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poky/documentation/getting-started/getting-started-concepts.xml
Scott Rifenbark 52b871825f getting-started: Changed chapter id 
(From yocto-docs rev: 52cc6ae12c5c92f6f8e508571b943443a6d133f1)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
2018-02-14 15:25:30 +00:00

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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
[<!ENTITY % poky SYSTEM "../poky.ent"> %poky; ] >
<chapter id='getting-started-concepts'>
<title>Yocto Project Concepts</title>
<para>
This chapter describes concepts for various areas of the Yocto Project.
Currently, topics include Yocto Project components, cross-development
generation, shared state (sstate) cache, runtime dependencies,
Pseudo and Fakeroot, x32 psABI, Wayland support, and Licenses.
</para>
<section id='yocto-project-components'>
<title>Yocto Project Components</title>
<para>
The
<ulink url='&YOCTO_DOCS_REF_URL;#bitbake-term'>BitBake</ulink>
task executor together with various types of configuration files
form the OpenEmbedded Core.
This section overviews these components by describing their use and
how they interact.
</para>
<para>
BitBake handles the parsing and execution of the data files.
The data itself is of various types:
<itemizedlist>
<listitem><para>
<emphasis>Recipes:</emphasis>
Provides details about particular pieces of software.
</para></listitem>
<listitem><para>
<emphasis>Class Data:</emphasis>
Abstracts common build information (e.g. how to build a
Linux kernel).
</para></listitem>
<listitem><para>
<emphasis>Configuration Data:</emphasis>
Defines machine-specific settings, policy decisions, and
so forth.
Configuration data acts as the glue to bind everything
together.
</para></listitem>
</itemizedlist>
</para>
<para>
BitBake knows how to combine multiple data sources together and
refers to each data source as a layer.
For information on layers, see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers'>Understanding and Creating Layers</ulink>"
section of the Yocto Project Development Tasks Manual.
</para>
<para>
Following are some brief details on these core components.
For additional information on how these components interact during
a build, see the
"<link linkend='development-concepts'>Development Concepts</link>"
section.
</para>
<section id='usingpoky-components-bitbake'>
<title>BitBake</title>
<para>
BitBake is the tool at the heart of the OpenEmbedded build
system and is responsible for parsing the
<ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink>,
generating a list of tasks from it, and then executing those
tasks.
</para>
<para>
This section briefly introduces BitBake.
If you want more information on BitBake, see the
<ulink url='&YOCTO_DOCS_BB_URL;#bitbake-user-manual'>BitBake User Manual</ulink>.
</para>
<para>
To see a list of the options BitBake supports, use either of
the following commands:
<literallayout class='monospaced'>
$ bitbake -h
$ bitbake --help
</literallayout>
</para>
<para>
The most common usage for BitBake is
<filename>bitbake <replaceable>packagename</replaceable></filename>,
where <filename>packagename</filename> is the name of the
package you want to build (referred to as the "target" in this
manual).
The target often equates to the first part of a recipe's
filename (e.g. "foo" for a recipe named
<filename>foo_1.3.0-r0.bb</filename>).
So, to process the
<filename>matchbox-desktop_1.2.3.bb</filename> recipe file, you
might type the following:
<literallayout class='monospaced'>
$ bitbake matchbox-desktop
</literallayout>
Several different versions of
<filename>matchbox-desktop</filename> might exist.
BitBake chooses the one selected by the distribution
configuration.
You can get more details about how BitBake chooses between
different target versions and providers in the
"<ulink url='&YOCTO_DOCS_BB_URL;#bb-bitbake-preferences'>Preferences</ulink>"
section of the BitBake User Manual.
</para>
<para>
BitBake also tries to execute any dependent tasks first.
So for example, before building
<filename>matchbox-desktop</filename>, BitBake would build a
cross compiler and <filename>glibc</filename> if they had not
already been built.
</para>
<para>
A useful BitBake option to consider is the
<filename>-k</filename> or <filename>--continue</filename>
option.
This option instructs BitBake to try and continue processing
the job as long as possible even after encountering an error.
When an error occurs, the target that failed and those that
depend on it cannot be remade.
However, when you use this option other dependencies can
still be processed.
</para>
</section>
<section id='usingpoky-components-metadata'>
<title>Metadata (Recipes)</title>
<para>
Files that have the <filename>.bb</filename> suffix are
"recipes" files.
In general, a recipe contains information about a single piece
of software.
This information includes the location from which to download
the unaltered source, any source patches to be applied to that
source (if needed), which special configuration options to
apply, how to compile the source files, and how to package the
compiled output.
</para>
<para>
The term "package" is sometimes used to refer to recipes.
However, since the word "package" is used for the packaged
output from the OpenEmbedded build system (i.e.
<filename>.ipk</filename> or <filename>.deb</filename> files),
this document avoids using the term "package" when referring
to recipes.
</para>
</section>
<section id='metadata-virtual-providers'>
<title>Metadata (Virtual Providers)</title>
<para>
Prior to the build, if you know that several different recipes
provide the same functionality, you can use a virtual provider
(i.e. <filename>virtual/*</filename>) as a placeholder for the
actual provider.
The actual provider would be determined at build time.
