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usb: the controller says how many device slots it has

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max_devices was 8, with the comment "QEMU presents a handful; a fuller
machine would grow this" — a number chosen against the test rig, waiting for
a real machine, which is the pattern docs/bounds-track-plan.md exists to
stop.

The driver already knew the true figure. It reads HCSPARAMS1.MaxSlots at
bring-up and writes it straight into op_config, so every slot the controller
offers has always been *enabled*; only the array tracking them was 8. QEMU's
xHCI reports 64, so seven eighths of the controller was live and invisible,
and the ninth device — a keyboard, mouse, webcam, headset, hub and two
sticks reach that without trying — disappeared on a hub-attached path that
logs nothing at all.

The array becomes a slice allocated from max_slots at bring-up. A controller
claiming zero slots cannot address anything, so that is now a dead
controller rather than an empty allocation failing mysteriously later. The
Device Context Base Address Array is a page, 511 usable entries, so it
already covered the 255-slot maximum.

The bring-up line reports both numbers, and usb-hid asserts they are equal
with a backreference rather than a magic number, so the test cannot drift
from the hardware. Pinning tracking back to 8 fails it: "64 slots,
tracking 8".

Suite 115/115.
This commit is contained in:
Daniel Samson
2026-08-08 11:39:49 +01:00
parent f4eb88e7d2
commit 69fbef40c0
5 changed files with 37 additions and 14 deletions
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docs/bounds-track-plan.md
+1 -1
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@@ -14,7 +14,7 @@ next one starts.*
|---|---|---|
| L1 | Reclamation: a dead task's registrations die with its claims | **stopped — the step was wrong; see open question 4** |
| L2 | Bounds build check + allowlist; declare what we have already touched | **done** — `zig build bounds`, 273 allowlisted, 5 declared |
| L3 | xHCI: slot count from `HCSPARAMS1.MaxSlots`, not 8 | not started |
| L3 | xHCI: slot count from `HCSPARAMS1.MaxSlots`, not 8 | **done** — QEMU reports 64; the driver tracked 8 |
| L4 | USB: configuration descriptor sized by `wTotalLength`, not 512 | not started |
| L5 | USB: interfaces from the descriptor, and the misattributed-endpoint bug | not started |
| L6 | xHCI: a failed `allocateDevice` stops leaking an enabled slot | not started |