Hi,

I’m running NixOS on a laptop with a MediaTek MT7921 (USB ID 13d3:3563). I’m debugging a Bluetooth regression affecting this device across multiple kernel versions.

Problem

Bluetooth is detected but does not initialize correctly on certain kernel versions.

Symptoms:

• hci0 exists but is unusable
• BD address shows 00:00:00:00:00:00
• no Bluetooth controller visible in bluetoothctl
• hciconfig shows empty features and DOWN state
• dmesg error: Failed to send wmt func ctrl (-22)

WiFi (mt7921e) works normally.

What I tested

• Kernel 7.0.5 → Bluetooth works
• Kernel 7.0.8 → Bluetooth breaks
• Kernel 6.18.28 → Bluetooth works
• Kernel 6.18.31 → Bluetooth breaks
• Downgrading kernel does not reliably fix the issue once broken
• Full power cycle (shutdown)
• Reloading btusb / btmtk
• No missing firmware errors in dmesg

System: NixOS (nixpkgs-managed kernels, reproducible configuration)

Question

Is this a known regression or instability in the MediaTek MT7921 Bluetooth driver (btmtk) across recent Linux kernel versions?

Is there a recommended stable kernel / firmware combination for this chipset?

Any known workaround besides pinning a specific working kernel version?

Thanks