mount -t debugfs none /sys/kernel/debug

echo "file net/bluetooth/* +pfl" > /sys/kernel/debug/dynamic_debug/control

Enable pr_debug

linux-4.18.6/net/bluetooth/Makefile

CFLAGS += -DDEBUG=1

cp /lib/modules/$(uname -r)/build/.config .
cp /lib/modules/$(uname -r)/build/Module.symvers .
 
make oldconfig
 
#make EXTRAVERSION=-my modules_prepare
make modules_prepare
make M=net/bluetooth
 
sudo cp net/bluetooth/bluetooth.ko /lib/modules/`uname -r`/updates
 
depmod -a
sudo rmmod btusb btrtl btintel btbcm bnep btusb bluetooth
 
modinfo bluetooth
filename:       /lib/modules/4.18.0-0.bpo.1-amd64/updates/bluetooth.ko
...
 
sudo modprobe bluetooth