doc/android/ab.txt

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_AB_SELECT=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 '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.

Command usage
-------------

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

for example:

    => ab_select slot_name mmc 1:4

or

    => 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.

References
----------

[1] https://source.android.com/devices/tech/ota/ab
[2] bootable/recovery/bootloader_message/include/bootloader_message/bootloader_message.h