diff options
| author | Lukas Wunner | 2021-12-18 10:58:56 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman | 2021-12-30 13:21:02 +0100 |
| commit | d3b3404df318504ec084213ab1065b73f49b0f1d (patch) | |
| tree | 3f5550e92e7ab19a46837b2db0b47f66cd24de25 /drivers/tty | |
| parent | 5021d709b31b8a14317998a33cbc78be0de9ab30 (diff) | |
serial: Fix incorrect rs485 polarity on uart open
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>
Diffstat (limited to 'drivers/tty')
| -rw-r--r-- | drivers/tty/serial/serial_core.c | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 29f4781db387..259f28ec6c8b 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); } } |