- 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 just U-Boot) is an open-source boot loader, which can be used on ST boards to initialize the platform and load the Linux® kernel.
- Official website: https://www.denx.de/wiki/U-Boot
- Official manual: project documentation and https://www.denx.de/wiki/DULG/Manual
- The official source code is available with git repository at git.denx.de
PC $> git clone git://git.denx.de/u-boot.git
Reading the README file is recommended. It covers the following topics:
- the source file tree structure
- the meaning of the CONFIG defines
- instructions for building U-Boot
- a brief description of the Hush shell
- a list of common environment variables
2 U-Boot overview
The same U-Boot source can generate 2 pieces of firmware used in the STM32 MPU boot chain: SPL and U-Boot
- Trusted boot chain: U-Boot as SSBL
- Basic boot chain: SPL as FSBL and U-Boot as SSBL
2.1 SPL: FSBL for basic boot
The U-Boot SPL or just SPL is the first stage boot loader (FSBL) for the basic boot chain.
It is a small binary (bootstrap utility), generated from the U-Boot source, which fits in the internal and limited embedded RAM:
- It is loaded by the ROM code
- it does the initial CPU and board configuration: clocks and DDR
- it loads the SSBL (U-Boot) into DDR memory
2.2 U-Boot: SSBL
U-Boot is the default second stage boot loader (SSBL) for the STM32 MPU platforms for the 2 boot chains, trusted and basic:
- it is configurable and expendable
- it has a simple command line interface (CLI), usually over a serial console port for interaction with the user
- it provides scripting capabilities
- it loads the kernel into RAM and passes control to the kernel
- it manages many internal and external devices like NAND, NOR, Ethernet, USB
- it has many supported features and commands for
- file systems: FAT, UBI/UBIFS, JFFS
- IP stack: FTP
- display: LCD, HDMI, BMP for splashcreen
- USB: host profile (mass storage) or device profile (DFU stack)
2.3 SPL phases
The SPL runs through the main following phases in SYSRAM:
- board_init_f(): init drivers up to DDR initialisation (mininimal stack and heap: CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN)
- configure heap in DDR (CONFIG_SPL_SYS_MALLOC_F_LEN)
- board_init_r(): init other drivers activated in the SPL device tree
- load U-Boot (or Kernel in Falcon mode: README.falcon ) and execute it
2.4 U-Boot phases
U-Boot runs through the following main phases in DDR:
- Pre-relocation initialization (common/board_f.c): minimal init (cpu, clock, reset, ddr, console,...) running at the load address CONFIG_SYS_TEXT_BASE
- Relocation: copy the code to the end of DDR
- Post-relocation initialization:(common/board_r.c): init all the drivers
- Execution of commands: through autoboot (CONFIG_AUTOBOOT) or console shell
- execute the boot command (bootcmd=CONFIG_BOOTCOMMAND by default):
for example, execute the command 'bootm' to:
- load and check images (kernel, device tree, ramdisk....)
- fixup kernel device tree
- install secure monitor (optional)
- pass control to the Linux kernel (or other target application)
- execute the boot command (bootcmd=CONFIG_BOOTCOMMAND by default):
3 U-Boot configuration
The U-Boot binary configuration is based on
- Kbuild infrastructure (as in Linux Kernel, you can use "make menuconfig" in 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 defines in include/configs/stm32mp*.h
the file name is configured by CONFIG_SYS_CONFIG_NAME
(these flags are progressively migrating to Kconfig)
for stm32mp157: we use include/configs/stm32mp1.h file
- DeviceTree = U-Boot and SPL binaries include a device tree blob which is parsed at run time
All the configuration flags (CONFIG_) are described in the source code: the README file or documentation directory
example: CONFIG_SPL => activate the SPL compilation.
Hence to compile U-Boot, you need to select the <target> and the device tree for the board to select a predefined configuration.
See #U-Boot_build for examples.
The U-Boot build system is based on configuration symbols as the kernel (defined in Kconfig files), and selected values are stored in a .config file in the build directory, with the same makefile target.
- select pre-defined configuration (defconfig file, in configs directory ) and generate the first .config
PC $> make <config>_defconfig
- change U-Boot compile configuration (modify .config) using one of the 5 make command
PC $> make menuconfig --> menu based program PC $> make config --> line-oriented configuration PC $> make xconfig --> QT program PC $> make gconfig --> GTK program PC $> make nconfig --> ncurse menu based program
You can then compile U-Boot with the updated .config.
