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@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v4.14+
Cc: Rafael Gago Castano <rgc@xxxxxx>
Cc: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
Cc: Su Bao Cheng <baocheng.su@xxxxxxxxxxx>
---
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
