Yocto project and open embedded training

Yocto Project and OpenEmbedded Training
Yocto Project and
OpenEmbedded Training
Free Electrons
©Copyright 2004-2015, Free Electrons.
Creative Commons BY-SA 3.0 license.
Latest update: May 11, 2015.
Document updates and sources:
http://free-electrons.com/doc/training/yocto
Corrections, suggestions, contributions and translations are welcome!
Send them to feedback@free-electrons.com
Embedded Linux
Developers
Free Electrons
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Hyperlinks in the document
There are many hyperlinks in the document
Regular hyperlinks:
http://kernel.org/
Kernel documentation links:
Documentation/kmemcheck.txt
Links to kernel source ﬁles and directories:
drivers/input
include/linux/fb.h
Links to the declarations, deﬁnitions and instances of kernel
symbols (functions, types, data, structures):
platform_get_irq()
GFP_KERNEL
struct file_operations

Free Electrons at a glance
Engineering company created in 2004
(not a training company!)
Locations: Orange, Toulouse, Lyon (France)
Serving customers all around the world
See http://free-electrons.com/company/customers/
Head count: 9
Only Free Software enthusiasts!
Focus: Embedded Linux, Linux kernel, Android Free Software
/ Open Source for embedded and real-time systems.
Activities: development, training, consulting, technical
support.
Added value: get the best of the user and development
community and the resources it oﬀers.

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Generic course information
Generic course
information
Free Electrons
Embedded Linux
Developers
Free Electrons

Hardware used in this training session
BeagleBone Black, from CircuitCo
Texas Instruments AM335x (ARM Cortex-A8)
Powerful CPU, with 3D acceleration,
additional processors (PRUs) and lots of
peripherals.
512 MB of RAM
2 GB of on-board eMMC storage
(4 GB in Rev C)
USB host and USB device ports
microSD slot
HDMI port
2 x 46 pins headers, with access to many
expansion buses (I2C, SPI, UART and more)
A huge number of expansion boards, called
capes. See http://beagleboardtoys.com/.

Do not damage your BeagleBone Black!
Do not remove power abruptly:
Boards components have been damaged by removing the
power or USB cable in an abrupt way, not leaving the PMIC
the time to switch off the components in a clean way. See
http://bit.ly/1FWHNZi
Reboot (reboot) or shutdown (halt) the board in software
when Linux is running.
You can also press the RESET button to reset and reboot.
When there is no software way, you can also switch off the
board by pressing the POWER button for 8 seconds.
Do not leave your board powered on a metallic surface (like a
laptop with a metal finish).

Course outline - Day 1
First dive into the Yocto Project.
Overview of an embedded Linux system architecture.
Organization of the Yocto Project source tree.
Customizing an image.
Building an image.
Labs: download the Yocto project sources, compile an image and
ﬂash the development board.

Recipes and layers details: write, use, customize.
Recipes syntax. Writing a recipe.
Development workﬂow in the Yocto Project with BitBake.
Adding packages to the generated image.
The Yocto Project layers. Adding a new layer.
Labs: add a custom application and its recipe to the build system,
create a new layer.

The Yocto Project as a BSP provider.
Extending a recipe.
Writing your own machine conﬁguration.
Adding a custom image.
Using the SDK with Eclipse.
Labs: integrate kernel changes into the build system, write a
machine conﬁguration, create a custom image, experiment with
the SDK.

Participate!
During the lectures...
Don’t hesitate to ask questions. Other people in the audience
may have similar questions too.
This helps the trainer to detect any explanation that wasn’t
clear or detailed enough.
Don’t hesitate to share your experience, for example to
compare Linux / Android with other operating systems used
in your company.
Your point of view is most valuable, because it can be similar
to your colleagues’ and diﬀerent from the trainer’s.
Your participation can make our session more interactive and
make the topics easier to learn.

Practical lab guidelines
During practical labs...
We cannot support more than 8 workstations at once (each
with its board and equipment). Having more would make the
whole class progress slower, compromising the coverage of the
whole training agenda (exception for public sessions: up to 10
people).
So, if you are more than 8 participants, please form up to 8
working groups.
Open the electronic copy of your lecture materials, and use it
throughout the practical labs to ﬁnd the slides you need again.
Don’t copy and paste from the PDF slides.
The slides contain UTF-8 characters that look the same as
ASCII ones, but won’t be understood by shells or compilers.

Cooperate!
As in the Free Software and Open Source community, cooperation
during practical labs is valuable in this training session:
If you complete your labs before other people, don’t hesitate
to help other people and investigate the issues they face. The
faster we progress as a group, the more time we have to
explore extra topics.
Explain what you understood to other participants when
needed. It also helps to consolidate your knowledge.
Don’t hesitate to report potential bugs to your instructor.
Don’t hesitate to look for solutions on the Internet as well.

Command memento sheet
This memento sheet gives
command examples for the most
typical needs (looking for ﬁles,
extracting a tar archive...)
It saves us 1 day of UNIX / Linux
command line training.
Our best tip: in the command line
shell, always hit the Tab key to
complete command names and ﬁle
paths. This avoids 95% of typing
mistakes.
Get an electronic copy on
http://free-electrons.com/
doc/training/embedded-
linux/command_memento.pdf

vi basic commands
The vi editor is very useful to
make quick changes to files in an
embedded target.
Though not very user friendly at
first, vi is very powerful and its
main 15 commands are easy to
learn and are sufficient for 99% of
everyone’s needs!
Get an electronic copy on
http://free-electrons.com/
doc/training/embedded-
linux/vi_memento.pdf
You can also take the quick tutorial
by running vimtutor. This is a
worthy investment!

Practical lab - Training Setup
Prepare your lab environment
Download the lab archive
Enforce correct permissions

Introduction to embedded Linux build systems
Introduction to
embedded Linux
build systems
Free Electrons
Embedded Linux
Developers
Free Electrons

Embedded Linux distribution projects

Build system deﬁnition
Purposes of a build system:
Compiling or cross-compiling applications.
Packaging applications.
Testing output binaries and ecosystem compatibility.
Deploying generated images.

Available system building tools
Large choice of tools
Buildroot, developed by the community
http://www.buildroot.org
PTXdist, developed by Pengutronix
http://pengutronix.de/software/ptxdist/
OpenWRT, originally a fork of Buildroot for wireless routers, now a
more generic project
http://www.openwrt.org
OpenEmbedded based build systems
http://www.openembedded.org:
Poky (from the Yocto Project)
Arago Project
˚Angstr¨om
Vendor speciﬁc tools (silicon vendor or embedded Linux vendor)

Comparison of distribution projects
Buildroot
Simple to use.
Adapted for small embedded devices.
Not perfect if you need advanced functionalities and multiple
machines support.
http://buildroot.org/

OpenWRT
Based on Buildroot.
Primarily used for embedded network devices like routers.
http://openwrt.org/

Poky
Part of the Yocto Project.
Using OpenEmbedded.
Suitable for more complex embedded systems.
Allows lots of customization.
Can be used for multiple targets at the same time.
http://yoctoproject.org/

Build system beneﬁts

Working without a build system
Each application has to be built manually, or using custom
and non stable scripts.
The root file system has to be created from scratch.
The applications configurations have to be done by hand.
Each dependency has to be matched manually.
Integrating softwares from different teams is painful.

Benefits
Build systems automate the process of building a target
system, including the kernel, and sometimes the toolchain.
They automatically download, configure, compile and install
all the components in the right order, sometimes after
applying patches to fix cross-compiling issues.
They make sure all the application dependencies are matched.
They already contain a large number of packages, that should
fit your main requirements, and are easily extensible.
The build becomes reproducible, which allows to easily change
the configuration of some components, upgrade them, fix
bugs, etc.
Several configurations can be handled in the same project. It
is possible to generate the same root file system for different
hardware targets or to have a debug image based on the
production one, with some more flags or debugging
applications.

Workﬂow
Development of each application is done out of the build
system!
Development is done on an external repository.
The build system downloads sources from this repository and
start the build following the instructions.
The build system is used to build the full system and to
provide a working image to the customer.

