serial: max310x: don't power down the port mid-transmission on close

max310x_shutdown() powers the port down regardless of what the
transmitter is doing. The power-off stops the UART clock mid-character,
truncating the final frame, and on the auto-RTS path it freezes the
RTS output at its current level: a close() with data still queued
leaves the RS485 transceiver asserted until the next open, which also
emits the interrupted character corrupted.

Waiting for the data to drain is no better: the tty layer's
wait-until-sent is bounded by closing_wait, configurable to none, and
a hangup arrives with no wait at all, so draining a full FIFO blocks
close() in uninterruptible sleep for seconds at low baud rates - and
forever when CTS flow control blocks the FIFO.

Instead, set MODE1 TxDisabl: the character in flight completes and the
transmitter ceases with TX_ at idle. Give that character one character
time (the chip has no transmitter-idle status), then reset the FIFOs
so the auto-RTS engine sees the transmitter empty and releases RTS
within the configured after-send hold; wait that hold plus one bit
time before powering off. Abandoned data was explicitly not waited
for, and startup() resets the FIFOs and TxDisabl on open anyway.

Measured on a MAX14830: a truncating close() at 50 baud takes 0.23 s,
RTS releases one character plus the hold after the last stop bit on
both polarities, and the reopen corruption is gone.

Fixes: f65444187a66 ("serial: New serial driver MAX310X")
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-v8-5-6ba881f81e1f@vaisala.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions