linux-kernel - Re: [PATCH v2] serial: imx: warn user when using unsupported configuration

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Date:   Wed, 18 Apr 2018 16:53:41 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Stefan Agner <stefan@...er.ch>
Cc:     gregkh@...uxfoundation.org, lukas@...ner.de, jslaby@...e.com,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] serial: imx: warn user when using unsupported
 configuration

On Wed, Apr 18, 2018 at 04:06:38PM +0200, Stefan Agner wrote:
> When using half-duplex mode (which disables receiver during txing)
> the RTS signal cannot be driven low during transmission. This seems
> to be a limitation of the i.MX UART IP: The RTS (CTS_B) signal is
> controlled by the receiver. When the receiver is disabled, the
> signal stays in UART logic idle state which is high...
> 
> If SER_RS485_RTS_ON_SEND is used, RTS needs to be high active during
> transmission. Since this is the default state of the RTS (CTS_B)
> signal when the receiver is off, half-duplex mode in this
> configuration works fine.
> 
> However, a low-active RTS signal (flag SER_RS485_RTS_ON_SEND not set)
> cannot be generated when the receiver is turned off.
> 
> Print an error if the user selects this unsupported configuration
> (both SER_RS485_RTS_ON_SEND and SER_RS485_RX_DURING_TX unset) and
> configure the closest working configuration (set the
> SER_RS485_RX_DURING_TX flag).
> 
> Signed-off-by: Stefan Agner <stefan@...er.ch>
> ---
> Changes since v1:
> - Consistently check for sport->have_rtscts && !(rs485conf->flags &
>   SER_RS485_RTS_ON_SEND)
> - Don't break printed message
> 
>  drivers/tty/serial/imx.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index 91f3a1a5cb7f..1c1080fc8084 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -1833,6 +1833,11 @@ static int imx_uart_rs485_config(struct uart_port *port,
>  		rs485conf->flags &= ~SER_RS485_ENABLED;
>  
>  	if (rs485conf->flags & SER_RS485_ENABLED) {
> +		/* Enable receiver if low-active RTS signal is requested */
> +		if (sport->have_rtscts &&
> +		    !(rs485conf->flags & SER_RS485_RTS_ON_SEND))
> +			rs485conf->flags |= SER_RS485_RX_DURING_TX;
> +

I wonder what should happen, if the device tree has both

	uart-has-rtscts;
	rts-gpios = <...>;

. I think the right thing would be to check for

	sport->have_rtscts && !sport->have_rtsgpio

.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |