Network boot

TFTP

On your PC, install a tftp server
```
apt-get install tftpd-hpa
```
Copy rootfs contents inside the tftp server root (/var/lib/tftpboot on debian/ubuntu systems). You can play with the access rights to allow your user to copy files directly (chown -R user:user /var/lib/tftpboot/).
```
echo "hello" > /var/lib/tftpboot/hello.txt
cp arch/arm/boot/zImage /var/lib/tftpboot/
cp $DTB /var/lib/tftpboot/
```
Set up your network interface (to a static ip address, say 192.168.11.1)

Setup u-boot and load the kernel and device tree from network

On raspberry we need to start the usb subsystem, because the network card is connected through usb:
```
usb start
```

Test it:

setenv ipaddr 192.168.11.2
setenv serverip 192.168.11.1
tftp $loadaddr /hello.txt
md.b $loadaddr 5

Now load & boot:

tftp $loadaddr zImage
tftp $fdt_addr_r $fdtfile
setenv bootargs ....
bootz $loadaddr - $fdt_addr_r

NFS

Install nfs server utilities (the server implementation is actually in-kernel):
```
apt-get install nfs-kernel-server
```

Setup export

cat /etc/exports
/home/nfsroot  192.168.11.2(rw,no_root_squash,no_subtree_check)

mkdir /home/nfsroot
chown yoyo:yoyo /home/nfsroot
cp hi /home/nfsroot
/etc/init.d/nfs-kernel-server restart

Boot the kernel from tftp as before, but instruct it to mount root filesystem from the network via nfs:
- root=/dev/nfs: this is a "special" device that tells kernel, we are using nfs.
- nfsroot=SERVER:MOUNT: tell kernel what to actually mount.
- ip=addr:[netmask]:[gateway]:[???]:[???]:[interface]: configure the ip stack.
```
setenv bootargs=console=ttyAMA0,115200 root=/dev/nfs nfsroot=192.168.11.1:/home/nfsroot ip=192.168.11.2:::::eth0
```

Let's automate it

You can use the U-Boot environment variables to automate things with the run command:

U-Boot> echo ahoj
ahoj
U-Boot> setenv x hi
U-Boot> setenv y 'echo ${x}'
U-Boot> run y
hi
U-Boot> setenv x hello
U-Boot> run y 
hello

Note that we used ' so that the variable x wasn't expanded when we executed the setenv command, but only expanded later, when run was interpreting the contents of that variable.

At startup, if not interrupted, U-Boot will "run" the contents of the bootcmd variable. You can use this to set up your default boot. (Note: bootcmd is already set to some default value that usually tries to boot from first available boot source).

To keep things a bit modular, we will set our netboot and netargs variables that will contain everything needed to boot from tftp/nfs.

Set up the boot command sequence:

setenv ipaddr 192.168.11.2
setenv serverip 192.168.11.1

setenv netboot 'usb start ; tftp ${kernel_addr_r} zImage ; tftp ${fdt_addr_r} ${fdtfile} ; setenv bootargs ${netargs} ; bootz ${kernel_addr_r} - ${fdt_addr_r}'

setenv netargs 'console=ttyAMA0,115200 root=/dev/nfs nfsroot=192.168.11.1:/home/nfsroot ip=192.168.11.2:::::eth0'

Now, set up bootcmd to run our netboot command:
```
  setenv bootcmd 'run netboot'
```
And now save the environment, so it persists between boots:
```
saveenv
```