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 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