Das U-Boot

U-Boot is a popular and powerful bootloader.

How it works

The boot process is devided into 4 stages:

First stage bootloader
Second stage bootloader
U-Boot
Kernel

First stage

The first stage bootloader is a hardcoded script inside the ROM chip. It can sometimes be modified by switches located on the board.

Second stage

The second stage bootloader (SPL or MLO) is the first stage of U-Boot. It still has very limited functionality and its purpose is setup the boot- process for U-Boot.

Third stage

U-Boot itself comes with a very powerful boot environment consisting of an integrated serial terminal and environment variables. The U-Boot environment variables can be accessed by typing

printenv [VARIABLE]

or

setenv VARIABLE VALUE

Environment variables can also be persisted

saveenv

and executed with the run command:

run bootcmd

This example starts the boot process and is the first command to be run when starting U-Boot.

Environment variables can also be predefined by using /boot/uEnv.txt. More about that here!

Fourth stage

The Fourth stage in this concept is the Kernel. It gets loaded into RAM by U-Boot and can be configured by adding options to the Kernel command line. Example from the Yocto Project:

KERNEL_ARGS += " console=ttyS0,115200 "

Sets the debug serial to /dev/ttyS0 with a baudrate of 115200. This example is the default for RaspberryPi boards.

U-Boot boot process

When the third stage of the boot process begins, the first thing happening is the execution of “bootcmd”.

$ printenv bootcmd

bootcmd=echo Booting from mmc ...; mmc dev ${mmcdev}; mmc dev ${mmcdev};
         if mmc rescan; then
             if run loadbootscript; then
                 run bootscript;
             else
                 if run loadimage; then
                     run mmcboot;
                 else
                     echo ERROR: Cannot run loadimage;
                 fi;
             fi;
         else
             echo ERROR: Cannot run mmc rescan;
         fi;

Note

This is not the actual output, but a pritty printed version to visualize what is going on.

The usually last command in the boot process is a form of “boot”, which executes the Kernel inside the image.itb file that has previously been loaded into the boards DDR main memory. The config file at $loadaddr sets the parameters, such as the ramdisk or kernel address in the .itb file. E.g.:

bootm ${loadaddr} - ${fdtaddr}