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