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

From: Einar Vading <einar.vading@axis.com>
To: Stefan Agner <stefan@agner.ch>
Cc: linux-arm-kernel@lists.infradead.org, s.hauer@pengutronix.de,
	baruch@tkos.co.il, linux-serial@vger.kernel.org,
	u.kleine-koenig@pengutronix.de
Subject: Re: serial: imx: half-duplex RS485 operation with RTS active low
Date: Mon, 16 Apr 2018 11:54:36 +0200	[thread overview]
Message-ID: <20180416095436.7mxh4cqdvlyleryr@lnxeinarv.se.axis.com> (raw)
In-Reply-To: <0e88706a05a302201be396d2f03c3e9e@agner.ch>

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.
> 
> 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
> 
> --
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