linux-kernel - [PATCH v1 15/31] serial: 8250_mxpcie: optimize Rx using memory-mapped FIFO access

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Message-ID: <20251130104222.63077-16-crescentcy.hsieh@moxa.com>
Date: Sun, 30 Nov 2025 18:42:06 +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 15/31] serial: 8250_mxpcie: optimize Rx using memory-mapped FIFO access

Add mxpcie8250_rx_chars() to retrieve data from the Rx FIFO via
memory-mapped FIFO access, using the UART-specific FIFO byte counter
and FIFO memory region.

This avoids per-byte I/O reads through serial_in(), improving receive
throughput on Moxa PCIe UART devices. For break and error conditions,
the handler falls back to serial8250_rx_chars() to retain standard
error handling logic.

Signed-off-by: Crescent Hsieh <crescentcy.hsieh@...a.com>
---
 drivers/tty/serial/8250/8250_mxpcie.c | 37 +++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_mxpcie.c b/drivers/tty/serial/8250/8250_mxpcie.c
index fe2906d16d80..cf519960d5d1 100644
--- a/drivers/tty/serial/8250/8250_mxpcie.c
+++ b/drivers/tty/serial/8250/8250_mxpcie.c
@@ -8,6 +8,7 @@
 
 #include <linux/module.h>
 #include <linux/pci.h>
+#include <linux/tty_flip.h>
 #include <linux/8250_pci.h>
 
 #include "8250.h"
@@ -35,6 +36,7 @@
 /* UART */
 #define MOXA_PUART_BASE_BAUD	921600
 #define MOXA_PUART_OFFSET	0x200
+#define MOXA_PUART_FIFO_SIZE	128
 
 /* Special Function Register (SFR) */
 #define MOXA_PUART_SFR		0x07
@@ -70,6 +72,9 @@
 #define MOXA_PUART_RTL		0x11	/* Rx Interrupt Trigger Level */
 #define MOXA_PUART_FCL		0x12	/* Flow Control Low Trigger Level */
 #define MOXA_PUART_FCH		0x13	/* Flow Control High Trigger Level */
+#define MOXA_PUART_RX_FIFO_CNT	0x15	/* Rx FIFO Data Counter */
+
+#define MOXA_PUART_RX_FIFO_MEM	0x100	/* Memory Space to Rx FIFO Data Register */
 
 #define MOXA_GPIO_DIRECTION	0x09
 #define MOXA_GPIO_OUTPUT	0x0A
@@ -253,6 +258,30 @@ static void mxpcie8250_unthrottle(struct uart_port *port)
 	uart_port_unlock_irqrestore(port, flags);
 }
 
+static void mxpcie8250_rx_chars(struct uart_8250_port *up)
+{
+	struct uart_port *port = &up->port;
+	struct tty_port *tport = &port->state->port;
+	int recv_room, gdl, i;
+	u8 buf[MOXA_PUART_FIFO_SIZE];
+
+	recv_room = tty_buffer_request_room(tport, port->fifosize);
+
+	if (recv_room) {
+		gdl = serial_in(up, MOXA_PUART_RX_FIFO_CNT);
+
+		if (gdl > recv_room)
+			gdl = recv_room;
+
+		for (i = 0; i < gdl; ++i)
+			buf[i] = *(port->membase + MOXA_PUART_RX_FIFO_MEM + i);
+
+		port->icount.rx += gdl;
+		tty_insert_flip_string(tport, buf, gdl);
+		tty_flip_buffer_push(tport);
+	}
+}
+
 static int mxpcie8250_handle_irq(struct uart_port *port)
 {
 	struct uart_8250_port *up = up_to_u8250p(port);
@@ -275,8 +304,12 @@ static int mxpcie8250_handle_irq(struct uart_port *port)
 	    !(port->read_status_mask & UART_LSR_DR))
 		skip_rx = true;
 
-	if (lsr & (UART_LSR_DR | UART_LSR_BI) && !skip_rx)
-		lsr = serial8250_rx_chars(up, lsr);
+	if (lsr & (UART_LSR_DR | UART_LSR_BI) && !skip_rx) {
+		if (lsr & UART_LSR_BRK_ERROR_BITS)
+			lsr = serial8250_rx_chars(up, lsr);
+		else
+			mxpcie8250_rx_chars(up);
+	}
 
 	serial8250_modem_status(up);
 
-- 
2.45.2