Document the BCB concept and U-Boot command, touching below aspects: - 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
Signed-off-by: Eugeniu Rosca erosca@de.adit-jv.com --- v4: - This is carved out from https://patchwork.ozlabs.org/patch/1104245/ ("[v3,3/3] doc: relocate/rename Android README and add BCB overview") with no modifications. --- doc/android/bcb.txt | 89 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 89 insertions(+) create mode 100644 doc/android/bcb.txt
diff --git a/doc/android/bcb.txt b/doc/android/bcb.txt new file mode 100644 index 000000000000..7b7177cacf21 --- /dev/null +++ b/doc/android/bcb.txt @@ -0,0 +1,89 @@ +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