This project contains the AOSP reference implementation of the Context Hub Runtime Environment (CHRE), which is Android’s platform for developing always-on applications, called nanoapps. CHRE runs in a vendor-specific processor that is independent of the main applications processor that runs Android. This enables CHRE and its nanoapps to be more power-efficient for use cases that require frequent background processing of contextual data, like sensor inputs. Nanoapps are written to the CHRE API, which is standardized across all platforms, enabling them to be code-compatible across different devices.

The CHRE reference implementation (“CHRE framework”) is designed to be portable across hardware and software platforms. While Android does not require a particular implementation (only compliance to the contracts given in the CHRE API) using the reference implementation helps to reduce the work needed to support CHRE, while also ensuring more consistent behavior for scenarios that can be difficult to enforce via testing.

Use the navigation bar at the top and/or the links below to move through the documentation. Raw files can also be found in the /doc folder.

• Documentation related to the CHRE framework:
  • Framework Overview
  • Porting Guide
  • Build System
  • Debugging
  • Testing
  • Vendor Extensions
• Documentation related to nanoapps:
  • Nanoapp Overview
  • Developer Guide
  • Interacting with Nanoapps
• General documentation:
  • Compatibility Design
  • Contributing

The CHRE API and PAL API are also extensively documented using Doxygen syntax. Run doxygen in the same folder as Doxyfile to generate a browseable HTML version of the documentation.

This repository (system/chre) contains an assortment of code, structured as follows:

• apps/: Public nanoapp source code, including sample nanoapps intended to showcase how to use the CHRE APIs, and test nanoapps used to validate API functionality
• build/: Files related to CHRE’s Makefile-based build system, which supports building the CHRE framework and nanoapps using a configurable toolchain
• chpp/: Context Hub Peripheral Protocol (CHPP) source code - see the nested README and associated documentation for details
• chre_api/: Contains the official definition of the CHRE API (current and past versions), which all CHRE implementations must adhere to
• core/: Common CHRE framework code, which is applicable to every platform (contrast to platform/)
• doc/: Contains documentation for the CHRE framework and associated topics
• external/: Code developed primarily outside of AOSP which (can be) leveraged by CHRE, and is potentially customized for CHRE (e.g. flatbuffers)
• host/: Reference code which supports the CHRE implementation, but runs on the applications processor (“host”), for example the Context Hub HAL
• java/: Java test code used in conjunction with test nanoapps
• pal/: The binary-stable Platform Abstraction Layer (PAL) C API definitions and tests (these PALs may optionally be used by the platform implementation)
• platform/: Code related to the implementation of CHRE on a particular platform/device (compare to core/), divided into sub-folders as follows:
  • platform/include: The interface between common code in core/ and platform-specific code implemented elsewhere in platform/
  • platform/shared: Code that may apply to multiple platforms, but is not necessarily applicable to all platforms (as in core/). For example, multiple platforms may choose to use the binary-stable PAL interface - this folder contains source files translating from the C++ platform abstractions to the C PAL APIs.
  • platform/<platform_name>: Code specific to the platform indicated by platform_name
• util/: Utility code that is not platform-specific, but not part of the core framework implementation. Includes code that is usable by nanoapps.
• variant/: Code/configuration that is specific to a particular device (more detail on variants can be found below). For example, multiple generations of a given chip may use the same platform code, but the code may be configured on a per-device basis in the build via variant.mk.

Code related to CHRE also exists in other parts of the Android tree, including:

• hardware/interfaces/contexthub/: The Context Hub HAL definition
• frameworks/base/core/java/android/hardware/location/ContextHub*.java: The APIs used by privileged apps to interact with CHRE and nanoapps
• frameworks/base/services/core/java/com/android/server/location/ContextHub*.java: The Context Hub service implementation in system server

If you’re unable to find the answers you’re looking for in CHRE documentation or are looking for specific guidance for your platform, device, or nanoapp, please reach out to the CHRE team via your TAM or through the Google Issue Tracker.