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")
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>

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 91d02c55c4707ae44f392b68ced3b289cc502abf..203ec3b46304acf5e28fe8633b2e9e035d27ddfb 100644 (file)
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -1484,6 +1484,19 @@ 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 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) {
@@ -1503,6 +1516,10 @@ 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;
 }