[Bug 221598] Bluetooth: btusb/btrtl RTL8821C 0bda:c821 HFP/mSBC microphone stutters with bursty SCO RX delivery

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=221598

--- Comment #8 from Alexander Paderin (apocarteres@gmail.com) ---
Additional update: loading the RTL8821CE Wi-Fi-side driver dramatically
improves Bluetooth HFP/mSBC RX stability on this RTL8821CE combo module.

Hardware topology:

- Bluetooth: Realtek `0bda:c821`, internal USB Full-Speed device, handled by
`btusb`
- Wi-Fi: Realtek RTL8821CE PCIe `10ec:c821`, handled by `rtw88_8821ce` /
`rtw_8821ce`

I ran the same near/far/near2 Bluetooth HFP microphone range test twice:

1. Wi-Fi driver not loaded.
2. Wi-Fi driver loaded manually, but Wi-Fi radio kept disabled in
NetworkManager.

State during the improved run:

```text
02:00.0 Network controller [0280]: Realtek Semiconductor Co., Ltd. RTL8821CE
802.11ac PCIe Wireless Network Adapter [10ec:c821]
        Kernel driver in use: rtw_8821ce
        Kernel modules: rtw88_8821ce

WIFI-HW  WIFI      WWAN-HW  WWAN
enabled  disabled  missing  enabled

wlp2s0 DOWN
```

Kernel log also showed the Wi-Fi firmware initialized:

```text
rtw_8821ce 0000:02:00.0: Firmware version 24.11.0, H2C version 12
```

Results:

| run | Wi-Fi driver | phase | RSSI avg | RSSI min | RX markers | RX bad seq/k
| RX bad dist/k | TX bad seq | TX bad dist |
|---|---|---|---:|---:|---:|---:|---:|---:|---:|
| `range-sco-test-20260531-005925` | not loaded | near | -38.9 | -42 | 1976 |
1.01 | 3.54 | 0 | 0 |
| `range-sco-test-20260531-005925` | not loaded | far | -58.1 | -70 | 3528 |
123.30 | 103.74 | 0 | 0 |
| `range-sco-test-20260531-005925` | not loaded | near2 | -49.6 | -54 | 1936 |
38.74 | 31.51 | 0 | 0 |
| `range-sco-test-20260531-010648` | loaded, Wi-Fi radio off | near | -46.7 |
-52 | 1975 | 1.01 | 1.01 | 0 | 0 |
| `range-sco-test-20260531-010648` | loaded, Wi-Fi radio off | far | -53.9 |
-60 | 3989 | 2.51 | 2.51 | 0 | 0 |
| `range-sco-test-20260531-010648` | loaded, Wi-Fi radio off | near2 | -53.1 |
-64 | 1984 | 6.55 | 6.55 | 0 | 0 |

The important result is the far phase:

- RX bad sequence rate dropped from `123.30/k` to `2.51/k`.
- RX bad distance rate dropped from `103.74/k` to `2.51/k`.
- TX remained clean in both runs.
- RSSI no longer collapsed to `-70 dBm`; the improved run bottomed at `-60
dBm`.

Because Wi-Fi remained disabled and the interface stayed down, this improvement
does not appear to come from active Wi-Fi traffic. It appears to come from the
Wi-Fi-side driver/firmware initialization of the same RTL8821CE combo chip.

This strongly suggests that the Linux Bluetooth-only initialization path for
`0bda:c821` leaves the combo chip in a worse Bluetooth RX/range state than the
state after `rtw_8821ce` initializes the Wi-Fi side. The likely area is Realtek
combo-chip coexistence/RF/antenna-path/shared firmware state.

Practical workaround on this machine:

- keep `rtw88_8821ce` loadable and loaded at boot;
- keep Wi-Fi radio disabled if Wi-Fi is not needed.

Local captures/reports:

- `captures/range-sco-test-20260531-005925.snoop`
- `captures/range-sco-test-20260531-005925-metrics.csv`
- `reports/range-sco-test-20260531-005925-analysis.md`
- `captures/range-sco-test-20260531-010648.snoop`
- `captures/range-sco-test-20260531-010648-metrics.csv`
- `reports/range-sco-test-20260531-010648-analysis.md`
- `reports/rtl8821ce-wifi-driver-coex-range-comparison-20260531.md`

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