# 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
![Bootloader stages](../img/bootloader-stages.png)

### 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
```bash
printenv [VARIABLE]
```
or
```bash
setenv VARIABLE VALUE
```

Environment variables can also be persisted
```bash
saveenv
```
and executed with the run command:
```bash
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!](https://linux-sunxi.org/UEnv.txt)

### 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:
```bash
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".
```bash
$ 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.:
```bash
bootm ${loadaddr} - ${fdtaddr}
```