Yocto Project and Poky reference system overview
Yocto Project and
Poky reference
system overview
Free Electrons
Embedded Linux
Developers
Free Electrons

The Yocto Project overview

About
The Yocto Project is a set of templates, tools and methods
that allow to build custom embedded Linux-based systems.
It is an open source project initiated by the Linux Foundation
in 2010 and is still managed by one of its fellows: Richard
Purdie.

The Yocto Project lexicon
The core components of the Yocto Project are:
BitBake, the build engine. It is a task scheduler, like make. It
interprets configuration files and recipes (also called metadata)
to perform a set of tasks, to download, configure and build
specified packages and filesystem images.
OpenEmbedded-Core, a set of base layers. It is a set of
recipes, layers and classes which are shared between all
OpenEmbedded based systems.
Poky, the reference system. It is a collection of projects and
tools, used to bootstrap a new distribution based on the Yocto
Project.

Organization of OpenEmbedded-Core:
Recipes describe how to fetch, conﬁgure, compile and package
applications and images. They have a speciﬁc syntax.
Layers are sets of recipes, matching a common purpose. For
Texas Instruments board support, the meta-ti layer is used.
Multiple layers are used within a same distribution, depending
on the requirements.
It supports the ARM, MIPS (32 and 64 bits), PowerPC and
x86 (32 and 64 bits) architectures.
It supports QEMU emulated machines for these architectures.

The Yocto Project is not used as a ﬁnite set of layers and
tools.
Instead, it provides a common base of tools and layers on
top of which custom and speciﬁc layers are added, depending
on your target.
The main required element is Poky, the reference system
which includes OpenEmbedded-Core. Other available tools
are optional, but may be useful in some cases.

Example of a Yocto Project based BSP
To build images for a BeagleBone Black, we need:
The Poky reference system, containing all common recipes and
tools.
The meta-ti layer, a set of Texas Instruments speciﬁc recipes.
All modiﬁcations are made in the meta-ti layer. Editing Poky
is a no-go!
We will set up this environment in the lab.

The Poky reference system overview

Download the Poky reference system
All oﬃcial projects part of the Yocto Project are available at
http://git.yoctoproject.org/cgit/
To download the Poky reference system:
git clone -b daisy git://git.yoctoproject.org/poky.git

Poky

Poky source tree 1/2
bitbake/ Holds all scripts used by the BitBake command.
Usually matches the stable release of the BitBake
project.
documentation/ All documentation sources for the Yocto Project
documentation. Can be used to generate nice
PDFs.
meta/ Contains the OpenEmbedded-Core metadata.
meta-skeleton/ Contains template recipes for BSP and kernel
development.

Poky source tree 2/2
meta-yocto/ Holds the configuration for the Poky reference
distribution.
meta-yocto-bsp/ Configuration for the Yocto Project reference
hardware board support package.
LICENSE The license under which Poky is distributed (a mix
of GPLv2 and MIT).
oe-init-build-env Script to set up the OpenEmbedded build
environment. It will create the build directory. It
takes an optional parameter which is the build
directory name. By default, this is build. This
script has to be sourced because it changes
environment variables.
scripts Contains scripts used to set up the environment,
development tools, and tools to flash the generated
images on the target.

Documentation
Documentation for the current sources, compiled as a ”mega
manual”, is available at:
http://www.yoctoproject.org/docs/current/mega-
manual/mega-manual.html
Variables in particular are described in the variable glossary:
http://www.yoctoproject.org/docs/current/ref-
manual/ref-manual.html#ref-variables-glossary

Using Yocto Project - basics
Using Yocto
Project - basics
Free Electrons
Embedded Linux
Developers
Free Electrons

Environment setup

Environment setup
All Poky files are left unchanged when building a custom
image.
Specific configuration files and build repositories are stored in
a separate build directory.
A script, oe-init-build-env, is provided to set up the build
directory and the environment variables (needed to be able to
use the bitbake command for example).

oe-init-build-env
Modiﬁes the environment: has to be sourced!
Adds environment variables, used by the build engine.
Allows you to use commands provided in Poky.
source ./oe-init-build-env [builddir]
Sets up a basic build directory, named builddir if it is not
found. If not provided, the default name is build.

Common targets
Common targets are listed when sourcing the script:
core-image-minimal A small image to boot a device and have
access to core command line commands and
services.
core-image-sato Image with Sato support. Sato is a GNOME
mobile-based user interface.
meta-toolchain Includes development headers and libraries to
develop directly on the target.
adt-installer Build the application development toolkit
installer.
meta-ide-support Generates the cross-toolchain. Useful when
working with the SDK.

Exported environment variables
BUILDDIR Absolute path of the build directory.
BB ENV EXTRAWHITE List of environment variables to load
from the user’s environment into BitBake data
store.
PATH Contains the directories where executable programs
are located. Absolute paths to scripts/ and
bitbake/bin/ are prepended.

Available commands
bitbake The main build engine command. Used to perform
tasks on available packages (download, conﬁgure,
compile. . . ).
bitbake-* Various speciﬁc commands related to the BitBake
build engine.
yocto-layer Command to create a new generic layer.
yocto-bsp Command to create a new generic BSP.

The build/ directory 1/2
conf/ Configuration files. Image specific and layer
configuration.
downloads/ Downloaded upstream tarballs of the packages used
in the builds.
sstate-cache/ Shared state cache. Used by all builds.
tmp/ Holds all the build system outputs.

The build/ directory 2/2
tmp/buildstats/ Build statistics for all packages built (CPU usage,
elapsed time, host, timestamps. . . ).
tmp/deploy/ Final output of the build.
tmp/deploy/images/ Contains the complete images built by the
OpenEmbedded build system. These images are used
to flash the target.
tmp/work/ Set of specific work directories, split by architecture.
They are used to unpack, configure and build the
packages. Contains the patched sources, generated
objects and logs.
tmp/sysroots/ Shared libraries and headers used to compile
packages for the target but also for the host.

Conﬁguring the build system

The build/conf/ directory
The conf/ directory in the build one holds build specific
configuration.
bblayers.conf Explicitly list the available layers.
local.conf Set up the configuration variables relative to the
current user for the build. Configuration
variables can be overridden there.

Configuring the build
The conf/local.conf configuration file holds local user
configuration variables:
BB NUMBER THREADS How many tasks BitBake should
perform in parallel.
PARALLEL MAKE How many processes should be used when
compiling.
MACHINE The machine the target is built for, e.g.
beaglebone.
PACKAGE CLASSES Packages format (deb, ipk or rpm).

Building an image

Compilation
The compilation is handled by the BitBake build engine.
Usage: bitbake [options] [recipename/target ...]
To build a target: bitbake [target]
Building a minimal image: bitbake core-image-minimal
This will run a full build for the selected target.

Practical lab - First Yocto build
Download the sources
Set up the environment
Conﬁgure the build
Build an image

Using Yocto Project - advanced usage
Using Yocto
Project - advanced
usage
Free Electrons
Embedded Linux
Developers
Free Electrons

Advanced build usage and conﬁguration
Select package variants.
Manually add packages to the generated image.
Run speciﬁc tasks with BitBake.

A little reminder
Recipes describe how to fetch, conﬁgure, compile and install
packages.
These tasks can be run independently (if their dependencies
are met).
All available packages in Poky are not selected by default in
the images.
Some packages may provide the same functionality, e.g.
OpenSSH and Dropbear.

Advanced conﬁguration

Overview
The OpenEmbedded build system uses configuration variables
to hold information.
Configuration settings are in upper-case by convention, e.g.
CONF_VERSION
To make configuration easier, it is possible to prepend,
append or define these variables in a conditional way.
All variables can be overridden or modified in
build/conf/local.conf

Methods and conditions 1/3
Append the keyword _append to a configuration variable to
add values after the ones previously defined (without space).
IMAGE_INSTALL_append = " dropbear" adds dropbear to
the packages installed on the image.
Append the keyword _prepend to add values before the ones
previously defined (without space).
FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
adds the folder to the set of paths where files are located (in a
recipe).
Append the machine name to only define a configuration
variable for a given machine. It tries to match with values
from MACHINEOVERRIDES which include MACHINE and
SOC_FAMILY.
KERNEL_DEVICETREE_beaglebone = "am335x-bone.dtb"
tells to use the kernel device tree am335x-bone.dtb only
when the machine is beaglebone.

