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authorLukas Wunner <lukas@wunner.de>2021-12-18 10:58:56 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2021-12-30 13:21:02 +0100
commitd3b3404df318504ec084213ab1065b73f49b0f1d (patch)
tree3f5550e92e7ab19a46837b2db0b47f66cd24de25 /drivers/tty
parent5021d709b31b8a14317998a33cbc78be0de9ab30 (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.c4
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);
}
}