The Embedded Linux Quick Start Guide

Kernel and user space
Chris Simmonds
Embedded Linux Conference Europe 2010
Copyright © 2010, 2net Limited

Embedded Linux Quick Start Guide
Kernel and user space

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Third element: kernel
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Version numbers

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About “BSPs”

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Configuring and cross compiling

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Booting

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Kernel and user space

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Kernel vs user space
Application code
Your program

User space
C library

System interface
Linux kernel

Generic services

Kernel space

Device drivers

Hardware
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Kernel version numbers
Example: 2.6.35.1
2: very unlikely to change
6: unlikely to change
2.6.0 released in December 2003
35: changes with each release,
every 12 weeks or so
1: bug fix number: changes every time
a bug is fixed, sometimes several times
per week

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Bug fix releases
●

●

●

●

Maintained by Greg Kroah-Hartman
Serious bugs are fixed in the current stable
version immediately
Sometimes older versions are fixed as well
Special note: the 2.6.27 and 2.6.32 stable
kernels maintained by Adrian Bunk
●

Current releases (October 2010)
–

2.6.27.54

–

2.6.32.24

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Board Support Packages
●

Mainline kernel works out-of-the-box for a
number of development boards
●

●

●

e.g. Beagleboard

But in most cases you will need a BSP from the
board or chip vendor
●

Lags mainline by a few versions

●

Levels of support vary between vendors

For custom boards you will have to write your
own BSP

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Levels of board support
●

Architecture
●

●

Chip (also known as System on Chip, SoC)
●

●

arm,mips, powerpc, x86,...
Atmel 91sam9, Freescale i.MX, TI OMAP, ...

Board
●

SoC manufacturer evaluation boards
–

●

Freescale Babbage, TI EVM, ...

COTS boards
–

Digi, Eurotech, ...

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Levels of board support (cont.)
●

Chip level support mostly done by manufacturer
●

●

often in own kernel tree: e.g. Freescale

Board level support done by board
manufacturer
●

based on SoC kernel

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Board support
●

Usually a kernel patch and a configuration file

●

Typical procedure is
tar xjf linux-2.6.34.tar.bz2
cd linux-2.6.34
patch -p 1 < ../linux-2.6.34-some_bsp.patch
cp ../some_bsp-kernel.config .config
make oldconfig

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Kernel modules
●

●

●

Kernel code that is loaded after the kernel has
booted
Advantages
●

Load drivers on demand (e.g. for USB devices)

●

Load drivers later – speed up initial boot

Disadvantages
●

Adds kernel version dependency to root file system

●

More files to manage

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Kernel configuration
●

Typical kernel has >> 1000 configuration options

●

Default configuration part of the BSP

●

Tweak configuration using

●

●

make menuconfig (ncurses text menu)

●

make xconfig (graphical menus using Qt)

●

make gconfig (graphical menus using Gtk+)

Files generated
●

.config

●

include/linux/autoconf.h

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Building the kernel
●

Set CROSS_COMPILE and ARCH
export ARCH=arm
export CROSS_COMPILE=arm-angstrom-linux-gnueabi-

●

●

Make targets
●

zImage - compressed kernel image

●

uImage - zImage plus U-Boot header

Files generated
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vmlinux

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arch/arm/boot/zImage

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arch/arm/boot/uImage

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Kernel command line
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Kernel behaviour set by “command line”
●

●

see Documentation/kernel-parameters.txt

Some examples
console: device to send kernel messages to, e.g.
console=ttyS0,115200

root: set device to load root file system from, e.g.
root=/dev/sda1

quiet: output fewer console messages
debug: output all console messages
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Fourth element: user space
●

What is user space?

●

Obtaining a root file system

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Busybox

●

Two types of init: Busybox and System V

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Managing device nodes: udev

●

Mounting a root file system over the network
and from flash memory

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What is user space?
●

●

A sane (POSIX) environment for applications
(unlike the kernel)
The main components are
●

Programs – e.g. init and a shell

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Libraries - e.g. libc

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Configuration files in /etc

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Device nodes in /dev

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User data in /home

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The root file system
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Mounted by the kernel during boot
●

●

requires a root=... kernel command line

Loaded from:
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ram disk (initramfs)

●

storage device: flash, SD, hard disk

●

network: nfs

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“I got a rootfs with my board”
●

●

As with the toolchain, this is usually a trap!
Board vendors usually over-configure to show
off the board
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bloated root file system

●

slow boot

●

… yet, they only offer a limited set of packages

●

and limited or no update service

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Other options for a root file system
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Roll-Your-Own (RYO)

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Use an integrated build tool

●

●

Buildroot

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OpenEmbedded

Use a binary distro
●

Ångström

●

Ubuntu or Debian

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Busybox
●

Web - http://www.busybox.net

●

Very common in embedded systems

●

Single binary that masquerades as many Linux
utilities, including
●

init

●

ash (a Bourne shell)

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file system utilities: mount, umount,...

