linux-kernel - [PATCH v1 04/31] serial: 8250_mxupci: enable auto RTS/CTS flow control

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Message-ID: <20251130104222.63077-5-crescentcy.hsieh@moxa.com>
Date: Sun, 30 Nov 2025 18:41:55 +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 04/31] serial: 8250_mxupci: enable auto RTS/CTS flow control

Implement a custom set_termios() to configure RTS/CTS hardware flow
control. When CRTSCTS is enabled in termios, the UART_MCR[5] AFE (Auto
Flow Control Enable) bit is set to allow automatic RTS/CTS signaling.

Signed-off-by: Crescent Hsieh <crescentcy.hsieh@...a.com>
---
 drivers/tty/serial/8250/8250_mxupci.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_mxupci.c b/drivers/tty/serial/8250/8250_mxupci.c
index c4fa52de8962..54e50cd3f472 100644
--- a/drivers/tty/serial/8250/8250_mxupci.c
+++ b/drivers/tty/serial/8250/8250_mxupci.c
@@ -78,6 +78,26 @@ static unsigned short mxupci8250_get_nports(unsigned short device)
 	return FIELD_GET(0x00F0, device);
 }
 
+static void mxupci8250_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;
+
+	serial8250_do_set_termios(port, new, old);
+
+	up->port.status &= ~(UPSTAT_AUTORTS | UPSTAT_AUTOCTS);
+
+	up->mcr &= ~UART_MCR_AFE;
+
+	if (cflag & CRTSCTS) {
+		up->mcr |= (UART_MCR_AFE | UART_MCR_RTS);
+		up->port.status |= (UPSTAT_AUTORTS | UPSTAT_AUTOCTS);
+	}
+	serial_out(up, UART_MCR, up->mcr);
+}
+
 static int mxupci8250_startup(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
@@ -152,6 +172,7 @@ static int mxupci8250_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_MU860;
 
+	up.port.set_termios = mxupci8250_set_termios;
 	up.port.startup = mxupci8250_startup;
 	up.port.shutdown = mxupci8250_shutdown;
 
-- 
2.45.2