- Last edited 2 years ago ago
Das U-Boot ("the Universal Boot Loader" or U-Boot) is an open-source bootloader that can be used to initialize the platform and load the Linux® kernel.
- 1 Das U-Boot
- 2 U-Boot overview
- 3 U-Boot configuration
- 4 U-Boot command line interface (CLI)
- 5 U-Boot build
- 6 References
1 Das U-Boot
- Official website: https://www.denx.de/wiki/U-Boot
- Official manual: project documentation and https://www.denx.de/wiki/DULG/Manual
- Official source code is available under git repository at 
git clone https://gitlab.denx.de/u-boot/u-boot.git
Read the README file before starting using U-Boot. It covers the following topics:
- source file tree structure
- description of CONFIG defines
- instructions for building U-Boot
- brief description of the Hush shell
- list of common environment variables
2 U-Boot overview
The same U-Boot source can generate two pieces of firmware used in SPL and U-Boot STM32 MPU boot chain:
- Trusted boot chain: TF-A as FSBL and U-Boot as SSBL
- Basic boot chain: SPL as FSBL and U-Boot as SSBL
It is provided only as an example of the simplest SSBL and to support upstream U-Boot development. However, several known limitations have been identified when SPL is used in conjunction with the minimal secure monitor provided within U-Boot for basic boot chain. They apply to:
- secure access to registers
- limited features (STM32CubeProgrammer / boot from NAND Flash memory).
No fix is planned for these limitations.
2.1 SPL: FSBL for basic boot
The U-Boot SPL or SPL is the first stage bootloader (FSBL) for the basic boot chain.
It is a small binary (bootstrap utility) generated from the U-Boot source and stored in the internal limited-size embedded RAM. SPL main features are the following:
- It is loaded by the ROM code.
- It performs the initial CPU and board configuration (clocks and DDR memory).
- It loads the SSBL (U-Boot) into the DDR memory.
2.2 U-Boot: SSBL
U-Boot is the default second-stage bootloader (SSBL) for STM32 MPU platforms. It is used both for trusted and basic boot chains. SSBL main features are the following:
- It is configurable and expendable.
- It features a simple command line interface (CLI), allowing users to interact over a serial port console.
- It provides scripting capabilities
- It loads the kernel into RAM and gives control to the kernel
- It manages several internal and external devices such as NAND and NOR Flash memories, Ethernet and USB.
- It supports the following features and commands:
- File systems: FAT, UBI/UBIFS, JFFS
- IP stack: FTP
- Display: LCD, HDMI, BMP for splashcreen
- USB: host (mass storage) or device (DFU stack)
2.3 SPL phases
SPL executes the following main phases in SYSRAM:
- board_init_f(): driver initialization including DDR initialization (mininimal stack and heap: CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN)
- configuration of heap in DDR memory (CONFIG_SPL_SYS_MALLOC_F_LEN)
- board_init_r(): initialization of the other drivers activated in the SPL device tree
- loading and execution of U-Boot (or Kernel in Falcon mode: README.falcon ).
2.4 U-Boot phases
U-Boot executes the following main phases in DDR memory:
- Pre-relocation initialization (common/board_f.c): minimal initialization (such as CPU, clock, reset, DDR and console) running at the CONFIG_SYS_TEXT_BASE load address.
- Relocation: copy of the code to the end of DDR memory.
- Post-relocation initialization:(common/board_r.c): initialization of all the drivers.
- Command execution through autoboot (CONFIG_AUTOBOOT) or console shell
- Execution of the boot command (by default bootcmd=CONFIG_BOOTCOMMAND):
for example, execution of the command
- load and check images (such as kernel, device tree and ramdisk)
- fixup the kernel device tree
- install the secure monitor (optional) or
- pass the control to the Linux kernel (or to another target application)
- Execution of the boot command (by default bootcmd=CONFIG_BOOTCOMMAND):
3 U-Boot configuration
The U-Boot binary configuration is based on
- Kbuild infrastructure (as in Linux Kernel, you can use
make menuconfigin U-Boot)
The configurations are based on:
- options defined in Kconfig files (CONFIG_ compilation flags)
- the selected configuration file: configs/stm32mp*_defconfig
- other compilation flags defined in include/configs/stm32mp*.h (these flags are progressively migrated to Kconfig)
The file name is configured through CONFIG_SYS_CONFIG_NAME.
For STM32MP15x lines , the include/configs/stm32mp1.h file is used.
- DeviceTree: U-Boot and SPL binaries include a device tree blob that is parsed at runtime
All the configuration flags (prefixed by CONFIG_) are described in the source code, either in the README file or in the documentation directory .
For example, CONFIG_SPL activates the SPL compilation.
