12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667 |
- 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 <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] bootable/recovery/bootloader_message/include/bootloader_message/bootloader_message.h
|