The previous methods can be combined.
If we deﬁne:
IMAGE_INSTALL = "busybox mtd-utils"
IMAGE_INSTALL_append = " dropbear"
IMAGE_INSTALL_append_beaglebone = " i2c-tools"
The resulting conﬁguration variable will be:
IMAGE_INSTALL = "busybox mtd-utils dropbear i2c-
tools" if the machine being built is beaglebone.
IMAGE_INSTALL = "busybox mtd-utils dropbear"
otherwise.

The most speciﬁc variable takes precedence.
Example:
IMAGE_INSTALL_beaglebone = "busybox mtd-utils i2c-tools"
If the machine is beaglebone:
IMAGE_INSTALL = "busybox mtd-utils i2c-tools"
Otherwise:

Operators 1/2
Various operators can be used to assign values to
conﬁguration variables:
= expand the value when using the variable
:= immediately expand the value
+= append (with space)
=+ prepend (with space)
.= append (without space)
=. prepend (without space)
?= assign if no other value was previously assigned
??= same as previous, with a lower precedence

Operators 2/2
Avoid using +=, =+, .= and =. in build/conf/local.conf
due to ordering issues.
If += is parsed before ?=, the latter will be discarded.
Using _append unconditionally appends the value.

Packages variants

Introduction to package variants
Some packages have the same purpose, and only one can be
used at a time.
The build system uses virtual packages to reﬂect this. A
virtual package describes functionalities and several packages
may provide it.
Only one of the packages that provide the functionality will be
compiled and integrated into the resulting image.

Variant examples
The virtual packages are often in the form virtual/<name>
Example of available virtual packages with some of their
variants:
virtual/bootloader: u-boot, u-boot-ti-staging. . .
virtual/kernel: linux-yocto, linux-yocto-tiny, linux-yocto-rt,
linux-ti-staging. . .
virtual/libc: eglibc, uclibc
virtual/xserver: xserver-xorg

Package selection
Variants are selected thanks to the PREFERRED_PROVIDER
conﬁguration variable.
The package names have to suﬃx this variable.
Examples:
PREFERRED_PROVIDER_virtual/kernel ?= "linux-ti-
staging"
PREFERRED_PROVIDER_virtual/libgl = "mesa"

Version selection
By default, Bitbake will try to build the provider with the
highest version number, unless the recipe deﬁnes
DEFAULT_PREFERENCE = "-1"
When multiple package versions are available, it is also
possible to explicitly pick a given version with
PREFERRED_VERSION.
The package names have to suﬃx this variable.
% can be used as a wildcard.
Example:
PREFERRED_VERSION_linux-yocto = "3.10%"
PREFERRED_VERSION_python = "2.7.3"

Packages

Selection
The set of packages installed into the image is defined by the
target you choose (e.g. core-image-minimal).
It is possible to have a custom set by defining our own target,
and we will see this later.
When developing or debugging, adding packages can be
useful, without modifying the recipes.
Packages are controlled by the IMAGE_INSTALL configuration
variable.

Exclusion
The list of packages to install is also ﬁltered using the
PACKAGE_EXCLUDE variable.
However, if a package needs installing to satisfy a dependency,
it will still be selected.

Format
By default, Poky uses the RPM package format
OpenEmbedded-Core uses the IPK package format
To select the generated package formats, use the
PACKAGE_CLASSES variable
Valid values are: package_rpm, package_deb, package_ipk
(multiple values are OK)

The power of BitBake

Common BitBake options
BitBake can be used to run a full build for a given target with
bitbake [target].
But it can be more precise, with optional options:
-c <task> execute the given task
-s list all locally available packages and their
versions
-f force the given task to be run by removing its
stamp ﬁle
world keyword for all packages
-b <recipe> execute tasks from the given recipe (without
resolving dependencies).

BitBake examples
bitbake -c listtasks virtual/kernel
Gives a list of the available tasks for the package
virtual/kernel. Tasks are preﬁxed with do_.
bitbake -c menuconfig virtual/kernel
Execute the task menuconfig on the kernel package.
bitbake -f dropbear
Force the dropbear package to be rebuilt from scratch.
bitbake -c fetchall world
Download all package sources and their dependencies.
For a full description: bitbake --help

shared state cache
BitBake stores the output of each task in a directory, the
shared state cache. Its location is controlled by the
SSTATE_DIR variable.
This cache is use to speed up compilation.
Over time, as you compile more recipes, it can grow quite big.
It is possible to clean old data with:
$ ./scripts/sstate-cache-management.sh --remove-duplicated -d
--cache-dir=<SSTATE_DIR>

Network usage

Keeping package sources on a local network
When fetching source code, BitBake will ﬁrst search in the
local download directory (DL_DIR).
It will then try to access mirrors:
First it tries locations from the PREMIRRORS variable, usually
set by the distribution layer. Poky uses:
PREMIRRORS_prepend = "
git://.*/.* http://www.yoctoproject.org/sources/ n
ftp://.*/.* http://www.yoctoproject.org/sources/ n
http://.*/.* http://www.yoctoproject.org/sources/ n
https://.*/.* http://www.yoctoproject.org/sources/ n"
Then it tries locations from the MIRRORS variable
Use the own-mirrors class to set your mirrors:
INHERIT += "own-mirrors"
SOURCE_MIRROR_URL = "http://example.com/my-source-mirror"
If all the mirrors fail, the build will fail.

Forbidding network access
You can use BB_GENERATE_MIRROR_TARBALLS = "1" to
generate tarballs of the git repositories in DL_DIR
You can also completely disable network access using
BB_NO_NETWORK = "1"

Practical lab - Advanced Yocto configuration
Modify the build configuration
Customize the package selection
Experiment with BitBake
Mount the root file system over
NFS

Writing recipes - basics
Writing recipes -
basics
Free Electrons
Embedded Linux
Developers
Free Electrons

Recipes: overview

Recipes

Basics
Recipes describe how to handle a given package.
A recipe is a set of instructions to describe how to retrieve,
patch, compile, install and generate binary packages for a
given application.
It also deﬁnes what build or runtime dependencies are
required.
The recipes are parsed by the BitBake build engine.
The format of a recipe ﬁle name is
<package-name>_<version>.bb

Content of a recipe
A recipe contains conﬁguration variables: name, license,
dependencies, path to retrieve the source code. . .
It also contains functions that can be run (fetch, conﬁgure,
compile. . . ) which are called tasks.
Tasks provide a set of actions to perform.
Remember the bitbake -c <task> <package> command?

Common variables
To make it easier to write a recipe, some variables are
automatically available:
PN package name, as specified in the recipe file
name
PV package version, as specified in the recipe file
name
PR package release, defaults to r0
PE package epoch: used to reorder package versions
when the numbering scheme has changed
When using the recipe bash_4.2.bb:
${PN} = "bash"
${PV} = "4.2"

Organization of a recipe

Many packages have more than one recipe, to support
different versions. In that case the common metadata is
included in each version specific recipe and is in a .inc file:
<package>.inc: version agnostic metadata.
<package>_<version>.bb: require <package>.inc and
version specific metadata.
We can divide a recipe into three main parts:
The header: what/who
The sources: where
The tasks: how

The header
Conﬁguration variables to describe the package:
DESCRIPTION describes what the software is about
HOMEPAGE URL to the project’s homepage
PRIORITY defaults to optional
SECTION package category (e.g. console/utils)
LICENSE the package’s license

The source locations 1/3
We need to retrieve both the raw sources from an official
location and the resources needed to configure, patch or
install the package.
SRC_URI defines where and how to retrieve the needed
elements. It is a set of URI schemes pointing to the resource
locations (local or remote).
URI scheme syntax: scheme://url;param1;param2
file://joystick-support.patch
${SOURCEFORGE_MIRROR}/<project-name>/${PN}-
${PV}.tar.gz
git://<url>;protocol=<protocol>;branch=<branch>
When using git, it is necessary to also define SRCREV. If
SRCREV is a hash or a tag not present in master, the branch
parameter is mandatory. When the tag is not in any branch, it
is possible to use nobranch=1

For the local files, the searched paths are defined in the
FILESPATH variable, custom ones can be added using
FILESEXTRAPATHS. BitBake will also search in subfolders
listed in the OVERRIDES variables in those paths.
Prepend the paths, as the order matters.
All local files found in SRC_URI are copied into the package’s
working directory, in build/tmp/work/.
Files ending in .patch, .diff or having the apply=yes
parameter will be applied after the sources are retrieved and
extracted.
Patches are applied in the order they are found.
LIC_FILES_CHKSUM defines the URI pointing to the license
file in the source code as well as its checksum. This allows to
track any license update: if the license changes, the build will
trigger a failure as the checksum won’t be valid anymore.
LIC_FILES_CHKSUM = "file://gpl.txt;md5=393a5ca..."

