RE: [PATCH] tty: serial: fsl_lpuart: disable transmitter before changing RS485 related registers

[Shenwei Wang]

> -----Original Message-----
> From: Sherry Sun <sherry.sun@xxxxxxx>
> Sent: Thursday, March 6, 2025 8:20 PM
> To: gregkh@xxxxxxxxxxxxxxxxxxx; jirislaby@xxxxxxxxxx
> Cc: linux-serial@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> imx@xxxxxxxxxxxxxxx; Shenwei Wang <shenwei.wang@xxxxxxx>
> 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@xxxxxxx>
> ---
>  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);

Since there may be a delay between writing to the register and the UARTCTRL_TE bit actually 
changing to 0, we should poll the UARTCTRL register and verify that UARTCTRL_TE has really 
become 0 before proceeding. Otherwise, subsequent operations would still execute while the
UARTCTRL_TE bit remains in the status of 1, which is not the intention of this patch.

Thanks,
Shenwei

> +	}
> +
>  	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