1Android A/B updates 2=================== 3 4Overview 5-------- 6 7A/B system updates ensures modern approach for system update. This feature 8allows one to use two sets (or more) of partitions referred to as slots 9(normally slot A and slot B). The system runs from the current slot while the 10partitions in the unused slot can be updated [1]. 11 12A/B enablement 13-------------- 14 15The A/B updates support can be activated by specifying next options in 16your board configuration file: 17 18 CONFIG_ANDROID_AB=y 19 CONFIG_CMD_AB_SELECT=y 20 21The disk space on target device must be partitioned in a way so that each 22partition which needs to be updated has two or more instances. The name of 23each instance must be formed by adding suffixes: _a, _b, _c, etc. 24For example: boot_a, boot_b, system_a, system_b, vendor_a, vendor_b. 25 26As a result you can use 'ab_select' command to ensure A/B boot process in your 27boot script. This command analyzes and processes A/B metadata stored on a 28special partition (e.g. "misc") and determines which slot should be used for 29booting up. 30 31Command usage 32------------- 33 34 ab_select <slot_var_name> <interface> <dev[:part_number|#part_name]> 35 36for example: 37 38 => ab_select slot_name mmc 1:4 39 40or 41 42 => ab_select slot_name mmc 1#misc 43 44Result: 45 46 => printenv slot_name 47 slot_name=a 48 49Based on this slot information, the current boot partition should be defined, 50and next kernel command line parameters should be generated: 51 52 - androidboot.slot_suffix= 53 - root= 54 55For example: 56 57 androidboot.slot_suffix=_a root=/dev/mmcblk1p12 58 59A/B metadata is organized according to AOSP reference [2]. On the first system 60start with A/B enabled, when 'misc' partition doesn't contain required data, 61the default A/B metadata will be created and written to 'misc' partition. 62 63References 64---------- 65 66[1] https://source.android.com/devices/tech/ota/ab 67[2] bootable/recovery/bootloader_message/include/bootloader_message/bootloader_message.h 68