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