1.. SPDX-License-Identifier: GPL-2.0+
2
3Android Bootloader Control Block (BCB)
4======================================
5
6The purpose behind this file is to:
7
8* give an overview of BCB w/o duplicating public documentation
9* describe the main BCB use-cases which concern U-Boot
10* reflect current support status in U-Boot
11* mention any relevant U-Boot build-time tunables
12* precisely exemplify one or more use-cases
13
14Additions and fixes are welcome!
15
16Overview
17--------
18
19Bootloader Control Block (BCB) is a well established term/acronym in
20the Android namespace which refers to a location in a dedicated raw
21(i.e. FS-unaware) flash (e.g. eMMC) partition, usually called ``misc``,
22which is used as media for exchanging messages between Android userspace
23(particularly recovery [1]_) and an Android-capable bootloader.
24
25On higher level, BCB provides a way to implement a subset of Android
26Bootloader Requirements [2]_, amongst which are:
27
28* Android-specific bootloader flow [3]_
29* Get the "reboot reason" (and act accordingly) [4]_
30* Get/pass a list of commands from/to recovery [1]_
31* TODO
32
33
34'bcb'. Shell command overview
35-----------------------------
36
37The ``bcb`` command provides a CLI to facilitate the development of the
38requirements enumerated above. Below is the command's help message::
39
40   => bcb
41   bcb - Load/set/clear/test/dump/store Android BCB fields
42
43   Usage:
44   bcb load  <dev> <part>       - load  BCB from mmc <dev>:<part>
45   bcb set   <field> <val>      - set   BCB <field> to <val>
46   bcb clear [<field>]          - clear BCB <field> or all fields
47   bcb test  <field> <op> <val> - test  BCB <field> against <val>
48   bcb dump  <field>            - dump  BCB <field>
49   bcb store                    - store BCB back to mmc
50
51   Legend:
52   <dev>   - MMC device index containing the BCB partition
53   <part>  - MMC partition index or name containing the BCB
54   <field> - one of {command,status,recovery,stage,reserved}
55   <op>    - the binary operator used in 'bcb test':
56             '=' returns true if <val> matches the string stored in <field>
57             '~' returns true if <val> matches a subset of <field>'s string
58   <val>   - string/text provided as input to bcb {set,test}
59             NOTE: any ':' character in <val> will be replaced by line feed
60             during 'bcb set' and used as separator by upper layers
61
62
63'bcb'. Example of getting reboot reason
64---------------------------------------
65
66.. code-block:: bash
67
68   if bcb load 1 misc; then
69       # valid BCB found
70       if bcb test command = bootonce-bootloader; then
71           bcb clear command; bcb store;
72           # do the equivalent of AOSP ${fastbootcmd}
73           # i.e. call fastboot
74       else if bcb test command = boot-recovery; then
75           bcb clear command; bcb store;
76           # do the equivalent of AOSP ${recoverycmd}
77           # i.e. do anything required for booting into recovery
78       else
79           # boot Android OS normally
80       fi
81   else
82       # corrupted/non-existent BCB
83       # report error or boot non-Android OS (platform-specific)
84   fi
85
86
87Enable on your board
88--------------------
89
90The following Kconfig options must be enabled::
91
92   CONFIG_PARTITIONS=y
93   CONFIG_MMC=y
94   CONFIG_BCB=y
95
96.. [1] https://android.googlesource.com/platform/bootable/recovery
97.. [2] https://source.android.com/devices/bootloader
98.. [3] https://patchwork.ozlabs.org/patch/746835/
99       ("[U-Boot,5/6] Initial support for the Android Bootloader flow")
100.. [4] https://source.android.com/devices/bootloader/boot-reason
101