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

[Lino Sanfilippo <LinoSanfilippo@gmx.de>]

On 23.12.23 13:49, Christoph Niedermaier wrote:
> From: Lukas Wunner [mailto:lukas@wunner.de]
> 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.
>

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()).

While a SER_RS485_MODE_RS422 flag set by userspace could work to switch to RS422, I
still think that the cleanest solution would be another ioctl TIOCSRS422 with a
parameter like

struct serial_rs422 {
       __u32   flags;
#define SER_RS422_ENABLED              (1 << 0)
#define SER_RS422_TERMINATE_BUS        (1 << 1)
	__u32 	padding[7]
};

The SER_RS485_MODE_RS422 flag could still be used internally as a hint to the driver.
That solution would also be expandable if needed. I planned to send a patch that implements this
as a RFC to the mailing list (maybe in the next few days).

Regards,
Lino