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USB report: the long outages were Android sleep, not the link

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Idle on battery, the tablet suspends ~60 s after screen-off and only
answers at its DHCP-renewal wakeups while the USB link stays up. Kept
awake with a video, a PD unplug costs 3.1 s and then nothing. The
earlier replug silences were the tablet waking from sleep.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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mikkeli
2026-10-09 00:43:37 +09:00
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docs/impl/reports/USB-TABLET-LINK.md
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@@ -111,6 +111,20 @@ Three PD unplug/replug cycles while pinging the tablet every 0.5 s:
- Exact replug time isn't known; the first failed ping is used as its marker. - Exact replug time isn't known; the first failed ping is used as its marker.
- Repeat (plug-in at ~00:27:12): the tablet was silent for 7.8 s with the Orin's link up, then one gadget reconnect (00:27:25.1–27.0) - Repeat (plug-in at ~00:27:12): the tablet was silent for 7.8 s with the Orin's link up, then one gadget reconnect (00:27:25.1–27.0)
and a fresh `DHCPDISCOVER`; about **15 s** to stable. Replug consistently costs more than unplug (3.5 s). and a fresh `DHCPDISCOVER`; about **15 s** to stable. Replug consistently costs more than unplug (3.5 s).
#### The "silence" is Android sleeping, not the link
| Test (2026-10-09) | Tablet state after PD unplug | Result |
|---|---|---|
| Idle, 3 min window | screen times out | Clean for ~60 s, then pings answered only in brief bursts right at its 30 s DHCP renewals, then silent. **No kernel link events**: USB stayed up throughout |
| Video playing, 150 s window | kept awake | 3.1 s outage at the unplug, then **zero loss for 2 min+** |
- Off charger and idle, Android suspends about 60 s after the screen times out ("keep awake on charge" no longer applies). The USB gadget link
stays electrically up, but the tablet only wakes for timers (DHCP renewal every ~30 s).
- The earlier "replug silence" was the same thing. Those windows started with the tablet already asleep, so plugging power in woke it, and the
5–8 s was wake-up plus re-creating its USB network.
- So the hub's PD change costs **~3.5 s** (the unplug detach). Everything beyond that came from tablet sleep.
- To the Orin, a sleeping tablet is indistinguishable from a dead link, except by heartbeat.
- At ignition, accessory power drops during cranking, so a start is likely to produce both edges back to back (not tested in the car yet). - At ignition, accessory power drops during cranking, so a start is likely to produce both edges back to back (not tested in the car yet).
Consequence: in the car, expect a link outage of up to about 12 s at every ignition change. That is a requirement for the services Consequence: in the car, expect a link outage of up to about 12 s at every ignition change. That is a requirement for the services