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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