[v2] USB: serial: cp210x: add minimun baud rate for CP2105 SCI

[Johanna Abrahamsson <johanna@mjao.org>]

From: Johanna Abrahamsson <johanna.abrahamsson@afconsult.com>

This patch adds 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>
---
 drivers/usb/serial/cp210x.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
index 260c875dd263..ceff735e961c 100644
--- a/drivers/usb/serial/cp210x.c
+++ b/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;
 };
@@ -1078,8 +1079,6 @@ static speed_t cp210x_get_actual_rate(struct usb_serial *serial, speed_t baud)
 	unsigned int prescale = 1;
 	unsigned int div;
 
-	baud = clamp(baud, 300u, priv->max_speed);
-
 	if (baud <= 365)
 		prescale = 4;
 
@@ -1122,7 +1121,7 @@ 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;
+	baud = clamp(tty->termios.c_ospeed, priv->min_speed, priv->max_speed);
 
 	/*
 	 * This maps the requested rate to the actual rate, a valid rate on
@@ -1134,8 +1133,6 @@ static void cp210x_change_speed(struct tty_struct *tty,
 		baud = cp210x_get_actual_rate(serial, 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)) {
@@ -1797,28 +1794,35 @@ 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;
 	speed_t max;
 
 	switch (priv->partnum) {
 	case CP210X_PARTNUM_CP2101:
+		min = 300;
 		max = 921600;
 		break;
 	case CP210X_PARTNUM_CP2102:
 	case CP210X_PARTNUM_CP2103:
+		min = 300;
 		max = 1000000;
 		break;
 	case CP210X_PARTNUM_CP2104:
 		use_actual_rate = true;
+		min = 300;
 		max = 2000000;
 		break;
 	case CP210X_PARTNUM_CP2108:
+		min = 300;
 		max = 2000000;
 		break;
 	case CP210X_PARTNUM_CP2105:
 		if (cp210x_interface_num(serial) == 0) {
 			use_actual_rate = true;
+			min = 300;
 			max = 2000000;	/* ECI */
 		} else {
+			min = 2400;
 			max = 921600;	/* SCI */
 		}
 		break;
@@ -1826,13 +1830,16 @@ static void cp210x_init_max_speed(struct usb_serial *serial)
 	case CP210X_PARTNUM_CP2102N_QFN24:
 	case CP210X_PARTNUM_CP2102N_QFN20:
 		use_actual_rate = true;
+		min = 300;
 		max = 3000000;
 		break;
 	default:
+		min = 300;
 		max = 2000000;
 		break;
 	}
 
+	priv->min_speed = min;
 	priv->max_speed = max;
 	priv->use_actual_rate = use_actual_rate;
 }