Create a Debian 8 filesystem image for OpenWRT

Linkit Smart 7688 board can be used as a MIPS-based Linux workstation, for generic development tasks. To get access to a console port, you can use any USB-to-serial adapter.

From a user point of view, an OpenWRT environment is quite different from Linux. But it's possible to build a Debian-based environment, located on microSD card and used via chroot.

• No modification to OpenWRT system is required
• All additional files placed on SD card
• Full traditional Linux environment is provided

To make a minimal root filesystem, you need a live MIPS Debian system. I had used Creator CI20 board. First you need to install a multistrap utility:

sudo apt-get install multistrap gpgv

Create a file multistrap.conf and put the following lines to it:

[General]
    arch=mipsel
    cleanup=true
    noauth=true
    unpack=true
    debootstrap=Debian
    aptsources=Debian

[Debian]
    packages=apt
    source=http://http.debian.net/debian
    keyring=debian-archive-keyring
    suite=jessie

Fetch a public key for Debian 8/jessie:

gpg --keyserver pgpkeys.mit.edu --recv-key 8B48AD6246925553
gpg -a --export 8B48AD6246925553 | sudo apt-key add -
gpg --keyserver pgpkeys.mit.edu --recv-key 7638D0442B90D010
gpg -a --export 7638D0442B90D010 | sudo apt-key add -
gpg --keyserver pgpkeys.mit.edu --recv-key CBF8D6FD518E17E1
gpg -a --export CBF8D6FD518E17E1 | sudo apt-key add -

Run multistrap utiltity to build a directory debian/ with a minimal set of installed packages:

sudo multistrap -a mipsel -d debian -f multistrap.conf

Put ftp.us.debian.org public key to a destination directory:

sudo rm debian/etc/apt/trusted.gpg.d/*
sudo cp /etc/apt/trusted.gpg debian/etc/apt

Add a default Google DNS resolver:

sudo bash -c "echo nameserver 8.8.8.8 > debian/etc/resolv.conf"

Fix a list of mounted filesystems:

sudo bash -c "ln -s /proc/mounts debian/etc/mtab"

Edit file debian/root/.bashrc and add the following lines to it:

export PATH=/sbin:/bin:/usr/sbin:/usr/bin
export TERM=xterm
unset HOSTNAME SHELL LOOP_MOUNTPOINT ASEC_MOUNTPOINT
unset ANDROID_PROPERTY_WORKSPACE ANDROID_ASSETS
unset ANDROID_BOOTLOGO LD_LIBRARY_PATH BOOTCLASSPATH
unset ANDROID_DATA MKSH ANDROID_ROOT EXTERNAL_STORAGE

Check that a minimal filesystem works as expected:

sudo chroot debian /bin/bash    -- start a shell in a new root filesystem
apt-get update                  -- check network access to Debian repository
apt-get clean
exit                            -- exit from chroot shell

Pack the root directory into a single tarball:

(cd debian; sudo tar cvzf ../debian.tgz .)
sudo chown $USER debian.tgz

The size of resulting file will be about 70 Mbytes. A resulting image is available for download here: ftp://ftp.vak.ru/unix/debian-jessie-mini.tgz

Create an empty Linux partition on SD card. Typically, a clean FS card contains a single partition of type FAT32. Use fdisk to change the partition type to Linux and create empty filesystem:

sudo umount /dev/mmcblk0p1
sudo sfdisk /dev/mmcblk0 << EOF
/dev/mmcblk0p2 : size=512M, type=82
/dev/mmcblk0p1 : type=83, bootable
EOF
sudo mke2fs /dev/mmcblk0p1

Install the debian package into the SD card:

sudo mount /dev/mmcblk0p1 /mnt
sudo tar xvzf debian.tgz -C /mnt
sudo umount /mnt

Build Linkit firmware with FPU and ext4 support

Follow instructions on page: https://github.com/MediaTek-Labs/linkit-smart-7688-feed

When doing menuconfig, enter Kernel Modules / Filesystems and enable kmod-fs-ext4 module. It makes sense to enable kmod-fuse as well, as it might come handy.

After build finished (it takes a few hours), copy the resulting image bin/ramips/openwrt-ramips-mt7688-LinkIt7688-squashfs-sysupgrade.bin to file lks7688.img on a microSD Flash drive. Plug the drive into the board, and reboot the board. Hold WiFi button for 5 seconds then release. Reflash process takes a few minutes to complete.

Start Debian on Linkit Smart 7688 board

Connect your board to Wi-Fi network:

uci set wireless.sta.ssid=IMG-Staff-BYOD
uci set wireless.sta.encryption=psk2
uci set wireless.sta.key=xxxxxxxx
uci set wireless.sta.disabled=0
uci commit
wifi

To start a shell in a new Debian environment, use this script:

mount /dev/mmcblk0p1 /mnt
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
HOME=/root chroot /mnt /bin/bash

You can install any needed packages, using a standard utility apt-get:

apt-get install vim build-essential git cvs subversion openssh-client
apt-get install dialog iputils-ping autoconf automake libtool gettext
apt-get install flex bison byacc gdb locate procps

To unmount, use the script:

umount /mnt/dev
umount /mnt/proc
umount /mnt/sys
umount /mnt

Enjoy!