linux-kernel - Re: [PATCH V2 1/2] dt-bindings: Add optional gpio property to uartps node to support rs485

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Message-ID: <c4d6ec9a13807866b7dbc7cbed478494@vanmierlo.com>
Date:   Thu, 12 Oct 2023 20:35:59 +0200
From:   m.brock@...mierlo.com
To:     Manikanta Guntupalli <manikanta.guntupalli@....com>
Cc:     git@....com, michal.simek@....com, gregkh@...uxfoundation.org,
        robh+dt@...nel.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 V2 1/2] dt-bindings: Add optional gpio property to uartps
 node to support rs485

Manikanta Guntupalli wrote on 2023-10-11 16:56:
> 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 shorts to each other,

s/shorts/are connected

> 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.

Please use DE and /RE to indicate DE is active high and /RE is active 
low.

> xlnx,phy-ctrl-gpios is optional property, because it is not required
> for uart console node.

How about introducing an rs485 generic gpios property instead of xlnx
private one? See also rs485-term-gpios and rs485-rx-during-tx-gpios.

Also note that every kernel driver expects to use RTS for this purpose.
So why not give this driver the option to choose a gpio instead of its
native RTS? And from there on use the rts route?
What if someone wants to use normal (non-rs485) RTS on a GPIO instead
of the native pin?

@Rob Herring
I am curious to know how the rs485 maintainers look at this.

Maarten