Re: [PATCH v8 5/9] serial: max310x: don't power down the port mid-transmission on close

[sashiko-bot@kernel.org]

> 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>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20261008-max310x-rs485-sw-delay-v8-0-6ba881f81e1f@vaisala.com?part=5