serial)
 		break;
 	}
 
+	priv->min_speed = min;
 	priv->max_speed = max;
 	priv->use_actual_rate = use_actual_rate;
 }