Android platform transplantation should be done in two steps

The purpose of Android system porting is to run the Android system on specific hardware. In the process of porting, grasping the key points and reducing the workload is an important aspect. From the perspective of work, the usual method is to first be familiar with the interface of the hardware abstraction layer, and then integrate and reuse the existing drivers. The main workload is in the implementation of the hardware abstraction layer. In order to better understand and debug the system, you should also properly understand the calls of the upper layer to the hardware abstraction layer.
There are two main parts of porting:
Linux driver
Android system hardware abstraction layer
The driver in Linux works in the kernel space, and the Android system hardware abstraction layer works in the user space. With the combination of these two parts, the huge Android system can run on a specific hardware platform.
The main work of Android porting is shown in Figure 1.

Figure 1 Main tasks of Android porting
After having a specific hardware system, it is usually necessary to implement its driver in Linux. These drivers are usually standard Linux drivers and are basically the same on the Android platform and other Linux platforms. The main implementation aspect is the Hardware Abstract Layer in the Android system. The Hardware Abstract Layer calls the driver in Linux downwards and provides an interface upwards for other parts of the Android system (usually the Android local framework layer) to call.
Tip: The interface of the Android hardware abstract layer is the interface of the local porting layer, which is not a standard API and has no forward or backward compatibility.
The content that needs to be ported in the Android system mainly includes the following parts:
Display part (Display)
includes framebuffer driver + Gralloc module (optional implementation)
User input part (Input)
includes Event driver + EventHub (Android standard content)
Multimedia codec (Codec)
includes hardware Codec driver + Codec plug-in (such as OpenMax)
3D accelerator part (3D Accelerator)
includes hardware OpenGL driver + OpenGL plug-in
Audio part (Audio)
includes Audio driver + Audio hardware abstraction layer
Video output part (Video Out)
includes video display driver + Overlay hardware abstraction layer
Camera part (Camera)
includes Camera driver (usually v4l2) + Camera hardware abstraction layer
Phone part (Phone)
Modem driver + RIL library
Global positioning system part (GPS)
includes GPS driver (usually serial port) + GPS hardware abstraction layer
Wireless LAN part (WIFI)
includes Wlan driver and protocol + WIFI adaptation layer (Android standard content)
Bluetooth part (Bluetooth ) Tooth
includes BT driver and protocol + BT adaptation layer (Android standard content)
Sensor part (Sensor)
includes Sensor driver + Sensor hardware abstraction layer
Vibrator part (Vibrator)
includes Vibrator driver + Vibrator hardware abstraction layer (Android standard content)
Backlight part (Light)
includes Light driver + Light hardware abstraction layer
Alarm part (Alarm)
includes Alarm driver and RTC system + user space call (Android standard content)
Battery part (Battery)
includes battery driver + battery hardware abstraction layer (Android standard content)
Android has many components, but not every component needs to be transplanted. For some pure software components, there is no need to transplant. For some components, such as the browser engine, although the support of the lower-level network is required, the network interface is not directly transplanted for it, but the standard network interface is completed through the wireless LAN or telephone system data connection.
Android porting can be divided into several categories: basic graphical user interface (GUI) part, including display part and user input part; hardware-related acceleration part, including media codec and OpenGL; audio and video input and output link, including audio, video output and camera part; connection part, including wireless LAN, Bluetooth, GPS; phone part; accessories: including sensors, backlight, vibrator, etc.
In addition to the above porting aspects, power management is also a very important aspect, which is related to each subsystem of Android.
The main parts that need to be ported in the Android system are shown in Figure 2.

Figure 2 Main components that need to be transplanted in the Android system
For most subsystems, the hardware abstraction layer and the driver need to be implemented according to the actual system conditions, such as: sensor part, audio part, video part, camera part, and phone part. There are also some subsystems where the hardware abstraction layer is standard and only the driver in the Linux kernel needs to be implemented, such as: input part, vibrator part, wireless LAN part, Bluetooth part, etc. For systems with standard hardware abstraction layers, sometimes some configuration work is usually required.
Today, with the development of the Android system, it is no longer just a platform for mobile devices, but can also be used for consumer electronics and smart home appliances, such as: netbooks, e-books, digital TVs, set-top boxes, fixed phones, etc. On these platforms, fewer components are usually required than on mobile devices. Generally speaking, the basic user interface part (including display and user input) needs to be transplanted, and other parts are optional. For example: the phone system, vibrator, backlight, sensor, etc. generally do not need to be implemented in non-mobile device systems; for some fixed-location devices, it is usually not necessary to implement the GPS system.