Re: [PATCH v2 RESEND] serial: sc16is7xx: fix TX gap caused by kfifo circular buffer wrap-around

[sashiko-bot@kernel.org]

> kfifo_out_linear_ptr() returns only one contiguous linear segment of the
> xmit kfifo. When transmit data wraps around the end of the kfifo, only
> the first segment up to the buffer end is sent. The remaining data at
> the start of the kfifo is not sent until the next TX interrupt fires,
> resulting in a visible mid-frame TX gap on the wire.
> 
> The resulting gap is unintended: data remains queued in the xmit kfifo,
> but the hardware TX FIFO drains empty before the remaining segment is
> sent. Such gaps can break timing-sensitive serial protocols such as
> Modbus RTU.
> 
> Modbus RTU requires a message to be transmitted as a continuous stream.
> For baud rates above 19200, the Modbus Serial Line guide recommends a
> fixed 750 us inter-character timeout. On the tested 115200-baud system,
> oscilloscope measurements showed mid-frame gaps exceeding that value.
> [ ... ]
> Cc: stable@vger.kernel.org
> Reported-by: Tobias Gannert <tobias.gannert@ziehl-abegg.de>
> Reviewed-by: Joachim Knorr <joachim.knorr@ziehl-abegg.de>
> Link: https://lore.kernel.org/linux-serial/20260623112225.82386-2-paultyson.mbewe@ziehl-abegg.de/
> Signed-off-by: Paul Mbewe <paultyson.mbewe@ziehl-abegg.de>

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

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