Hence to compile U-Boot, select the <target> and the device tree for the board in order to choose a predefined configuration.
Refer to #U-Boot_build for examples.
Like the kernel, the U-Boot build system is based on configuration symbols (defined in Kconfig files). The selected values are stored in a .config file in the build directory, with the same makefile target. .
Proceed as follows:
- Select a pre-defined configuration (defconfig file in configs directory ) and generate the first .config:
- Change the U-Boot compile configuration (modify .config) by using one of the following five
make menuconfig --> menu based program make config --> line-oriented configuration make xconfig --> QT program make gconfig --> GTK program make nconfig --> ncurse menu based program
You can then compile U-Boot with the updated .config.
Warning: the modification is performed locally in the build directory. It will be lost after a
Save your configuration to be able to use it as a defconfig file:
This target saves the current config as a defconfig file in the build directory. It can then be compared with the predefined configuration (configs/stm32mp*defconfig).
The other makefile targets are the following:
make help .... Configuration targets: config - Update current config utilising a line-oriented program nconfig - Update current config utilising a ncurses menu based program menuconfig - Update current config utilising a menu based program xconfig - Update current config utilising a Qt based front-end gconfig - Update current config utilising a GTK+ based front-end oldconfig - Update current config utilising a provided .config as base localmodconfig - Update current config disabling modules not loaded localyesconfig - Update current config converting local mods to core defconfig - New config with default from ARCH supplied defconfig savedefconfig - Save current config as ./defconfig (minimal config) allnoconfig - New config where all options are answered with no allyesconfig - New config where all options are accepted with yes allmodconfig - New config selecting modules when possible alldefconfig - New config with all symbols set to default randconfig - New config with random answer to all options listnewconfig - List new options olddefconfig - Same as oldconfig but sets new symbols to their default value without prompting
3.2 Device tree
Refer to doc/README.fdt-control for details.
The board device tree has the same binding as the kernel. It is integrated within the SPL and U-Boot binaries:
- By default, it is appended at the end of the code (CONFIG_OF_SEPARATE).
- It is embedded in the U-Boot binary (CONFIG_OF_EMBED). This is useful for debugging since it enables easy .elf file loading.
A default device tree is available in the defconfig file (by setting CONFIG_DEFAULT_DEVICE_TREE).
You can either select another supported device tree using the DEVICE_TREE make flag. For stm32mp boards, the corresponding file is: arch/arm/dts/stm32mp*.dts .
or provide a precompiled device tree blob (using EXT_DTB option):
The SPL device tree is also generated from this device tree. However to reduce its size, the U-Boot makefile uses the fdtgrep tool to parse the full U-Boot DTB and identify all the drivers required by SPL.
To do this, U-Boot uses specific device-tree flags to define if the associated driver is initialized prior to U-Boot relocation and/or if the associated node is present in SPL :
- u-boot,dm-pre-reloc => present in SPL, initialized before relocation in U-Boot
- u-boot,dm-pre-proper => initialized before relocation in U-Boot
- u-boot,dm-spl => present in SPL
In the device tree used by U-Boot, these flags need to be added in each node used in SPL or in U-Boot before relocation and for each used handle (clock, reset, pincontrol).
4 U-Boot command line interface (CLI)
Refer to U-Boot Command Line Interface.
If CONFIG_AUTOBOOT is activated, you have CONFIG_BOOTDELAY seconds (2s by default) to enter the console by pressing any key, after the line below is displayed and bootcmd is executed (CONFIG_BOOTCOMMAND):
Hit any key to stop autoboot: 2
The commands are defined in cmd/*.c . They are activated through the corresponding CONFIG_CMD_* configuration flag.
