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

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Message-ID: <Z8sQVNaLCxl3x9eG@lizhi-Precision-Tower-5810>
Date: Fri, 7 Mar 2025 10:27:16 -0500
From: Frank Li <Frank.li@....com>
To: Sherry Sun <sherry.sun@....com>
Cc: gregkh@...uxfoundation.org, jirislaby@...nel.org,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	imx@...ts.linux.dev, shenwei.wang@....com
Subject: Re: [PATCH] tty: serial: fsl_lpuart: disable transmitter before
 changing RS485 related registers

On Fri, Mar 07, 2025 at 10:19:50AM +0800, Sherry Sun wrote:
> 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>

Reviewed-by: Frank Li <Frank.Li@....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
>