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  <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

<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

‘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)

Enable on your board

The following Kconfig options must be enabled:

[1](1, 2)
[3] (“[U-Boot,5/6] Initial support for the Android Bootloader flow”)