help command in the U-Boot shell to list the commands available on your device:
Below the list of all commands extracted from U-Boot Manual (not-exhaustive):
- Information Commands
- bdinfo - prints Board Info structure
- coninfo - prints console devices and information
- flinfo - prints Flash memory information
- iminfo - prints header information for application image
- help - prints online help
- Memory Commands
- base - prints or sets the address offset
- crc32 - checksum calculation
- cmp - memory compare
- cp - memory copy
- md - memory display
- mm - memory modify (auto-incrementing)
- mtest - simple RAM test
- mw - memory write (fill)
- nm - memory modify (constant address)
- loop - infinite loop on address range
- Flash Memory Commands
- cp - memory copy
- flinfo - prints Flash memory information
- erase - erases Flash memory
- protect - enables or disables Flash memory write protection
- mtdparts - defines a Linux compatible MTD partition scheme
- Execution Control Commands
- source - runs a script from memory
- bootm - boots application image from memory
- go - starts application at address 'addr'
- Download Commands
- bootp - boots image via network using BOOTP/TFTP protocol
- dhcp - invokes DHCP client to obtain IP/boot params
- loadb - loads binary file over serial line (kermit mode)
- loads - loads S-Record file over serial line
- rarpboot- boots image via network using RARP/TFTP protocol
- tftpboot- boots image via network using TFTP protocol
- Environment Variables Commands
- printenv- prints environment variables
- saveenv - saves environment variables to persistent storage
- setenv - sets environment variables
- run - runs commands in an environment variable
- bootd - boots default, i.e., run 'bootcmd'
- Flattened Device Tree support
- fdt addr - selects the FDT to work on
- fdt list - prints one level
- fdt print - recursive printing
- fdt mknode - creates new nodes
- fdt set - sets node properties
- fdt rm - removes nodes or properties
- fdt move - moves FDT blob to new address
- fdt chosen - fixup dynamic information
- Special Commands
- i2c - I2C sub-system
- Storage devices
- Miscellaneous Commands
- echo - echoes args to console
- reset - Performs a CPU reset
- sleep - delays the execution for a predefined time
- version - prints the monitor version
To add a new command, refer to doc/README.commands .
4.2 U-Boot environment variables
The U-Boot behavior is configured through environment variables.
On the first boot, U-Boot uses a default environment embedded in the U-Boot binary. You can modify it by changing the content of CONFIG_EXTRA_ENV_SETTINGS in your configuration file (for example ./include/configs/stm32mp1.h) (see README / - Default Environment).
This environment can be modified and saved in the boot device. When it is present, it is loaded during U-Boot initialization:
- for e•MMC/SD card boot (CONFIG_ENV_IS_IN_EXT4), in the bootable ext4 partition "bootfs" in
in file CONFIG_ENV_EXT4_FILE="uboot.env".
- for NAND boot (CONFIG_ENV_IS_IN_UBI), in the two UBI volumes "config" (CONFIG_ENV_UBI_VOLUME) and "config_r" (CONFIG_ENV_UBI_VOLUME_REDUND).
- for NOR boot (CONFIG_ENV_IS_IN_SPI_FLASH), in the u-boot_env mtd parttion (at offset CONFIG_ENV_OFFSET).
4.2.1 env command
env command allows displaying, modifying and saving the environment in U-Boot console.
help env env - environment handling commands Usage: env default [-f] -a - [forcibly] reset default environment env default [-f] var [...] - [forcibly] reset variable(s) to their default values env delete [-f] var [...] - [forcibly] delete variable(s) env edit name - edit environment variable env exists name - tests for existence of variable env print [-a | name ...] - print environment env print -e [name ...] - print UEFI environment env run var [...] - run commands in an environment variable env save - save environment env set -e name [arg ...] - set UEFI variable; unset if 'arg' not specified env set [-f] name [arg ...]
Example: proceed as follows to restore the default environment and save it. This is useful after a U-Boot upgrade:
env default -a env save
"bootcmd" variable is the autoboot command. It defines the command executed when U-Boot starts (CONFIG_BOOTCOMMAND).
For stm32mp, CONFIG_BOOTCOMMAND="run bootcmd_stm32mp":
env print bootcmd bootcmd=run bootcmd_stm32mp
"bootcmd_stm32mp" is a script that selects the command to be executed for each boot device (see ./include/configs/stm32mp1.h), based on generic distro scripts:
- for serial/usb: execute the
- for mmc boot (e•MMC, SD card), boot only on the same device (bootcmd_mmc...).
- for nand boot, boot with on ubifs partition on nand (bootcmd_ubi0).
- for nor boot, use the default order e•MMC (SDMMC 1)/ NAND / SD card (SDMMC 0) / SDMMC2 (the default bootcmd: distro_bootcmd).
env print bootcmd_stm32mp
You can then change this configuration:
- either permanently in your board file (default environment by CONFIG_EXTRA_ENV_SETTINGS or change CONFIG_BOOTCOMMAND value) or
- temporarily in the saved environment:
env set bootcmd run bootcmd_mmc0 env save
Note: To reset the environment to its default value:
env default bootcmd env save
4.3 Generic Distro configuration
Refer to doc/README.distro for details.
This feature is activated by default on ST boards (CONFIG_DISTRO_DEFAULTS):
- one boot command (bootmcd_xxx) exists for each bootable device.
- U-Boot is independent of the Linux distribution used.
- bootcmd is defined in ./include/config_distro_bootcmd.h
When DISTRO is enabled, the command that is executed by default is include/config_distro_bootcmd.h :
This script tries any device found in the 'boot_targets' variable and executes the associated bootcmd.
