cc1: harden RTL8821CU Wi-Fi across warm reboots

[pdscomp]

Problem

The Elegoo Centauri Carbon 1 RTL8821CU Wi-Fi adapter intermittently failed after warm reboot:

• Early failures occurred during USB enumeration, including SET_ADDRESS returning -EPROTO.
• Later captures showed the adapter enumerate and bind, then fail rtw88 vendor control transfers with -EPROTO during firmware startup.
• Once wedged, the old path issued more than 1,000 failing register controls and could remain broken across soft resets.

The original failure reproduced every 3-5 soft reboot/flash cycles on the CC1 original mainboard (not the updated wifi chip one).

Changes

linux: add slow-reset quirk for the CC1 USB hub

Marks the CC1 QinHeng 1a86:8091 parent hub with USB_QUIRK_HUB_SLOW_RESET. USB core gives downstream devices an extra 100 ms after port reset, directly improving the pre-enumeration warm-reset failure without cycling the whole hub.

The initial hub-quirk plus shutdown-unbind experiment reached roughly 10 cycles before a different, post-bind -EPROTO failure was captured.

rtw88: retry failed USB register controls

Retries only register-control -EPROTO failures after 5, 20, 80, 320, and 1280 ms. Successful transfers have no added delay. A terminal failure latches the control transport as broken, preserving the triggering operation/address and suppressing the previous thousand-message error storm.

This replaces a discarded blanket 5 ms delay experiment, which added about 18 seconds to Wi-Fi startup and still failed after 19 loops.

rtw88: serialize USB vendor controls

Adds the vendor-request mutex used by Realtek's 8821cu driver. It keeps a register request and its RTL8821C 0x4e0 reg-sec companion atomic and serializes firmware-page controls on the same mutex, preventing control requests from interleaving. Firmware fragments are not individually retried because the full download is checksummed.

Validation

• Clean rtw88 build: 1,143 tasks passed; both patches applied without fuzz.
• Full CC1 SWU build on current main: 6,521 tasks passed.
• Combined production fixes plus an experimental shutdown hook: 44 consecutive reboot loops.
• Final image with only the kernel/rtw88 changes and no shutdown hook: 540 consecutive successful reboot loops on carbon-devu.
• No fixed successful-path delay remains.

The reboot counts validate the combined final patch set; the driver patches were not individually ablated.

[github-actions]

✅ Build Artifacts

Branch: doom/oldwifibroke
Build: 80a77c2 (merge into main)

Artifact	Size
CC1 Firmware	91.31 MB

View workflow run

[pdscomp]

Full Wi-Fi investigation log and test history:

https://gist.github.com/pdscomp/c207506c250913d83c703178d089b901

[github-actions]

✅ Build Artifacts

Branch: doom/oldwifibroke
Build: 085eb29 (merge into main)

Artifact	Size
CC1 Firmware	91.31 MB

View workflow run

[github-actions]

✅ Build Artifacts

Branch: doom/oldwifibroke
Build: 9073ece (merge into main)

Artifact	Size
CC1 Firmware	91.31 MB

View workflow run