Re: [PATCH v1 4/4] usb: serial: mxuport: add sysfs control for UART FIFO

[Crescent Hsieh <crescentcy.hsieh@moxa.com>]

Hi Greg and Johan,

I agree that USB-to-serial cannot provide deterministic latency.
However, the embedded UART used by Moxa UPort has a 512-byte FIFO, so
its FIFO/trigger configuration still has a measurable effect on the
typical latency on this hardware.

RQ_VENDOR_SET_FIFO_DISABLE is a historical name. On the current
firmware, it does not literally turn the UART FIFO on or off, but
adjusts the UART RX/TX trigger levels instead. Its semantics are close
to the trigger-level control I am currently working on for the PCI
serial cards [1].

In a 1-byte loopback latency test under the same wiring and host setup,
with 10,000 samples for each configuration, I measured the following
results (in us):

       | Enable | Disable |
--------------------------|
Min    |    637 |     210 |
Median |    764 |     235 |
P95    |    811 |     238 |
P99    |    820 |     260 |
Max    |  4,214 |   3,807 |
Avg    |    744 |     235 |
--------------------------|

So while this does not make USB deterministic, it does show that this
setting has a significant effect on the typical latency on this hardware
and also matters for compatibility with some legacy serial equipment.

[1]
https://lore.kernel.org/all/20260504084900.22380-16-crescentcy.hsieh@moxa.com

---
Sincerely,
Crescent Hsieh