- Last edited 4 months ago ago
- 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
Das U-Boot ("the Universal Boot Loader" or U-Boot) is an open-source bootloader that can be used on STMicroelectronics boards to initialize the platform and load the Linux® kernel.
- Official source code is available under git repository  at https://source.denx.de/u-boot/u-boot, also available in https://github.com/u-boot/u-boot
- source.denx.de also hosts the Custodian git trees;
the STMicroelectronics U-Boot Custodian Tree is available in https://source.denx.de/u-boot/custodians/u-boot-stm
- the fork of "U-Boot" Source Tree used in OpenSTLinux is available in ST GITHUB with associated releases : https://github.com/STMicroelectronics/u-boot
- source.denx.de also hosts the Custodian git trees;
- Official manual:
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
U-Boot is included in a TF-A FIP.
U-Boot SPL is not supported in OpenSTLinux.
2.1.1 SPL description
The U-Boot SPL or SPL is an alternate first stage bootloader (FSBL).
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.
2.1.2 SPL execution sequence
SPL executes the following main steps 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 ).
2.2 U-Boot: SSBL
2.2.1 U-Boot description
U-Boot is the second-stage bootloader (SSBL) of boot chain for STM32 Arm® Cortex® MPUs .
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)
2.2.2 U-Boot execution sequence
U-Boot executes the following main steps 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 example on STMicroelectronics boards:
- DeviceTree: U-Boot 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 located in the build directory, with the same makefile target. .
Proceed as follows:
- Select a predefined 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
The board device tree has the same binding as the kernel. By default it is integrated within the U-Boot binaries: u-boot.bin: appended at the end of the code (CONFIG_OF_SEPARATE) or embedded in the U-Boot binary (CONFIG_OF_EMBED).
In OpenSTLinux, the U-Boot device tree (u-boot.dtb) is provided as external file loaded by FSBL=TF-A when U-Boot code is started (u-boot-nodtb.bin: code without device tree): device tree address is provided as boot parameter (in r2 register).
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 STM32 Arm® Cortex® MPUs boards, the corresponding file is <dts-file-name>.dts in arch/arm/dts/stm32mp*.dts , with <dts-file-name> set to the full name of the board:
or provide a device tree blob (dtb file) resulting from the dts file compilation, by using the EXT_DTB option:
To obtain a device tree file <dts-file-name>.dts that is identical to the Linux kernel one, these U-Boot properties are only added for ST boards in the add-on file <dts-file-name>-u-boot.dtsi. This file is automatically included in <dts-file-name>.dts during device tree compilation (this is a generic U-Boot Makefile behavior).
Device tree in SPL (not used in OpenSTLinux):
The SPL device tree is also generated from this U-Boot 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 determine 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 all the nodes used in SPL or in U-Boot before relocation, and for all used handles (clock, reset, pincontrol).
4 U-Boot command line interface (CLI)
U-Boot has a set of built-in commands for managing the system and booting the device.By modifying U-Boot source code, you can create your own built-in commands.
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:
The U-Boot usage is described in U-Boot Documentation but only few commands are described here.
Below the list of all commands extracted from an old U-Boot Manual] (obsolete and 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 - default boot, that is 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
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 STMicroelectronics the enviromnent is according the boot device:
- To boot from e•MMC/SD card (CONFIG_ENV_IS_IN_MMC): at the end of the partition indicated by config field "u-boot,mmc-env-partition" in device-tree (for ST boards: partition named "u-boot-env" in ecosystem release ≥ v4.0.0 ).
- To boot from NAND Flash memory (CONFIG_ENV_IS_IN_UBI): in the two UBI volumes "config" (CONFIG_ENV_UBI_VOLUME) and "config_r" (CONFIG_ENV_UBI_VOLUME_REDUND).
- To boot from NOR Flash memory (CONFIG_ENV_IS_IN_SPI_FLASH): the "u-boot-env" mtd partition 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 ...]
See U-Boot Documentation for details.
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
You can also use the command activated by CONFIG_CMD_ERASEENV:
"bootcmd" variable is the autoboot command (see doc/README.autoboot ). 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/stm32mp13_st_common.h or ./include/configs/stm32mp15_st_common.h for STMicroelectronics board), based on generic distro scripts:
- To boot from a serial/usb device: execute the
- To boot from an e•MMC, SD card: boot only on the same device (bootcmd_mmc...).
