Understand the HAL hardware abstraction layer in the Android system architecture

Introduction to HAL

HAL refers to the Hardware Abstraction Layer, which is an abstract interface between the operating system and the hardware. The function of HAL is to abstract the functions of the hardware into a unified interface so that the upper-level software can access the hardware by calling these interfaces without having to care about the details of the specific hardware.

In the Android system , the existence of HAL allows hardware from different manufacturers to be uniformly called by upper-level applications, thereby improving the compatibility and portability of the system. HAL can also help developers develop applications more conveniently, because they do not need to write different drivers for different hardware, but can access the hardware by calling the unified HAL interface.

Android's HAL plays an extremely important role, making the Android system more flexible and extensible.

System Architecture

1. Application layer: includes applications that users interact directly with, such as social media applications, games, tool applications, etc.
2. Application framework layer: provides various APIs required for application development, including content providers, view systems, notification managers, activity managers, etc.
3. System runtime layer: includes core libraries used to support the running of Android applications, such as graphics library, media library, database library, etc.
4. HAL hardware abstraction layer: provides a unified interface for the Android system to access hardware functions, such as camera devices, Bluetooth devices, audio devices, etc.
5. Linux kernel : provides the underlying hardware abstraction layer of the Android system, including drivers, security, memory management, etc.

The entire architecture is shown in the figure below:

picture

This architecture makes the Android system scalable and flexible, while also providing developers with a wealth of APIs and tools to develop various types of applications.

HAL layer example

Suppose we want to implement a simple HAL layer interface for a sensor device:

 // sensors.h #ifndef ANDROID_SENSORS_INTERFACE_H #define ANDROID_SENSORS_INTERFACE_H #include <hardware/hardware.h> __BEGIN_DECLS #define SENSORS_HARDWARE_MODULE_ID "sensors" struct sensors_module_t { struct hw_module_t common; }; struct sensors_poll_device_t { struct hw_device_t common; int (*activate)(struct sensors_poll_device_t *dev, int handle, int enabled); int (*setDelay)(struct sensors_poll_device_t *dev, int handle, int64_t ns); int (*poll)(struct sensors_poll_device_t *dev, sensors_event_t* data, int count); }; __END_DECLS #endif // ANDROID_SENSORS_INTERFACE_H

This is a simple HAL layer interface example for a sensor device, which defines the structure of the sensor module and the sensor polling device, and contains function pointers for activating sensors, setting delays, and polling sensor data. In actual HAL layer development, it is also necessary to implement the specific function codes corresponding to these function pointers and register them with the hardware module of the HAL layer.

The above is a simple example to demonstrate how to define the HAL layer interface of the sensor device. In actual development, it is necessary to define and implement the corresponding interface according to the specific hardware device and function.