When the sources are provided in a tarball:
An md5 or an sha256 sum must be provided:
SRC_URI[md5sum] =
"97b2c3fb082241ab5c56ab728522622b"
SRC_URI[sha256sum] = "..."
BitBake expects to ﬁnd the extracted sources in a directory
called <package-name>-<version>. This is controlled by
the S variable. If the directory has another name, you must
explicitly deﬁne S.
By default, sources are fetched in $BUILDDIR/downloads.
Change it with the DL_DIR variable in conf/local.conf
Use the PREMIRRORS and MIRRORS variables if you want to
use local mirrors or fallback mirrors.
To ensure no network access will be done when building, use
BB_NO_NETWORK = "1".

Dependencies
A package can have dependencies during the build or at
runtime. To reﬂect these requirements in the recipe, two
variables are used:
DEPENDS List of the package build-time dependencies.
RDEPENDS List of the package runtime dependencies. Must
be package speciﬁc (e.g. with _${PN}).
DEPENDS = "package_b": the local do_configure task
depends on the do_populate_sysroot task of package b.
RDEPENDS_${PN} = "package_b": the local do_build task
depends on the do_package_write_<archive-format>
task of package b.

Tasks
Default tasks already exists, they are deﬁned in classes:
do fetch
do unpack
do patch
do conﬁgure
do compile
do install
do package
do rootfs
You can get a list of existing tasks for a recipe with:
bitbake <recipe> -c listtasks

Writing tasks 1/3
Functions use the sh shell syntax, with available
OpenEmbedded variables and internal functions available.
D The destination directory (root directory of
where the ﬁles are installed, before creating the
image).
WORKDIR the package’s working directory
Syntax of a task:
do_task() {
action0
action1
...
}

Writing tasks 2/3
Example:
do_compile() {
${CC} ${CFLAGS} ${LDFLAGS} -o hello ${WORKDIR}/hello.c
}
do_install() {
install -d ${D}${bindir}
install -m 0755 hello ${D}${bindir}
}

Writing tasks 3/3
Or using a Makeﬁle:
do_compile() {
oe_runmake
}
do_install() {
}

Modifying existing tasks
Tasks can be extended with _prepend or _append
do_install_append() {
install -d ${D}${sysconfdir}
install -m 0755 hello.conf ${D}${sysconfdir}
}

Adding new tasks
Tasks can be added with addtask
do_mkimage () {
uboot-mkimage ...
}
addtask mkimage after do_compile before do_install

Example of a recipe

Hello world recipe
DESCRIPTION = "Hello world program"
HOMEPAGE = "http://example.net/helloworld/"
PRIORITY = "optional"
SECTION = "examples"
LICENSE = "GPLv2"
SRC_URI = "file://hello.c"
do_compile() {
${CC} ${CFLAGS} ${LDFLAGS} -o hello ${WORKDIR}/hello.c
}
do_install() {
}

Example of a recipe with a version
agnostic part

tar.inc
SUMMARY = "GNU file archiving program"
HOMEPAGE = "http://www.gnu.org/software/tar/"
SECTION = "base"
SRC_URI = "${GNU_MIRROR}/tar/tar-${PV}.tar.bz2"
do_configure() { ... }
do_compile() { ... }
do_install() { ... }

tar 1.17.bb
require tar.inc
LICENSE = "GPLv2"
LIC_FILES_CHKSUM =
"file://COPYING;md5=59530bdf33659b29e73d4adb9f9f6552"
SRC_URI += "file://avoid_heap_overflow.patch"
SRC_URI[md5sum] = "c6c4f1c075dbf0f75c29737faa58f290"

tar 1.26.bb
require tar.inc
LICENSE = "GPLv3"
LIC_FILES_CHKSUM =
"file://COPYING;md5=d32239bcb673463ab874e80d47fae504"
SRC_URI[md5sum] = "2cee42a2ff4f1cd4f9298eeeb2264519"

Practical lab - Add a custom application
Write a recipe for a custom
application
Integrate it in the image

Writing recipes - advanced
Writing recipes -
advanced
Free Electrons
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Developers
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Extending a recipe

Introduction to recipe extensions
It is a good practice not to modify recipes available in Poky.
But it is sometimes useful to modify an existing recipe, to
apply a custom patch for example.
The BitBake build engine allows to modify a recipe by
extending it.
Multiple extensions can be applied to a recipe.

Introduction to recipe extensions
Metadata can be changed, added or appended.
Tasks can be added or appended.
Operators are used extensively, to add, append, prepend or
assign values.

Extend a recipe

Extend a recipe
The recipe extensions end in .bbappend
Append files must have the same root name as the recipe they
extend.
example_0.1.bbappend applies to example_0.1.bb
Append files are version specific. If the recipe is updated to
a newer version, the append files must also be updated.
If adding new files, the path to their directory must be
prepended to the FILESEXTRAPATHS variable.
Files are looked up in paths referenced in FILESEXTRAPATHS,
from left to right.
Prepending a path makes sure it has priority over the recipe’s
one. This allows to override recipes’ files.

Extend a recipe: compatibility
When using a Yocto Project release older than 1.5, the
Metadata revision number must explicitly be incremented in
each append ﬁle.
The revision number is stored in the PRINC variable.
At the end of the recipe, you must increment it:
PRINC := "${@int(PRINC) + 1"}
Since version 1.5, PRINC is automatically taken care of unless
you are building on multiple machines. In that case, use the
PR server, with bitbake-prserv

Append ﬁle example

Hello world append ﬁle
FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
SRC_URI += "file://custom-modification-0.patch
file://custom-modification-1.patch
"

Advanced recipe conﬁguration

Advanced conﬁguration
In the real word, more complex conﬁgurations are often
needed because recipes may:
Provide virtual packages
Inherit generic functions from classes

Providing virtual packages
BitBake allows to use virtual names instead of the actual
package name. We saw a use case with package variants.
The virtual name is speciﬁed through the PROVIDES variable.
Several packages can provide the same virtual name. Only one
will be built and installed into the generated image.
PROVIDES = "virtual/kernel"

Classes

Introduction to classes
Classes provide an abstraction to common code, which can be
re-used in multiple packages.
Common tasks do not have to be re-developed!
Any metadata and task which can be put in a recipe can be
used in a class.
Classes extension is .bbclass
Classes are located in the classes folder of a layer.
Packages can use this common code by inheriting a class:
inherits <class>

Common classes
Common classes can be found in meta/classes/
base.bbclass
kernel.bbclass
autotools.bbclass
update-alternatives.bbclass
useradd.bbclass
. . .

The base class 1/2
Every recipe inherits the base class automatically.
Contains a set of basic common tasks to fetch, unpack or
compile packages.
Inherits other common classes, providing:
Mirrors deﬁnitions: DEBIAN_MIRROR, GNU_MIRROR,
KERNELORG_MIRROR. . .
The ability to ﬁlter patches by SRC_URI
Some tasks: clean, listtasks or fetchall.

The base class 2/2
Defines oe_runmake, using EXTRA_OEMAKE to use custom
arguments.
In Poky, EXTRA_OEMAKE defaults to -e MAKEFLAGS=.
The -e option to give variables taken from the environment
precedence over variables from makefiles.
Upstream libraries or softwares often embed their own
Makefile.
Helps not using hardcoded CC or CFLAGS variables in makefiles.

