How To Modify Android's Bluetooth Stack to Enable A2dp Sink

Question 1

This is actually what I'm trying to do for a long time... The reason you can't find the configuration file is because Google replaced the Bluetooth stack from BlueZ with a new stack built by Google and Broadcom. The new stack uses a different configuration file, which I don't know how to tinker with.

If you are serious about it, the closest thing I found to start with is the official introduction for the bluetooth framework on Android: https://source.android.com/devices/bluetooth.html

Question 2

So the above answer isn't totally correct.

The following is how it breaks down:

HAL, is the hardware abstraction layer that implements the actual Bluetooth state machines in c/cpp code, as such it controls the various state machines for A2dp, HFP, GATT, SPP, AVRCP, etc. services. Each of these services also reference SMP and ATT files for controlling the actual Bluetooth server or client databases, and there security.

HCI, is where the actual work gets done. the HAL doesn't really do anything, it assembles the complex data messages that get sent along a tty serial (either spi, or UART) to an inter-network connected chip on the PCBA via the methods used in the HCI layer, which can be found in the "BTE" layer in /external/bluetooth/bluedroid/ directory of an android compiling trunk from AOSP 4.2.2 to current. - currently there are several manufacturers of these chips but they are mostly all Broadcom based ic's packaged in a dual, or triple radio package that contains a wifi, Bluetooth 4.0 Smart, and Bluetooth 4.0 radio.

It is possible to do what you are trying to do, but you would need to include hardware.so, and bluetooth_jni.so into an NDK/JNI package/project that goes with your apps, and registers via the calls from the .cpp files for each of the Bluetooth services found in "Packages/apps/Bluetooth/jni", you would then handle the registration in your NDK library of 'com_android_bluetooth_a2dp.cpp', and 'com_android_bluetooth_avrcp.cpp', as their appropriately typed objects.

The other issue is, you will need to implement your own custom A2DP stack, as the Android Bluedroid stack only has bit's and piece's of the Sink role implemented in the frameworks while the A2DP role has a full implementation of Source role. Additionally, depending what you actually intend to do with your Bluetooth A2DP sink implementation you will need to implement AVRCP as well - as per the Bluetooth SIG (special interest group), there are inter-connectivity requirements between Bluetooth devices that will lead to major issues if you implement sink role, without AVRCP "remote control target device" and "remote control control device", as the sink role ATT commands from Bluetooth over A2DP (or any Bluetooth service/profile) execute certain handshakes during the service discovery process, when the associated gateway ( the connecting device ), executes a capabilities request the A2DP service is expected to implement i/o capabilities for Start Stop commands, and possibly skip/track advance commands.

Additional to all of this, when implementing A2DP you will need to choose whether you will be handling PCM streams or AAC streams. If you are handling AAC streams (or DRM protected PCM streams for that matter, which anything like Pandora, spotify, etc. uses), you need to implement the SBC Encoder or Decoder appropriate to you implementation, else all you will have is a bunch of encrypted data. Also, be sure to implement the bitrate at the appropriate speed for your devices AudioManager implementation, some phones use 48,000hZ and some us 44,100hZ, this is important if you want high quality audio as generally most PCM A2DP implementations that are utilizing Surround Sound 7.1+ will require 48,000hZ as well as AAC encoding/decoding.

I hope this provides you with some insight.

Question 3

https://android-review.googlesource.com/#/c/98161/ implements A2DP Sink. It works on Nexus 5. You can try it.

Question 4

There are lots of changes happened in Android OS over the time.

As of Android O (Android 8.*), sink profiles are partially supported by Google. Such as Audio sink will easily work if enabled in profile. It looks like the higher layer of BT is kind of implementation complete at the framework, which is in the form of App i.e. packages/app/Bluetooth (with some bugs but still works). But all profiles are not completed at framework lower layer via HAL interfaces which is btif framework (such as btif_rc.cpp, etc which you can look at Android source) and which is the replacement to older Bluez stack by Google.

As I said BT sink is partially implemented and is in work in progress state. BT sink such as Audio will easily work if enabled as sink profile but not all such as AVRCP will not work. At present, I saw the issue with AOSP code that incoming traffic from the remote device to Android works but outgoing traffic from Android to the remote device doesn't work (upon which AVRCP profile works) as remote device object is not maintained in the stack and so JNI calls from app/Bluetooth fails with null device at btif_*.cpp files. For example, send pass-through commands doesn't work.

So, we may see Bluetooth sink profiles working in future.

If you want to explore more check AOSP,

Services at packages/app/Bluetooth/
HAL's at system/bt/btif/