[PATCH 6.6 20/26] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

6.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Sherry Sun <sherry.sun@nxp.com>

commit f5cb528d6441eb860250a2f085773aac4f44085e upstream.

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")
Cc: stable <stable@kernel.org>
Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20250312022503.1342990-1-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/tty/serial/fsl_lpuart.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -1488,6 +1488,19 @@ static int lpuart32_config_rs485(struct
 
 	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 for the transmit engine to complete */
+		lpuart32_wait_bit_set(&sport->port, UARTSTAT, UARTSTAT_TC);
+		lpuart32_write(&sport->port, ctrl & ~UARTCTRL_TE, UARTCTRL);
+
+		while (lpuart32_read(&sport->port, UARTCTRL) & UARTCTRL_TE)
+			cpu_relax();
+	}
+
 	lpuart32_write(&sport->port, modem, UARTMODIR);
 
 	if (rs485->flags & SER_RS485_ENABLED) {
@@ -1507,6 +1520,10 @@ static int lpuart32_config_rs485(struct
 	}
 
 	lpuart32_write(&sport->port, modem, UARTMODIR);
+
+	if (ctrl & UARTCTRL_TE)
+		lpuart32_write(&sport->port, ctrl, UARTCTRL);
+
 	return 0;
 }