The kernel class
Used to build Linux kernels.
Defines tasks to configure, compile and install a kernel and its
modules.
The kernel is divided into several packages: kernel,
kernel-base, kernel-dev, kernel-modules. . .
Automatically provides the virtual package virtual/kernel.
Configuration variables are available:
KERNEL_IMAGETYPE, defaults to zImage
KERNEL_EXTRA_ARGS
INITRAMFS_IMAGE

The autotools class
Defines tasks and metadata to handle packages using the
autotools build system (autoconf, automake and libtool):
do_configure: generates the configure script using
autoreconf and loads it with standard arguments or
cross-compilation.
do_compile: runs make
do_install: runs make install
Extra configuration parameters can be passed with
EXTRA_OECONF.
Compilation flags can be added thanks to the EXTRA_OEMAKE
variable.

Example: use the autotools class
DESCRIPTION = "Print a friendly, customizable greeting"
HOMEPAGE = "https://www.gnu.org/software/hello/"
LICENSE = "GPLv3"
SRC_URI = "${GNU_MIRROR}/hello/hello-${PV}.tar.gz"
SRC_URI[md5sum] = "67607d2616a0faaf5bc94c59dca7c3cb"
SRC_URI[sha256sum] = "ecbb7a2214196c57ff9340aa71458e1559abd38f6d8d169666846935df191ea7"
LIC_FILES_CHKSUM = "file://COPYING;md5=d32239bcb673463ab874e80d47fae504"
inherits autotools

The update-alternative class
Allows to install multiple binaries having the same
functionality, avoiding conﬂicts by renaming the binaries.
Four variables are used to conﬁgure the class:
ALTERNATIVE NAME The name of the binary.
ALTERNATIVE LINK The path of the resulting binary.
ALTERNATIVE PATH The path of the real binary.
ALTERNATIVE PRIORITY The alternative priority.
The command with the highest priority will be used.

The useradd class
This class helps to add users to the resulting image.
Adding custom users is required by many services to avoid
running them as root.
USERADD_PACKAGES must be deﬁned when the useradd class
is inherited. Deﬁnes the list of packages which needs the user.
Users and groups will be created before the packages using it
perform their do_install.
At least one of the two following variables must be set:
USERADD_PARAM: parameters to pass to useradd.
GROUPADD_PARAM: parameters to pass to groupadd.

Example: use the useradd class
DESCRIPTION = "useradd class usage example"
LICENSE = "MIT"
SRC_URI = "file://file0"
LIC_FILES_CHKSUM = "file://${COREBASE}/meta/files/common-licenses/MIT;md5=0835ade698e0bc..."
inherits useradd
USERADD_PACKAGES = "${PN}"
USERADD_PARAM = "-u 1000 -d /home/user0 -s /bin/bash user0"
do_install() {
install -m 644 file0 ${D}/home/user0/
chown user0:user0 ${D}/home/user0/file0
}

Binary packages

Specifics for binary packages
It is possible to install binaries into the generated root
filesystem.
Set the LICENSE to CLOSED.
Use the do_install task to copy the binaries into the root
file system.

Debugging recipes

Debugging recipes
For each task, logs are available in the temp directory in the
work folder of a recipe.
A development shell, exporting the full environment can be
used to debug build failures:
$ bitbake -c devshell <recipe>
To understand what a change in a recipe implies, you can
activate build history in local.conf:
INHERIT += "buildhistory"
BUILDHISTORY_COMMIT = "1"
Then use the buildhistory-diff tool to examine
diﬀerences between two builds.
./scripts/buildhistory-diff

Layers
Layers
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Developers
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Layers
Introduction to layers

Layers’ principles
The OpenEmbedded build system manipulates metadata.
Layers allow to isolate and organize the metadata.
A layer is a collection of packages and build tasks.
It is a good practice to begin a layer name with the preﬁx
meta-.

Layers in Poky

Layers in Poky
The Poky reference system is a set of basic common layers:
meta
meta-oe
meta-skeleton
meta-yocto
meta-yocto-bsp
Poky is not a ﬁnal set of layers. It is the common base.
Layers are added when needed.
When making modiﬁcations to the existing recipes or when
adding new packages, it is a good practice not to modify
Poky. Instead you can create your own layers!

Poky

Integrate and use a layer 1/2
A list of existing and maintained layers can be found at
http://layers.openembedded.org/layerindex/branch/
master/layers/
Instead of redeveloping layers, always check the work hasn’t
been done by others.
It takes less time to download a layer providing a package you
need and to add an append ﬁle if some modiﬁcations are
needed than to do it from scratch.

Integrate and use a layer 2/2
The location where a layer is saved on the disk doesn’t
matter.
But a good practice is to save it where all others layers are
stored.
The only requirement is to let BitBake know about the new
layer:
The list of layers BitBake uses is defined in
build/conf/bblayers.conf
To include a new layer, add its absolute path to the BBLAYERS
variable.
BitBake parses each layer specified in BBLAYERS and adds the
recipes, configurations files and classes it contains.

Some useful layers
Many SoC specific layers are available, providing support for
the boards using these SoCs. Some examples: meta-ti,
meta-fsl-arm and meta-raspberrypi.
Other layers offer to support packages not available in the
Poky reference system:
meta-browser: web browsers (Chromium, Firefox).
meta-filesystems: support for additional filesystems.
meta-gstreamer10: support for GStreamer 1.0.
meta-java and meta-oracle-java: Java support.
meta-linaro-toolchain: Linaro toolchain recipes.
meta-qt5: QT5 modules.
meta-realtime: real time tools and test programs.
meta-telephony and many more. . .

Layers
Creating a layer

Custom layer

Create a custom layer 1/2
A layer is a set of ﬁles and directories and can be created by
hand.
However, the yocto-layer command helps us create new
layers and ensures this is done right.
meta- is automatically prepended to the layer name.
By default yocto-layer creates the new layer in the current
directory.
yocto-layer create <layer_name> -o <dest_dir>

Create a custom layer 2/2
The layer created will be pre-filled with the following files:
conf/layer.conf The layer’s configuration. Holds its priority
and generic information. No need to modify it in
many cases.
COPYING.MIT The license under which a layer is released.
By default MIT.
README A basic description of the layer. Contains a
contact e-mail to update.
By default, all metadata matching ./recipes-*/*/*.bb will
be parsed by the BitBake build engine.

Use a layer: best practices
Do not copy and modify existing recipes from other layers.
Instead use append files.
Avoid duplicating files. Use append files or explicitly use a
path relative to other layers.
Save the layer alongside other layers, in OEROOT.
Use LAYERDEPENDS to explicitly define layer dependencies.

Practical lab - Create a custom layer
Create a layer from scratch
Add recipes to the new layer
Integrate it to the build

Practical lab - Extend a recipe
Apply patches to an existing recipe
Use a custom configuration file for
an existing recipe
Extend a recipe to fit your needs

BSP Layers
BSP Layers
Free Electrons
Embedded Linux
Developers
Free Electrons

BSP Layers
Introduction to BSP layers in the
Yocto Project

BSP layers

Overview
BSP layers are device speciﬁc layers. They hold metadata
with the purpose of supporting speciﬁc hardware devices.
BSP layers describe the hardware features and often provide a
custom kernel and bootloader with the required modules and
drivers.
BSP layers can also provide additional software, designed to
take advantage of the hardware features.
As a layer, it is integrated into the build system as we
previously saw.
A good practice is to name it meta-<bsp_name>.

BSP layers Specifics
BSP layers are a subset of the layers.
In addition to package recipes and build tasks, they often
provide:
Hardware configuration files (machines).
Bootloader, kernel and display support and configuration.
Pre-built user binaries.

BSP Layers
Generating a new BSP layer

Creating a new BSP 1/3
A dedicated command is provided to create BSP layers:
yocto-bsp.
As for the layers, meta- is automatically prepended to the
BSP layer’s name.
yocto-bsp create <name> <karch>
karch stands for ”kernel architecture”. You can dump a list
of the available ones by running: yocto-bsp list karch.
yocto-bsp create felabs arm

yocto-bsp will prompt a few questions to help conﬁgure the
kernel, bootloader and X support if needed.
You will also need to choose compiler tuning (cortexa9,
cortexa15, cortexm3, cortexm5. . . ).
And enable some functionalities (keyboard and mouse
support).

