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@moxa.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

On Sun, Nov 30, 2025 at 12:44 PM Crescent Hsieh
<crescentcy.hsieh@moxa.com> wrote:
>
> 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.

...

> +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);

> +

Unneeded blank line

> +       if (recv_room) {

  if (!recv_room)
    return;

> +               gdl = serial_in(up, MOXA_PUART_RX_FIFO_CNT);

> +

Unneeded blank line.

> +               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);
> +       }
> +}

...

> -       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);
> +       }

And again, please try not to modify the code you just added in the
previous changes.

-- 
With Best Regards,
Andy Shevchenko