mikkeli
66eab916bc
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz
64 lines
2.7 KiB
Markdown
64 lines
2.7 KiB
Markdown
---
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type: report
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date: 2026-10-11
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---
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# USB link test: plain cable vs PD hub
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Does the PD hub degrade the tablet ⇄ Orin USB (NCM) link? Measured from the app
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(the only place on the tablet whose sockets use the link) with the link test,
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`lib/linktest.dart` + `tool/linktest_server.py`.
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## Short version
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- **No measurable effect of the PD hub**, with or without the charger connected.
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All three setups: 0% loss at every size and rate up to 500 packets/s, the same
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latencies, 0 TCP retransmits.
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- **The loss seen earlier the same day did not come back.** Before these runs,
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ICMP ping from the Orin lost 2.5–32% at 20 packets/s through the hub. After a
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replug, the same ping sweep is 0% through the hub too. That connection was in
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a bad state; the hub is not shown to be the cause. Cause unknown; worth
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watching for (the soak below is the app-shaped check).
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- **The link is asymmetric:** tablet → Orin ~22 MB/s, Orin → tablet 1.9–5.6 MB/s
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and variable, with ~30 ms RTT under load and no loss (the Orin's gadget TX side
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queues rather than drops).
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- **App-shaped traffic is fine:** 30 s at 10 packets/s, longest silence ≤ 144 ms,
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against a 5 s heartbeat timeout.
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- **adb push through the SSH tunnel stays slow (0.14 MB/s, 223 Orin retransmits
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per 20 MB) on a healthy link**, while the app's TCP over the same link runs at
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5.6 MB/s with none. Specific to the adbd connection; not the hub, not routing
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(`tcp_fwmark_accept=1`). Unexplained.
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## Results
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Link USB 2.0 high-speed in all runs. TCP: 5 s each way; UDP: 3 s per cell, RTT
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p50 / p99 / max in ms.
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| | Plain C-to-C | Hub + charger | Hub, no charger |
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|---|---|---|---|
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| UDP loss, 64 B and 1400 B, 10–500 /s | 0% | 0% | 0% |
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| UDP 64 B 10/s RTT | 4.1 / 24.2 / 24.2 | 4.0 / 10.5 / 10.5 | 4.1 / 5.6 / 5.6 |
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| UDP 1400 B 500/s RTT | 2.8 / 7.3 / 39.7 | 2.7 / 9.4 / 42.5 | 2.7 / 16.6 / 60.2 |
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| Soak 64 B 10/s 30 s: loss, max gap | 0%, 144 ms | 0%, 135 ms | 0%, 104 ms |
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| TCP Orin → tablet | 1.85 MB/s | 4.06 MB/s | 5.57 MB/s |
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| TCP tablet → Orin | 21.8 MB/s | 22.4 MB/s | 22.6 MB/s |
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| Orin TCP retransmits | 0 | 0 | 0 |
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| ICMP ping from the Orin, 20/s, 56–1472 B | 0% | 0% | 0% |
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For reference, the same test over WiFi to the Orin's LAN address: 0% loss, about
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25 MB/s each way.
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Raw records: `~/linktest/results.jsonl` on the car Orin; tabulate with
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`python3 tool/linktest_summary.py ~/linktest/results.jsonl [label…]`.
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## Running it
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On the Orin: `python3 linktest_server.py` (from `tool/`, copied to `~/linktest/`).
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On the tablet: **Link test** in the link bar, or remotely:
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```bash
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adb -s localhost:15555 shell am start -n dev.yukart.display/.MainActivity --es linktest <label>[@<host>]
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```
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The host defaults to the gateway's. A run takes about 75 s and reports each
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result to the Orin as it goes.
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