In this case, you should add <filename>virtual/*</filename>
to
<ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>,
rather than listing the specified provider.
You would select the actual provider by setting the
<ulink url='&YOCTO_DOCS_REF_URL;#var-PREFERRED_PROVIDER'><filename>PREFERRED_PROVIDER</filename></ulink>
variable (i.e.
<filename>PREFERRED_PROVIDER_virtual/*</filename>)
in the build's configuration file (e.g.
<filename>poky/build/conf/local.conf</filename>).
<note>
Any recipe that PROVIDES a <filename>virtual/*</filename>
item that is ultimately not selected through
<filename>PREFERRED_PROVIDER</filename> does not get built.
Preventing these recipes from building is usually the
desired behavior since this mechanism's purpose is to
select between mutually exclusive alternative providers.
</note>
</para>
<para>
The following lists specific examples of virtual providers:
<itemizedlist>
<listitem><para>
<filename>virtual/mesa</filename>:
Provides <filename>gbm.pc</filename>.
</para></listitem>
<listitem><para>
<filename>virtual/egl</filename>:
Provides <filename>egl.pc</filename> and possibly
<filename>wayland-egl.pc</filename>.
</para></listitem>
<listitem><para>
<filename>virtual/libgl</filename>:
Provides <filename>gl.pc</filename> (i.e. libGL).
</para></listitem>
<listitem><para>
<filename>virtual/libgles1</filename>:
Provides <filename>glesv1_cm.pc</filename>
(i.e. libGLESv1_CM).
</para></listitem>
<listitem><para>
<filename>virtual/libgles2</filename>:
Provides <filename>glesv2.pc</filename>
(i.e. libGLESv2).
</para></listitem>
</itemizedlist>
</para>
</section>
<section id='usingpoky-components-classes'>
<title>Classes</title>
<para>
Class files (<filename>.bbclass</filename>) contain information
that is useful to share between
<ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink>
files.
An example is the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-autotools'><filename>autotools</filename></ulink>
class, which contains common settings for any application that
Autotools uses.
The
"<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes'>Classes</ulink>"
chapter in the Yocto Project Reference Manual provides
details about classes and how to use them.
</para>
</section>
<section id='usingpoky-components-configuration'>
<title>Configuration</title>
<para>
The configuration files (<filename>.conf</filename>) define
various configuration variables that govern the OpenEmbedded
build process.
These files fall into several areas that define machine
configuration options, distribution configuration options,
compiler tuning options, general common configuration options,
and user configuration options in
<filename>local.conf</filename>, which is found in the
<ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>.
</para>
</section>
</section>
<section id="cross-development-toolchain-generation">
<title>Cross-Development Toolchain Generation</title>
<para>
The Yocto Project does most of the work for you when it comes to
creating
<ulink url='&YOCTO_DOCS_REF_URL;#cross-development-toolchain'>cross-development toolchains</ulink>.
This section provides some technical background on how
cross-development toolchains are created and used.
For more information on toolchains, you can also see the
<ulink url='&YOCTO_DOCS_SDK_URL;'>Yocto Project Application Development and the Extensible Software Development Kit (eSDK)</ulink>
manual.
</para>
<para>
In the Yocto Project development environment, cross-development
toolchains are used to build the image and applications that run
on the target hardware.
With just a few commands, the OpenEmbedded build system creates
these necessary toolchains for you.
</para>
<para>
The following figure shows a high-level build environment regarding
toolchain construction and use.
</para>
<para>
<imagedata fileref="figures/cross-development-toolchains.png" width="8in" depth="6in" align="center" />
</para>
<para>
Most of the work occurs on the Build Host.
This is the machine used to build images and generally work within the
the Yocto Project environment.
When you run BitBake to create an image, the OpenEmbedded build system
uses the host <filename>gcc</filename> compiler to bootstrap a
cross-compiler named <filename>gcc-cross</filename>.
The <filename>gcc-cross</filename> compiler is what BitBake uses to
compile source files when creating the target image.
You can think of <filename>gcc-cross</filename> simply as an
automatically generated cross-compiler that is used internally within
BitBake only.
<note>
The extensible SDK does not use
<filename>gcc-cross-canadian</filename> since this SDK
ships a copy of the OpenEmbedded build system and the sysroot
within it contains <filename>gcc-cross</filename>.
</note>
</para>
<para>
The chain of events that occurs when <filename>gcc-cross</filename> is
bootstrapped is as follows:
<literallayout class='monospaced'>
gcc -> binutils-cross -> gcc-cross-initial -> linux-libc-headers -> glibc-initial -> glibc -> gcc-cross -> gcc-runtime
</literallayout>
<itemizedlist>
<listitem><para>
<filename>gcc</filename>:
The build host's GNU Compiler Collection (GCC).
</para></listitem>
<listitem><para>
<filename>binutils-cross</filename>:
The bare minimum binary utilities needed in order to run
the <filename>gcc-cross-initial</filename> phase of the
bootstrap operation.
</para></listitem>
<listitem><para>
<filename>gcc-cross-initial</filename>:
An early stage of the bootstrap process for creating
the cross-compiler.
This stage builds enough of the <filename>gcc-cross</filename>,
the C library, and other pieces needed to finish building the
final cross-compiler in later stages.
