Add support for Moxa PCIe multiport serial boards to switch serial
interfaces via the ioctl() command "TIOCSRS485" using the following
flags in the "serial_rs485" struct:

    - RS232                  = (no flags set)
    - RS422                  = SER_RS485_ENABLED | SER_RS485_MODE_RS422
    - RS485_2W (half-duplex) = SER_RS485_ENABLED
    - RS485_4W (full-duplex) = SER_RS485_ENABLED | SER_RS485_RX_DURING_TX

This allows users to dynamically configure the serial mode without
requiring additional hardware modifications.

Signed-off-by: Crescent Hsieh <crescentcy.hsieh@moxa.com>
---
 drivers/tty/serial/8250/8250_mxpcie.c | 45 +++++++++++++++++++++++++--
 1 file changed, 43 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_mxpcie.c b/drivers/tty/serial/8250/8250_mxpcie.c
index 90ce522fe40f..5d1097c166e4 100644
--- a/drivers/tty/serial/8250/8250_mxpcie.c
+++ b/drivers/tty/serial/8250/8250_mxpcie.c
@@ -105,6 +105,10 @@ enum {
 	MOXA_SUPP_RS485 = BIT(2),
 };
 
+static const struct serial_rs485 mxpcie8250_rs485_supported = {
+	.flags = SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND | SER_RS485_RX_DURING_TX | SER_RS485_MODE_RS422,
+};
+
 static bool mxpcie8250_is_mini_pcie(unsigned short device)
 {
 	if (device == PCI_DEVICE_ID_MOXA_CP102N ||
@@ -166,6 +170,38 @@ static int mxpcie8250_set_interface(struct mxpcie8250 *priv,
 	return 0;
 }
 
+/*
+ * Moxa PCIe multiport serial boards support switching serial interfaces
+ * via the ioctl() command "TIOCSRS485". Supported modes and corresponding
+ * flags in "serial_rs485":
+ *
+ *	RS232			= (no flags set)
+ *	RS422			= SER_RS485_ENABLED | SER_RS485_MODE_RS422
+ *	RS485_2W (half-duplex)	= SER_RS485_ENABLED
+ *	RS485_4W (full-duplex)	= SER_RS485_ENABLED | SER_RS485_RX_DURING_TX
+ */
+static int mxpcie8250_rs485_config(struct uart_port *port,
+				   struct ktermios *termios,
+				   struct serial_rs485 *rs485)
+{
+	struct pci_dev *pdev = to_pci_dev(port->dev);
+	struct mxpcie8250 *priv = pci_get_drvdata(pdev);
+	u8 mode = MOXA_UIR_RS232;
+
+	if (rs485->flags & SER_RS485_ENABLED) {
+		if (rs485->flags & SER_RS485_MODE_RS422)
+			mode = MOXA_UIR_RS422;
+		else if (rs485->flags & SER_RS485_RX_DURING_TX)
+			mode = MOXA_UIR_RS485_4W;
+		else
+			mode = MOXA_UIR_RS485_2W;
+	} else if (!(priv->supp_rs & MOXA_SUPP_RS232)) {
+		return -ENODEV;
+	}
+
+	return mxpcie8250_set_interface(priv, port->port_id, mode);
+}
+
 static void mxpcie8250_set_termios(struct uart_port *port,
 				   struct ktermios *new,
 				   const struct ktermios *old)
@@ -392,9 +428,14 @@ static int mxpcie8250_setup(struct pci_dev *pdev,
 	int offset = idx * MOXA_PUART_OFFSET;
 	u8 init_mode = MOXA_UIR_RS232;
 
-	if (!(priv->supp_rs & MOXA_SUPP_RS232))
+	if (priv->supp_rs & MOXA_SUPP_RS485) {
+		up->port.rs485_config = mxpcie8250_rs485_config;
+		up->port.rs485_supported = mxpcie8250_rs485_supported;
+	}
+	if (!(priv->supp_rs & MOXA_SUPP_RS232)) {
 		init_mode = MOXA_UIR_RS422;
-
+		up->port.rs485.flags = SER_RS485_ENABLED | SER_RS485_MODE_RS422;
+	}
 	mxpcie8250_set_interface(priv, idx, init_mode);
 
 	if (idx == 3 &&
-- 
2.45.2