[PATCH V2 1/4] dt_bindings: rs485: Add binding for GPIO that controls Rx enable during Tx

[Christoph Niedermaier]

This patch adds a binding for a generic definition of an output GPIO that
indicates the state of rs485-rx-during-tx. The idea is that the hardware
already controls the option receiving during sending before it gets to the
signal receiving hardware. The standard RS485 is a half-duplex bus that in
most cases is driven by an UART controller. The advantage of using this
GPIO is that it is independent of the capabilities of the UART core and
the UART driver. On the hardware side the interface to the bus is
controlled by a transceiver, that has a pin called RE (RX enable) or
similar, which connects the bus to the RX signal of the UART controller.
The GPIO can switch between two states to control the RE pin via an
electrical circuit:
- Active:
  The RE pin is always active. The UART RX see everything on the bus and
  therefore also what happens with the TX signal on the bus.
- Inactive:
  The RE pin is always active, but during sending on the bus the pin RE is
  inactive. So basically the receiving during sending is suppressed.

A possible circuit diagram could look like this:
                                  ┌──────────────────┐
                                  │       RS485      │
                TX ───────────────┤D                 │
                                  │    Transceiver   │
               RTS ────┬──────────┤DE                │
                       │          │                  │
                       │ ┌─────┐  │                  │
                       └─┤&    │  │                  │
                         │     ├──┤!RE               │
!rx_during_tx_gpio ──────┤     │  │                  │
                         └─────┘  │                  │
                                  │                  │
                RX ───────────────┤R                 │
                                  │                  │
                                  └──────────────────┘

Here the RTS pin of the UART core is used to control TX via the transceiver
pin DE (Drive Enable). RE and rx_during_tx_gpio are active low.

Signed-off-by: Christoph Niedermaier <cniedermaier at dh-electronics.com>
---
Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Cc: Rob Herring <robh+dt at kernel.org>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt at linaro.org>
Cc: Alexander Dahl <ada at thorsis.com>
Cc: Marek Vasut <marex at denx.de>
Cc: kernel at dh-electronics.com
Cc: devicetree at vger.kernel.org
To: linux-serial at vger.kernel.org
To: linux-arm-kernel at lists.infradead.org
---
V2: - Rework of the commit message
    - Rework GPIO property comment
---
 Documentation/devicetree/bindings/serial/rs485.yaml | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/devicetree/bindings/serial/rs485.yaml b/Documentation/devicetree/bindings/serial/rs485.yaml
index 90a1bab40f05..6d780911e342 100644
--- a/Documentation/devicetree/bindings/serial/rs485.yaml
+++ b/Documentation/devicetree/bindings/serial/rs485.yaml
@@ -51,6 +51,10 @@ properties:
     description: GPIO pin to enable RS485 bus termination.
     maxItems: 1
 
+  rs485-rx-during-tx-gpios:
+    description: Output GPIO pin that indicates the state of rs485-rx-during-tx.
+    maxItems: 1
+
 additionalProperties: true
 
 ...
-- 
2.11.0