Introduction to Embedded Linux. http://tusharkute.com http://snashlug.org

1. Module-4 Introduction Embedded
Linux
Tushar B Kute
tushar@tusharkute.com
http://snashlug.org
contact@snashlug.org

2. What is Embedded Linux?
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porting the Linux kernel to run on a particular
CPU and board which will be put into an
embedded device.
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There are many companies that sell embedded
Linux solutions.
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These usually include a ported Linux kernel with
cross-development tools, and sometimes with
real time extensions.
●
the APIs and kernel codebase are the same for
embedded Linux as desktop Linux
2

3. Why Embedded Linux?
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Royalty-free
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Strong networking support
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Has already been ported to many different CPU
architectures
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Relatively small for its feature set
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Easy to configure
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Huge application base
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Modern OS (eg. memory management, kernel
modules, etc.)
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4. Distributions
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Commercial:
– MontaVista
– BlueCat Linux
– …
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MMU less
– uCLinux
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Hard Real-time support
– RTLinux
4

5. Embedded Linux System
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Boot loader
– U-boot
– GRUB
– …
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Kernel
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File system
– Many types
5

6. Building system
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Configure and compile Toolchain
– Better idea: use already made
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Configure and make the boot-loader
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Install the boot loader on the target
– Use special hardware or older board boot loader
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Configure and compile the kernel
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Build file system
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7. Development
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Choose C library
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Use the toolchain to create
– User space applications
– Kernel modules (drivers)
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Many free tools
7

8. Free tools
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C library
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Toolchains
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Emulators
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Root file systems
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Graphical toolkits
●
….
8

9. C library
http://www.gnu.org/software/libc/
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License: LGPL
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C library from the GNU project
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Designed for performance, standards compliance and
portability
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Found on all GNU / Linux host systems
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Quite big for small embedded systems: about 1.7 MB
on Familiar Linux iPAQs (libc: 1.2 MB, libm: 500
KB)

10. uClibc
http://www.uclibc.org/ for CodePoet Consulting
License: LGPL
Lightweight C library for small embedded systems, with most
features though.
The whole Debian Woody was ported to it...
You can assume it satisfied most needs!
Size (arm): 4 times smaller than glibc!
uClibc: approx. 400 KB (libuClibc: 300 KB, libm: 55KB)
glibc: approx 1700 KB (libc: 1.2 MB, libm: 500 KB)
Now supported by MontaVista and TimeSys.

11. newlib
http://sources.redhat.com/newlib/
Minimal C library for very small embedded systems
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Lets you remove floating point support wherever you
don't need it. Also provides an integer only
iprintf() function. Much smaller!
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Provides single precision math library functions. Much
faster than the standard IEEE compliant ones.

12. klibc
http://www.kernel.org/pub/linux/libs/klibc/
“Kernel C library”
Tiny and minimalistic C library designed for use in an
initramfs at boot time (alternative to initrds).
Fine for the creation of simple shell scripts.
Not elaborate enough to support BusyBox
applications.

13. uClibc toolchains
Free Electrons uClibc toolchains
http://free-electrons.com/community/tools/uclibc
Run on i386 GNU/Linux
Supported platforms
arm, armeb, i386, m68k, ppc, mips, mipsel, sh

14. Platform specific toolchains
ARM
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Code Sourcery (glibc only, used by many):
http://www.codesourcery.com/gnu_toolchains/arm/
Also available for Solaris and Windows workstations.
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ftp://ftp.handhelds.org/projects/toolchain/ (glibc only)
MIPS
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http://www.linux-mips.org/wiki/Toolchains (useful links)

15. Toolchain building utilities
Buildroot: http://buildroot.uclibc.org/
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Dedicated Makefile to build uClibc based toolchains
and even entire root filesystems.
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Downloads sources and applies patches.
Crosstool: http://www.kegel.com/crosstool/
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Dedicated script to build glibc based toolchains
Doesn’t support uClibc yet.
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Downloads sources and applies patches.

16. Scratchbox
http://scratchbox.org/ - A cross-compiling toolkit project
Makes it easier to cross-compile
a complete embedded Linux system.
Works by allowing tools to be cross-compiled in a
transparent way, making building tools believe they are
doing a native compile job.
Supported platforms: arm, x86
Uses the qemu emulator to transparently run built arm
binaries.
Experimental support for ppc, mips and cris.

