USB: serial: cp210x: improve baudrate support for CP2102N

CP2102N devices support a lot more baudrates than earlier chips by
SiLabs. These devices are not constrained anymore by the table in AN205,
and are able to generate almost any baudrate in the supported range
with only minimal errors. This has also been verified with a scope on
a physical device. This patch adds support for all baudrates supported
by the CP2102N.

Signed-off-by: Karoly Pados <pados@pados.hu>
[johan: rework on top of an205 and max-speed patches ]
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -355,6 +355,9 @@ static struct usb_serial_driver * const serial_drivers[] = {
 #define CP210X_PARTNUM_CP2104	0x04
 #define CP210X_PARTNUM_CP2105	0x05
 #define CP210X_PARTNUM_CP2108	0x08
+#define CP210X_PARTNUM_CP2102N_QFN28	0x20
+#define CP210X_PARTNUM_CP2102N_QFN24	0x21
+#define CP210X_PARTNUM_CP2102N_QFN20	0x22
 #define CP210X_PARTNUM_UNKNOWN	0xFF
 
 /* CP210X_GET_COMM_STATUS returns these 0x13 bytes */
@@ -454,6 +457,15 @@ 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.
  */
@@ -1053,20 +1065,32 @@ 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);
+	struct usb_serial *serial = port->serial;
+	struct cp210x_serial_private *priv = usb_get_serial_data(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, max_speed]
+	/*
+	 * 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].
 	 *
 	 * NOTE: B0 is not implemented.
 	 */
-	if (baud < 1000000)
+	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) {
 		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)) {
@@ -1520,6 +1544,11 @@ static void cp210x_init_max_speed(struct usb_serial *serial)
 		else
 			max = 921600;	/* SCI */
 		break;
+	case CP210X_PARTNUM_CP2102N_QFN28:
+	case CP210X_PARTNUM_CP2102N_QFN24:
+	case CP210X_PARTNUM_CP2102N_QFN20:
+		max = 3000000;
+		break;
 	default:
 		max = 2000000;
 		break;