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

[Lino Sanfilippo]

Hi,

On 14.12.23 14:41, Christoph Niedermaier wrote:
> From: Crescent CY Hsieh <crescentcy.hsieh@xxxxxxxx>
> Sent: Thursday, December 14, 2023 11:25 AM
>> On Mon, Dec 11, 2023 at 03:07:59PM +0200, Andy Shevchenko wrote:
>>> On Sat, Dec 09, 2023 at 12:47:47PM +0100, Lino Sanfilippo wrote:
>>>> On 06.12.23 16:42, Lino Sanfilippo wrote:
>>>
>>>>>>>> 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@xxxxxxxx/
>>>>>>>>
>>>>>>>
>>>>>>> That new flag was suggested by me instead of using SER_RS422_ENABLED, which
>>>>>>> would mostly be redundant to SER_RS485_ENABLED.
>>>>
>>>> A cleaner solution would probably be to not handle RS422 with the RS485 settings at
>>>> all, but to introduce another set of ioctls to set and read it.
>>>>
>>>> An own RS422 structure like
>>>>
>>>> struct serial_rs422 {
>>>>     __u32   flags;
>>>> #define SER_RS422_ENABLED           (1 << 0)
>>>> #define SER_RS422_TERMINATE_BUS             (1 << 1)
>>>> };
>>>>
>>>>
>>>> could be used as the parameter for these new ioctls.
>>>>
>>>> Any comments on this?
>>>
>>> I have (maybe not so constructive) a comment. Please, at all means try to not
>>> extend the existing serial data structures, we have too many ones with too many
>>> fields already. For user space, though, one may use unions and flags, but for
>>> internal ones it might be better ways, I think.
>>
>> How about revising the name of 'TIOCSRS485' and 'serial_rs485' to a
>> general one, and put RS422 and RS485 configuration flags into that
>> structure?
>>
>> So that in userspace it could set RS422 or RS485 configurations using a
>> single ioctl command and one structure.
>>
>> In this way, it won't be confused in userspace and won't add new data
>> structure internally as well.
>>
>
> 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.
>

Thats why I vote for a  RS422 mode.

> 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.

How can the driver distinguish between RS485 full duplex and half duplex then?
In full duplex RTS control is not needed AFAIU.

>
> I think the GPIOs reflect the flag states and are meaningful:
> - SER_RS485_TERMINATE_BUS: Switch bus termination on/off by GPIO
> - SER_RS485_RX_DURING_TX:  Used to enable/disable RX during TX
>                            in hardware by GPIO (for 2-wire)
> - SER_RS485_ENABLED:       Muxing between RS-232 and RS-485 by GPIO
>
> Switching RS-485 on during boot could also be handled by a devicetree
> overlay. Evaluate the GPIO and load a DTO accordingly before booting.
>
> Please correct me if I have misrepresented something...
>
> If I looked at it in this new way, I would discard my idea with the
> FULL_DUPLEX and HALF_DUPLEX. For a better use of RS-422 it would be
> good to disable transceiver control via RTS. It can be done by clearing
> the existing flags SER_RS485_RTS_ON_SEND and SER_RS485_RTS_AFTER_SEND
> at the same time, but I think it is confusing. Better would be a flag
> for RS-422:
>
> RS-422:                     Set SER_RS422_MODE for disabling
>                             transceiver control via RTS.
> RS-485 (2-wire and 4-wire): Clear SER_RS422_MODE for enabling
>                             transceiver control via RTS.
>
> Finally, at present it is also not possible to distinguish between RS485
> 2-wire and 4-wire operation. I think it isn't necessary, as different
> hardware has to be used anyway. The hardware then determines the
> configuration, see above.

But the driver should somehow be informed that there exists a full
duplex hardware setup, so that it does not need to control the RTS line.
Maybe by means of a device tree property?

Regards,
Lino


>
>
> Regards
> Christoph