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Subject: Re: [PATCH 1/2] dt-bindings: serial: rs485: add rs485-mux-gpios
 binding
To:     Lukas Wunner <lukas@wunner.de>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>, devicetree@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org,
        Crescent CY Hsieh <crescentcy.hsieh@moxa.com>,
        =?UTF-8?Q?Ilpo_J=c3=a4rvinen?= <ilpo.jarvinen@linux.intel.com>,
        brenda.streiff@ni.com, cniedermaier@dh-electronics.com,
        Tomas Paukrt <tomaspaukrt@email.cz>
References: <20231120151056.148450-1-linux@rasmusvillemoes.dk>
 <20231120151056.148450-2-linux@rasmusvillemoes.dk>
 <20231122145344.GA18949@wunner.de>
From:   Lino Sanfilippo <LinoSanfilippo@gmx.de>
Message-ID: <3b8548b1-b8a9-0c9e-4040-5cfda06a85c6@gmx.de>
Date:   Sun, 26 Nov 2023 00:40:07 +0100
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Precedence: bulk

Hi,

On 22.11.23 at 15:53, Lukas Wunner wrote:
> 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/
>

That new flag was suggested by me instead of using SER_RS422_ENABLED, whic=
h
would mostly be redundant to SER_RS485_ENABLED.
I dont know if it is a good choice in the long term to handle both modes w=
ithin
the RS485 configuration. It would be cleaner to have an own RS422 structur=
e with
its own flags and properties. And until now the only flag that seems to ma=
ke sense
for both RS422 and RS485 is AFAICS SER_RS485_TERMINATE_BUS.

On the other hand the bus termination is at least a property that both mod=
es have
in common. And handling RS422 in its own structure would require another i=
octl
to set and get the the RS422 settings.

But maybe there are more or better possibilities to handle RS4822 support.=
 I would like to
hear other ideas.


> 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.

Actually we would have four (as Brenda already wrote,
see https://lore.kernel.org/all/c6ea912f-d5ab-4761-813d-3b6b6be141cb@ni.co=
m/),
and with the propose SER_RS485_MODE_RS422 flag these modes would be used l=
ike

RS-232:                       rs485->flags =3D 0
RS-422:                       rs485->flags =3D SER_RS485_ENABLED|SER_RS485=
_MODE_RS422
RS-485 (2-wire half-duplex):  rs485->flags =3D SER_RS485_ENABLED
RS-485 (4-wire full-duplex):  rs485->flags =3D SER_RS485_ENABLED|SER_RS485=
_RX_DURING_TX


>  A single GPIO seems insufficient to handle that.

GPIOs for RS485 is another thing.

I mean, currently we have a GPIO for RS485 termination (I introduced it wi=
th commit 44b27aec9d9680875).
Christoph introduced support for a rx-during-tx GPIO (see commit 163f080eb=
717). Tomas intends
to add a GPIO which enables RS485 if asserted
(see https://lore.kernel.org/all/3Za.ZZs%7D.ndXI8CMee4.1bN6eQ@seznam.cz/) =
and with Rasmus patches
we are about to add a MUX-GPIO which is to be asserted if RS485 is enabled=
.

I wonder where this will end and if we really have to support every possib=
le GPIO
in the serial core.

Regards,
Lino