How to do it...

We now have the boot binaries and root filesystem ready for network booting, and we need to configure U-Boot to perform the network boot.

Boot the Wandboard and stop at the U-Boot prompt by pressing any key on the serial console. Make sure it has an Ethernet cable plugged in and connected to your local network. You should see the U-Boot banner and prompt as follows:

The Yocto 2.4 version of U-Boot for the Wandboard has introduced changes in the default environment so that there is less platform-specific customization made in the source. As such, previous versions used to have a default environment ready to perform a network boot just by setting a few environmental variables and running the netboot script.

The current U-Boot has instead replaced it with a network boot mechanism that looks for a U-Boot script called extlinux.conf on the configured TFTP server and executes it. In that way, platform-specific booting options are isolated into the boot script which is compiled with the U-Boot source.

The Yocto Project prepares an extlinux.conf boot script and copies it to the deploy directory along with the images. We can add kernel command line arguments to pass to the Linux kernel in this boot script by using the UBOOT_EXTLINUX_KERNEL_ARGS configuration variable. More details about customizing the extlinux.conf script is provided in Chapter 2, The BSP Layer.

However, for development purposes, it is more flexible to restore the previous network boot environment variables:

> env set netload 'tftpboot ${loadaddr} ${image};tftpboot ${fdt_addr} ${fdt_file}'
> env set netargs 'setenv bootargs console=${console} ${optargs} root=/dev/nfs ip=${ipaddr} nfsroot=${serverip}:${nfsroot},v3,tcp'
> env set netboot 'echo Booting from net ...;run netargs;run netload;bootz ${loadaddr} - ${fdt_addr}'  

The netload script loads the Linux kernel binary and the device tree blob into memory. The netargs script prepares the bootargs environmental variable to pass the correct kernel command line parameters for a network boot, and the netboot command executes the network boot by running netargs and using the bootz command.

Now we will prepare the rest of the environmental variables it needs:

1. Configure a static IP address with:

> env set ipaddr <static_ip>

1. Configure the IP address of your host system, where the TFTP and NFS servers have been set up:

> env set serverip <host_ip>

1. Configure the root filesystem mount:

> env set nfsroot /nfsroot

1. Configure the Linux kernel and device tree filenames:

> env set image zImage-wandboard.bin
> env set fdt_file zImage-imx6qp-wandboard-revd1.dtb

1. Save the U-Boot environment to the microSD card:

> env save

1. Perform a network boot:

> run netboot

The Linux kernel and device tree will be fetched from the TFTP server, and the root filesystem will be mounted by the kernel from the NFS share.

You should be able to log in with the root user without a password prompt.

Once booted, we can find out the kernel command line arguments used to boot by doing:

$ cat /proc/cmdline
console=ttymxc0,115200 root=/dev/nfs ip=192.168.1.15 nfsroot=192.168.1.115:/nfsroot,v3,tcp