init.sh

#!/busybox sh

# The SD card will contain the kernel image with initramfs (boot.img) and the
# rootfs image (rootfs.cramfs). In order to mount the rootfs, the kernel is
# booted with this init script first, and the init script performs the mounting
# and switching.
#
# Since the rootfs image is on the SD card initially, we first need to mount
# the SD card itself. After SD card is mounted, the rootfs image is mounted.
# Since rootfs is read-only, we must also mount the tmpfs and make a unionfs
# mount that creates a writable overlay over the read-only rootfs using the
# tmpfs.
#
# When all filesystems are mounted, we move the mount points to appropriate
# locations within the new rootfs, and perform switch_root to make the new
# rootfs active. The init script from the new rootfs is then executed.

MOUNT="/busybox mount"
MOVE="$MOUNT --move"
MKDIR="/busybox mkdir"
LOSETUP="/busybox losetup"
SWITCH_ROOT="/busybox switch_root"
SH="/busybox sh"
SLEEP="/busybox sleep"

# Mount devtmpfs for nodes
$MOUNT -t devtmpfs devtmpfs /dev

# Setup console
exec 0</dev/console
exec 1>/dev/console
exec 2>/dev/console

echo "========================================================================"
echo "Initramfs booting..."
echo "========================================================================"

$MKDIR -p /rootfs /tmpfs /root /sdcard

# Wait for SDCARD device to appear
while ! [ -e /dev/mmcblk0p1 ]
do
    $SLEEP 3s
done

# Mount the SD card
$MOUNT -t vfat -o utf8,uid=1000 /dev/mmcblk0p1 /sdcard

# Mount the cramfs volume
$LOSETUP /dev/loop0 /sdcard/rootfs.cramfs
$MOUNT -t cramfs /dev/loop0 /rootfs

# Create a tmpfs to hold transient data
$MOUNT -t tmpfs -o size=80m tmpfs /tmpfs

# Create a unionfs mount
$MOUNT -t unionfs -o dirs=/tmpfs=rw:/rootfs=ro unionfs /root

# Move mount points to new root
$MKDIR /root/mnt/tmpfs /root/mnt/rootfs /root/mnt/sdcard
$MOVE /dev /root/dev
$MOVE /tmpfs /root/mnt/tmpfs
$MOVE /rootfs /root/mnt/rootfs
$MOVE /rootfs /root/mnt/sdcard

# Switch to new root
exec $SWITCH_ROOT /root /sbin/init

# If the switch root failed, drop to a shell
exec $SH