blob: 7b7177cacf212a1b405c9e4a36e2c65ffc35973d (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
|
Android Bootloader Control Block (BCB)
The purpose behind this file is to:
- give an overview of BCB w/o duplicating public documentation
- describe the main BCB use-cases which concern U-Boot
- reflect current support status in U-Boot
- mention any relevant U-Boot build-time tunables
- precisely exemplify one or more use-cases
Additions and fixes are welcome!
1. OVERVIEW
---------------------------------
Bootloader Control Block (BCB) is a well established term/acronym in
the Android namespace which refers to a location in a dedicated raw
(i.e. FS-unaware) flash (e.g. eMMC) partition, usually called "misc",
which is used as media for exchanging messages between Android userspace
(particularly recovery [1]) and an Android-capable bootloader.
On higher level, BCB provides a way to implement a subset of Android
Bootloader Requirements [2], amongst which are:
- Android-specific bootloader flow [3]
- Get the "reboot reason" (and act accordingly) [4]
- Get/pass a list of commands from/to recovery [1]
- TODO
2. 'BCB'. SHELL COMMAND OVERVIEW
-----------------------------------
The 'bcb' command provides a CLI to facilitate the development of the
requirements enumerated above. Below is the command's help message:
=> bcb
bcb - Load/set/clear/test/dump/store Android BCB fields
Usage:
bcb load <dev> <part> - load BCB from mmc <dev>:<part>
bcb set <field> <val> - set BCB <field> to <val>
bcb clear [<field>] - clear BCB <field> or all fields
bcb test <field> <op> <val> - test BCB <field> against <val>
bcb dump <field> - dump BCB <field>
bcb store - store BCB back to mmc
Legend:
<dev> - MMC device index containing the BCB partition
<part> - MMC partition index or name containing the BCB
<field> - one of {command,status,recovery,stage,reserved}
<op> - the binary operator used in 'bcb test':
'=' returns true if <val> matches the string stored in <field>
'~' returns true if <val> matches a subset of <field>'s string
<val> - string/text provided as input to bcb {set,test}
NOTE: any ':' character in <val> will be replaced by line feed
during 'bcb set' and used as separator by upper layers
3. 'BCB'. EXAMPLE OF GETTING REBOOT REASON
-----------------------------------
if bcb load 1 misc; then
# valid BCB found
if bcb test command = bootonce-bootloader; then
bcb clear command; bcb store;
# do the equivalent of AOSP ${fastbootcmd}
# i.e. call fastboot
else if bcb test command = boot-recovery; then
bcb clear command; bcb store;
# do the equivalent of AOSP ${recoverycmd}
# i.e. do anything required for booting into recovery
else
# boot Android OS normally
fi
else
# corrupted/non-existent BCB
# report error or boot non-Android OS (platform-specific)
fi
4. ENABLE ON YOUR BOARD
-----------------------------------
The following Kconfig options must be enabled:
CONFIG_PARTITIONS=y
CONFIG_MMC=y
CONFIG_BCB=y
[1] https://android.googlesource.com/platform/bootable/recovery
[2] https://source.android.com/devices/bootloader
[3] https://patchwork.ozlabs.org/patch/746835/
("[U-Boot,5/6] Initial support for the Android Bootloader flow")
[4] https://source.android.com/devices/bootloader/boot-reason
|
