The i.MX UART peripheral uses the RST_B signal as input, and CTS_B as
output. This is just like the CDE role in RS-232. This is true
regardless of the "DTE mode" setting of this peripheral.
As a result, rs485 support hardware must use the CTS_B signal to control
the RS-485 transceiver. This is in contrast to generic rs485 kernel
code, documentation, and DT property names that consistently refer to
the RST as transceiver control signal.
Add a note in the DT binding document about that, to reduce the
confusion somewhat.
Signed-off-by: Baruch Siach <baruch@xxxxxxxxxx>
---
Documentation/devicetree/bindings/serial/fsl-imx-uart.txt | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/Documentation/devicetree/bindings/serial/fsl-imx-uart.txt b/Documentation/devicetree/bindings/serial/fsl-imx-uart.txt
index afcfbc34e243..35957cbf1571 100644
--- a/Documentation/devicetree/bindings/serial/fsl-imx-uart.txt
+++ b/Documentation/devicetree/bindings/serial/fsl-imx-uart.txt
@@ -9,7 +9,11 @@ Optional properties:
- fsl,dte-mode : Indicate the uart works in DTE mode. The uart works
in DCE mode by default.
- rs485-rts-delay, rs485-rts-active-low, rs485-rx-during-tx,
- linux,rs485-enabled-at-boot-time: see rs485.txt
+ linux,rs485-enabled-at-boot-time: see rs485.txt. Note that for RS485
+ you must enable either the "uart-has-rtscts" or the "rts-gpios"
+ properties. In case you use "uart-has-rtscts" the signal that controls
+ the transceiver is actually CTS_B, not RTS_B. CTS_B is always output,
+ and RTS_B is input, regardless of dte-mode.
Please check Documentation/devicetree/bindings/serial/serial.txt
for the complete list of generic properties.
--
2.18.0
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