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([2a0b:e7c0:0:107::49]) by smtp.gmail.com with ESMTPSA id w6sm4937097wrs.52.2021.12.19.22.28.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 19 Dec 2021 22:28:01 -0800 (PST) Message-ID: <8a10bea0-fb8a-c25a-6828-ab907b336d0b@kernel.org> Date: Mon, 20 Dec 2021 07:28:00 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH] serial: Fix incorrect rs485 polarity on uart open Content-Language: en-US To: Lukas Wunner , Greg Kroah-Hartman Cc: Su Bao Cheng , baocheng_su@163.com, Jan Kiszka , Chao Zeng , linux-serial@vger.kernel.org, Lino Sanfilippo , Philipp Rosenberger , Rafael Gago Castano References: <9395767847833f2f3193c49cde38501eeb3b5669.1639821059.git.lukas@wunner.de> From: Jiri Slaby In-Reply-To: <9395767847833f2f3193c49cde38501eeb3b5669.1639821059.git.lukas@wunner.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org 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 > Cc: stable@vger.kernel.org # v4.14+ > Cc: Rafael Gago Castano Rafael, can you ack/test this, please? > Cc: Jan Kiszka > Cc: Su Bao Cheng > --- > 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 suse labs