This tool is a "native" package (i.e. it is designed to run on
the build host).
</para></listitem>
<listitem><para>
<filename>linux-libc-headers</filename>:
Headers needed for the cross-compiler.
</para></listitem>
<listitem><para>
<filename>glibc-initial</filename>:
An initial version of the Embedded GLIBC needed to bootstrap
<filename>glibc</filename>.
</para></listitem>
<listitem><para>
<filename>gcc-cross</filename>:
The final stage of the bootstrap process for the
cross-compiler.
This stage results in the actual cross-compiler that
BitBake uses when it builds an image for a targeted
device.
<note>
If you are replacing this cross compiler toolchain
with a custom version, you must replace
<filename>gcc-cross</filename>.
</note>
This tool is also a "native" package (i.e. it is
designed to run on the build host).
</para></listitem>
<listitem><para>
<filename>gcc-runtime</filename>:
Runtime libraries resulting from the toolchain bootstrapping
process.
This tool produces a binary that consists of the
runtime libraries need for the targeted device.
</para></listitem>
</itemizedlist>
</para>
<para>
You can use the OpenEmbedded build system to build an installer for
the relocatable SDK used to develop applications.
When you run the installer, it installs the toolchain, which contains
the development tools (e.g., the
<filename>gcc-cross-canadian</filename>),
<filename>binutils-cross-canadian</filename>, and other
<filename>nativesdk-*</filename> tools,
which are tools native to the SDK (i.e. native to
<ulink url='&YOCTO_DOCS_REF_URL;#var-SDK_ARCH'><filename>SDK_ARCH</filename></ulink>),
you need to cross-compile and test your software.
The figure shows the commands you use to easily build out this
toolchain.
This cross-development toolchain is built to execute on the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SDKMACHINE'><filename>SDKMACHINE</filename></ulink>,
which might or might not be the same
machine as the Build Host.
<note>
If your target architecture is supported by the Yocto Project,
you can take advantage of pre-built images that ship with the
Yocto Project and already contain cross-development toolchain
installers.
</note>
</para>
<para>
Here is the bootstrap process for the relocatable toolchain:
<literallayout class='monospaced'>
gcc -> binutils-crosssdk -> gcc-crosssdk-initial -> linux-libc-headers ->
glibc-initial -> nativesdk-glibc -> gcc-crosssdk -> gcc-cross-canadian
</literallayout>
<itemizedlist>
<listitem><para>
<filename>gcc</filename>:
The build host's GNU Compiler Collection (GCC).
</para></listitem>
<listitem><para>
<filename>binutils-crosssdk</filename>:
The bare minimum binary utilities needed in order to run
the <filename>gcc-crosssdk-initial</filename> phase of the
bootstrap operation.
</para></listitem>
<listitem><para>
<filename>gcc-crosssdk-initial</filename>:
An early stage of the bootstrap process for creating
the cross-compiler.
This stage builds enough of the
<filename>gcc-crosssdk</filename> and supporting pieces so that
the final stage of the bootstrap process can produce the
finished cross-compiler.
This tool is a "native" binary that runs on the build host.
</para></listitem>
<listitem><para>
<filename>linux-libc-headers</filename>:
Headers needed for the cross-compiler.
</para></listitem>
<listitem><para>
<filename>glibc-initial</filename>:
An initial version of the Embedded GLIBC needed to bootstrap
<filename>nativesdk-glibc</filename>.
</para></listitem>
<listitem><para>
<filename>nativesdk-glibc</filename>:
The Embedded GLIBC needed to bootstrap the
<filename>gcc-crosssdk</filename>.
</para></listitem>
<listitem><para>
<filename>gcc-crosssdk</filename>:
The final stage of the bootstrap process for the
relocatable cross-compiler.
The <filename>gcc-crosssdk</filename> is a transitory compiler
and never leaves the build host.
Its purpose is to help in the bootstrap process to create the
eventual relocatable <filename>gcc-cross-canadian</filename>
compiler, which is relocatable.
This tool is also a "native" package (i.e. it is
designed to run on the build host).
</para></listitem>
<listitem><para>
<filename>gcc-cross-canadian</filename>:
The final relocatable cross-compiler.
When run on the
<ulink url='&YOCTO_DOCS_REF_URL;#var-SDKMACHINE'><filename>SDKMACHINE</filename></ulink>,
this tool
produces executable code that runs on the target device.
Only one cross-canadian compiler is produced per architecture
since they can be targeted at different processor optimizations
using configurations passed to the compiler through the
compile commands.
This circumvents the need for multiple compilers and thus
reduces the size of the toolchains.
</para></listitem>
</itemizedlist>
</para>
<note>
For information on advantages gained when building a
cross-development toolchain installer, see the
"<ulink url='&YOCTO_DOCS_SDK_URL;#sdk-building-an-sdk-installer'>Building an SDK Installer</ulink>"
section in the Yocto Project Application Development and the
Extensible Software Development Kit (eSDK) manual.
</note>
</section>
<section id="shared-state-cache">
<title>Shared State Cache</title>
<para>
By design, the OpenEmbedded build system builds everything from
scratch unless BitBake can determine that parts do not need to be
rebuilt.
