Deploying Embedded Linux Systems

Info Box Applies to Linux

Introduction

Deployment of Embedded Linux systems is the typical operation which follows the development phase. When the application is ready and fully tested in the develoment environment, it's time to take the system to the field for the “real work”. This phase brings a lot of concerns to cope with, for example creating a suitable root filesystem, saving the data properly, implement successful on-the-field update strategies. This how-to guide explains how to solve the problems connected to the deployment of an embedded linux system.

The development environment

The following figure illustrates the typical developing environment for an Embedded Linux system: it is composed by a host machine and a target machine.

The host (usually a PC or a virtual machine running the Linux operating system) is used by the developer to (cross-)compile the code that will run on the target, for example a Dave ARM CPU module such as Lizard or Naon. The Linux kernel running on the target is able to mount the root file system from different physical media. During the software development, it is very common to use a directory exported via NFS by the host for this purpose. Moreover, the linux kernel is usually retrieved by a simple network transfer protocol like tftp.

Moving to the field

When the system is ready to move to the field, most of the times the particular link between host and target must be removed. In the worst case, the system must run without any NFS filesystem or file transfer services, relying only on its hardware resources (for example, the on-board ram and flash memories). Resources that are obviously limited, due to the nature of the embedded systems. Generally speaking, the procedure used to deploy the system configuration highly depends on the specific application; however, some topics are quite common. The following sections shine a light on these topics.

Root file systems

Linux needs a root file system: a root file system must contain everything needed to support the Linux system (applications, settings, data, ..). The root file system is the file system that is contained on the same partition on which the root directory is located. The Linux kernel, at the end of its startup stage, mounts the root file system on the configured root device and finally launches the /sbin/init, the first user space process and "father" of all the other processes. An example of root file system is shown below:

drwxr-xr-x  2 root     root     4096 2011-05-03 11:23 bin/
drwxr-xr-x  2 root     root     4096 2011-04-01 17:20 boot/
drwxr-xr-x  3 root     root     4096 2011-07-07 12:17 dev/
drwxr-xr-x 44 root     root     4096 2011-05-03 19:02 etc/
drwxr-xr-x  4 root     root     4096 2011-04-01 17:35 home/
drwxr-xr-x  5 root     root     4096 2011-05-03 11:23 lib/
drwxr-xr-x 12 root     root     4096 2011-07-07 12:03 media/
drwxr-xr-x  6 root     root     4096 2011-05-19 16:39 mnt/
drwxr-xr-x  2 root     root     4096 2011-03-11 05:21 proc/
drwxr-xr-x  2 root     root     4096 2011-05-03 11:23 sbin/
drwxr-xr-x  2 root     root     4096 2011-03-11 05:21 sys/
lrwxrwxrwx  1 root     root        8 2011-05-30 12:19 tmp -> /var/tmp/
drwxr-xr-x 11 root     root     4096 2010-11-05 19:36 usr/
drwxr-xr-x  8 root     root     4096 2010-12-12 11:30 var/