[PATCH] serial: Fix incorrect rs485 polarity on uart open

From: Lukas Wunner <lukas@wunner.de>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jirislaby@kernel.org>
Cc: "Su Bao Cheng" <baocheng.su@siemens.com>,
	baocheng_su@163.com, Jan Kiszka <jan.kiszka@siemens.com>,
	Chao Zeng <chao.zeng@siemens.com>,
	linux-serial@vger.kernel.org,
	Lino Sanfilippo <LinoSanfilippo@gmx.de>,
	Philipp Rosenberger <p.rosenberger@kunbus.com>,
	Rafael Gago Castano <rgc@hms.se>
Subject: [PATCH] serial: Fix incorrect rs485 polarity on uart open
Date: Sat, 18 Dec 2021 10:58:56 +0100	[thread overview]
Message-ID: <9395767847833f2f3193c49cde38501eeb3b5669.1639821059.git.lukas@wunner.de> (raw)

Commit a6845e1e1b78 ("serial: core: Consider rs485 settings to drive
RTS") sought to deassert RTS when opening an rs485-enabled uart port.
That way, the transceiver does not occupy the bus until it transmits
data.

Unfortunately, the commit mixed up the logic and *asserted* RTS instead
of *deasserting* it:

The commit amended uart_port_dtr_rts(), which raises DTR and RTS when
opening an rs232 port.  "Raising" actually means lowering the signal
that's coming out of the uart, because an rs232 transceiver not only
changes a signal's voltage level, it also *inverts* the signal.  See
the simplified schematic in the MAX232 datasheet for an example:
https://www.ti.com/lit/ds/symlink/max232.pdf

So, to raise RTS on an rs232 port, TIOCM_RTS is *set* in port->mctrl
and that results in the signal being driven low.

In contrast to rs232, the signal level for rs485 Transmit Enable is the
identity, not the inversion:  If the transceiver expects a "high" RTS
signal for Transmit Enable, the signal coming out of the uart must also
be high, so TIOCM_RTS must be *cleared* in port->mctrl.

The commit did the exact opposite, but it's easy to see why given the
confusing semantics of rs232 and rs485.  Fix it.

Fixes: a6845e1e1b78 ("serial: core: Consider rs485 settings to drive RTS")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v4.14+
Cc: Rafael Gago Castano <rgc@hms.se>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Su Bao Cheng <baocheng.su@siemens.com>
---
 drivers/tty/serial/serial_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 29f4781..259f28e 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -162,7 +162,7 @@ static void uart_port_dtr_rts(struct uart_port *uport, int raise)
 	int RTS_after_send = !!(uport->rs485.flags & SER_RS485_RTS_AFTER_SEND);
 
 	if (raise) {
-		if (rs485_on && !RTS_after_send) {
+		if (rs485_on && RTS_after_send) {
 			uart_set_mctrl(uport, TIOCM_DTR);
 			uart_clear_mctrl(uport, TIOCM_RTS);
 		} else {
@@ -171,7 +171,7 @@ static void uart_port_dtr_rts(struct uart_port *uport, int raise)
 	} else {
 		unsigned int clear = TIOCM_DTR;
 
-		clear |= (!rs485_on || !RTS_after_send) ? TIOCM_RTS : 0;
+		clear |= (!rs485_on || RTS_after_send) ? TIOCM_RTS : 0;
 		uart_clear_mctrl(uport, clear);
 	}
 }
-- 
2.33.0