Example for mmc0, mmc1, mmc2, pxe and ubifs devices:
bootcmd_mmc0=setenv devnum 0; run mmc_boot bootcmd_mmc1=setenv devnum 1; run mmc_boot bootcmd_mmc2=setenv devnum 2; run mmc_boot bootcmd_pxe=run boot_net_usb_start; dhcp; if pxe get; then pxe boot; fi bootcmd_ubifs0=setenv devnum 0; run ubifs_boot
U-Boot searches for a extlinux.conf configuration file for each bootable device. This file defines the kernel configuration to be used:
- kernel + device tree + ramdisk files (optional)
- FIT image
4.4 U-Boot scripting capabilities
"Script files" are command sequences that are executed by the U-Boot command interpreter. This feature is particularly useful to configure U-Boot to use a real shell (hush) as command interpreter.
See U-Boot script manual for an example.
5 U-Boot build
- a PC with Linux and tools:
- U-Boot source code
- from the Mainline U-Boot in official GIT repository 
git clone https://gitlab.denx.de/u-boot/u-boot.git
5.1.1 ARM cross compiler
A cross compiler  must be installed on your Host (X86_64, i686, ...) for the ARM targeted Device architecture. In addition, the $PATH and $CROSS_COMPILE environment variables must be configured in your shell.
You can use gcc for ARM, available in:
- the SDK toolchain (see Cross-compile with OpenSTLinux SDK)
PATH and CROSS_COMPILE are automatically updated.
- an existing package
For example, install gcc-arm-linux-gnueabihf on Ubuntu/Debian: ( sudo apt-get.
- an existing toolchain:
- latest gcc toolchain provided by arm (https://developer.arm.com/open-source/gnu-toolchain/gnu-a/downloads/)
- gcc v7 toolchain provided by linaro: (https://www.linaro.org/downloads/)
For example, to use gcc-arm-9.2-2019.12-x86_64-arm-none-linux-gnueabihf.tar.xz from arm, extract the toolchain in $HOME and update your environment with:
export PATH=$HOME/gcc-arm-9.2-2019.12-x86_64-arm-none-linux-gnueabihf/bin:$PATH export CROSS_COMPILE=arm-none-linux-gnueabihf-
For example, to use gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabi.tar.xz
Unzip the toolchain in $HOME and update your environment with:
export PATH=$HOME/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabi/bin:$PATH export CROSS_COMPILE=arm-linux-gnueabi-
In the U-Boot source directory, select the <target> and the <device tree> for your configuration and then execute the
make all command:
make <target>_defconfig make DEVICE_TREE=<device-tree> all
Optionally KBUILD_OUTPUT can be used to change the output directory to compile several targets or not to compile in the source directory. For example:
DEVICE_TREE can also be exported to your environment when only one board is supported. For example:
Examples from STM32MP15 U-Boot:
- stm32mp15_trusted_defconfig: trusted boot chain, U-Boot (without SPL) is unsecure and uses Secure monitor from TF-A
- stm32mp15_optee_defconfig: trusted boot chain, U-Boot (without SPL) is unsecure and uses Secure monitor from SecureOS = OP-TEE
- stm32mp15_basic_defconfig: basic boot chain, with an SPL as FSBL, U-BOOT is secure and installs monitor with PSCI
The board diversity is only managed with the device tree.
export KBUILD_OUTPUT=../build/trusted make stm32mp15_trusted_defconfig make DEVICE_TREE=stm32mp157c-<board> all
export KBUILD_OUTPUT=../build/optee export DEVICE_TREE=stm32mp157c-<board> make stm32mp15_optee_defconfig make all
make stm32mp15_basic_defconfig make DEVICE_TREE=stm32mp157c-<board> all
Use help to list other targets:
5.3 Output files
The resulting U-Boot files are located in your build directory (U-Boot or KBUILD_OUTPUT).
Two binary formats are used for stm32mp devices:
- STM32 image format (*.stm32), managed by mkimage U-Boot tools and Signing_tool. It is requested by ROM code and TF-A (see STM32 header for binary files for details).
- uImage (*.img) format, file including a U-Boot header, managed by SPL and U-Boot (for kernel load)
The U-Boot generated files are the following
- For Trusted boot chain (TF-A is used as FSBL, with or without OP-TEE)
- u-boot.stm32 : U-Boot binary with STM32 image header, loaded by TF-A
- For Basic boot chain (SPL is used as FSBL)
- u-boot-spl.stm32 : SPL binary with STM32 image header, loaded by ROM code
- u-boot.img : U-Boot binary with uImage header, loaded by SPL
The files used to debug with gdb are
- u-boot : elf file for U-Boot
- spl/u-boot-spl : elf file for SPL