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Building Embedded Linux - Full Custom

Debian Root File System

Summary

There are several flavours of rootfs to choose from. This one focuses on the Debian rootfs. This will give you an environment similar to the Rasbperry Pi OS for your DE10-Nano. There is a small note in the Appendix about BuildRoot and Yocto if you're interested in the differences.

Debootstrap and QEMU

Debootstrap is a utility that makes it easy to create a rootfs on an existing Debian based machine. However, since our host machine is most likely x86_64 and we are targeting an armhf architecture for the DE10-Nano, we will need to install an emulator from the QEMU project. To install both on your host OS:

sudo apt install debootstrap qemu-user-static

First Stage

In the first stage, we will create a directory to hold the rootfs. Note that almost all the commands in this part will be done as root using sudo so take care not to make any mistakes.

cd $DEWD
mkdir rootfs

# buster is the latest debian version at the time of writing.
# Replace it with whatever is the latest.
sudo debootstrap --arch=armhf --foreign buster rootfs

Second Stage

First, we have to copy over qemu to the target file system and chroot to target. Without copying over qemu we cannot chroot.

cd $DEWD
sudo cp /usr/bin/qemu-arm-static rootfs/usr/bin/

Now we should be able to chroot:

sudo chroot rootfs /usr/bin/qemu-arm-static /bin/bash -i

Once in the chroot-ed environment, we can kick off the second stage of debootstrap:

/debootstrap/debootstrap --second-stage

This will take about 5 minutes to complete.

NOTE - Manjaro/Arch

If your development machine is running Manjaro/Arch linux then the following commands are needed:

# Install debootstrap.
sudo pacman -S debootstrap

# Install qemu-arm-static. Note that qemu-arm won't work and we need
# qemu-arm-static.
cd /tmp
git clone https://aur.archlinux.org/qemu-arm-static.git
cd qemu-arm-static
makepkg -si

Also, when creating the rootfs directory, it needs to be done from a folder which is owned by root:

sudo mkdir -p /var/tmp/rootfs
cd /var/tmp

And proceed with the same steps as above to create the rootfs.

Configuration

While still in the chroot environment, let's do some setup so that our rootfs is more convenient to use.

  • Ugh, where's vim? - Vim is my preferred editor, so I'll install it first before I do anything else. If you're comfortable using nano as the editor, then you can skip this.

    apt install vim -y

  • Hostname - Change the name to be different from the current host distro in /etc/hostname. I call mine de10-nano.

  • Root password - Set the root password so it is not blank. I just set it to 'root'.

    passwd

  • fstab - Let's update fstab so that the system auto mounts the drives. Copy the following lines as is into /etc/fstab:

    none		/tmp	tmpfs	defaults,noatime,mode=1777	0	0
/dev/mmcblk0p2	/	ext4	defaults	0	1

  • Enable the serial console - This allows you to see all the messages at boot time without having to ssh into the device using a simple serial console (Putty, minicom etc)

    systemctl enable serial-getty@ttyS0.service

  • Locales - Configure and install the locales you will need. For me, this is just en_US.UTF-8:

    apt install locales -y
dpkg-reconfigure locales

  • Ethernet - To get the ethernet on the DE10-Nano working, we need to add the following to the file/etc/network/interfaces under the line that says source-directory /etc/network/interfaces.d. This will enable DHCP. A note for folks attempting this with a more recent version of Linux. You will need to replace 'eth0' with 'end0' in the code below, or the systemd networking.service will fail on boot. This is a result of the change to the Predictable Interface Names naming scheme.

    auto lo eth0
iface lo inet loopback

allow-hotplug eth0
iface eth0 inet dhcp

  • Sources.list - Use a more complete apt sources.list. Edit the file /etc/apt/sources.list and add the following. Replace buster with whatever version of debian you are using:

    deb http://deb.debian.org/debian/ buster main contrib non-free
deb-src http://deb.debian.org/debian/ buster main contrib non-free
deb http://deb.debian.org/debian/ buster-updates main contrib non-free
deb-src http://deb.debian.org/debian/ buster-updates main contrib non-free
deb http://deb.debian.org/debian-security/ buster/updates main contrib non-free
deb-src http://deb.debian.org/debian-security/ buster/updates main contrib non-free

  • Openssh-Server - Install openssh-server so that you can ssh into the device:

    apt install openssh-server -y

  • (Optional) Root login over ssh - If you want to ssh as root, add/uncomment the following line in /etc/ssh/sshd_config:

    PermitRootLogin yes

  • (Optional) Add a user - TODO - add a user and enable sudo.

  • PRNG entropy seeding speedups - This speeds up the ssh server startup time on debian buster. See here for more details.

    apt install haveged -y

  • Install any other packages - You can install any other packages you need as well:

    apt install net-tools build-essential device-tree-compiler -y

    Explanation:

    • net-tools makes the ifconfig command available.
    • build-essential install gcc and allows you to compile programs on the DE10-Nano.
    • device-tree-compiler is needed to compile the device tree when flashing the FPGA directly from the HPS.

(Optional) Setup WIFI

You can also do the necessary steps for setting up WIFI as detailed here. This is optional and can be done later on also if you can connect your de10-nano to the LAN.

Clean up

Run the following for some basic clean up:

# Remove cached packages.
apt clean

# Remove QEMU.
rm /usr/bin/qemu-arm-static

# Exit from the chroot.
exit

Create a tarball

The last step is to create a tarball that we will extract into our SD Card partition:

cd $DEWD
cd rootfs

# Don't forget the dot at the end.
# Also has to be run as root.
sudo tar -cjpf $DEWD/rootfs.tar.bz2 .

# One level up to see the file.
cd ..

And that completes the Debian rootfs. Be careful when extracting this as it will extract everything within the same directory (without creating another directory).

References

Quick and easy bootstrap of Debian for armhf

Debootstrap - Linux-Sunxi

Appendix

BuildRoot and Yocto

BuildRoot and Yocto are two of the most popular embedded linux generating platforms out there. No doubt you would have heard of them. They are mature and well supported and used by many device manufacturers globally. However, in this guide, I chose to go with Debian instead of either of these. And the reason for that is the target audience. Both BuildRoot and Yocto are designed to build a highly customised and restricted linux OS that the manufacturer wants to have on their device (smartwatch, router, microwave etc). And some of the benefits are:

  • Optimized for tiny footprint
  • Packages installed are restricted
  • Well supported steps for firmware updates

I wanted a general purpose OS for my DE10-Nano which is why I chose to go with Debian. But if you're doing this for work, chances are you're pretty much looking for BuildRoot or Yocto.

Next | Creating the SD Card Image
Back | RootFS - Choose one:

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