1 .. SPDX-License-Identifier: GPL-2.0+
9 A/B system updates ensures modern approach for system update. This feature
10 allows 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
12 partitions in the unused slot can be updated [1]_.
17 The A/B updates support can be activated by specifying next options in
18 your board configuration file::
21 CONFIG_CMD_AB_SELECT=y
23 The disk space on target device must be partitioned in a way so that each
24 partition which needs to be updated has two or more instances. The name of
25 each instance must be formed by adding suffixes: ``_a``, ``_b``, ``_c``, etc.
26 For example: ``boot_a``, ``boot_b``, ``system_a``, ``system_b``, ``vendor_a``,
29 As a result you can use ``ab_select`` command to ensure A/B boot process in your
30 boot script. This command analyzes and processes A/B metadata stored on a
31 special partition (e.g. ``misc``) and determines which slot should be used for
39 ab_select <slot_var_name> <interface> <dev[:part_number|#part_name]>
43 => ab_select slot_name mmc 1:4
47 => ab_select slot_name mmc 1#misc
54 Based on this slot information, the current boot partition should be defined,
55 and next kernel command line parameters should be generated:
57 * ``androidboot.slot_suffix=``
62 androidboot.slot_suffix=_a root=/dev/mmcblk1p12
64 A/B metadata is organized according to AOSP reference [2]_. On the first system
65 start with A/B enabled, when ``misc`` partition doesn't contain required data,
66 the default A/B metadata will be created and written to ``misc`` partition.
71 .. [1] https://source.android.com/devices/tech/ota/ab
72 .. [2] https://android.googlesource.com/platform/bootable/recovery/+/refs/tags/android-10.0.0_r25/bootloader_message/include/bootloader_message/bootloader_message.h