USB: serial: cp210x: fix alternate function for CP2102N QFN20

The QFN20 part has a different GPIO/port function assignment. The
configuration struct bit field ordered as TX/RX/RS485/WAKEUP/CLK
which exactly matches GPIO0-3 for QFN24/28. However, QFN20 has a
different GPIO to primary function assignment.

Special case QFN20 to follow to properly detect which GPIOs are
available.
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://lore.kernel.org/r/51830b2b24118eb0f77c5c9ac64ffb2f519dbb1d.1622218300.git.stefan@agner.ch
Fixes: c8acfe0a ("USB: serial: cp210x: implement GPIO support for CP2102N")
Cc: stable@vger.kernel.org	# 4.19
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/cp210x.c

@@ -537,6 +537,12 @@ struct cp210x_single_port_config {
 #define CP210X_2NCONFIG_GPIO_RSTLATCH_IDX	587
 #define CP210X_2NCONFIG_GPIO_CONTROL_IDX	600
 
+/* CP2102N QFN20 port configuration values */
+#define CP2102N_QFN20_GPIO2_TXLED_MODE		BIT(2)
+#define CP2102N_QFN20_GPIO3_RXLED_MODE		BIT(3)
+#define CP2102N_QFN20_GPIO1_RS485_MODE		BIT(4)
+#define CP2102N_QFN20_GPIO0_CLK_MODE		BIT(6)
+
 /* CP210X_VENDOR_SPECIFIC, CP210X_WRITE_LATCH call writes these 0x2 bytes. */
 struct cp210x_gpio_write {
 	u8	mask;
@@ -1733,7 +1739,19 @@ static int cp2102n_gpioconf_init(struct usb_serial *serial)
 	priv->gpio_pushpull = (gpio_pushpull >> 3) & 0x0f;
 
 	/* 0 indicates GPIO mode, 1 is alternate function */
-	priv->gpio_altfunc = (gpio_ctrl >> 2) & 0x0f;
+	if (priv->partnum == CP210X_PARTNUM_CP2102N_QFN20) {
+		/* QFN20 is special... */
+		if (gpio_ctrl & CP2102N_QFN20_GPIO0_CLK_MODE)   /* GPIO 0 */
+			priv->gpio_altfunc |= BIT(0);
+		if (gpio_ctrl & CP2102N_QFN20_GPIO1_RS485_MODE) /* GPIO 1 */
+			priv->gpio_altfunc |= BIT(1);
+		if (gpio_ctrl & CP2102N_QFN20_GPIO2_TXLED_MODE) /* GPIO 2 */
+			priv->gpio_altfunc |= BIT(2);
+		if (gpio_ctrl & CP2102N_QFN20_GPIO3_RXLED_MODE) /* GPIO 3 */
+			priv->gpio_altfunc |= BIT(3);
+	} else {
+		priv->gpio_altfunc = (gpio_ctrl >> 2) & 0x0f;
+	}
 
 	if (priv->partnum == CP210X_PARTNUM_CP2102N_QFN28) {
 		/*