[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 #7 from Alexander Paderin (apocarteres@gmail.com) ---
Additional update after isolating user-space HCI traffic:

I found a strong practical mitigation/confounder: stopping/removing Blueman
significantly improves the HFP/mSBC microphone quality on this machine.

Before this test, Blueman UI processes were running:

- `blueman-manager`
- `blueman-applet`
- `blueman-tray`

The bad traces with Blueman running contained frequent HCI control traffic,
especially:

- `Read RSSI` (`0x1405`)
- `Read Transmit Power Level` (`0x0c2d`)

After stopping/removing Blueman, the same headset/machine produced much cleaner
captures. The subjective microphone stutter also became noticeably better.

Summary of RX mSBC/H2 continuity results:

| run | extra HCI polling | RX H2 markers | bad seq | bad seq/k | bad dist |
bad dist/k |
|---|---:|---:|---:|---:|---:|---:|
| old control | yes | 3486 | 21 | 6.02 | 23 | 6.60 |
| no-Blueman control 1 | no | 4301 | 6 | 1.39 | 15 | 3.49 |
| no-Blueman control 2 | no | 4312 | 6 | 1.39 | 6 | 1.39 |
| bad noisy watch-only run | yes | 3949 | 454 | 114.97 | 425 | 107.62 |
| bad noisy raw-open repeat | yes | 4812 | 453 | 94.14 | 385 | 80.01 |

TX remained clean in all runs (`bad_sequence=0`, `bad_distance=0`).

Important caveat: I then tried to reproduce the issue without Blueman by
injecting the same polling manually after eSCO setup. The stress script sent 25
pairs of:

- `Read RSSI` (`0x1405`)
- `Read Transmit Power Level` (`0x0c2d`)

That did not reproduce the catastrophic bad runs:

| run | polling | RX H2 markers | bad seq | bad seq/k | bad dist | bad dist/k |
|---|---:|---:|---:|---:|---:|---:|
| no-Blueman control 2 | no | 4312 | 6 | 1.39 | 6 | 1.39 |
| artificial HCI polling stress | 25 RSSI + 25 TX-power reads | 3778 | 16 |
4.24 | 13 | 3.44 |

So `Read RSSI` / `Read Transmit Power Level` alone is not proven to be the
direct root cause. The stronger conclusion is:

- Removing Blueman is a real practical mitigation on this RTL8821C/0bda:c821
system.
- Extra user-space HCI management/control traffic is a strong confounder and
can correlate with much worse RX mSBC continuity.
- The underlying failure still appears to be RX-only H2/mSBC continuity loss:
host TX stays clean, eSCO setup parameters remain normal, and RX still has some
breaks even in the clean no-Blueman baseline.

Relevant local captures/reports:

- `captures/sco-control-test-20260531-002158.snoop`
- `captures/sco-control-test-20260531-002431.snoop`
- `captures/hci-poll-stress-test-20260531-003222.snoop`
- `reports/control-no-blueman-20260531-002158-findings.md`
- `reports/control-no-blueman-repeat-20260531-002431-findings.md`
- `reports/hci-poll-stress-20260531-003222-findings.md`

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