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

From: Jiri Slaby <jirislaby@kernel.org>
To: Lukas Wunner <lukas@wunner.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.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: Re: [PATCH] serial: Fix incorrect rs485 polarity on uart open
Date: Mon, 20 Dec 2021 07:28:00 +0100	[thread overview]
Message-ID: <8a10bea0-fb8a-c25a-6828-ab907b336d0b@kernel.org> (raw)
In-Reply-To: <9395767847833f2f3193c49cde38501eeb3b5669.1639821059.git.lukas@wunner.de>

On 18. 12. 21, 10:58, Lukas Wunner wrote:
> 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>

Rafael, can you ack/test this, please?

> 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);
>   	}
>   }

-- 
js
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