linux-kernel - [PATCH v1 13/31] serial: 8250_mxpcie: enable on-chip software flow control

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Message-ID: <20251130104222.63077-14-crescentcy.hsieh@moxa.com>
Date: Sun, 30 Nov 2025 18:42:04 +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 13/31] serial: 8250_mxpcie: enable on-chip software flow control

Enable on-chip XON/XOFF software flow control via EFR[3:0], and configure
XON/XOFF characters using dedicated registers.

This patch also implements throttle() and unthrottle() to handle receive
pause and resume based on software flow control.

Signed-off-by: Crescent Hsieh <crescentcy.hsieh@...a.com>
---
 drivers/tty/serial/8250/8250_mxpcie.c | 62 ++++++++++++++++++++++++++-
 1 file changed, 61 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_mxpcie.c b/drivers/tty/serial/8250/8250_mxpcie.c
index 197d016354c9..ceef6a5e677e 100644
--- a/drivers/tty/serial/8250/8250_mxpcie.c
+++ b/drivers/tty/serial/8250/8250_mxpcie.c
@@ -41,13 +41,31 @@
 #define MOXA_PUART_SFR_950	BIT(5)
 
 /* Enhanced Function Register (EFR) */
+/*
+ * EFR[1:0] - In-Band Receive Flow Control Mode (Compare XON/XOFF):
+ *	00b (0x00) = Disabled
+ *	01b (0x01) = Recognize XON2 & XOFF2 as XOFF character
+ *	10b (0x02) = Recognize XON1 & XOFF1 as XOFF character
+ *	11b (0x03) = Depends on EFR[3:2]
+ * EFR[3:2] - In-Band Transmit Flow Control Mode (Insert XON/XOFF):
+ *	00b (0x00) = Disabled
+ *	01b (0x04) = Use XON2 & XOFF2 as XOFF character
+ *	10b (0x08) = Use XON1 & XOFF1 as XOFF character
+ *	11b (0x0C) = Reserved
+ */
 #define MOXA_PUART_EFR			0x0A
+#define MOXA_PUART_EFR_RX_FLOW		0x02	/* Recognize XON1 & XOFF1 as XOFF character */
+#define MOXA_PUART_EFR_TX_FLOW		0x08	/* Use XON1 & XOFF1 as XOFF character */
 #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_XON1		0x0B
+#define MOXA_PUART_XON2		0x0C
+#define MOXA_PUART_XOFF1	0x0D
+#define MOXA_PUART_XOFF2	0x0E
 #define MOXA_PUART_TTL		0x10	/* Tx Interrupt Trigger Level */
 #define MOXA_PUART_RTL		0x11	/* Rx Interrupt Trigger Level */
 #define MOXA_PUART_FCL		0x12	/* Flow Control Low Trigger Level */
@@ -152,7 +170,7 @@ static void mxpcie8250_set_termios(struct uart_port *port,
 
 	serial8250_do_set_termios(port, new, old);
 
-	up->port.status &= ~(UPSTAT_AUTORTS | UPSTAT_AUTOCTS);
+	up->port.status &= ~(UPSTAT_AUTORTS | UPSTAT_AUTOCTS | UPSTAT_AUTOXOFF);
 
 	efr = serial_in(up, MOXA_PUART_EFR);
 	efr &= ~(MOXA_PUART_EFR_AUTO_RTS | MOXA_PUART_EFR_AUTO_CTS);
@@ -161,6 +179,21 @@ static void mxpcie8250_set_termios(struct uart_port *port,
 		efr |= (MOXA_PUART_EFR_AUTO_RTS | MOXA_PUART_EFR_AUTO_CTS);
 		up->port.status |= (UPSTAT_AUTORTS | UPSTAT_AUTOCTS);
 	}
+	/* Set on-chip software flow control character */
+	serial_out(up, MOXA_PUART_XON1, START_CHAR(tty));
+	serial_out(up, MOXA_PUART_XON2, START_CHAR(tty));
+	serial_out(up, MOXA_PUART_XOFF1, STOP_CHAR(tty));
+	serial_out(up, MOXA_PUART_XOFF2, STOP_CHAR(tty));
+
+	efr &= ~(MOXA_PUART_EFR_RX_FLOW_MASK | MOXA_PUART_EFR_TX_FLOW_MASK);
+
+	if (I_IXON(tty))
+		efr |= MOXA_PUART_EFR_RX_FLOW;
+
+	if (I_IXOFF(tty)) {
+		efr |= MOXA_PUART_EFR_TX_FLOW;
+		up->port.status |= UPSTAT_AUTOXOFF;
+	}
 	serial_out(up, MOXA_PUART_EFR, efr);
 }
 
@@ -195,6 +228,31 @@ static void mxpcie8250_shutdown(struct uart_port *port)
 	serial8250_do_shutdown(port);
 }
 
+static void mxpcie8250_throttle(struct uart_port *port)
+{
+	unsigned long flags;
+
+	uart_port_lock_irqsave(port, &flags);
+
+	port->ops->stop_rx(port);
+
+	uart_port_unlock_irqrestore(port, flags);
+}
+
+static void mxpcie8250_unthrottle(struct uart_port *port)
+{
+	struct uart_8250_port *up = up_to_u8250p(port);
+	unsigned long flags;
+
+	uart_port_lock_irqsave(port, &flags);
+
+	up->ier |= UART_IER_RLSI | UART_IER_RDI;
+	port->read_status_mask |= UART_LSR_DR;
+	serial_out(up, UART_IER, up->ier);
+
+	uart_port_unlock_irqrestore(port, flags);
+}
+
 static int mxpcie8250_init(struct pci_dev *pdev)
 {
 	resource_size_t iobar_addr = pci_resource_start(pdev, 2);
@@ -284,6 +342,8 @@ static int mxpcie8250_probe(struct pci_dev *pdev, const struct pci_device_id *id
 	up.port.set_termios = mxpcie8250_set_termios;
 	up.port.startup = mxpcie8250_startup;
 	up.port.shutdown = mxpcie8250_shutdown;
+	up.port.throttle = mxpcie8250_throttle;
+	up.port.unthrottle = mxpcie8250_unthrottle;
 
 	for (i = 0; i < num_ports; i++) {
 		if (mxpcie8250_setup(pdev, priv, &up, i))
-- 
2.45.2