Protocol not supported error when BT_RFCOMM is in the kernel config

[devsk]

$ bluetooth_battery AC:12:2F:0B:3D:84
Couldn't find the RFCOMM port number
Traceback (most recent call last):
File "/home/devsk/.local/bin/bluetooth_battery", line 33, in
sys.exit(load_entry_point('bluetooth-battery==1.2.0', 'console_scripts', 'bluetooth_battery')())
File "/home/devsk/.local/lib/python3.7/site-packages/bluetooth_battery/bluetooth_battery.py", line 90, in main
s = bluetooth.BluetoothSocket(bluetooth.RFCOMM)
File "/home/devsk/.local/lib/python3.7/site-packages/bluetooth/bluez.py", line 165, in init
_sock = _bt.btsocket (proto)
_bluetooth.error: (93, 'Protocol not supported')

$ zgrep RFCOMM /proc/config.gz
CONFIG_BT_RFCOMM=y
CONFIG_BT_RFCOMM_TTY=y

Running Gentoo Linux with kernel 5.10.2

$ bluetoothctl devices
Device AC:12:2F:0B:3D:84 Soundcore Life Q30

[TheWeirdDev]

Is rfcomm kernel module loaded at startup?

[devsk]

As you can see from the zgrep I posted, the module is built into the kernel. So, its loaded for sure.

[TheWeirdDev]

Just to make sure, what is the output of lsmod | grep rfcomm

[devsk]

If a module is built-in, it won't show up in lsmod. In fact, it just doesn't exist as a .ko file that you can modprobe or insmod to load it.

# modprobe rfcomm
modprobe: FATAL: Module rfcomm not found in directory /lib/modules/5.10.2

# lsmod|grep -i rfcom

$ zgrep RFCOMM /proc/config.gz
CONFIG_BT_RFCOMM=y
CONFIG_BT_RFCOMM_TTY=y

[ghost]

If a module is built-in, it won't show up in lsmod. In fact, it just doesn't exist as a .ko file that you can modprobe or insmod to load it.

# modprobe rfcomm
modprobe: FATAL: Module rfcomm not found in directory /lib/modules/5.10.2

# lsmod|grep -i rfcom

$ zgrep RFCOMM /proc/config.gz
CONFIG_BT_RFCOMM=y
CONFIG_BT_RFCOMM_TTY=y

This is a bug in the Linux kernel. Try CONFIG_BT_RFCOMM=m.

[devsk]

Do you have a link for the bug?

How is bluetooth working if there is a bug with basic RFCOMM protocol missing? I can see and connect all devices fine. blueman has no issues.

[TheWeirdDev]

This is a bug in the Linux kernel. Try CONFIG_BT_RFCOMM=m.

Now that makes sense since my arch machine also has CONFIG_BT_RFCOMM set to m by default.

➜ zgrep RFCOMM /proc/config.gz
CONFIG_BT_RFCOMM=m
CONFIG_BT_RFCOMM_TTY=y

[ghost]

Do you have a link for the bug?

How is bluetooth working if there is a bug with basic RFCOMM protocol missing? I can see and connect all devices fine. blueman has no issues.

I haven't investigated this issue further. I only know that when I recompiled kernel with the following options then I see a "Protocol not supported" message:

CONFIG_BT_RFCOMM=y
CONFIG_BT_BNEP=y
CONFIG_BT_HIDP=y

[devsk]

ok, that's evidence.

Do we need to make the other two (HIDP and BNEP) also as modules?

[ghost]

Do we need to make the other two (HIDP and BNEP) also as modules?

I don't know.

[TheWeirdDev]

Mine is like this: (I didn't change these)

➜ zgrep BT_BNEP /proc/config.gz
CONFIG_BT_BNEP=m
CONFIG_BT_BNEP_MC_FILTER=y
CONFIG_BT_BNEP_PROTO_FILTER=y

➜ zgrep BT_HIDP /proc/config.gz
CONFIG_BT_HIDP=m

[devsk]

I can confirm. This is a weird kernel bug. It works when RFCOMM=m is selected.

Closing this because its not this program's fault.