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

Signed-off-by: Stefan Agner <>
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..1ba7e98ddc76 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -1833,6 +1833,10 @@ 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 (rs485conf->flags & SER_RS485_RTS_AFTER_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 +2269,17 @@ 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 */
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