1. Embedded Linux @ Information Technology Institute 1
Embedded Linux
From Scratch To Yocto
Information Technology Institute
Sherif Mousa
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Linux
●
Unix (Ken Thompson & Dennis Ritchie) AT&T
company 1969
●
FSF & GNU (Gnu is Not Unix) & GPL (Richard
Stallman) 1984
●
Hurd Kernel
●
Linux (LinusTorvalds) 1991
●
GNU/Linux 1992 (complete, open source, UNIX-
like operating system)
– https://www.gnu.org/gnu/linux-and-gnu.html
–
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Embedded Linux
●
Embedded Linux is the use of Linux in
embedded computer systems.
●
Adapting the Linux kernel and customizing the
user-land libraries and utilities to embedded
applications such as those in use in consumer
electronics, military, medical, industrial,
network, and auto industries.
●
Creating an Embedded Linux based system is
like a puzzle. Putting the right pieces together
will create the final image.
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Some Whys ...
●
Why OS for Embedded ?
– Make use of micro-processor capabilities (Multi-Tasking …).
– Easy to program.
– System scalability.
●
Why Embedded not Normal OS ?
– Systems with small and limited resources.
– Special-Purpose systems.
– Real-Time systems.
●
Why Linux ?
– Inexpensive, robust, easy to program, open source.
– Ported to a variety of CPU architectures.
– Large device drivers coverage.
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Why build from scratch ?
●
“When you bake it at the factory that’s what it
does forever. “
Tim Bird, Sony Entertainment
●
You can use ready-made distributions as well,
but you will face some restrictions if you need
to modify something.
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Some Facts ...
●
Is it easy?
– NO, IT’S NOT EASY.
– To finish a complete OS, it’s not just Linux kernel to
compile, you need to build and integrate the other OS
components (BootLoader, Libraries, Commands, …….)
– And you will do all of this without getting a PhD degree
in OS building :)
●
Do I need more knowledge ?
– If you are a Linux super user, and you think
that you need to learn more things to enter
the Embedded Linux field, THAT’S WRONG.
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Embedded Linux Development
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Some definitions
●
Cross Compiler
– A cross compiler is a compiler capable of creating
executable code for a platform other than the one on
which the compiler is running.
●
Toolchain
– Set of programming tools that are used to create a
product (typically another computer program), used in
chain.
– A simple software development ToolChain consists of a
compiler and linker to transform the source code into an
executable program, libraries to provide interfaces to
the operating system, and a debugger.
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Some definitions (cont'd)
●
Cross-Toolchain
– The usual development tools available on a
GNU/Linux workstation is a native ToolChain.
– For embedded systems, it is usually impossible or
not interesting to use a ToolChain installed on it
●
The target is restricted in terms of storage and memory
●
The target is very slow compared to your workstation
– Therefore, Cross-ToolChain is used. Runs on your
workstation but generates binaries for your target.
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Cross-Toolchain
X86 BinaryX86 Binary Target BinaryTarget Binary
X86
X86 Target CPU
Source CodeSource Code
Native ToolChainNative ToolChain Cross ToolChainCross ToolChain
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Embedded Linux System Components
●
Cross-compilation ToolChain
– Compiler, debugger, libraries, headers, extra tools.
●
Boot Loader (grub, lilo, uboot)
– Started by the hardware.
– Responsible for basic initialization, loading and executing the kernel.
●
Linux Kernel (Normal or Real-Time)
– Contains the process and memory management, network stack, device drivers, and
many other core OS responsibilities, and provides services to userspace applications.
●
C Library (glibc, uclibc, ….)
– The interface between the kernel and the userspace applications.
●
User Land
– Configuration files
– Common system commands
– User application(s)
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Linux System Running
HardwareHardware
Linux KernelLinux Kernel
User
Land
C LibraryC Library
app Aapp A app Bapp B
Call
a service
Manage
Hardware
Event
Handler
Event
notification
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Booting Sequence
●
Bootloader
– Executed by the hardware at a fixed location in ROM / Flash
– Initializes support for the device where the kernel image is found (local
storage, network, removable media)
– Loads the kernel image in RAM and execute.
●
Linux Kernel
– Uncompresses itself
– Initializes the kernel core and statically compiled drivers (required to access
the root filesystem)
– Mounts the root filesystem (specified by the root kernel parameter)
– Executes the first userspace program (specified by the init kernel parameter)
●
First userspace program Configures userspace and starts up system
services and any installed user interface (graphical or terminal based).
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Building Process
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How to build?
●
From scratch
– The original way, you need to build every single
component from scratch, from its source code.
Applying your configurations, compiler options,
building each component of the system, then
integrate all these parts together.
●
Auto-Build tools
– Automated tools in the form of “build scripts”,
written by the experts in this field to make it easy
for everyone to build a full embedded Linux system
with the minimal knowledge of building process.
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Building from scratch
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Components you need to build
– Cross-Toolchain (ready or build)
– BootLoader
– Linux Kernel configuration and compiling
– Root filesystem hierarchy
– System command (BusyBox)
– Startup scripts and variables
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Two approaches
– Bottom-Up approach (start with minimal system)
– Top-Down approach (start with ready base system)
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Auto-Build Tools
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Buildroot
– It’s a set of Makefiles and patches that simplifies
and automates the process of building a complete
Linux system for an embedded system, by
using cross-compilation.
– Useful mainly while working with small or
embedded systems, using various CPU
architectures – including x86, ARM, MIPS, and
PowerPC.
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Auto-Build Tools
●
Yocto
– It is a Linux Foundation workgroup
– It's a complete embedded Linux development
environment with tools, metadata, and
documentation - everything you need
– The Yocto Project has the objective of attempting
to improve the lives of developers of
customized Linux systems supporting
the ARM, MIPS, PowerPC and x86/x86 64
architectures.