Warning: modification is only done locally in the build directory, it is lost after a "make distclean"
So if you want to use your configuration as defconfig:
PC $> make savedefconfig
This target saves the current config as a defconfig file in the build directory, and can be compared with the predefined configuration (configs/stm32mp*defconfig).
The other makefile targets are :
PC $> 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, with the same binding as the kernel, is integrated with the SPL and U-Boot binaries:
- appended at the end of the code by default (CONFIG_OF_SEPARATE)
- embedded in binary (CONFIG_OF_EMBED): useful for debug, allows easy elf file loading
A default device tree is defined in the defconfig file (with CONFIG_DEFAULT_DEVICE_TREE).
You can also select another supported device tree with the make flag DEVICE_TREE
for stm32mp32 boards the file are: arch/arm/dts/stm32mp*.dts
PC $> make DEVICE_TREE=<dts-file-name>
or you can provide a precompiled device tree blob (with EXT_DTB option)
PC $> make EXT_DTB=boot/<dts-file-name>.dtb
The SPL device tree is also generated from this device tree; but to reduce its size, the U-Boot makefile uses the fdtgrep tool to parse the full U-Boot DTB and identify all the drivers needed by SPL.
To do this, U-Boot uses some specific device-tree flags to specify 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-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 but also for each used handle (clock, reset, pincontrol).
4 U-Boot command line interface (CLI)
If CONFIG_AUTOBOOT is activated, to enter in this console, you have CONFIG_BOOTDELAY seconds (2s by default) before bootcmd execution (CONFIG_BOOTCOMMAND) by pressing any key when the line below is displayed.
Hit any key to stop autoboot: 2
The commands are defined cmd/*.c , they are activated under associated configuration flag CONFIG_CMD_*.
Use the command help in the U-Boot shell to list the available commands on your device.
List of commands extracted from U-Boot Manual (not-exhaustive):
- Information Commands
- bdinfo - print Board Info structure
- coninfo - print console devices and informations
- flinfo - print FLASH memory information
- iminfo - print header information for application image
- help - print online help
- Memory Commands
- base - print or set 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 - print FLASH memory information
- erase - erase FLASH memory
- protect - enable or disable FLASH write protection
- mtdparts - define a Linux compatible MTD partition scheme
- Execution Control Commands
- source - run script from memory
- bootm - boot application image from memory
- go - start application at address 'addr'
- Download Commands
- bootp - boot image via network using BOOTP/TFTP protocol
- dhcp - invoke DHCP client to obtain IP/boot params
- loadb - load binary file over serial line (kermit mode)
- loads - load S-Record file over serial line
- rarpboot- boot image via network using RARP/TFTP protocol
- tftpboot- boot image via network using TFTP protocol
- Environment Variables Commands
- printenv- print environment variables
- saveenv - save environment variables to persistent storage
- setenv - set environment variables
- run - run commands in an environment variable
- bootd - boot default, i.e., run 'bootcmd'
- Flattened Device Tree support
- fdt addr - select FDT to work on
- fdt list - print one level
- fdt print - recursive print
- fdt mknode - create new nodes
- fdt set - set node properties
- fdt rm - remove nodes or properties
- fdt move - move FDT blob to new address
- fdt chosen - fixup dynamic info
- Special Commands
- i2c - I2C sub-system
- Storage devices
- Miscellaneous Commands
- echo - echo args to console
- reset - Perform RESET of the CPU
- sleep - delay execution for some time
- version - print monitor version
To add a new command, see doc/README.commands
4.2 U-Boot environment variables
The U-Boot behavior is configured with environment variables.
By default the env is NOT saved (CONFIG_ENV_IS_NOWHERE), only the default environment is used (saveenv command is not working)
You can modify this default environment by changing the content of CONFIG_EXTRA_ENV_SETTINGS in your configuration file (for example ./include/configs/stm32mp1.h) (see README / - Default Environment).
You can also choose one location with configuration flags:
- CONFIG_ ENV_IS_IN_NAND
Autoboot command: defines the command executed when U-Boot starts (CONFIG_BOOTCOMMAND).
But you can change this variable in CONFIG_EXTRA_ENV_SETTINGS (after BOOTENV macro needed for #Generic Distro configuration).
