mirror of
https://xff.cz/git/u-boot/
synced 2025-10-31 18:35:42 +01:00
ae954baf3b
Add a short documentation for A/B enablement and 'ab_select' command usage. Signed-off-by: Ruslan Trofymenko <ruslan.trofymenko@linaro.org> Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com> Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org> Reviewed-by: Alistair Strachan <astrachan@google.com> Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org> Reviewed-by: Simon Glass <sjg@chromium.org>
68 lines
1.9 KiB
Plaintext
68 lines
1.9 KiB
Plaintext
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
|