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

[Uwe Kleine-König]

Hi Stefan,

On Mon, Apr 16, 2018 at 11:14:32AM +0200, Stefan Agner wrote:
> Using upstream I noticed that RS-485 does not work in the default
> configuration for our platforms (Toradex Apalis/Colibri). Closer

This is an i.MX6?

> debugging shows that it is related to "serial: imx: default to half
> duplex rs485".
> 
> 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;

That means you're not using a GPIO for RTS signaling, right?
 
> 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).
> 
> 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...
> 
> Is this a known problem? Any idea where that could come from? It looks
> as if the receiver part is actually enabling RTS...?

Which kernel version do you use? My latest rs485 related patches went
into v4.17-rc1. With that I managed to make rs485 half duplex work on
several customer boards.

> Also, isn't enabling RX even in half-duplex mode quite common in order
> to detect collisions?

I don't know.

Best regards
Uwe

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Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |