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serial: imx: warn user when using unsupported configuration

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When using half-duplex mode (which disables receiver during txing)
the RTS signal cannot be driven low during transmission when using
i.MX UART RTS/CTS control. 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>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Stefan Agner 2018-04-19 17:39:16 +02:00 committed by Greg Kroah-Hartman
parent 46c6975a1f
commit 6d215f83e5
1 changed files with 17 additions and 0 deletions
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17
drivers/tty/serial/imx.c
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@ -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 && !sport->have_rtsgpio &&
!(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 +2270,18 @@ 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");
/*
* If using the i.MX UART RTS/CTS control then 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->have_rtsgpio &&
(!(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 */