[PATCH v2] serial: imx: warn user when using unsupported configuration

[PATCH v2] serial: imx: warn user when using unsupported configuration

[Stefan Agner]

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@agner.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;
+
 		/* disable transmitter */
 		ucr2 = imx_uart_readl(sport, UCR2);
 		if (rs485conf->flags & SER_RS485_RTS_AFTER_SEND)
@@ -2265,6 +2270,16 @@ static int imx_uart_probe(struct platform_device *pdev)
 	    (!sport->have_rtscts && !sport->have_rtsgpio))
 		dev_err(&pdev->dev, "no RTS control, disabling rs485\n");
 
+	/*
+	 * The RTS (CTS_B) signal cannot be set low during transmission
+	 * in case the receiver is off (limitation of the i.MX UART IP).
+	 */
+	if (sport->port.rs485.flags & SER_RS485_ENABLED && sport->have_rtscts &&
+	    (!(sport->port.rs485.flags & SER_RS485_RTS_ON_SEND) &&
+	     !(sport->port.rs485.flags & SER_RS485_RX_DURING_TX)))
+		dev_err(&pdev->dev,
+			"low-active RTS not possible when receiver is off, enabling receiver\n");
+
 	imx_uart_rs485_config(&sport->port, &sport->port.rs485);
 
 	/* Disable interrupts before requesting them */
-- 
2.17.0

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

[Uwe Kleine-König]

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@agner.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/  |

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

[Stefan Agner]

On 18.04.2018 16:53, Uwe Kleine-König wrote:
> 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@agner.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 = <...>;

Hm, it seems that the code controls both in that case.

> 
> . I think the right thing would be to check for
> 
> 	sport->have_rtscts && !sport->have_rtsgpio

I agree, since it controls both one can use the GPIO to have a working
half duplex with low-active RTS configuration.

Will send v3.

--
Stefan