1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889 |
- 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
|