Fundamentally, building from scratch is attractive as it means all
parts are built fresh and there is no possibility of stale data
causing problems.
When developers hit problems, they typically default back to
building from scratch so they know the state of things from the
start.
</para>
<para>
Building an image from scratch is both an advantage and a
disadvantage to the process.
As mentioned in the previous paragraph, building from scratch
ensures that everything is current and starts from a known state.
However, building from scratch also takes much longer as it
generally means rebuilding things that do not necessarily need
to be rebuilt.
</para>
<para>
The Yocto Project implements shared state code that supports
incremental builds.
The implementation of the shared state code answers the following
questions that were fundamental roadblocks within the OpenEmbedded
incremental build support system:
<itemizedlist>
<listitem><para>
What pieces of the system have changed and what pieces have
not changed?
</para></listitem>
<listitem><para>
How are changed pieces of software removed and replaced?
</para></listitem>
<listitem><para>
How are pre-built components that do not need to be rebuilt
from scratch used when they are available?
</para></listitem>
</itemizedlist>
</para>
<para>
For the first question, the build system detects changes in the
"inputs" to a given task by creating a checksum (or signature) of
the task's inputs.
If the checksum changes, the system assumes the inputs have changed
and the task needs to be rerun.
For the second question, the shared state (sstate) code tracks
which tasks add which output to the build process.
This means the output from a given task can be removed, upgraded
or otherwise manipulated.
The third question is partly addressed by the solution for the
second question assuming the build system can fetch the sstate
objects from remote locations and install them if they are deemed
to be valid.
<note>
The OpenEmbedded build system does not maintain
<ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink>
information as part of the shared state packages.
Consequently, considerations exist that affect maintaining
shared state feeds.
For information on how the OpenEmbedded build system
works with packages and can track incrementing
<filename>PR</filename> information, see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#automatically-incrementing-a-binary-package-revision-number'>Automatically Incrementing a Binary Package Revision Number</ulink>"
section in the Yocto Project Development Tasks Manual.
</note>
</para>
<para>
The rest of this section goes into detail about the overall
incremental build architecture, the checksums (signatures), shared
state, and some tips and tricks.
</para>
<section id='overall-architecture'>
<title>Overall Architecture</title>
<para>
When determining what parts of the system need to be built,
BitBake works on a per-task basis rather than a per-recipe
basis.
You might wonder why using a per-task basis is preferred over
a per-recipe basis.
To help explain, consider having the IPK packaging backend
enabled and then switching to DEB.
In this case, the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-install'><filename>do_install</filename></ulink>
and
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-package'><filename>do_package</filename></ulink>
task outputs are still valid.
However, with a per-recipe approach, the build would not
include the <filename>.deb</filename> files.
Consequently, you would have to invalidate the whole build and
rerun it.
Rerunning everything is not the best solution.
Also, in this case, the core must be "taught" much about
specific tasks.
This methodology does not scale well and does not allow users
to easily add new tasks in layers or as external recipes
without touching the packaged-staging core.
</para>
</section>
<section id='overview-checksums'>
<title>Checksums (Signatures)</title>
<para>
The shared state code uses a checksum, which is a unique
signature of a task's inputs, to determine if a task needs to
be run again.
Because it is a change in a task's inputs that triggers a
rerun, the process needs to detect all the inputs to a given
task.
For shell tasks, this turns out to be fairly easy because
the build process generates a "run" shell script for each task
and it is possible to create a checksum that gives you a good
idea of when the task's data changes.
</para>
<para>
To complicate the problem, there are things that should not be
included in the checksum.
First, there is the actual specific build path of a given
task - the
<ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink>.
It does not matter if the work directory changes because it
should not affect the output for target packages.
Also, the build process has the objective of making native
or cross packages relocatable.
<note>
Both native and cross packages run on the build host.
However, cross packages generate output for the target
architecture.
</note>
The checksum therefore needs to exclude
<filename>WORKDIR</filename>.
The simplistic approach for excluding the work directory is to
set <filename>WORKDIR</filename> to some fixed value and
create the checksum for the "run" script.
</para>
<para>
Another problem results from the "run" scripts containing
functions that might or might not get called.
The incremental build solution contains code that figures out
dependencies between shell functions.
This code is used to prune the "run" scripts down to the
minimum set, thereby alleviating this problem and making the
"run" scripts much more readable as a bonus.
</para>
<para>
So far we have solutions for shell scripts.
What about Python tasks?
The same approach applies even though these tasks are more
difficult.
The process needs to figure out what variables a Python
function accesses and what functions it calls.
Again, the incremental build solution contains code that first
figures out the variable and function dependencies, and then
creates a checksum for the data used as the input to the task.
</para>
<para>
Like the <filename>WORKDIR</filename> case, situations exist
where dependencies should be ignored.
For these cases, you can instruct the build process to
ignore a dependency by using a line like the following:
<literallayout class='monospaced'>
PACKAGE_ARCHS[vardepsexclude] = "MACHINE"
</literallayout>
This example ensures that the
<ulink url='&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCHS'><filename>PACKAGE_ARCHS</filename></ulink>
variable does not depend on the value of
<ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink>,
even if it does reference it.
</para>
<para>
Equally, there are cases where we need to add dependencies
BitBake is not able to find.
