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

[Lukas Wunner <lukas@wunner.de>]

On Sat, Dec 23, 2023 at 02:40:58PM +0100, Lino Sanfilippo wrote:
> On 23.12.23 13:49, Christoph Niedermaier wrote:
> > From: Lukas Wunner [mailto:lukas@wunner.de]
> > Sent: Thursday, December 21, 2023 4:53 PM
> > > Well why don't we just allow enabling or disabling RS-485 termination
> > > independently from the SER_RS485_ENABLED bit in struct serial_rs485?
> > >
> > > Just let the user issue a TIOCSRS485 ioctl to toggle termination even
> > > if in RS-232 mode and use that mode for RS-422.
> >
> > Sounds not bad. The termination should only depend on whether the GPIO is
> > given or not.
> >
> > Irrespective of this, I think the Linos idea of an RS-422 mode is not bad.
> > This allows you to take care of special features that were previously
> > overlooked. For example, hardware flow control can be switched off so that
> > this does not cause any problems.
> 
> Also note, that RS232 and RS422 may NOT always be the same from driver
> perspective.
> Take a look at 8250_excar.c for example. That driver has to configure
> the hardware accordingly when switching from RS232 to RS422
> (see iot2040_rs485_config()).

Actually it seems there are a bunch of GPIOs on the IOT2040 board
(called MPIO instead of GPIO by the driver).  See the documentation
of the wiring at line 87 in drivers/tty/serial/8250/8250_exar.c.

So "configure the hardware" seems to just boil down to toggling the
right GPIO (aka MPIO) pins to mux the UART signals to the right
(RS232/RS485/RS422) transceiver.

IOT2040 is an ACPI-based platform, so no devicetree to describe
the RS232/RS485/RS422 mux GPIOs, but the underlying concept is the same.

Thanks,

Lukas