A new layer is created, named meta-<bsp_name> and
contains the following information:
binary/ Contains bootable images or build filesystem, if
needed.
conf/layer.conf The BSP layer’s configuration.
conf/machine/ Holds the machine configuration files. One is
created by default: <bsp_name>.conf
recipes-* A few recipes are created, thanks to the user
input gathered by the yocto-bsp command.
README The layer’s documentation. This file needs to be
updated.

BSP Layers
Hardware conﬁguration ﬁles

Overview 1/2
A layer provides one machine file (hardware configuration file)
per machine it supports.
These configuration files are stored under
meta-<bsp_name>/conf/machine/*.conf
The file names correspond to the values set in the MACHINE
configuration variable.
meta-ti/conf/machine/beaglebone.conf
MACHINE = "beaglebone"
Each machine should be described in the README file of the
BSP.

Overview 2/2
The hardware configuration file contains configuration
variables related to the architecture and to the machine
features.
Some other variables help customize the kernel image or the
filesystems used.

Machine conﬁguration
TARGET ARCH The architecture of the device being built.
PREFERRED PROVIDER virtual/kernel The default kernel.
MACHINE FEATURES List of hardware features provided by the
machine, e.g. usbgadget usbhost screen wifi
SERIAL CONSOLE Speed and device for the serial console to
attach. Passed to the kernel as the console
parameter, e.g. 115200 ttyS0
KERNEL IMAGETYPE The type of kernel image to build, e.g.
zImage
IMAGE FSTYPES Format of the root ﬁlesystem images to be
created, e.g. tar.bz2 squashfs

MACHINE_FEATURES
Lists the hardware features provided by the machine.
These features are used by package recipes to enable or
disable functionalities.
Some packages are automatically added to the resulting root
ﬁlesystem depending on the feature list.
The feature bluetooth:
Adds the bluez daemon to be built and added to the image.
Enables bluetooth support in ConnMan.

conf/machine/include/cfa10036.inc
# Common definitions for cfa-10036 boards
include conf/machine/include/mxs-base.inc
SOC_FAMILY = "mxs:mx28:cfa10036"
PREFERRED_PROVIDER_virtual/kernel ?= "linux-cfa"
IMAGE_BOOTLOADER = "barebox"
BAREBOX_BINARY = "barebox"
IMXBOOTLETS_MACHINE = "cfa10036"
KERNEL_IMAGETYPE = "zImage"
KERNEL_DEVICETREE = "imx28-cfa10036.dtb"
# we need the kernel to be installed in the final image
IMAGE_INSTALL_append = " kernel-image kernel-devicetree"
SDCARD_ROOTFS ?= "${DEPLOY_DIR_IMAGE}/${IMAGE_NAME}.rootfs.ext3"
IMAGE_FSTYPES ?= "tar.bz2 ext3 barebox.mxsboot-sdcard sdcard"
SERIAL_CONSOLE = "115200 ttyAMA0"
MACHINE_FEATURES = "usbgadget usbhost vfat"

conf/machine/cfa10057.conf
#@TYPE: Machine
#@NAME: Crystalfontz CFA-10057
#@SOC: i.MX28
#@DESCRIPTION: Machine configuration for CFA-10057, also called CFA-920
#@MAINTAINER: Alexandre Belloni <alexandre.belloni@free-electrons.com>
include conf/machine/include/cfa10036.inc
KERNEL_DEVICETREE += "imx28-cfa10057.dtb"
MACHINE_FEATURES += "touchscreen"

BSP Layers
Image types

IMAGE_FSTYPES
Configures the resulting root filesystem image format.
If more than one format is specified, one image per format will
be generated.
Image formats instructions are delivered in Poky, thanks to
meta/classes/image_types.bbclass
Common image formats are: ext2, ext3, ext4, squashfs,
squashfs-xz, cpio, jffs2, ubifs, tar.bz2, tar.gz. . .

Creating an image type
If you have a particular layout on your storage (for example
bootloader location on an SD card), you may want to create
your own image type.
This is done through a class that inherits from image_types.
It has to deﬁne a function named IMAGE_CMD_<type>.

wic
A new way of creating images has been introduced recently:
wic
It is a tool that can create a flashable image from the
compiled packages and artifacts.
It can create partitions
It can select which files are located in which partition through
the use of plugins.
The final image layout is described in a .wks file.
It can be extended in any layer.
Usage example:
$ wic create mkefidisk -e core-image-base

BSP Layers
Formfactor

Overview
The yocto-bsp command generates a formfactor recipe.
recipes-bsp/formfactor/formfactor_0.0.bbappend
formfactor is a recipe providing information about the
hardware that is not described by other sources such as as the
kernel.
This configuration is defined in the recipe in: recipes-
bsp/formfactor/formfactor/<machine>/machconfig
Default values are defined in:
meta/recipes-bsp/formfactor/files/config

Formfactor example
HAVE_TOUCHSCREEN=1
HAVE_KEYBOARD=1
DISPLAY_CAN_ROTATE=0
DISPLAY_ORIENTATION=0
DISPLAY_WIDTH_PIXELS=640
DISPLAY_HEIGHT_PIXELS=480
DISPLAY_BPP=16
DISPLAY_DPI=150
DISPLAY_SUBPIXEL_ORDER=vrgb

BSP Layers
Bootloader

Default bootloader 1/2
By default the bootloader used is the mainline version of
U-Boot, with a fixed version (per Poky release).
All the magic is done in
meta/recipes-bsp/u-boot/u-boot.inc
Some configuration variables used by the U-Boot recipe can
be customized, in the machine file.

Default bootloader 2/2
SPL BINARY If an SPL is built, describes the name of the output
binary. Defaults to an empty string.
UBOOT SUFFIX bin (default) or img.
UBOOT MACHINE The target used to build the conﬁguration.
UBOOT ENTRYPOINT The bootloader entry point.
UBOOT LOADADDRESS The bootloader load address.
UBOOT MAKE TARGET Make target when building the
bootloader. Defaults to all.

Customize the bootloader
By default no recipe is added to customize the bootloader.
It is possible to do so by creating an extended recipe and to
append extra metadata to the original one.
This works well when using a mainline version of U-Boot.
Otherwise it is possible to create a custom recipe.
Try to still use meta/recipes-bsp/u-boot/u-boot.inc

BSP Layers
Kernel

Linux kernel recipes in Yocto
There are basically two ways of compiling a kernel in the
Yocto Project:
By using the linux-yocto packages, provided in Poky.
By using a fully custom kernel recipe.
The kernel used is selected in the machine ﬁle thanks to:
PREFERRED_PROVIDER_virtual/kernel
Its version if deﬁned with:
PREFERRED_VERSION_<kernel_provider>

Linux Yocto 1/4
linux-yocto is a generic set of recipes for building mainline
Linux kernel images.
The yocto-bsp tool creates basic appended recipes to allow
to extend the linux-yocto ones.
meta-<bsp_name>/recipes-kernel/linux/linux-
yocto_*.bbappend
PREFERRED_PROVIDER_virtual/kernel = "linux-yocto"
PREFERRED_VERSION_linux-yocto = "3.14%"

Linux Yocto 2/4
Like other appended recipes, patches can be added by filling
SRC_URI with .patch and/or .diff files.
The kernel configuration must also be provided, and the file
containing it must be called defconfig.
This can be generated from a Linux source tree, by using
make savedefconfig
The configuration can be split in several files, by using the
.cfg extension. It is the best practice when adding new
features:
SRC_URI += "file://defconfig
file://nand-support.cfg
file://ethernet-support.cfg"

Linux Yocto 3/4
Configuration fragments can be generated directly with the
bitbake command:
1. Configure the kernel following its recipe instructions:
bitbake -c kernel_configme linux-yocto
2. Edit the configuration:
bitbake -c menuconfig linux-yocto
3. Save the configuration differences:
bitbake -c diffconfig linux-yocto
The differences will be saved at $WORKDIR/fragment.cfg
After integrating configuration fragments into the appended
recipe, you can check everything is fine by running:
bitbake -c kernel_configcheck -f linux-yocto

Linux Yocto 4/4
Another way of configuring linux-yocto is by using
Advanced Metadata.
It is a powerful way of splitting the configuration and the
patches into several pieces.
It is designed to provide a very configurable kernel.
The full documentation can be found at
https://www.yoctoproject.org/docs/1.6.1/kernel-
dev/kernel-dev.html#kernel-dev-advanced

Linux Yocto: Kernel Metadata 1/4
Kernel Metadata is a way to organize and to split the kernel
configuration and patches in little pieces each providing
support for one feature.
Two main configuration variables help taking advantage of
this:
LINUX KERNEL TYPE standard (default), tiny or
preempt-rt
standard: generic Linux kernel policy.
tiny: bare minimum configuration, for small
kernels.
preempt-rt: applies the PREEMPT_RT patch.
KERNEL FEATURES List of features to enable. Features are
sets of patches and configuration fragments.

