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https://github.com/AsahiLinux/u-boot
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68 lines
1.9 KiB
Text
68 lines
1.9 KiB
Text
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Android A/B updates
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===================
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Overview
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--------
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A/B system updates ensures modern approach for system update. This feature
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allows one to use two sets (or more) of partitions referred to as slots
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(normally slot A and slot B). The system runs from the current slot while the
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partitions in the unused slot can be updated [1].
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A/B enablement
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--------------
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The A/B updates support can be activated by specifying next options in
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your board configuration file:
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CONFIG_ANDROID_AB=y
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CONFIG_CMD_AB_SELECT=y
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The disk space on target device must be partitioned in a way so that each
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partition which needs to be updated has two or more instances. The name of
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each instance must be formed by adding suffixes: _a, _b, _c, etc.
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For example: boot_a, boot_b, system_a, system_b, vendor_a, vendor_b.
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As a result you can use 'ab_select' command to ensure A/B boot process in your
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boot script. This command analyzes and processes A/B metadata stored on a
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special partition (e.g. "misc") and determines which slot should be used for
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booting up.
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Command usage
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-------------
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ab_select <slot_var_name> <interface> <dev[:part_number|#part_name]>
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for example:
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=> ab_select slot_name mmc 1:4
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or
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=> ab_select slot_name mmc 1#misc
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Result:
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=> printenv slot_name
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slot_name=a
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Based on this slot information, the current boot partition should be defined,
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and next kernel command line parameters should be generated:
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- androidboot.slot_suffix=
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- root=
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For example:
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androidboot.slot_suffix=_a root=/dev/mmcblk1p12
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A/B metadata is organized according to AOSP reference [2]. On the first system
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start with A/B enabled, when 'misc' partition doesn't contain required data,
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the default A/B metadata will be created and written to 'misc' partition.
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References
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----------
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[1] https://source.android.com/devices/tech/ota/ab
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[2] bootable/recovery/bootloader_message/include/bootloader_message/bootloader_message.h
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