In this article, we will discuss the process of updating a partition scheme for A/B OTA (Over-The-Air) updates on an Android device. The update process involves modifying multiple partitions to ensure that the system remains functional and up-to-date.

AB_OTA_PARTITIONS

The first step is to define the partition scheme using the AB_OTA_PARTITIONS variable. This variable specifies the names of the partitions that will be updated:

AB_OTA_PARTITIONS := \
 boot \
 system \
 bootloader

These three partitions are essential for A/B OTA updates: boot, system, and bootloader. The boot partition contains the kernel, the system partition contains the system image, and the bootloader partition contains the firmware that boots the device.

Update Engine

The update engine is responsible for updating all partitions simultaneously. This process is critical to ensure that the system remains functional after an update. If any of the partitions are not updated correctly, the system may become unstable or even fail to boot.

Post-Installation Configuration

After the update process is complete, it's essential to configure the post-installation steps. The POSTINSTALL_PATH_system variable specifies the path to the script that will be executed after the update. In this case, we are using a script called postinst located in the /system/usr/bin/ directory.

AB_OTA_POSTINSTALL_CONFIG += \
 RUN_POSTINSTALL_system=true \
 POSTINSTALL_PATH_system=usr/bin/postinst \
 FILESYSTEM_TYPE_system=ext4

This script will be executed after the update to perform any necessary post-installation tasks, such as updating system files or recompiling native libraries.

Compilation

To compile the postinst script and its dependencies, we need to include it in the build configuration. We can do this by adding the following line to the device's .mk file:

PRODUCT_PACKAGES += otapreopt_script

This will compile the otapreopt_script package and include it in the system image.

DEX_PREOPT

To optimize the DEX files (Dalvik Executable) for faster execution, we can use the DEX_PREOPT variable. This variable specifies the path to the pre-optimization script that will be executed before the update:

DEX_PREOPT := /system/bin/otapreopt_script

This script will optimize the DEX files by recompiling them with just-in-time (JIT) compilation.

In this article, we have discussed the process of updating a partition scheme for A/B OTA updates. We defined the AB_OTA_PARTITIONS variable to specify the names of the partitions that will be updated, configured the post-installation steps using the POSTINSTALL_PATH_system variable, and compiled the postinst script and its dependencies.

Additional Resources

For more information on A/B OTA updates and how to implement them in your device's configuration file, please refer to the official Android documentation or the source code of the Android Open Source Project (AOSP).

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