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