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- .. SPDX-License-Identifier: GPL-2.0+
- 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!
- 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
- '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
- 'bcb'. Example of getting reboot reason
- ---------------------------------------
- .. code-block:: bash
- 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
- 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
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