From: Lukas Wunner lukas@wunner.de
commit d3b3404df318504ec084213ab1065b73f49b0f1d upstream.
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") 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 Signed-off-by: Lukas Wunner lukas@wunner.de Link: https://lore.kernel.org/r/9395767847833f2f3193c49cde38501eeb3b5669.163982105... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/serial/serial_core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -159,7 +159,7 @@ static void uart_port_dtr_rts(struct uar 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 { @@ -168,7 +168,7 @@ static void uart_port_dtr_rts(struct uar } 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); } }