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

[Christoph Niedermaier]

From: Lukas Wunner [mailto:lukas@xxxxxxxxx]
Sent: Thursday, December 21, 2023 4:53 PM
> 
> On Thu, Dec 14, 2023 at 01:41:47PM +0000, Christoph Niedermaier wrote:
>> I will summarize the current situation from my point of view, maybe it helps:
>>
>> RS-232:
>>   - Full Duplex Point-to-Point connection
>>   - No transceiver control with RTS
>>   - No termination
>>   - No extra struct in use
>>
>> RS-422:
>>   - Full Duplex Point-to-Point connection
>>   - No transceiver control with RTS needed
>>   - Termination possible
>>   - Extra struct serial_rs485 needed if termination is used
>>  => RS-422 can be used in RS-232 operation, but if a termination should be
>>     switchable the RS485 flag has to be enabled. But then also transceiver
>>     control will be enabled. Not a very satisfying situation.
> 
> 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.
> 
> Looks like the simplest solution to me.

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.

>> RS-485 (2-wire) very common:
>>   - Half Duplex RS-485 bus
>>   - Transceiver control with RTS is needed
>>   - Termination possible
>>   - Extra struct serial_rs485 is needed
>>  => RS-485 has to be enabled and configured:
>>     - Set SER_RS485_ENABLED
>>     - Set SER_RS485_RTS_ON_SEND or SER_RS485_RTS_AFTER_SEND
>>     - Set/clear SER_RS485_RX_DURING_TX depending on whether
>>       the receiver path should be on or off during sending.
>>       If it's set it allows to monitor the sending on the bus
>>       and detect whether another bus device is transmitting
>>       at the same time.
>>     - Set/clear SER_RS485_TERMINATE_BUS for bus termination.
>>
>> RS-485 (4-wire) little used:
>>   - Full Duplex RS-485 bus
>>   - Transceiver control with RTS is needed
>>   - Termination possible
>>   - Extra struct serial_rs485 is needed
>>  => RS-485 has to be enabled and configured:
>>     - Set SER_RS485_ENABLED
>>     - Set SER_RS485_RTS_ON_SEND or SER_RS485_RTS_AFTER_SEND
>>     - Set SER_RS485_RX_DURING_TX, as the receiver should always
>>       be enabled independently of TX, because TX and RX are
>>       separated from each other by their own wires.
>>     - Set/clear SER_RS485_TERMINATE_BUS for bus termination.
> 
> Thanks for that overview, I found it very helpful.
> 
> One small addendum:  Hardware flow control.  Only possible with
> RS-232.  Doesn't work in any of the other modes, right?

You are right I forgot to mention it.

Regards and Merry Christmas
Christoph