You can accomplish this by using a line like the following:
<literallayout class='monospaced'>
PACKAGE_ARCHS[vardeps] = "MACHINE"
</literallayout>
This example explicitly adds the <filename>MACHINE</filename>
variable as a dependency for
<filename>PACKAGE_ARCHS</filename>.
</para>
<para>
Consider a case with in-line Python, for example, where
BitBake is not able to figure out dependencies.
When running in debug mode (i.e. using
<filename>-DDD</filename>), BitBake produces output when it
discovers something for which it cannot figure out dependencies.
The Yocto Project team has currently not managed to cover
those dependencies in detail and is aware of the need to fix
this situation.
</para>
<para>
Thus far, this section has limited discussion to the direct
inputs into a task.
Information based on direct inputs is referred to as the
"basehash" in the code.
However, there is still the question of a task's indirect
inputs - the things that were already built and present in the
<ulink url='&YOCTO_DOCS_REF_URL;#build-directory'>Build Directory</ulink>.
The checksum (or signature) for a particular task needs to add
the hashes of all the tasks on which the particular task
depends.
Choosing which dependencies to add is a policy decision.
However, the effect is to generate a master checksum that
combines the basehash and the hashes of the task's
dependencies.
</para>
<para>
At the code level, there are a variety of ways both the
basehash and the dependent task hashes can be influenced.
Within the BitBake configuration file, we can give BitBake
some extra information to help it construct the basehash.
The following statement effectively results in a list of
global variable dependency excludes - variables never
included in any checksum:
<literallayout class='monospaced'>
BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH DL_DIR \
SSTATE_DIR THISDIR FILESEXTRAPATHS FILE_DIRNAME HOME LOGNAME SHELL TERM \
USER FILESPATH STAGING_DIR_HOST STAGING_DIR_TARGET COREBASE PRSERV_HOST \
PRSERV_DUMPDIR PRSERV_DUMPFILE PRSERV_LOCKDOWN PARALLEL_MAKE \
CCACHE_DIR EXTERNAL_TOOLCHAIN CCACHE CCACHE_DISABLE LICENSE_PATH SDKPKGSUFFIX"
</literallayout>
The previous example excludes
<ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink>
since that variable is actually constructed as a path within
<ulink url='&YOCTO_DOCS_REF_URL;#var-TMPDIR'><filename>TMPDIR</filename></ulink>,
which is on the whitelist.
</para>
<para>
The rules for deciding which hashes of dependent tasks to
include through dependency chains are more complex and are
generally accomplished with a Python function.
The code in <filename>meta/lib/oe/sstatesig.py</filename> shows
two examples of this and also illustrates how you can insert
your own policy into the system if so desired.
This file defines the two basic signature generators
<ulink url='&YOCTO_DOCS_REF_URL;#oe-core'>OE-Core</ulink>
uses: "OEBasic" and "OEBasicHash".
By default, there is a dummy "noop" signature handler enabled
in BitBake.
This means that behavior is unchanged from previous versions.
OE-Core uses the "OEBasicHash" signature handler by default
through this setting in the <filename>bitbake.conf</filename>
file:
<literallayout class='monospaced'>
BB_SIGNATURE_HANDLER ?= "OEBasicHash"
</literallayout>
The "OEBasicHash" <filename>BB_SIGNATURE_HANDLER</filename>
is the same as the "OEBasic" version but adds the task hash to
the stamp files.
This results in any
<ulink url='&YOCTO_DOCS_REF_URL;#metadata'>Metadata</ulink>
change that changes the task hash, automatically
causing the task to be run again.
This removes the need to bump
<ulink url='&YOCTO_DOCS_REF_URL;#var-PR'><filename>PR</filename></ulink>
values, and changes to Metadata automatically ripple across
the build.
</para>
<para>
It is also worth noting that the end result of these
signature generators is to make some dependency and hash
information available to the build.
This information includes:
<itemizedlist>
<listitem><para>
<filename>BB_BASEHASH_task-</filename><replaceable>taskname</replaceable>:
The base hashes for each task in the recipe.
</para></listitem>
<listitem><para>
<filename>BB_BASEHASH_</filename><replaceable>filename</replaceable><filename>:</filename><replaceable>taskname</replaceable>:
The base hashes for each dependent task.
</para></listitem>
<listitem><para>
<filename>BBHASHDEPS_</filename><replaceable>filename</replaceable><filename>:</filename><replaceable>taskname</replaceable>:
The task dependencies for each task.
</para></listitem>
<listitem><para>
<filename>BB_TASKHASH</filename>:
The hash of the currently running task.
</para></listitem>
</itemizedlist>
</para>
</section>
<section id='shared-state'>
<title>Shared State</title>
<para>
Checksums and dependencies, as discussed in the previous
section, solve half the problem of supporting a shared state.
The other part of the problem is being able to use checksum
information during the build and being able to reuse or rebuild
specific components.
</para>
<para>
The
<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-sstate'><filename>sstate</filename></ulink>
class is a relatively generic implementation of how to
"capture" a snapshot of a given task.
The idea is that the build process does not care about the
source of a task's output.
Output could be freshly built or it could be downloaded and
unpacked from somewhere - the build process does not need to
worry about its origin.
</para>
<para>
There are two types of output, one is just about creating a
directory in
<ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink>.
A good example is the output of either
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-install'><filename>do_install</filename></ulink>
or
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-package'><filename>do_package</filename></ulink>.
The other type of output occurs when a set of data is merged
into a shared directory tree such as the sysroot.
</para>
<para>
The Yocto Project team has tried to keep the details of the
implementation hidden in <filename>sstate</filename> class.
From a user's perspective, adding shared state wrapping to a task
is as simple as this
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-deploy'><filename>do_deploy</filename></ulink>
example taken from the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-deploy'><filename>deploy</filename></ulink>
class:
<literallayout class='monospaced'>
DEPLOYDIR = "${WORKDIR}/deploy-${PN}"
SSTATETASKS += "do_deploy"
do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"
do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"
python do_deploy_setscene () {
sstate_setscene(d)
}
addtask do_deploy_setscene
do_deploy[dirs] = "${DEPLOYDIR} ${B}"
</literallayout>
The following list explains the previous example:
<itemizedlist>
<listitem><para>
Adding "do_deploy" to <filename>SSTATETASKS</filename>
adds some required sstate-related processing, which is
implemented in the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-classes-sstate'><filename>sstate</filename></ulink>
class, to before and after the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-deploy'><filename>do_deploy</filename></ulink>
task.
</para></listitem>
<listitem><para>
The
<filename>do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"</filename>
declares that <filename>do_deploy</filename> places its
output in <filename>${DEPLOYDIR}</filename> when run
normally (i.e. when not using the sstate cache).
This output becomes the input to the shared state cache.
</para></listitem>
<listitem><para>
The
<filename>do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"</filename>
line causes the contents of the shared state cache to be
copied to <filename>${DEPLOY_DIR_IMAGE}</filename>.
<note>
If <filename>do_deploy</filename> is not already in
the shared state cache or if its input checksum
(signature) has changed from when the output was
cached, the task will be run to populate the shared
state cache, after which the contents of the shared
state cache is copied to
<filename>${DEPLOY_DIR_IMAGE}</filename>.
If <filename>do_deploy</filename> is in the shared
state cache and its signature indicates that the
cached output is still valid (i.e. if no
relevant task inputs have changed), then the
contents of the shared state cache will be copied
directly to
<filename>${DEPLOY_DIR_IMAGE}</filename> by the
<filename>do_deploy_setscene</filename> task
instead, skipping the
<filename>do_deploy</filename> task.
</note>
</para></listitem>
<listitem><para>
The following task definition is glue logic needed to
make the previous settings effective:
<literallayout class='monospaced'>
python do_deploy_setscene () {
sstate_setscene(d)
}
addtask do_deploy_setscene
</literallayout>
<filename>sstate_setscene()</filename> takes the flags
above as input and accelerates the
<filename>do_deploy</filename> task through the
shared state cache if possible.
If the task was accelerated,
<filename>sstate_setscene()</filename> returns True.
Otherwise, it returns False, and the normal
<filename>do_deploy</filename> task runs.
For more information, see the
"<ulink url='&YOCTO_DOCS_BB_URL;#setscene'>setscene</ulink>"
section in the BitBake User Manual.
</para></listitem>
<listitem><para>
The <filename>do_deploy[dirs] = "${DEPLOYDIR} ${B}"</filename>
line creates <filename>${DEPLOYDIR}</filename> and
<filename>${B}</filename> before the
<filename>do_deploy</filename> task runs, and also sets
the current working directory of
<filename>do_deploy</filename> to
<filename>${B}</filename>.
For more information, see the
"<ulink url='&YOCTO_DOCS_BB_URL;#variable-flags'>Variable Flags</ulink>"
section in the BitBake User Manual.
<note>
In cases where
<filename>sstate-inputdirs</filename> and
<filename>sstate-outputdirs</filename> would be the
same, you can use
<filename>sstate-plaindirs</filename>.
For example, to preserve the
<filename>${PKGD}</filename> and
<filename>${PKGDEST}</filename> output from the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-package'><filename>do_package</filename></ulink>
task, use the following:
<literallayout class='monospaced'>
do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}"
</literallayout>
</note>
</para></listitem>
<listitem><para>
<filename>sstate-inputdirs</filename> and
<filename>sstate-outputdirs</filename> can also be used
with multiple directories.
For example, the following declares
<filename>PKGDESTWORK</filename> and
<filename>SHLIBWORK</filename> as shared state
input directories, which populates the shared state
cache, and <filename>PKGDATA_DIR</filename> and
<filename>SHLIBSDIR</filename> as the corresponding
shared state output directories:
<literallayout class='monospaced'>
do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}"
do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}"
</literallayout>
</para></listitem>
<listitem><para>
These methods also include the ability to take a
lockfile when manipulating shared state directory
structures, for cases where file additions or removals
are sensitive:
<literallayout class='monospaced'>
do_package[sstate-lockfile] = "${PACKAGELOCK}"
</literallayout>
</para></listitem>
</itemizedlist>
</para>
<para>
Behind the scenes, the shared state code works by looking in
<ulink url='&YOCTO_DOCS_REF_URL;#var-SSTATE_DIR'><filename>SSTATE_DIR</filename></ulink>
and
<ulink url='&YOCTO_DOCS_REF_URL;#var-SSTATE_MIRRORS'><filename>SSTATE_MIRRORS</filename></ulink>
for shared state files.
