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    Subject[PATCH 5.10 449/563] serial: Fix incorrect rs485 polarity on uart open
    Date
    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/9395767847833f2f3193c49cde38501eeb3b5669.1639821059.git.lukas@wunner.de
    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
    @@ -162,7 +162,7 @@ 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 {
    @@ -171,7 +171,7 @@ 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);
    }
    }

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