Real-hardware finding: a SuperSpeed hub (the user's Genesys, 4 SS
downstream ports) HUNG the bus driver at boot — it stopped logging with
no fault, and the hub's port power dropped. Cause: a SuperSpeed hub has
change bits a USB2 hub lacks (link-state, BH-reset), and the B4 servicing
— written and tested against QEMU's USB2 hub — never cleared them, so
the hub's status-change endpoint re-reported the same port forever and
the driver spun servicing it, never reaching the USB2 companion hub
where the full-speed keyboard actually lives.
hubPortStatusAck now clears the SuperSpeed-only change features too
(gated on hub speed; a USB2 hub is unaffected), and the tick services a
bounded batch of hub changes (32) before yielding — so even if a hub's
change bits misbehave, the driver cannot spin the machine. A per-port
status log line surfaces exactly what a hub reports, so the next real
boot shows the SS hub's port states and whether the USB2 companion
enumerates. QEMU usb-hub cases still green (the SS clears are gated off
for QEMU's USB2 hub).
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