Wrong RS485 RTS polarity in 8250 OMAP UART Driver vs DT binding?

From: Francesco Dolcini <francesco@dolcini.it>
To: Lukas Wunner <lukas@wunner.de>,
	Vignesh Raghavendra <vigneshr@ti.com>, Bin Liu <b-liu@ti.com>,
	linux-serial@vger.kernel.org, Zeng Chao <chao.zeng@siemens.com>
Cc: Rob Herring <robh@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>,
	ilpo.jarvinen@linux.intel.com, tony@atomide.com,
	andriy.shevchenko@linux.intel.com,
	matthias.schiffer@ew.tq-group.com, linux-kernel@vger.kernel.org,
	Max Krummenacher <max.krummenacher@toradex.com>
Subject: Wrong RS485 RTS polarity in 8250 OMAP UART Driver vs DT binding?
Date: Wed, 15 Mar 2023 21:41:56 +0100	[thread overview]
Message-ID: <ZBItlBhzo+YETcJO@francesco-nb.int.toradex.com> (raw)

Hello all,
I have been recently working on RS485 on a TI K3 AM62 (using 8250_omap.c
driver) and there is something that I do not really understand about
the RTS polarity.

According to the DT binding (Documentation/devicetree/bindings/serial/rs485.yaml)
when `rs485-rts-active-low` is present the behavior should be "drive RTS
low when sending", however in the code it seems that this is handled the
opposite way (drivers/tty/serial/8250/8250_omap.c:861).

/*
 * Retain same polarity semantics as RS485 software emulation,
 * i.e. SER_RS485_RTS_ON_SEND means driving RTS low on send.
 */

We have working hardware, using another UART that expect
rs485-rts-active-low, and the exact same HW, when using OMAP uart,
requires rs485-rts-active-high (the HW is modular, so we are really just
changing the uart, keeping the same RS485 transceiver / connections).

What's going on there? Is the semantic of the 8250_omap driver just the
opposite as it should be? Am I doing something wrong in the device tree?

This is the relevant DTS part.

&main_uart1 {
	linux,rs485-enabled-at-boot-time;
	rs485-rts-active-high;
	rs485-rx-during-tx;
	status = "okay";
};

Francesco