Realtek bluetooth controller malfunctions after upgrading from the 5.15.0-94-generic non-HWE kernel

[林博仁(Buo-ren, Lin)]

In a troubleshooting thread[1] I assisted within the local Ubuntu community a user encountered an issue where Bluetooth devices(e.g. HID mouse) no longer function after a certain update on a Xubuntu 22.04 system that uses an HWE kernel.

bluetoothd logs appear to have some error messages like:

```
Bluetooth management interface 1.22 initialized
src/adapter.c:reset_adv_monitors_complete() Failed to reset Adv monitors: Failed (0x03)
Failed to clear UUIDs: Failed (0x03)
Failed to add UUID: Failed (0x03)
Failed to add UUID: Failed (0x03)
Failed to add UUID: Failed (0x03)
Endpoint registered: sender=1.32 path=/MediaEndpoint/A2DPSource/ldac
```

```
Endpoint registered: sender=1.46 path=/MediaEndpoint/A2DPSource/sbc_xq_552
src/device.c:load_gatt_db() no cache for D1:2B:38:RE:DA:CT
src/service.c:service_accept() input-hog profile accept failed for D1:2B:38:RE:DA:CT
src/device.c:load_gatt_db() no cache for D1:2B:38:RE:DA:CT
src/service.c:service_accept() input-hog profile accept failed for D1:2B:38:RE:DA:CT
src/device.c:load_gatt_db() no cache for D1:2B:38:RE:DA:CT
src/service.c:service_accept() input-hog profile accept failed for D1:2B:38:RE:DA:CT
```

```
Bluetooth: hci0: unexpected cc 0x0c7c length: 1 < 3
Bluetooth: hci0: unexpected SMP command 0x06 from D1:2B:38:RE:DA:CT
Bluetooth: hci0: unexpected SMP command 0x07 from D1:2B:38:RE:DA:CT
Bluetooth: hci0: unexpected SMP command 0x08 from D1:2B:38:RE:DA:CT
Bluetooth: hci0: unexpected SMP command 0x09 from D1:2B:38:RE:DA:CT
```

The Bluetooth controller seems to be manufactured by Realtek(VendorID=0bda), however I am unable to find the exact model number of the hardware(ProductID=b00a).

Downgrade the linux-firmware package to 20220329.git681281e4-0ubuntu3.14 doesn't fix the issue, however, when the kernel is downgraded from 6.5.0-17-generic to 5.15.0-94-generic the issue is no longer reproduced, which appears to be a regression in the kernel and/or the Bluetooth stack instead of the device firmware.

As I'm not the one who reproduces this issue I can only document the gathered information here, though there are users in other distros who encountered similar problems on the 6.x kernel:

Bluetoothd errors since kernel 6.x : archlinux
https://www.reddit.com/r/archlinux/comments/16e9aks/bluetoothd_errors_since_kernel_6x/

[1] https://t.me/x_ubuntu_taiwan_community/111326