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!


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

‘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

bcb load <interface> <dev> <part>  - load  BCB from <interface> <dev>:<part>
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 <interface>

<interface> - storage device interface (virtio, mmc, etc)
<dev>       - storage device index containing the BCB partition
<part>      - 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

‘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
        # boot Android OS normally
    # corrupted/non-existent BCB
    # report error or boot non-Android OS (platform-specific)

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