linux-kernel - [PATCH] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

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Message-Id: <20250307021950.1000221-1-sherry.sun@nxp.com>
Date: Fri,  7 Mar 2025 10:19:50 +0800
From: Sherry Sun <sherry.sun@....com>
To: gregkh@...uxfoundation.org,
	jirislaby@...nel.org
Cc: linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	imx@...ts.linux.dev,
	shenwei.wang@....com
Subject: [PATCH] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

According to the LPUART reference manual, TXRTSE and TXRTSPOL of MODIR
register only can be changed when the transmitter is disabled.
So disable the transmitter before changing RS485 related registers and
re-enable it after the change is done.

Fixes: 67b01837861c ("tty: serial: lpuart: Add RS485 support for 32-bit uart flavour")
Signed-off-by: Sherry Sun <sherry.sun@....com>
---
 drivers/tty/serial/fsl_lpuart.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 91d02c55c470..4dc2f3e2b8e0 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -1484,6 +1484,16 @@ static int lpuart32_config_rs485(struct uart_port *port, struct ktermios *termio
 
 	unsigned long modem = lpuart32_read(&sport->port, UARTMODIR)
 				& ~(UARTMODIR_TXRTSPOL | UARTMODIR_TXRTSE);
+	u32 ctrl;
+
+	/* TXRTSE and TXRTSPOL only can be changed when transmitter is disabled. */
+	ctrl = lpuart32_read(&sport->port, UARTCTRL);
+	if (ctrl & UARTCTRL_TE) {
+		/* wait transmit engin complete */
+		lpuart32_wait_bit_set(&sport->port, UARTSTAT, UARTSTAT_TC);
+		lpuart32_write(&sport->port, ctrl & ~UARTCTRL_TE, UARTCTRL);
+	}
+
 	lpuart32_write(&sport->port, modem, UARTMODIR);
 
 	if (rs485->flags & SER_RS485_ENABLED) {
@@ -1503,6 +1513,10 @@ static int lpuart32_config_rs485(struct uart_port *port, struct ktermios *termio
 	}
 
 	lpuart32_write(&sport->port, modem, UARTMODIR);
+
+	if (ctrl & UARTCTRL_TE)
+		lpuart32_write(&sport->port, ctrl, UARTCTRL);
+
 	return 0;
 }
 
-- 
2.34.1