[PATCH v2] USB: serial: cp210x: Improve baudrate support for CP2102N

[Karoly Pados]

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@xxxxxxxx>
---
 drivers/usb/serial/cp210x.c | 104 +++++++++++++++++++++++-------------
 1 file changed, 68 insertions(+), 36 deletions(-)

diff --git a/drivers/usb/serial/cp210x.c b/drivers/usb/serial/cp210x.c
index b1849f657e01..5c943e181d68 100644
--- a/drivers/usb/serial/cp210x.c
+++ b/drivers/usb/serial/cp210x.c
@@ -357,6 +357,9 @@ 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 */
@@ -456,6 +459,19 @@ struct cp210x_gpio_write {
 	u8	state;
 } __packed;
 
+/*
+ * Helper to determine if a specific serial device belongs to the cp2102n
+ * family of devices.
+ */
+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.
  */
@@ -757,43 +773,59 @@ static int cp210x_get_line_ctl(struct usb_serial_port *port, u16 *ctl)
 
 /*
  * cp210x_quantise_baudrate
- * Quantises the baud rate as per AN205 Table 1
+ * Determine a valid baudrate nearest to the requested one.
  */
-static unsigned int cp210x_quantise_baudrate(unsigned int baud)
+static unsigned int cp210x_quantise_baudrate(struct usb_serial *serial,
+					     unsigned int baud)
 {
-	if (baud <= 300)
-		baud = 300;
-	else if (baud <= 600)      baud = 600;
-	else if (baud <= 1200)     baud = 1200;
-	else if (baud <= 1800)     baud = 1800;
-	else if (baud <= 2400)     baud = 2400;
-	else if (baud <= 4000)     baud = 4000;
-	else if (baud <= 4803)     baud = 4800;
-	else if (baud <= 7207)     baud = 7200;
-	else if (baud <= 9612)     baud = 9600;
-	else if (baud <= 14428)    baud = 14400;
-	else if (baud <= 16062)    baud = 16000;
-	else if (baud <= 19250)    baud = 19200;
-	else if (baud <= 28912)    baud = 28800;
-	else if (baud <= 38601)    baud = 38400;
-	else if (baud <= 51558)    baud = 51200;
-	else if (baud <= 56280)    baud = 56000;
-	else if (baud <= 58053)    baud = 57600;
-	else if (baud <= 64111)    baud = 64000;
-	else if (baud <= 77608)    baud = 76800;
-	else if (baud <= 117028)   baud = 115200;
-	else if (baud <= 129347)   baud = 128000;
-	else if (baud <= 156868)   baud = 153600;
-	else if (baud <= 237832)   baud = 230400;
-	else if (baud <= 254234)   baud = 250000;
-	else if (baud <= 273066)   baud = 256000;
-	else if (baud <= 491520)   baud = 460800;
-	else if (baud <= 567138)   baud = 500000;
-	else if (baud <= 670254)   baud = 576000;
-	else if (baud < 1000000)
-		baud = 921600;
-	else if (baud > 2000000)
-		baud = 2000000;
+	if (cp210x_is_cp2102n(serial)) {
+		int clk_div;
+		int prescaler;
+
+		/*
+		 * Limit to valid range, then
+		 * calculate actual baudrate as per cp2102n datasheet.
+		 */
+		baud = clamp(baud, 300u, 3000000u);
+		prescaler = baud <= 365 ? 4 : 1;
+		clk_div = DIV_ROUND_CLOSEST(48000000, 2 * prescaler * baud);
+		baud = 48000000 / (2 * prescaler * clk_div);
+	} else {
+		/* Quantise baudrate as per AN205 Table 1 */
+		if (baud <= 300)
+			baud = 300;
+		else if (baud <= 600)      baud = 600;
+		else if (baud <= 1200)     baud = 1200;
+		else if (baud <= 1800)     baud = 1800;
+		else if (baud <= 2400)     baud = 2400;
+		else if (baud <= 4000)     baud = 4000;
+		else if (baud <= 4803)     baud = 4800;
+		else if (baud <= 7207)     baud = 7200;
+		else if (baud <= 9612)     baud = 9600;
+		else if (baud <= 14428)    baud = 14400;
+		else if (baud <= 16062)    baud = 16000;
+		else if (baud <= 19250)    baud = 19200;
+		else if (baud <= 28912)    baud = 28800;
+		else if (baud <= 38601)    baud = 38400;
+		else if (baud <= 51558)    baud = 51200;
+		else if (baud <= 56280)    baud = 56000;
+		else if (baud <= 58053)    baud = 57600;
+		else if (baud <= 64111)    baud = 64000;
+		else if (baud <= 77608)    baud = 76800;
+		else if (baud <= 117028)   baud = 115200;
+		else if (baud <= 129347)   baud = 128000;
+		else if (baud <= 156868)   baud = 153600;
+		else if (baud <= 237832)   baud = 230400;
+		else if (baud <= 254234)   baud = 250000;
+		else if (baud <= 273066)   baud = 256000;
+		else if (baud <= 491520)   baud = 460800;
+		else if (baud <= 567138)   baud = 500000;
+		else if (baud <= 670254)   baud = 576000;
+		else if (baud < 1000000)
+			baud = 921600;
+		else if (baud > 2000000)
+			baud = 2000000;
+	}
 	return baud;
 }
 
@@ -1054,7 +1086,7 @@ static void cp210x_change_speed(struct tty_struct *tty,
 	 *
 	 * NOTE: B0 is not implemented.
 	 */
-	baud = cp210x_quantise_baudrate(baud);
+	baud = cp210x_quantise_baudrate(port->serial, baud);
 
 	dev_dbg(&port->dev, "%s - setting baud rate to %u\n", __func__, baud);
 	if (cp210x_write_u32_reg(port, CP210X_SET_BAUDRATE, baud)) {
-- 
2.17.1