linux-kernel - Re: [PATCH V3 1/2] dt-bindings: Add optional gpio property to uartps node to support rs485

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Message-ID: <20231026180724.GA4108223-robh@kernel.org>
Date:   Thu, 26 Oct 2023 13:07:24 -0500
From:   Rob Herring <robh@...nel.org>
To:     Manikanta Guntupalli <manikanta.guntupalli@....com>
Cc:     git@....com, michal.simek@....com, gregkh@...uxfoundation.org,
        krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
        linux-serial@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, jirislaby@...nel.org,
        linux-arm-kernel@...ts.infradead.org, radhey.shyam.pandey@....com,
        srinivas.goud@....com, shubhrajyoti.datta@....com,
        manion05gk@...il.com
Subject: Re: [PATCH V3 1/2] dt-bindings: Add optional gpio property to uartps
 node to support rs485

On Tue, Oct 24, 2023 at 08:18:46PM +0530, Manikanta Guntupalli wrote:
> Add optional gpio property to uartps node and reference to rs485.yaml
> 
> On Xilinx/AMD Kria SOM KD240 board rs485 connects via TI ISOW1432
> Transceiver device, where one GPIO is used for driving DE/RE signals.
> With rs485 half duplex configuration, DE and RE are shorted to each other,
> and at a time, any node acts as either a driver or a receiver.
> 
> Here,
> DE - Driver enable. If pin is floating, driver is disabled.
> RE - Receiver enable. If pin is floating, receiver buffer is disabled.

What happens when pin is not floating? Is floating (i.e. open drain) for 
RTS a requirement? And floating doesn't define high or low because it 
could be pulled either way.

> 
> For more deatils, please find below link which contains Transceiver
> device(ISOW1432) datasheet
> https://www.ti.com/lit/ds/symlink/isow1432.pdf?ts=1682607122706&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FISOW1432%252Fpart-details%252FISOW1432DFMR%253FkeyMatch%253DISOW1432DFMR%2526tisearch%253Dsearch-everything%2526usecase%253DOPN
> 
> rts-gpios is optional property, because it is not required
> for uart console node.
> 
> Signed-off-by: Manikanta Guntupalli <manikanta.guntupalli@....com>
> ---
> Changes for V2:
> Modify optional gpio name to xlnx,phy-ctrl-gpios.
> Update commit description.
> Changes for V3:
> Modify optional gpio name to rts-gpios.
> Update commit description.
> ---
>  Documentation/devicetree/bindings/serial/cdns,uart.yaml | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/serial/cdns,uart.yaml b/Documentation/devicetree/bindings/serial/cdns,uart.yaml
> index e35ad1109efc..7ee305f9a45f 100644
> --- a/Documentation/devicetree/bindings/serial/cdns,uart.yaml
> +++ b/Documentation/devicetree/bindings/serial/cdns,uart.yaml
> @@ -46,6 +46,11 @@ properties:
>    power-domains:
>      maxItems: 1
>  
> +  rts-gpios:
> +    description: Optional GPIO to control transmit/receive on RS485 phy
> +      in halfduplex mode.

You need to define what 'active' means here because -gpios all have an 
active flag. Is it always active low (or high) or depends on the board?


> +    maxItems: 1
> +
>  required:
>    - compatible
>    - reg
> @@ -55,6 +60,7 @@ required:
>  
>  allOf:
>    - $ref: serial.yaml#
> +  - $ref: rs485.yaml#
>    - if:
>        properties:
>          compatible:
> -- 
> 2.25.1
>