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

[Rob Herring <robh@kernel.org>]

On Wed, Nov 23, 2022 at 01:30:01PM +0100, Christoph Niedermaier wrote:
> 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@dh-electronics.com>
> ---
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Rob Herring <robh+dt@kernel.org>
> Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
> Cc: Alexander Dahl <ada@thorsis.com>
> Cc: Marek Vasut <marex@denx.de>
> Cc: kernel@dh-electronics.com
> Cc: devicetree@vger.kernel.org
> To: linux-serial@vger.kernel.org
> To: linux-arm-kernel@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.

An output sets the state. An input samples or indicates the state.

This should include something about the active state: The active state 
enables RX during TX.

> +    maxItems: 1
> +
>  additionalProperties: true
>  
>  ...
> -- 
> 2.11.0
> 
>