linux-kernel - [PATCH v1 12/31] serial: 8250_mxpcie: enable auto RTS/CTS flow control

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Message-ID: <20251130104222.63077-13-crescentcy.hsieh@moxa.com>
Date: Sun, 30 Nov 2025 18:42:03 +0800
From: Crescent Hsieh <crescentcy.hsieh@...a.com>
To: gregkh@...uxfoundation.org,
	jirislaby@...nel.org,
	ilpo.jarvinen@...ux.intel.com,
	andy.shevchenko@...il.com
Cc: linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org,
	crescentcy.hsieh@...a.com
Subject: [PATCH v1 12/31] serial: 8250_mxpcie: enable auto RTS/CTS flow control

Implement a custom set_termios() to configure RTS/CTS hardware flow
control using the Enhanced Feature Register (EFR). This enables support
for automatic RTS/CTS signaling when CRTSCTS is enabled in termios.

Signed-off-by: Crescent Hsieh <crescentcy.hsieh@...a.com>
---
 drivers/tty/serial/8250/8250_mxpcie.c | 28 +++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_mxpcie.c b/drivers/tty/serial/8250/8250_mxpcie.c
index 39a178dc0519..197d016354c9 100644
--- a/drivers/tty/serial/8250/8250_mxpcie.c
+++ b/drivers/tty/serial/8250/8250_mxpcie.c
@@ -43,6 +43,10 @@
 /* Enhanced Function Register (EFR) */
 #define MOXA_PUART_EFR			0x0A
 #define MOXA_PUART_EFR_ENHANCED		BIT(4)
+#define MOXA_PUART_EFR_AUTO_RTS		BIT(6)
+#define MOXA_PUART_EFR_AUTO_CTS		BIT(7)
+#define MOXA_PUART_EFR_RX_FLOW_MASK	GENMASK(1, 0)
+#define MOXA_PUART_EFR_TX_FLOW_MASK	GENMASK(3, 2)
 
 #define MOXA_PUART_TTL		0x10	/* Tx Interrupt Trigger Level */
 #define MOXA_PUART_RTL		0x11	/* Rx Interrupt Trigger Level */
@@ -137,6 +141,29 @@ static int mxpcie8250_set_interface(struct mxpcie8250 *priv,
 	return 0;
 }
 
+static void mxpcie8250_set_termios(struct uart_port *port,
+				   struct ktermios *new,
+				   const struct ktermios *old)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+	struct tty_struct *tty = port->state->port.tty;
+	unsigned int cflag = tty->termios.c_cflag;
+	u8 efr;
+
+	serial8250_do_set_termios(port, new, old);
+
+	up->port.status &= ~(UPSTAT_AUTORTS | UPSTAT_AUTOCTS);
+
+	efr = serial_in(up, MOXA_PUART_EFR);
+	efr &= ~(MOXA_PUART_EFR_AUTO_RTS | MOXA_PUART_EFR_AUTO_CTS);
+
+	if (cflag & CRTSCTS) {
+		efr |= (MOXA_PUART_EFR_AUTO_RTS | MOXA_PUART_EFR_AUTO_CTS);
+		up->port.status |= (UPSTAT_AUTORTS | UPSTAT_AUTOCTS);
+	}
+	serial_out(up, MOXA_PUART_EFR, efr);
+}
+
 static int mxpcie8250_startup(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
@@ -254,6 +281,7 @@ static int mxpcie8250_probe(struct pci_dev *pdev, const struct pci_device_id *id
 	up.port.flags = UPF_SKIP_TEST | UPF_BOOT_AUTOCONF | UPF_SHARE_IRQ | UPF_FIXED_TYPE;
 	up.port.type = PORT_MUEX50;
 
+	up.port.set_termios = mxpcie8250_set_termios;
 	up.port.startup = mxpcie8250_startup;
 	up.port.shutdown = mxpcie8250_shutdown;
 
-- 
2.45.2