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

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Date:   Thu, 23 Nov 2023 11:38:02 +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 Thu, Nov 23, 2023 at 11:07:16AM +0100, Rasmus Villemoes wrote:
> On 22/11/2023 15.53, Lukas Wunner wrote:
> > 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.
> 
> I don't see Crescent introducing any new gpio that needs to be handled.
> In fact, I can't even see why from the perspective of the software that
> rs422 isn't just rs232; there's no transmit enable pin that needs to be
> handled. But maybe the uart driver does something different in rs422
> mode; I assume he must have some update of some driver, since otherwise
> the new rs422 bit should be rejected by the core. So I can't really see
> the whole picture of that rs422 story.

The question is, could we conceivably have the need to support
switching between the three modes RS-232, RS-485, RS-422.
If yes, then the GPIO mux interface should probably allow for that.

As a case in point, the Siemens IOT 2040 has two serial ports
which can be set to either of those three modes.  The signals
are routed to the same D-sub socket, but the pins used are
different.  See page 46 and 47 of this document:

https://cache.industry.siemens.com/dl/files/658/109741658/att_899623/v1/iot2000_operating_instructions_enUS_en-US.pdf

The driver for this product is 8250_exar.c.  It's an Intel-based
product, so no devicetree, but it shows that such use cases exist.

Thanks,

Lukas