linux-kernel - Re: [PATCH 1/2] dt-bindings: serial: rs485: add rs485-mux-gpios binding

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Message-ID: <20231122145344.GA18949@wunner.de>
Date:   Wed, 22 Nov 2023 15:53:44 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>, devicetree@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        Crescent CY Hsieh <crescentcy.hsieh@...a.com>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Subject: Re: [PATCH 1/2] dt-bindings: serial: rs485: add rs485-mux-gpios
 binding

On Mon, Nov 20, 2023 at 04:10:54PM +0100, Rasmus Villemoes wrote:
> Some boards are capable of both rs232 and rs485, and control which
> external terminals are active via a gpio-controlled mux. Allow
> describing that gpio in DT so that the kernel can transparently handle
> the proper setting when the uart is switched between rs232 and rs485
> modes.

Crescent CY Hsieh (+cc) is in parallel trying to add an RS-422 mode bit
to struct serial_rs485:

https://lore.kernel.org/all/20231121095122.15948-1-crescentcy.hsieh@moxa.com/

I don't know whether that makes sense at all (I had thought RS-422 is
the same as RS-485 with full-duplex, i.e. SER_RS485_ENABLED plus
SER_RS485_RX_DURING_TX).

But if that patch gets accepted, we'd have *three* different modes:
RS-232, RS-485, RS-422.  A single GPIO seems insufficient to handle that.
You'd need at least two GPIOs.


> --- a/Documentation/devicetree/bindings/serial/rs485.yaml
> +++ b/Documentation/devicetree/bindings/serial/rs485.yaml
> @@ -61,6 +61,11 @@ properties:
>        the active state enables RX during TX.
>      maxItems: 1
>  
> +  rs485-mux-gpios:
> +    description: GPIO pin to control muxing of the SOC signals to the RS485
> +      transceiver.
> +    maxItems: 1

The description doesn't really add much to the name "rs485-mux-gpios".

Suggestion:

    description: selects whether the UART is connect to an RS-232 driver (low)
      or an RS-485 transceiver (high)

Thanks,

Lukas