1.. SPDX-License-Identifier: GPL-2.0+
2
3Android A/B updates
4===================
5
6Overview
7--------
8
9A/B system updates ensures modern approach for system update. This feature
10allows one to use two sets (or more) of partitions referred to as slots
11(normally slot A and slot B). The system runs from the current slot while the
12partitions in the unused slot can be updated [1]_.
13
14A/B enablement
15--------------
16
17The A/B updates support can be activated by specifying next options in
18your board configuration file::
19
20    CONFIG_ANDROID_AB=y
21    CONFIG_CMD_AB_SELECT=y
22
23The disk space on target device must be partitioned in a way so that each
24partition which needs to be updated has two or more instances. The name of
25each instance must be formed by adding suffixes: ``_a``, ``_b``, ``_c``, etc.
26For example: ``boot_a``, ``boot_b``, ``system_a``, ``system_b``, ``vendor_a``,
27``vendor_b``.
28
29As a result you can use ``ab_select`` command to ensure A/B boot process in your
30boot script. This command analyzes and processes A/B metadata stored on a
31special partition (e.g. ``misc``) and determines which slot should be used for
32booting up.
33
34If the A/B metadata partition has a backup bootloader_message block that is used
35to ensure one is always valid even in the event of interruption when writing, it
36can be enabled in your board configuration file::
37
38    CONFIG_ANDROID_AB_BACKUP_OFFSET=0x1000
39
40Command usage
41-------------
42
43.. code-block:: none
44
45    ab_select <slot_var_name> <interface> <dev[:part_number|#part_name]>
46
47for example::
48
49    => ab_select slot_name mmc 1:4
50
51or::
52
53    => ab_select slot_name mmc 1#misc
54
55Result::
56
57    => printenv slot_name
58    slot_name=a
59
60Based on this slot information, the current boot partition should be defined,
61and next kernel command line parameters should be generated:
62
63* ``androidboot.slot_suffix=``
64* ``root=``
65
66For example::
67
68    androidboot.slot_suffix=_a root=/dev/mmcblk1p12
69
70A/B metadata is organized according to AOSP reference [2]_. On the first system
71start with A/B enabled, when ``misc`` partition doesn't contain required data,
72the default A/B metadata will be created and written to ``misc`` partition.
73
74References
75----------
76
77.. [1] https://source.android.com/devices/tech/ota/ab
78.. [2] https://android.googlesource.com/platform/bootable/recovery/+/refs/tags/android-10.0.0_r25/bootloader_message/include/bootloader_message/bootloader_message.h
79