90 lines
3.3 KiB
Plaintext
90 lines
3.3 KiB
Plaintext
|
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
|