- To boot from a NAND Flash memory: boot on ubifs partition on the NAND memory (bootcmd_ubi0).
- To boot from a NOR Flash memory: use the SD card (on SDMMC 0 on ST boards with bootcmd_mmc0)
env print bootcmd_stm32mp
You can then change this configuration:
- either permanently in your board file
- default environment by CONFIG_EXTRA_ENV_SETTINGS (see ./include/configs/stm32mp1.h)
- change CONFIG_BOOTCOMMAND value in your defconfig
- 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 U-Boot scripting capabilities
"Script" is made up of variables (
bootcmd for example) that contain a set of commands that are executed by the U-Boot command interpreter one after another. You run this script with the
run command, for example:
"Script image" is external binary file with U-Boot header and containing a sequence of commands. It is loaded by U-Boot in memory and executed using the
source command, it allows the user to execute a set of predefined U-Boot commands automatically.
To convert an text file commands.txt into a script image for U-Boot, you have to use the mkimage tool as follows:
mkimage -T script -C none -n 'Demo Script File' -d commands.txt setenv.img
If CONFIG_AUTOBOOT is activated, you have some delay established in
bootdelay variable or CONFIG_BOOTDELAY (2s by default, 1s for ST configuration) to enter the console by pressing any key, after the line below is displayed.
Hit any key to stop autoboot: 2
If U-Boot is not interrupted, bootcmd is executed (CONFIG_BOOTCOMMAND), in other words automatic booting have the same effect that the command:
4.5 Generic Distro configuration
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 from 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 an extlinux.conf configuration file for each bootable device. This file defines the kernel configuration to be used with
- files to start the OS:
- kernel (uImage) + device tree + ramdisk files (optional)
- FIT image, including all these needed files (for details see doc/uImage.FIT/howto.txt )
5 U-Boot build
See U-Boot Documentation.
- a PC with Linux and tools:
- U-Boot source code
- from the Mainline U-Boot in official GIT repository 
git clone https://source.denx.de/u-boot/u-boot.git
5.2 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 install gcc-arm-linux-gnueabihf
- an existing toolchain:
- latest gcc toolchain provided by arm (https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-a/downloads)
- Linaro also provides monthly build: (https://www.linaro.org/downloads/)
1. gcc-arm-9.2-2019.12-x86_64-arm-none-linux-gnueabihf.tar.xz provided by 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-
2.gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabi.tar.xz, extract 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 defconfig for the <target> and the <device tree> for your board and then execute the
make all command:
make <target>_defconfig make DEVICE_TREE=<device tree> all
Use help to list other targets than all:
- KBUILD_OUTPUT can be used to change the output build directory in order to compile several targets in the source directory. For example:
- DEVICE_TREE can also be exported to your environment when only one board is supported. For example:
The result is the following:
export KBUILD_OUTPUT=<path> export DEVICE_TREE=<device tree> make <target>_defconfig make all
5.4 Output files
The resulting U-Boot files are located in your build directory (U-Boot or KBUILD_OUTPUT).
For OpenSTLinux, two U-Boot files are used by ST boards to generate the FIP file used byTF-A BL2:
- BL33_CFG=u-boot.dtb : the U-Boot device tree, selected by DEVICE_TREE, loaded by TF-A BL2
- BL33=u-boot-nodtb.bin: the U-Boot executable, loaded by TF-A BL2 started by secure world = OP-TEE with BL33_CFG as parameter
Nota: All the compiled device tree are available in $KBUILD_OUTPUT/arch/arm/dts/*.dtb.
You can select them as BL33_CFG parameter without U-Boot recompilation.
See TF-A BL2 overview for FIP details.
The file used to debug with gdb is u-boot, the elf file for U-Boot.
The file u-boot.stm32, i.e. the U-Boot binary with STM32 header, is no more generated and used in OpenSTlinux.
This file is generated only with stm32mp15_trusted_defconfig, when CONFIG_STM32MP15x_STM32IMAGE is activated. This defconfig is kept for compatibility with upstream, it is not supported in OpenSTlinux.
The STM32 image format (*.stm32) is also managed by mkimage U-Boot tools and Signing_tool to manage this compatibility, stm32mp15_trusted_defconfig and stm32mp15_basic_defconfig.
- https://source.denx.de/u-boot/u-boot.git or https://github.com/u-boot/u-boot