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network utilities: ifconfig, route,...

●

and of course, the vi editor

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Busybox example
# ls -l /bin
lrwxrwxrwx 1
lrwxrwxrwx 1
lrwxrwxrwx 1
-rwxr-xr-x 1
lrwxrwxrwx 1
...

root
root
root
root
root

root
root
root
root
root

7
7
7
744480
7

2008-08-06
2008-08-06
2008-08-06
2008-05-16
2008-08-06

11:44
11:44
11:44
15:46
11:44

addgroup -> busybox
adduser -> busybox
ash -> busybox
busybox
cat -> busybox

So when you type (for example)
cat /etc/inittab

… launches /bin/busybox with

argv [0] = “/bin/cat”

Busybox main() parses argv[0] and jumps to cat applet

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init
●

/sbin/init is the first program to be run
●

●

change by setting kernel parameter “init=...”

Two common versions of init
●

Busybox init
–

●

e.g. by buildroot

System V init
–

e.g. by Angstrom

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Busybox init
●

Begins by reading /etc/inittab, for example:
/etc/inittab
::sysinit:/etc/init.d/rcS
::respawn:-/sbin/getty -L ttyS0 115200 vt100
::ctrlaltdel:/sbin/reboot
::shutdown:/bin/umount -a -r
::restart:/sbin/init

/etc/init.d/rcS
#! /bin/sh
echo "Starting rcS"
mount -t proc proc /proc
mount -t sysfs sysfs /sys
ifconfig lo 127.0.0.1
ifconfig eth0 192.168.1.101

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System V init
●

Also begins by reading /etc/inittab
●

●

More complex format than Busybox

System V runlevels
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A runlevel defines a system state
–

0 is halt

–

1 is single user

–

2-5 are multi-user

–

6 is reboot

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System V inittab
Format:
id:runlevels:action:process

Default runlevel = 5

id:5:initdefault:
si::sysinit:/etc/init.d/rcS

Boot script = /etc/init.d/rcS

~~:S:wait:/sbin/sulogin

Single-user mode: add 'S' to
kernel command line

l0:0:wait:/etc/init.d/rc
l1:1:wait:/etc/init.d/rc
l2:2:wait:/etc/init.d/rc
l3:3:wait:/etc/init.d/rc
l4:4:wait:/etc/init.d/rc
l5:5:wait:/etc/init.d/rc
l6:6:wait:/etc/init.d/rc

0
1
2
3
4
5
6

z6:6:respawn:/sbin/sulogin
S:2345:respawn:/sbin/getty 38400 ttyS1

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Scripts for each runlevel

Launch a login on the
console

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Initialisation scripts
●

Each service is controlled by a script in /etc/init.d:

# ls /etc/init.d
alignment.sh
banner
bootmisc.sh
checkroot
devpts.sh
dropbear
finish.sh
functions
halt
hostname.sh
hwclock.sh
●

modutils.sh
mountall.sh
mountnfs.sh
networking
populate-volatile.sh
ramdisk
rc
rcS
reboot
rmnologin
save-rtc.sh

sendsigs
single
sysfs.sh
syslog
syslog.busybox
udev
udev-cache
umountfs
umountnfs.sh
urandom

Most take parameters start and stop, e.g.
/etc/init.d/syslog stop

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/dev: device nodes
●

Most hardware appears as nodes in /dev

●

Create by hand:
mknod /dev/ttyS0 c 4 64

●

Or, use a dynamic device manager (udev)

●

udev pros
●

●

less hassle; handles removable devices (e.g. USB)

udev cons
●

slow

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The rootfs during development
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Advantages of mounting rootfs over NFS
●

easy to access and modify the rootfs

●

No limit on size

Step 1. Export a directory on the development host with a line like this
in /etc/exports
/home/chris/rootdir *(rw,sync,no_subtree_check,no_root_squash)

Step 2. Set kernel parameters
root=/dev/nfs rw nfsroot=192.168.1.1:/home/chris/rootdir ip=192.168.1.101

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The rootfs in production
●

Usually stored in a partition of flash memory
Flash file system:
jffs2, yaffs2, ubifs
MTD
(Memory Technology Devices)
Flash driver
Flash chip(s)
Typical kernel parameters:
root=/dev/mtdblock1 rootfstype=jffs2

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Flash file systems
●

●

●

jffs2 (Journalling Flash File System 2)
●

This is the most common Linux flash fs

●

Robust, but slow (especially mount time)

yaffs2 (Yet Another Flash File System 2)
●

Optimised for NAND flash memory

●

Not part of main-line kernel

ubifs (Unsorted Block Image File System)
●

Fast and robust

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Summary
●

●

Kernel
●

Your choice of kernel is limited by BSP

●

Many build-time kernel configuration options

●

Boot-time configuration via command line

User space
●

Starts when kernel mounts rootfs

●

First program to run is (default) /sbin/init

●

Both Busybox init and System V init are common

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