serial: max310x: fix breaks not reaching the wire in RS485 mode

The chip's auto-RTS (MODE1.TRNSCVCTRL) asserts the RS485 transceiver
only while FIFO data is shifting out, and a break is not FIFO data: on
an RS485 port a requested break sets the TX break bit but the
transceiver is never enabled, so the break never reaches the wire.
Break-led protocols cannot work at all.

Disable auto-RTS for the break duration and drive RTS manually via the
LCR RTS bit, then restore auto-RTS when the break ends. Track the break
in tx_break and leave MODE1 alone in the rs485-config worker while it
is set - a TIOCSRS485 arriving mid-break would otherwise re-enable
auto-RTS on top of the manual RTS and release the transceiver before
the break ends; break_ctl() restores auto-RTS from the then-current
configuration when the break completes. The worker runs under
port->mutex - break_ctl() and set_termios() already do - so the
tx_break test and the MODE1 write cannot straddle a break starting or
ending, and startup() clears tx_break alongside the latched TXBREAK
bit, since a port can be closed with a break still asserted.

Only the break assertion is gated on RS485 being enabled; the restore
at break end runs unconditionally and derives MODE1 from the current
configuration. Returning early for a port whose RS485 was disabled
mid-break - that reconfigure is deferred like any other - would leak
the manually driven RTS and leave the transceiver holding the bus
indefinitely after the break ends. On a non-RS485 port the restore is
a no-op: nothing else writes the LCR RTS bit.

Fixes: 55367c620aed ("serial: max310x: Add support for RS-485 mode")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Tapio Reijonen <tapio.reijonen@vaisala.com>
Link: https://patch.msgid.link/20261008-max310x-rs485-sw-delay-v8-2-6ba881f81e1f@vaisala.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions