Re: serial: imx: half-duplex RS485 operation with RTS active low

[Stefan Agner <stefan@agner.ch>]

On 16.04.2018 11:54, Einar Vading wrote:
> On Mon, Apr 16, 2018 at 11:14:32AM +0200, Stefan Agner wrote:
>> Hi,
>>
>> Using upstream I noticed that RS-485 does not work in the default
>> configuration for our platforms (Toradex Apalis/Colibri). Closer
>> debugging shows that it is related to "serial: imx: default to half
>> duplex rs485".
> 
> We where struggling a bit with half duplex rs485 too a while ago.
>>
>> We use the i.MX UART in DTE mode and control the RS-485 transceiver
>> using the RTS signal in low-active mode.
>>
>> 	uart-has-rtscts;
>> 	fsl,dte-mode;
>> 	linux,rs485-enabled-at-boot-time;
>> 	rs485-rts-active-low;
>>
>> Using this setting leads to the RTS signal not getting asserted (the
>> oscilloscope only shows a very short fluke before the start bit is
>> sent).
> 
> I think this is what we had too. But if I recall we where supposed to be
> active-high but I think the behavior is the same either way.

With v4.17-rc1, active high RTS, DTE in half duplex mode seems to work
fine (looks good on an oscilloscope). However, active low seems to be
broken.

>>
>> However, using
>>
>> 	uart-has-rtscts;
>> 	fsl,dte-mode;
>> 	linux,rs485-enabled-at-boot-time;
>> 	rs485-rts-active-low;
>> 	rs485-rx-during-tx;
>>
>> Asserts the RTS signal low active just fine...
> 
> We have a (RTS active high) configuration working for us now with only
> 
>     uart-has-rtscts
> 
> in DT, and then when using the port we do
> 
>     struct serial_rs485 rs485conf;
>     memset(&rs485conf, 0x00, sizeof(struct serial_rs485));
> 
>     rs485conf.flags |= SER_RS485_ENABLED;
>     rs485conf.flags |= SER_RS485_RTS_ON_SEND;
>     rs485conf.flags &= ~(SER_RS485_RTS_AFTER_SEND);

>From what I can tell, in v4.17-rc1 this should be equivalent to just:

 	linux,rs485-enabled-at-boot-time;

> 
>     f_debug("Calling ioctl...");
>     if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
>       f_critical("ioctl serial_rs485 failed");
>       return -1;
>     }
> 
> Hope it helps.

Thanks, I will check using RS485 flags via ioctrl.

--
Stefan

>>
>> Is this a known problem? Any idea where that could come from? It looks
>> as if the receiver part is actually enabling RTS...?
>>
>> Also, isn't enabling RX even in half-duplex mode quite common in order
>> to detect collisions?
>>
>> --
>> Stefan
> 
> // Einar
>>
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