Kernel Metadata can be stored in the linux-yocto recipe
space.
It must be under $FILESEXTRAPATHS. A best practice is to
follow this directory hierarchy:
bsp/
cfg/
features/
ktypes/
patches/
Kernel Metadata are divided into 3 file types:
Description files, ending in .scc
Configuration fragments
Patches

Kernel Metadata description ﬁles have their own syntax, used
to describe the feature provided and which patches and
conﬁguration fragments to use.
Simple example, features/smp.scc
define KFEATURE_DESCRIPTION "Enable SMP"
kconf hardware smp.cfg
patch smp-support.patch
To integrate the feature into the kernel image:
KERNEL_FEATURES += "features/smp.scc"

.scc syntax description:
branch <ref> Create a new branch relative to the current
one.
define Defines variables.
include <scc file> Include another description file.
Parsed inline.
kconf [hardware|non-hardware] <cfg file> Queues a
configuration fragment, to merge it into Linux’s
.config
git merge <branch> Merge branch into the current git
branch.
patch <patch file> Applies patch file to the current
git branch.

Practical lab - Create a custom machine conﬁguration
Write a machine conﬁguration
Understand how the target
architecture is chosen

Distro Layers
Distro Layers
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Distro Layers
Distro Layers

Distro layers

Distro layers
You can create a new distribution by using a Distro layer.
This allows to change the defaults that are used by Poky.
It is useful to distribute changes that have been made in
local.conf

Best practice
A distro layer is used to provides policy configurations for a
custom distribution.
It is a best practice to separate the distro layer from the
custom layers you may create and use.
It often contains:
Configuration files.
Specific classes.
Distribution specific recipes: initialization scripts, splash screen
packages. . .

Creating a Distro layer
The configuration file for the distro layer is
conf/distro/<distro>.conf
This file must define the DISTRO variable.
It is possible to inherit configuration from an existing distro
layer.
You can also use all the DISTRO_* variables.
Use DISTRO = "<distro>" in local.conf to use your
distro configuration.
require conf/distro/poky.conf
DISTRO = "distro"
DISTRO_NAME = "distro description"
DISTRO_VERSION = "1.0"
MAINTAINER = "..."

Images
Images
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Images
Introduction to images

Overview 1/3
An image is the top level recipe and is used alongside the
machine definition.
Whereas the machine describes the hardware used and its
capabilities, the image is architecture agnostic and defines
how the root filesystem is built, with what packages.
By default, several images are provided in Poky:
meta*/recipes*/images/*.bb

Overview 2/3
Common images are:
core-image-base Console-only image, with full support of the
hardware.
core-image-minimal Small image, capable of booting a device.
core-image-minimal-dev Small image with extra debug
symbols, tools and libraries.
core-image-x11 Image with basic X11 support.
core-image-rt core-image-minimal with real time tools and
test suite.

Overview 3/3
An image is no more than a recipe.
It has a description, a license and inherits the core-image
class.

Organization of an image recipe
Some special configuration variables are used to describe an
image:
IMAGE BASENAME The name of the output image files.
Defaults to ${PN}.
IMAGE INSTALL List of packages and package groups to
install in the generated image.
IMAGE ROOTFS SIZE The final root filesystem size.
IMAGE FEATURES List of features to enable in the image.
IMAGE FSTYPES List of formats the OpenEmbedded build
system will use to create images.
IMAGE LINGUAS List of the locales to be supported in the
image.
IMAGE PKGTYPE Package type used by the build system.
One of deb, rpm, ipk and tar.
IMAGE POSTPROCESS COMMAND Shell commands to
run at post process.

Example of an image
require recipes-core/images/core-image-minimal.bb
DESCRIPTION = "Example image"
IMAGE_INSTALL += " ninvaders"
IMAGE_FSTYPES = "tar.bz2 cpio squashfs"
IMAGE_PKGTYPE = "deb"
LICENSE = "MIT"

Images
Package groups

Overview
Package groups are a way to group packages by
functionality or common purpose.
Package groups are used in image recipes to help building the
list of packages to install.
They can be found under
meta*/recipes-core/packagegroups/
A package group is yet another recipe.
The preﬁx packagegroup- is always used.

Common package groups
packagegroup-core-boot
packagegroup-core-buildessential
packagegroup-core-nfs
packagegroup-core-ssh-dropbear
packagegroup-core-ssh-openssh
packagegroup-core-tools-debug
packagegroup-core-tools-proﬁle

Example
meta/recipes-core/packagegroups/packagegroup-core-
nfs.bb:
DESCRIPTION = "NFS package groups"
LICENSE = "MIT"
PR = "r2"
inherit packagegroup
PACKAGES = "${PN}-server"
SUMMARY_${PN}-server = "NFS server"
RDEPENDS_${PN}-server = "
nfs-utils
nfs-utils-client
"

Practical lab - Create a custom image
Write an image recipe
Choose the packages to install

Creating and using an SDK
Creating and using
an SDK
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Introduction to the SDK

Overview
An SDK (Software Development Kit) is a set of tools allowing
the development of applications for a given target (operating
system, platform, environment. . . ).
It generally provides a set of of tools including:
Compilers or cross-compilers.
Linkers.
Library headers.
Debuggers.
Custom utilities.

Advantages
The Yocto Project is often used to build images for embedded
targets.
This often requires a special toolchain, to cross compile the
software.
Some libraries headers may be speciﬁc to the target and not
available on the developers’ computers.
A self-suﬃcient environment makes development easier and
avoids many errors.
Long manuals are not necessary, the only thing required is the
SDK!
Using the SDK to develop an application limits the risks of
dependency issues when running it on the target.

The Poky SDK
The Poky reference system is used to generate images, by
building many applications and doing a lot conﬁguration work.
When developing an application, we only care about the
application itself.
We want to be able to develop, test and debug easily.
The Poky SDK is an application development SDK, which can
be generated to provide a full environment compatible with
the target.
It includes a toolchain, libraries headers and all the needed
tools.
This SDK can be installed on any computer and is
self-contained. The presence of Poky is not required for the
SDK to fully work.

Generating an SDK

Available SDKs
Two diﬀerent SDKs can be generated:
A generic SDK, including:
A toolchain.
Common tools.
A collection of basic libraries.
An image-based SDK, including:
The generic SDK.
The sysroot matching the target root ﬁlesystem.
Its toolchain is self-contained (linked to an SDK embedded
libc).
The SDKs generated with Poky are distributed in the form of
a shell script.
Executing this script extracts the tools and sets up the
environment.

The generic SDK
Mainly used for low-level development, where only the
toolchain is needed:
Bootloader development.
Kernel development.
The recipe meta-toolchain generates this SDK:
bitbake meta-toolchain
The generated script, containing all the tools for this SDK, is
in:
build/tmp/deploy/sdk
Example: poky-eglibc-x86_64-meta-toolchain-i586-
toolchain-1.5.3.sh
The SDK will be conﬁgured to be compatible with the
speciﬁed MACHINE.

