Android provides a default Bluetooth stack, BlueDroid, that is divided into two layers: The Bluetooth Embedded System (BTE), which implements the core Bluetooth functionality and the Bluetooth Application Layer (BTA), which communicates with Android framework applications. A Bluetooth system service communicates with the Bluetooth stack through JNI and with applications through Binder IPC. The system service provides developers access to various Bluetooth profiles. The following diagram shows the general structure of the Bluetooth stack:

Application framework

At the application framework level is the app's code, which utilizes the android.bluetooth APIs to interact with the bluetooth hardware. Internally, this code calls the Bluetooth process through the Binder IPC mechanism.

Bluetooth system service

The Bluetooth system service, located in packages/apps/Bluetooth, is packaged as an Android app and implements the Bluetooth service and profiles at the Android framework layer. This app calls into the HAL layer via JNI.

JNI

The JNI code associated with android.bluetooth is located in packages/apps/Bluetooth/jni. The JNI code calls into the HAL layer and receives callbacks from the HAL when certain Bluetooth operations occur, such as when devices are discovered.

HAL

The hardware abstraction layer defines the standard interface that the android.bluetooth APIs and Bluetooth process calls into and that you must implement to have your bluetooth hardware function correctly. The header files for the Bluetooth HAL is located in the hardware/libhardware/include/hardware/bluetooth.h and hardware/libhardware/include/hardware/bt_*.h files.

Bluetooth stack

The default Bluetooth stack is provided for you and is located in external/bluetooth/bluedroid. The stack implements the generic Bluetooth HAL as well as customizes it with extensions and configuration changes.

Vendor extensions

To add custom extensions and an HCI layer for tracing, you can create a libbt-vendor module and specify these components.

Implementing the HAL

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The Bluetooth HAL is located in the hardware/libhardware/include/hardware/ directory and consists of the following header files:

• bluetooth.h: Contains the HAL for the Bluetooth hardware on the device
• bt_av.h: Contains the HAL for the advanced audio profile.
• bt_hf.h: Contains the HAL for the handsfree profile.
• bt_hh.h: Contains the HAL for the HID host profile
• bt_hl.h: Contains the HAL for the health profile
• bt_pan.h: Contains the HAL for the pan profile
• bt_sock.h: Contains the HAL for the socket profile.

Keep in mind that your Bluetooth implementation is not constrained to the features and profiles exposed in the HAL. You can find the default implementation located in the BlueDroid Bluetooth stack in the external/bluetooth/bluedroid directory, which implements the default HAL and also extra features and customizations.

Customizing the BlueDroid Stack

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If you are using the default BlueDroid stack, but want to make a few customizations, you can do the following things:

• Custom Bluetooth profiles - If you want to add Bluetooth profiles that do not have HAL interfaces provided by Android, you must supply an SDK add-on download to make the profile available to app developers, make the APIs available in the Bluetooth system process app (packages/apps/Bluetooth), and add them to the BlueDroid stack (external/bluetooth/bluedroid).
• Custom vendor extensions and configuration changes - You can add things such as extra AT commands or device-specific configuration changes by creating a libbt-vendor module. See the vendor/broadcom/libbt-vendor directory for an example.
• Host Controller Interface (HCI) - You can provide your own HCI by creating a libbt-hci module, which is mainly used for debug tracing. See the external/bluetooth/hci directory for an example.