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

[Einar Vading]

On Mon, Apr 16, 2018 at 11:54:36AM +0200, Einar Vading wrote:
> On Mon, Apr 16, 2018 at 11:14:32AM +0200, Stefan Agner wrote:

Forgot to mention that we are not using DTE-de mode. We tried to but failed.

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