17. qemu
http://qemu.org
Fast processor emulator
using a portable dynamic translator.
Full system emulation
Emulates the processor and various peripherals
Supported: x86, x86_64, ppc, arm, sparc, mips
To know which machine types are supported:
qemu­system­arm ­M ?
i386, x86_64 system emulation: now close to native speeds
thanks to the kqemu kernel module (now GPL v2!).

18. ARM emulators
Only Free Software, of course!
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SkyEye: http://skyeye.sourceforge.net
Emulates several ARM platforms (AT91, Xscale...) and
can boot several operating systems (Linux, uClinux, and
others)
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Softgun: http://softgun.sourceforge.net
Virtual ARM system with many virtual on-board
peripherals. Boots Linux.
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SWARM - Software ARM - arm7 emulator
http://www.cl.cam.ac.uk/~mwd24/phd/swarm.html
Can run uClinux

19. Other emulators
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ColdFire emulator
http://www.slicer.ca/coldfire/
Can boot uClinux

20. Minicom
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Definition: serial communication program
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Available in all GNU / Linux distributions
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Capabilities (all through a serial link):
– Serial console to a remote Unix system
– File transfer
– Modem control and dial-up
– Serial port configuration

21. General purpose toolbox: BusyBox
http://www.busybox.net/
Most Unix command line utilities within a single executable!
It even includes a web server!
Sizes less than < 500 KB (statically compiled with uClibc) or less
than 1 MB (statically compiled with glibc).
Easy to configure which features to include.
The best choice for
Initramfs / initrd with complex scripts
Small and medium size embedded systems
See http://www-128.ibm.com/developerworks/linux/library/l-busybox/
for a nice introduction.

22. ssh server and client: dropbear
http://matt.ucc.asn.au/dropbear/dropbear.html
Very small memory footprint ssh server for embedded systems
Satisfies most needs. Both client and server!
Size: 110 KB, statically compiled with uClibc on i386.
(OpenSSH client and server: approx 1200 KB,
dynamically compiled with glibc on i386)
Useful to:
– Get a remote console on the target device
– Copy files to and from the target device (scp or rsync
­e ssh).

23. Benefits of a web server interface
Many network enabled devices can just have a network interface
Examples: modems / routers, IP cameras, printers...
No need to develop drivers and applications for computers
connected to the device. No need to support multiple operating
systems!
Just need to develop static or dynamic HTML pages
(possibly with powerful client-side JavaScript).
Easy way of providing access to device information and
parameters.
Reduced hardware costs (no LCD, very little storage space
needed)

24. Linux porting projects
Useful to find patches, binaries, documentation,
toolchains...
Only ports for embedded systems are listed
arm: http://www.arm.linux.org.uk/
See also http://www.linux-arm.org/ (from ARM Limited)
m68k: http://www.linux-m68k.org/
mips: http://www.linux-mips.org/
ppc: http://penguinppc.org/embedded/
sh: http://linuxsh.sourceforge.net/
xtensa: http://xtensa.sourceforge.net/

25. Useful web sites
LinuxDevices.com: http://linuxdevices.com
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Weekly newsletter with news and announcements
about embedded devices running Linux.
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Articles, whitepapers, and Linux embedded devices
catalog.
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An excellent site to follow industry news!

26. Example:
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Download CodeSourcery's toolchain installer for
GNU/Linux target for IA32 host
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Install it: sh arm-2008q3-72-arm-none-linux-gnueabi.bin
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The toolchain provides the cross compiler
arm-none-linux-gnueabi-gcc. You need to put it's
directory in your $PATH. Once you have the toolchain,
you can easily compile your hello world program:
arm-none-linux-gnueabi-gcc -o hello -static hello.c
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Copy the binary to your phone and run it from an adb
shell prompt: ./hello

27. Installation
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sudo apt-get install gcc-arm-linux-gnueabi
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sudo apt-get install android-tools-adb

28. Engineers are not users, they are creators!

29. This presentation is created using LibreOffice Writer 4.1.0.4 available freely
under GNU public license.
Thank you