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https://github.com/AsahiLinux/u-boot
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90 lines
3.3 KiB
Text
90 lines
3.3 KiB
Text
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Android Bootloader Control Block (BCB)
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The purpose behind this file is to:
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- give an overview of BCB w/o duplicating public documentation
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- describe the main BCB use-cases which concern U-Boot
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- reflect current support status in U-Boot
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- mention any relevant U-Boot build-time tunables
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- precisely exemplify one or more use-cases
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Additions and fixes are welcome!
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1. OVERVIEW
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---------------------------------
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Bootloader Control Block (BCB) is a well established term/acronym in
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the Android namespace which refers to a location in a dedicated raw
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(i.e. FS-unaware) flash (e.g. eMMC) partition, usually called "misc",
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which is used as media for exchanging messages between Android userspace
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(particularly recovery [1]) and an Android-capable bootloader.
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On higher level, BCB provides a way to implement a subset of Android
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Bootloader Requirements [2], amongst which are:
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- Android-specific bootloader flow [3]
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- Get the "reboot reason" (and act accordingly) [4]
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- Get/pass a list of commands from/to recovery [1]
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- TODO
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2. 'BCB'. SHELL COMMAND OVERVIEW
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-----------------------------------
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The 'bcb' command provides a CLI to facilitate the development of the
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requirements enumerated above. Below is the command's help message:
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=> bcb
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bcb - Load/set/clear/test/dump/store Android BCB fields
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Usage:
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bcb load <dev> <part> - load BCB from mmc <dev>:<part>
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bcb set <field> <val> - set BCB <field> to <val>
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bcb clear [<field>] - clear BCB <field> or all fields
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bcb test <field> <op> <val> - test BCB <field> against <val>
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bcb dump <field> - dump BCB <field>
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bcb store - store BCB back to mmc
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Legend:
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<dev> - MMC device index containing the BCB partition
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<part> - MMC partition index or name containing the BCB
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<field> - one of {command,status,recovery,stage,reserved}
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<op> - the binary operator used in 'bcb test':
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'=' returns true if <val> matches the string stored in <field>
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'~' returns true if <val> matches a subset of <field>'s string
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<val> - string/text provided as input to bcb {set,test}
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NOTE: any ':' character in <val> will be replaced by line feed
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during 'bcb set' and used as separator by upper layers
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3. 'BCB'. EXAMPLE OF GETTING REBOOT REASON
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-----------------------------------
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if bcb load 1 misc; then
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# valid BCB found
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if bcb test command = bootonce-bootloader; then
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bcb clear command; bcb store;
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# do the equivalent of AOSP ${fastbootcmd}
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# i.e. call fastboot
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else if bcb test command = boot-recovery; then
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bcb clear command; bcb store;
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# do the equivalent of AOSP ${recoverycmd}
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# i.e. do anything required for booting into recovery
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else
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# boot Android OS normally
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fi
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else
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# corrupted/non-existent BCB
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# report error or boot non-Android OS (platform-specific)
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fi
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4. ENABLE ON YOUR BOARD
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-----------------------------------
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The following Kconfig options must be enabled:
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CONFIG_PARTITIONS=y
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CONFIG_MMC=y
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CONFIG_BCB=y
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[1] https://android.googlesource.com/platform/bootable/recovery
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[2] https://source.android.com/devices/bootloader
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[3] https://patchwork.ozlabs.org/patch/746835/
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("[U-Boot,5/6] Initial support for the Android Bootloader flow")
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[4] https://source.android.com/devices/bootloader/boot-reason
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