// -*- mode:doc; -*- // vim: set syntax=asciidoc: Running OpenADK created Linux firmware ====================================== Bootloader ~~~~~~~~~~~ The Bootloader is used to initialize the machine and load the Linux kernel. A list of popular Bootloaders can be found on http://elinux.org/Bootloader. OpenADK provides the Bootloader if necessary for a target system. You can find them in +make menuconfg+ under +Packages/Bootloader+. Some Bootloaders require the Linux kernel in a special format (SREC, ELF, ..), compressed or with a special header. This will be automatically done by OpenADK in +target//Makefile+ while creating the firmware archives or images. Linux kernel ~~~~~~~~~~~~ The kernel is a program that constitutes the central core of a computer operating system. It has complete control over everything that occurs in the system. The Bootloader can provide some basic runtime configuration parameters via the kernel commandline feature. The Linux kernel in OpenADK is intended to be very small in size and will be by default compressed with xz compression algorithm, if available for the target system. You can configure the compression algorithm used for the compression of the Linux kernel and if choosen the initramfs filesystem in +make menuconfig+. In +Kernel configuration+ you have the choice between dfferent kernel versions. The latest version will be automatically used. There you can choose any needed addon drivers or any supported runtime and debugging features. The kernel expands itself on boot, if compressed, and then initialize the hardware. The additional kernel modules are loaded later by a init script. The kernel will autoamtically mount the virtual filesystem /dev as devtmpfs and then will execute +/sbin/init+ in userspace. init system ~~~~~~~~~~~ The _init_ program is the first userspace program started by the kernel (it carries the PID number 1), and is responsible for starting the userspace services and programs (for example: web server, graphical applications, other network servers, etc.). OpenADK uses *Busybox* init. Amongst many programs, Busybox has an implementation of a basic +init+ program, which is sufficient for most embedded systems. The Busybox +init+ program will read the +/etc/inittab+ file at boot to know what to do. The syntax of this file can be found in http://git.busybox.net/busybox/tree/examples/inittab (note that Busybox +inittab+ syntax is special: do not use a random +inittab+ documentation from the Internet to learn about Busybox +inittab+). The default +inittab+ in OpenADK is generated while producing the +base-files+ package. The main job the default inittab does is to start the +/etc/init.d/rcS+ shell script, and start one or more +getty+ programs (which provides a login prompt). /dev management ~~~~~~~~~~~~~~~ On a Linux system, the +/dev+ directory contains special files, called _device files_, that allow userspace applications to access the hardware devices managed by the Linux kernel. Without these _device files_, your userspace applications would not be able to use the hardware devices, even if they are properly recognized by the Linux kernel. OpenADK uses *dynamic device nodes using devtmpfs and mdev*. This method relies on the _devtmpfs_ virtual filesystem in the kernel, which is enabled by default for all OpenADK generated kernels, and adds the +mdev+ userspace utility on top of it. +mdev+ is a program part of Busybox that the kernel will call every time a device is added or removed. Thanks to the +/etc/mdev.conf+ configuration file, +mdev+ can be configured to for example, set specific permissions or ownership on a device file, call a script or application whenever a device appears or disappear, etc. Basically, it allows _userspace_ to react on device addition and removal events. +mdev+ is also important if you have devices that require a firmware, as it will be responsible for pushing the firmware contents to the kernel. +mdev+ is a lightweight implementation (with fewer features) of +udev+. For more details about +mdev+ and the syntax of its configuration file, see http://git.busybox.net/busybox/tree/docs/mdev.txt. initscripts ~~~~~~~~~~~ The /etc/init.d/rcS script will execute all shell scripts in /etc/init.d in order with the parameter +autostart+. The order is identified by the +#INIT+ comment in the script. All scripts are sourcing the +/etc/rc.conf+ file to determine if a service should be started on boot and which flags if any are used for the service. By default all services are disabled. If the variable for a service is set to "DAEMON" and mksh is installed, the service starts asynchronously in the background. Most scripts provided by OpenADK via +package//files/.init+ are like: --------------------- #!/bin/sh #PKG foo #INIT 60 . /etc/rc.conf case $1 in autostop) ;; autostart) test x"${foo:-NO}" = x"NO" && exit 0 test x"$foo" = x"DAEMON" && test -x /bin/mksh && exec mksh -T- $0 start exec sh $0 start ;; start) /usr/sbin/foo $foo_flags ;; stop) kill $(pgrep -f /usr/sbin/foo ) ;; restart) sh $0 stop sh $0 start ;; *) echo "usage: $0 (start|stop|restart)" exit 1 esac exit $? --------------------- cfgfs - configuration file system ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The cfgfs application for the OpenADK system uses a special small partition on the block device of your embedded system (f.e. flash, sd card, compact flash or hard disk). Only changes made to /etc on your embedded system are saved in a compressed form (using LZO1 compression algorithm) in this partition. There is no Linux filesystem on this partition. The embedded system initialization process will setup /etc correctly on boot up, when cfgfs application is found. After making any changes to /etc, which should survive a reboot of the embedded system must be written to the cfgfs partition via “cfgfs commit”. Trying to reboot, shutdown or halt an embedded system with unsaved changes will generate an error, which can be circumvented. Updates to /etc via the ipkg package manager will be reported. --------------------- cfgfs Configuration Filesystem Utility (cfgfs), Version 1.09 Syntax: /sbin/cfgfs commit [-f] /sbin/cfgfs erase /sbin/cfgfs setup [-N] /sbin/cfgfs status [-rq] /sbin/cfgfs { dump | restore } [] --------------------- network configuration ~~~~~~~~~~~~~~~~~~~~~ On bootup +/etc/network/interfaces+ is used to find out which network configuration should be used. The default is to use DHCP (via busybox +udhcpc+) on the first found ethernet device to configure the network. See network configuration for detailed syntax of +/etc/network/interfaces+. It is similar to Debian network configuration and uses +ifupdown+ from +busybox+. See Appendix xref:network-configuration[] getting a shell on the system ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There are two method available to get a shell on your embedded system created with OpenADK. You can either login locally via serial console or graphical console or you can login remotely via secure shell. In both cases the default user is +root+ and the default password is +linux123+. *You should always change the default password!!* You can do this either via +passwd+ on the system or you can preconfigure a password via +make menuconfig+ under +Runtime configuration+. The default shell used in OpenADK is +mksh+ from http://www.mirbsd.org/mksh/. You can change the shell in +make menuconfig+ under +Runtime configuration+. Be aware of the fact that the bootup process might use some +mksh+ features to speedup the system start. When you change the shell for system +/bin/sh+ the slower startup is used as a fallback.