Here is an example:
<literallayout class='monospaced'>
SSTATE_MIRRORS ?= "\
file://.* http://someserver.tld/share/sstate/PATH;downloadfilename=PATH \n \
file://.* file:///some/local/dir/sstate/PATH"
</literallayout>
<note>
The shared state directory
(<filename>SSTATE_DIR</filename>) is organized into
two-character subdirectories, where the subdirectory
names are based on the first two characters of the hash.
If the shared state directory structure for a mirror has the
same structure as <filename>SSTATE_DIR</filename>, you must
specify "PATH" as part of the URI to enable the build system
to map to the appropriate subdirectory.
</note>
</para>
<para>
The shared state package validity can be detected just by
looking at the filename since the filename contains the task
checksum (or signature) as described earlier in this section.
If a valid shared state package is found, the build process
downloads it and uses it to accelerate the task.
</para>
<para>
The build processes use the <filename>*_setscene</filename>
tasks for the task acceleration phase.
BitBake goes through this phase before the main execution
code and tries to accelerate any tasks for which it can find
shared state packages.
If a shared state package for a task is available, the
shared state package is used.
This means the task and any tasks on which it is dependent
are not executed.
</para>
<para>
As a real world example, the aim is when building an IPK-based
image, only the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_ipk'><filename>do_package_write_ipk</filename></ulink>
tasks would have their shared state packages fetched and
extracted.
Since the sysroot is not used, it would never get extracted.
This is another reason why a task-based approach is preferred
over a recipe-based approach, which would have to install the
output from every task.
</para>
</section>
<section id='tips-and-tricks'>
<title>Tips and Tricks</title>
<para>
The code in the build system that supports incremental builds
is not simple code.
This section presents some tips and tricks that help you work
around issues related to shared state code.
</para>
<section id='overview-debugging'>
<title>Debugging</title>
<para>
Seeing what metadata went into creating the input signature
of a shared state (sstate) task can be a useful debugging
aid.
This information is available in signature information
(<filename>siginfo</filename>) files in
<ulink url='&YOCTO_DOCS_REF_URL;#var-SSTATE_DIR'><filename>SSTATE_DIR</filename></ulink>.
For information on how to view and interpret information in
<filename>siginfo</filename> files, see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#dev-viewing-task-variable-dependencies'>Viewing Task Variable Dependencies</ulink>"
section in the Yocto Project Development Tasks Manual.
</para>
</section>
<section id='invalidating-shared-state'>
<title>Invalidating Shared State</title>
<para>
The OpenEmbedded build system uses checksums and shared
state cache to avoid unnecessarily rebuilding tasks.
Collectively, this scheme is known as "shared state code."
</para>
<para>
As with all schemes, this one has some drawbacks.
It is possible that you could make implicit changes to your
code that the checksum calculations do not take into
account.
These implicit changes affect a task's output but do not
trigger the shared state code into rebuilding a recipe.
Consider an example during which a tool changes its output.
Assume that the output of <filename>rpmdeps</filename>
changes.
The result of the change should be that all the
<filename>package</filename> and
<filename>package_write_rpm</filename> shared state cache
items become invalid.
However, because the change to the output is
external to the code and therefore implicit,
the associated shared state cache items do not become
invalidated.
In this case, the build process uses the cached items
rather than running the task again.
Obviously, these types of implicit changes can cause
problems.
</para>
<para>
To avoid these problems during the build, you need to
understand the effects of any changes you make.
Realize that changes you make directly to a function
are automatically factored into the checksum calculation.
Thus, these explicit changes invalidate the associated
area of shared state cache.
However, you need to be aware of any implicit changes that
are not obvious changes to the code and could affect
the output of a given task.
</para>
<para>
When you identify an implicit change, you can easily
take steps to invalidate the cache and force the tasks
to run.
The steps you can take are as simple as changing a
function's comments in the source code.
For example, to invalidate package shared state files,
change the comment statements of
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-package'><filename>do_package</filename></ulink>
or the comments of one of the functions it calls.
Even though the change is purely cosmetic, it causes the
checksum to be recalculated and forces the OpenEmbedded
build system to run the task again.
<note>
For an example of a commit that makes a cosmetic
change to invalidate shared state, see this
<ulink url='&YOCTO_GIT_URL;/cgit.cgi/poky/commit/meta/classes/package.bbclass?id=737f8bbb4f27b4837047cb9b4fbfe01dfde36d54'>commit</ulink>.
</note>
</para>
</section>
</section>
</section>
<section id='automatically-added-runtime-dependencies'>
<title>Automatically Added Runtime Dependencies</title>
<para>
The OpenEmbedded build system automatically adds common types of
runtime dependencies between packages, which means that you do not
need to explicitly declare the packages using
<ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>.
Three automatic mechanisms exist (<filename>shlibdeps</filename>,
<filename>pcdeps</filename>, and <filename>depchains</filename>)
that handle shared libraries, package configuration (pkg-config)
modules, and <filename>-dev</filename> and
<filename>-dbg</filename> packages, respectively.
