linux-kernel - Re: [PATCH] tty: serial: imx: fix rs485 rx after tx

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Date:   Fri, 16 Jun 2023 15:40:29 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Martin Fuzzey <martin.fuzzey@...wbird.group>
cc:     linux-serial@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Fabio Estevam <festevam@...il.com>,
        Marek Vasut <marex@...x.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: serial: imx: fix rs485 rx after tx

On Fri, 16 Jun 2023, Martin Fuzzey wrote:

> Since commit 79d0224f6bf2 ("tty: serial: imx: Handle RS485 DE signal
> active high") RS485 reception no longer works after a transmission.
> 
> The following scenario shows the problem:
> 	1) Open a port in RS485 mode
> 	2) Receive data from remote (OK)
> 	3) Transmit data to remote (OK)
> 	4) Receive data from remote (Nothing received)
> 
> In RS485 mode, imx_uart_start_tx() calls imx_uart_stop_rx() and, when the
> transmission is complete, imx_uart_stop_tx() calls imx_uart_start_rx().
> 
> Since the above commit imx_uart_stop_rx() now sets the loopback bit but
> imx_uart_start_rx() does not clear it causing the hardware to remain in
> loopback mode and not receive external data.
> 
> Fix this by moving the existing loopback disable code to a helper function
> and calling it from imx_uart_start_rx() too.
> 
> Fixes: 79d0224f6bf2 ("tty: serial: imx: Handle RS485 DE signal active high")
> Cc: stable@...r.kernel.org
> Signed-off-by: Martin Fuzzey <martin.fuzzey@...wbird.group>
> ---
>  drivers/tty/serial/imx.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index c5e17569c3ad..3fe8ff241522 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -369,6 +369,16 @@ static void imx_uart_soft_reset(struct imx_port *sport)
>  	sport->idle_counter = 0;
>  }
>  
> +static void imx_uart_disable_loopback_rs485(struct imx_port *sport)
> +{
> +	unsigned int uts;
> +
> +	/* See SER_RS485_ENABLED/UTS_LOOP comment in imx_uart_probe() */
> +	uts = imx_uart_readl(sport, imx_uart_uts_reg(sport));
> +	uts &= ~UTS_LOOP;
> +	imx_uart_writel(sport, uts, imx_uart_uts_reg(sport));
> +}
> +
>  /* called with port.lock taken and irqs off */
>  static void imx_uart_start_rx(struct uart_port *port)
>  {
> @@ -390,6 +400,7 @@ static void imx_uart_start_rx(struct uart_port *port)
>  	/* Write UCR2 first as it includes RXEN */
>  	imx_uart_writel(sport, ucr2, UCR2);
>  	imx_uart_writel(sport, ucr1, UCR1);
> +	imx_uart_disable_loopback_rs485(sport);
>  }
>  
>  /* called with port.lock taken and irqs off */
> @@ -1422,7 +1433,7 @@ static int imx_uart_startup(struct uart_port *port)
>  	int retval;
>  	unsigned long flags;
>  	int dma_is_inited = 0;
> -	u32 ucr1, ucr2, ucr3, ucr4, uts;
> +	u32 ucr1, ucr2, ucr3, ucr4;
>  
>  	retval = clk_prepare_enable(sport->clk_per);
>  	if (retval)
> @@ -1521,10 +1532,7 @@ static int imx_uart_startup(struct uart_port *port)
>  		imx_uart_writel(sport, ucr2, UCR2);
>  	}
>  
> -	/* See SER_RS485_ENABLED/UTS_LOOP comment in imx_uart_probe() */
> -	uts = imx_uart_readl(sport, imx_uart_uts_reg(sport));
> -	uts &= ~UTS_LOOP;
> -	imx_uart_writel(sport, uts, imx_uart_uts_reg(sport));
> +	imx_uart_disable_loopback_rs485(sport);
>  
>  	spin_unlock_irqrestore(&sport->port.lock, flags);

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>

-- 
 i.