Android A/B updates

Overview

A/B system updates ensures modern approach for system update. This feature allows one to use two sets (or more) of partitions referred to as slots (normally slot A and slot B). The system runs from the current slot while the partitions in the unused slot can be updated [1].

A/B enablement

The A/B updates support can be activated by specifying next options in your board configuration file:

CONFIG_ANDROID_AB=y
CONFIG_CMD_BCB=y

The disk space on target device must be partitioned in a way so that each partition which needs to be updated has two or more instances. The name of each instance must be formed by adding suffixes: _a, _b, _c, etc. For example: boot_a, boot_b, system_a, system_b, vendor_a, vendor_b.

As a result you can use bcb ab_select command to ensure A/B boot process in your boot script. This command analyzes and processes A/B metadata stored on a special partition (e.g. misc) and determines which slot should be used for booting up.

If the A/B metadata partition has a backup bootloader_message block that is used to ensure one is always valid even in the event of interruption when writing, it can be enabled in your board configuration file:

CONFIG_ANDROID_AB_BACKUP_OFFSET=0x1000

Command usage

bcb ab_select <slot_var_name> <interface> <dev[:part_number|#part_name]>

for example:

=> bcb ab_select slot_name mmc 1:4

or:

=> bcb ab_select slot_name mmc 1#misc

Result:

=> printenv slot_name
slot_name=a

Based on this slot information, the current boot partition should be defined, and next kernel command line parameters should be generated:

androidboot.slot_suffix=
root=

For example:

androidboot.slot_suffix=_a root=/dev/mmcblk1p12

A/B metadata is organized according to AOSP reference [2]. On the first system start with A/B enabled, when misc partition doesn’t contain required data, the default A/B metadata will be created and written to misc partition.