The image-based SDK
Used to develop applications running on the target.
One task is dedicated to the process. The task behavior can
vary between the images.
populate_sdk
To generate an SDK for core-image-minimal:
bitbake -c populate_sdk core-image-minimal
The generated script, containing all the tools for this SDK, is
in:
build/tmp/deploy/sdk
Example: poky-eglibc-x86_64-core-image-minimal-
i586-toolchain-1.5.3.sh
The SDK will be conﬁgured to be compatible with the
speciﬁed MACHINE.

Use the SDK

SDK format
Both SDKs are distributed as bash scripts.
These scripts self extract themselves to install the toolchains
and the ﬁles they provide.
To install an SDK, retrieve the generated script and execute
it.
The script asks where to install the SDK. Defaults to
/opt/poky/<version>
Example: /opt/poky/1.5.3
$ ./poky-eglibc-x86_64-meta-toolchain-i586-toolchain-1.5.3.sh
Enter target directory for SDK (default: /opt/poky/1.5.3):
You are about to install the SDK to "/opt/poky/1.5.3". Proceed[Y/n]?
Extracting SDK...done
Setting it up...done
SDK has been successfully set up and is ready to be used.

Use the SDK
To use the SDK, a script is available to set up the
environment:
$ cd /opt/poky/1.5.3
$ source ./environment-setup-i586-poky-linux
The PATH is updated to take into account the binaries
installed alongside the SDK.
Environment variables are exported to help using the tools.

SDK installation
environment-setup-i586-poky-linux Exports environment variables.
site-conﬁg-i586-poky-linux Variables used during the toolchain
creation
sysroots SDK binaries, headers and libraries. Contains one
directory for the host and one for the target.
version-i586-poky-linux Version information.

SDK environment variables
CC Full path to the C compiler binary.
CFLAGS C flags, used by the C compiler.
CXX C++ compiler.
CXXFLAGS C++ flags, used by CPP
LD Linker.
LDFLAGS Link flags, used by the linker.
ARCH For kernel compilation.
CROSS COMPILE For kernel compilation.
GDB SDK GNU Debugger.
OBJDUMP SDK objdump.
To see the full list, open the environment script.

Examples
To build an application for the target:
$ $CC -o example example.c
The LDFLAGS variables is set to be used with the C compiler
(gcc).
When building the Linux kernel, unset this variable.
$ unset LDFLAGS
$ make menuconfig
$ make

Eclipse integration

The Yocto Project ADT
Eclipse can be used as a graphical interface to the Yocto
SDK.
To modify and develop recipes and applications.
To compile packages and build images.
To debug the system.
This is done thanks to an Eclipse Yocto plugin.
The SDK and the Eclipse plugin is called the Yocto
Application Development Toolkit.

Installation
The Eclipse Yocto plugin can be installed from a pre-built
version available at:
http://downloads.yoctoproject.org/releases/
eclipse-plugin/1.6.1/kepler
Or it can be compiled from the oﬃcial repository:
git://git.yoctoproject.org/eclipse-poky-kepler
The full explanations to install and use the Eclipse Yocto
plugin are given in the labs.

Practical lab - Create and use a Poky SDK
Generate an SDK
Compile an application for the
target in the SDK

Practical lab - Use the Yocto SDK through Eclipse
Install the Yocto Eclipse plugin
Conﬁgure the plugin to work with
the previously used environment
Use Eclipse to modify recipes
Build an image from Eclipse

Licensing
Licensing
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Licensing
Managing licenses

Tracking license changes
The license of an external project may change at some point.
The LIC_FILES_CHKSUM tracks changes in the license ﬁles.
If the license’s checksum changes, the build will fail.
The recipe needs to be updated.
LIC_FILES_CHKSUM = "
file://COPYING;md5=...
file://src/file.c;beginline=3;endline=21;md5=..."
LIC_FILES_CHKSUM is mandatory in every recipe, unless
LICENSE is set to CLOSED.

Package exclusion
We may not want some packages due to their licenses.
To exclude a speciﬁc license, use INCOMPATIBLE_LICENSE
To exclude all GPLv3 packages:
INCOMPATIBLE_LICENSE = "GPLv3"
License names are the ones used in the LICENSE variable.

Commercial licenses
By default the build system does not include commercial
components.
Packages with a commercial component deﬁne:
LICENSE_FLAGS = "commercial"
To build a package with a commercial component, the
package must be in the LICENSE_FLAGS_WHITELIST variable.
Example, gst-plugins-ugly:
LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly"

Writing recipes - going further
Writing recipes -
going further
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Splitting packages

Beneﬁts
Packages can be split.
Useful when a single remote repository provides multiple
binaries or libraries.
The list of packages to provide is deﬁned by the PACKAGES
variable.

Example
The kexec tools provides kexec and kdump:
require kexec-tools.inc
export LDFLAGS = "-L${STAGING_LIBDIR}"
EXTRA_OECONF = " --with-zlib=yes"
SRC_URI[md5sum] = "b9f2a3ba0ba9c78625ee7a50532500d8"
SRC_URI[sha256sum] = "..."
PACKAGES =+ "kexec kdump"
FILES_kexec = "${sbindir}/kexec"
FILES_kdump = "${sbindir}/kdump"

Packages features

Beneﬁts
Features can be built depending on the needs.
This allows to avoid compiling all features in a software
component when only a few are required.
A good example is ConnMan: Bluetooth support is built only if
there is Bluetooth on the target.
The PACKAGECONFIG variable is used to conﬁgure the build on
a per feature granularity, for packages.

PACKAGECONFIG
PACKAGECONFIG takes the list of features to enable.
PACKAGECONFIG[feature] takes up to four arguments,
separated by commas:
1. Argument used by the conﬁguration task if the feature is
enabled (EXTRA_OECONF).
2. Argument added to EXTRA_OECONF if the feature is disabled.
3. Additional build dependency (DEPENDS), if enabled.
4. Additional runtime dependency (RDEPENDS), if enabled.
Unused arguments can be omitted or left blank.

Example: from ConnMan
PACKAGECONFIG ??= "wifi openvpn"
PACKAGECONFIG[wifi] = "--enable-wifi,
--disable-wifi,
wpa-supplicant"
PACKAGECONFIG[bluetooth] = "--enable-bluetooth,
--disable-bluetooth,
bluez4"
PACKAGECONFIG[openvpn] = "--enable-openvpn,
--with-openvpn=...,
--disable-openvpn,
,
openvpn"

Conditional features

Conditional features
Some values can be set dynamically, thanks to a set of
functions:
base_contains(variable, checkval, trueval,
falseval, d): if checkval is found in variable, trueval
is returned; otherwise falseval is used.
Example:
PACKAGECONFIG ??= "
${@base_contains(’DISTRO_FEATURES’, ’wifi’,’wifi’, ’’, d)}
${@base_contains(’DISTRO_FEATURES’, ’bluetooth’,’bluetooth’, ’’, d)}
${@base_contains(’DISTRO_FEATURES’, ’3g’,’3g’, ’’, d)}"

Applying patches

Applying patches
Files ending in .patch, .diff or with the apply=yes
parameter will be applied during the do_patch task.
It is possible to select which tool will be used to apply the
patches listed in SRC_URI variable with PATCHTOOL.
By default, PATCHTOOL = ’quilt’ in Poky.
Possible values: git, patch and quilt.
See meta/classes/patch.bbclass

Resolving conflicts
The PATCHRESOLVE variable defines how to handle conflicts
when applying patches.
It has two valid values:
noop: the build fails if a patch cannot be successfully applied.
user: a shell is launched to resolve manually the conflicts.
By default, PATCHRESOLVE = "noop" in meta-yocto.

Yocto Project Resources
Yocto Project
Resources
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Yocto Project documentation
https://www.yoctoproject.org/documentation
Wiki: https://wiki.yoctoproject.org/wiki/Main_Page
http://packages.yoctoproject.org

Useful Reading
Embedded Linux Development with Yocto
Project, July 2014
https://www.packtpub.com/
application-development/embedded-
linux-development-yocto-project
By Otavio Salvador and Daiane Angolini
From basic to advanced usage, helps
writing better, more ﬂexible recipes. A
good reference to jumpstart your Yocto
Project development.

Last slides
Last slides
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Last slide
Thank you!
And may the Source be with you

Yocto project and open embedded training

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