#define CONFIG_EXTRA_ENV_SETTINGS \ "stdin=serial\0" \ "stdout=serial\0" \ "stderr=serial\0" \ "kernel_addr_r=0xc2000000\0" \ "fdt_addr_r=0xc4000000\0" \ "scriptaddr=0xc4100000\0" \ "pxefile_addr_r=0xc4200000\0" \ "splashimage=0xc4300000\0" \ "ramdisk_addr_r=0xc4400000\0" \ "fdt_high=0xffffffff\0" \ "initrd_high=0xffffffff\0" \ BOOTENV \ "bootcmd=run bootcmd_mmc0\0"
4.3 Generic Distro configuration
This feature is activated for 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
With DISTRO the default command executed: include/config_distro_bootcmd.h
This script will try any device found in the variable 'boot_targets' and execute the associated bootcmd.
Example for device mmc0, mmc1, mmc2, pxe and ubifs:
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 searchs for a configuration file extlinux.conf in a bootable device, this file defines the kernel configuration to use:
- kernel + device tree + ramdisk files (optional)
- FIT image
4.4 U-Boot scripting capabilities
"Script files" are command sequences that will be executed by U-Boot's command interpreter; this feature is especially useful when you configure U-Boot to use a real shell (hush) as command interpreter.
See U-Boot script manual for example.
5 U-Boot build
- a PC with Linux and tools:
- U-Boot source code
PC $> git clone https://github.com/STMicroelectronics/u-boot
- from the Mainline U-Boot in official GIT repository 
PC $> git clone http://git.denx.de/u-boot.git
5.1.1 ARM cross compiler
You need to have a cross compiler  installed on your Host (X86_64, i686, ...) for the targeted Device architecture = ARM, the environment variables ($PATH and $CROSS_COMPILE) need to 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, on Ubuntu/Debian: (PC $> sudo apt-get install gcc-arm-linux-gnueabihf)
- an existing toolchain:
- gcc v8 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/)
from https://releases.linaro.org/components/toolchain/binaries/7.2-2017.11/arm-linux-gnueabi/ unzip it in $HOME,
and you need to update your environment:
PC $> export PATH=$HOME/gcc-linaro-7.2.1-2017.11-x86_64_arm-linux-gnueabi/bin:$PATH PC $> export CROSS_COMPILE=arm-linux-gnueabi-
In the U-Boot source directory, you need to select the <target> and the <device tree> for your configuration and then execute the "make all" command.
PC $> make <target>_defconfig PC $> make DEVICE_TREE=<device-tree> all
KBUILD_OUTPUT can be used optionally to change the output directory if you want to compile several targets or don't compile in the source directory, for example:
PC $> export KBUILD_OUTPUT=../build/basic
DEVICE_TREE can be also exported to your environment when you support only one board, for example:
PC $> export DEVICE_TREE=stm32mp157c-ev1
For all the stm32mp15 family, we manage 3 configurations:
- 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.
Examples from STM32MP15 U-Boot:
PC $> export KBUILD_OUTPUT=../build/basic PC $> make stm32mp15_basic_defconfig PC $> make DEVICE_TREE=stm32mp157c-<board> all
PC $> export KBUILD_OUTPUT=../build/trusted PC $> make stm32mp15_trusted_defconfig PC $> make DEVICE_TREE=stm32mp157c-<board> all
PC $> export KBUILD_OUTPUT=../build/trusted PC $> export DEVICE_TREE=stm32mp157c-ev1 PC $> make stm32mp15_trusted_defconfig PC $> make all
Use help to list other targets:
PC $> make help
5.3 Output files
The resulting U-Boot files are present in your build directory (U-Boot or KBUILD_OUTPUT) and SPL Images are in the spl subdirectory.
STM32 image format (*.stm32) is managed by mkimage U-Boot tools and is requested by boot ROM (for basic boot chain) or by TF-A (for trusted boot chain).
- u-boot.stm32 : U-Boot binary with STM32 image header => SSBL for Trusted boot chain
- u-boot.img : U-Boot binary with uImage header => SSBL for Basic boot chain
- u-boot : elf file, used to debug with gdb
- spl/u-boot-spl.stm32 : SPL binary with STM32 image header => FSBL for Basic boot chain
- spl/u-boot-spl : elf file, used to debug with gdb
Secondary Program Loader, Also known as U-Boot SPL
Second Stage Boot Loader
First Stage Boot Loader
Random Access Memory
Read Only Memory
Central processing unit
Doubledata rate (memory domain)
High-Definition Multimedia Interface (HDMI standard)
Device Firmware Upgrade
Device Tree Binary (or Blob)
Memory Technology Device
Serial Peripheral Interface
Electrically-erasable programmable read-only memory
Software development kit
Trusted Firmware for Arm Cortex-A
Power State Coordination Interface