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- .. SPDX-License-Identifier: GPL-2.0+
- 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
- -------------
- .. code-block:: none
- 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] https://android.googlesource.com/platform/bootable/recovery/+/refs/tags/android-10.0.0_r25/bootloader_message/include/bootloader_message/bootloader_message.h
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