How to build bootloaders for Android

This article explains how to rebuild the primary and the secondary bootloaders for the STM32 device families. It is intended for Distribution Package users.

1 Prerequisites[edit]

The environment must be installed using the Distribution Package adapted to the selected microprocessor device. See the list of AndroidTM Distribution Packages.

To be able to execute following instructions, work from the distribution root directory and initialize the environment with the following instructions:

 source build/
 lunch aosp_<BoardId>-userdebug
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The bspsetup instruction only needs to be run once for each distribution

2 Partition layout[edit]

The Trusted Firmware (TF-A) is used as the primary bootloader in the STM32MPU distribution for Android™. The TF-A binary file is part of the fsbl partition.

The Universal Bootloader (U-Boot) is used as the secondary bootloader in the STM32MPU distribution for Android™. The U-Boot binary file is part of the ssbl partition.

The binaries are available in device/stm/<STM32Series>-bootloader/prebuilt.

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For information on the complete partition layout for the corresponding development platform, refer to the corresponding Flash mapping article named Your_development_platform Flash mapping for AndroidTM. Example: STM32MP15 Flash mapping for Android for the STM32MP15 boards.

All Flash mapping for AndroidTM articles belong to the Category:Flash mapping for Android.

3 Load the bootloader sources[edit]

By default, the bootloader sources are not part of the STM32MPU distribution for AndroidTM. To load the sources, execute the following instruction:


It will load both the TF-A and U-Boot sources.

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The load_bootloader script uses the configuration file android_fsbl.config (resp. android_ssbl.config) available in the device/stm/<STM32Series>-bootloader/source/patch/<fsbl-version>/ (resp. .../<ssbl-version>/) directory.

The bootloader sources are located in:

  • TF-A: device/stm/<STM32Series>-bootloader/tf-a-<STM32Series>
  • U-Boot: device/stm/<STM32Series>-bootloader/u-boot-<STM32Series>
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By default, only the archive is loaded from the remote repository (reducing size). If required the remote repository with all the git history can be cloned instead.

For that purpose, BOOTLOADER_ARCHIVE_PATH must be replaced by BOOTLOADER_GIT_PATHin the file android_fsbl.config (resp. android_ssbl.config) . Then execute load_bootloader again with -f option if previously loaded sources have to be replaced.

4 Build the bootloaders[edit]

To build both the FSBL and the SSBL, run:


This instruction generates both the TF-A and the U-Boot for every available board, memory and boot mode in the optee and trusted configuration.

To generate the FSBL and the SSBL for a single configuration set:

 build_bootloader --<BootMode> -b <BoardId> -m <MemoryType>

BootMode option examples:

  • optee
  • trusted

MemoryType option examples:

  • sd
  • emmc

To get back more details on available options, run:

 build_bootloader -h

Here after, this the result for the version 1.4:

Usage: build_bootloader [Options] [Mode Options] [Board Options]

  This script allows building the bootloaders source (TF-A and/or U-Boot)

  -h / --help: print this message
  -v / --version: get script version
  -i / --install: update prebuilt images
  --verbose=<level>: enable verbosity (1 or 2 depending on level of verbosity required)

Mode options (exclusive, default = both optee and trusted):
  -o / --optee: set optee mode for bootloaders
  -t / --trusted: set trusted mode for bootloaders (non op-tee option)
  -p / --programmer: build dedicated programmer version (-i option forced)

Board options: (default = all possibilities)
  -b <name> / --board=<name>: set board name from following list = eval (default: all)
  -m <config> / --mem=<config>: set memory configuration from following list = sd emmc (default: all)

5 Integrate the bootloaders[edit]

To update prebuilt images add -i option:

 build_bootloader -i

Or use the specific configuration method with -i option.

 build_bootloader -i --<BootMode> -b <BoardId> -m <MemoryType>

To integrate the updated prebuilt images, execute from the root directory:

 make -j

Then update associated partitions on your device. Refer to How to populate boards for Android