For other types of runtime dependencies, you must manually declare
the dependencies.
<itemizedlist>
<listitem><para>
<filename>shlibdeps</filename>:
During the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-package'><filename>do_package</filename></ulink>
task of each recipe, all shared libraries installed by the
recipe are located.
For each shared library, the package that contains the
shared library is registered as providing the shared
library.
More specifically, the package is registered as providing
the
<ulink url='https://en.wikipedia.org/wiki/Soname'>soname</ulink>
of the library.
The resulting shared-library-to-package mapping
is saved globally in
<ulink url='&YOCTO_DOCS_REF_URL;#var-PKGDATA_DIR'><filename>PKGDATA_DIR</filename></ulink>
by the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-packagedata'><filename>do_packagedata</filename></ulink>
task.</para>
<para>Simultaneously, all executables and shared libraries
installed by the recipe are inspected to see what shared
libraries they link against.
For each shared library dependency that is found,
<filename>PKGDATA_DIR</filename> is queried to
see if some package (likely from a different recipe)
contains the shared library.
If such a package is found, a runtime dependency is added
from the package that depends on the shared library to the
package that contains the library.</para>
<para>The automatically added runtime dependency also
includes a version restriction.
This version restriction specifies that at least the
current version of the package that provides the shared
library must be used, as if
"<replaceable>package</replaceable> (>= <replaceable>version</replaceable>)"
had been added to
<ulink url='&YOCTO_DOCS_REF_URL;#var-RDEPENDS'><filename>RDEPENDS</filename></ulink>.
This forces an upgrade of the package containing the shared
library when installing the package that depends on the
library, if needed.</para>
<para>If you want to avoid a package being registered as
providing a particular shared library (e.g. because the library
is for internal use only), then add the library to
<ulink url='&YOCTO_DOCS_REF_URL;#var-PRIVATE_LIBS'><filename>PRIVATE_LIBS</filename></ulink>
inside the package's recipe.
</para></listitem>
<listitem><para>
<filename>pcdeps</filename>:
During the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-package'><filename>do_package</filename></ulink>
task of each recipe, all pkg-config modules
(<filename>*.pc</filename> files) installed by the recipe
are located.
For each module, the package that contains the module is
registered as providing the module.
The resulting module-to-package mapping is saved globally in
<ulink url='&YOCTO_DOCS_REF_URL;#var-PKGDATA_DIR'><filename>PKGDATA_DIR</filename></ulink>
by the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-packagedata'><filename>do_packagedata</filename></ulink>
task.</para>
<para>Simultaneously, all pkg-config modules installed by
the recipe are inspected to see what other pkg-config
modules they depend on.
A module is seen as depending on another module if it
contains a "Requires:" line that specifies the other module.
For each module dependency,
<filename>PKGDATA_DIR</filename> is queried to see if some
package contains the module.
If such a package is found, a runtime dependency is added
from the package that depends on the module to the package
that contains the module.
<note>
The <filename>pcdeps</filename> mechanism most often
infers dependencies between <filename>-dev</filename>
packages.
</note>
</para></listitem>
<listitem><para>
<filename>depchains</filename>:
If a package <filename>foo</filename> depends on a package
<filename>bar</filename>, then <filename>foo-dev</filename>
and <filename>foo-dbg</filename> are also made to depend on
<filename>bar-dev</filename> and
<filename>bar-dbg</filename>, respectively.
Taking the <filename>-dev</filename> packages as an
example, the <filename>bar-dev</filename> package might
provide headers and shared library symlinks needed by
<filename>foo-dev</filename>, which shows the need
for a dependency between the packages.</para>
<para>The dependencies added by
<filename>depchains</filename> are in the form of
<ulink url='&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS'><filename>RRECOMMENDS</filename></ulink>.
<note>
By default, <filename>foo-dev</filename> also has an
<filename>RDEPENDS</filename>-style dependency on
<filename>foo</filename>, because the default value of
<filename>RDEPENDS_${PN}-dev</filename> (set in
<filename>bitbake.conf</filename>) includes
"${PN}".
</note></para>
<para>To ensure that the dependency chain is never broken,
<filename>-dev</filename> and <filename>-dbg</filename>
packages are always generated by default, even if the
packages turn out to be empty.
See the
<ulink url='&YOCTO_DOCS_REF_URL;#var-ALLOW_EMPTY'><filename>ALLOW_EMPTY</filename></ulink>
variable for more information.
</para></listitem>
</itemizedlist>
</para>
<para>
The <filename>do_package</filename> task depends on the
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-packagedata'><filename>do_packagedata</filename></ulink>
task of each recipe in
<ulink url='&YOCTO_DOCS_REF_URL;#var-DEPENDS'><filename>DEPENDS</filename></ulink>
through use of a
<filename>[</filename><ulink url='&YOCTO_DOCS_BB_URL;#variable-flags'><filename>deptask</filename></ulink><filename>]</filename>
declaration, which guarantees that the required
shared-library/module-to-package mapping information will be available
when needed as long as <filename>DEPENDS</filename> has been
correctly set.
</para>
</section>
<section id='fakeroot-and-pseudo'>
<title>Fakeroot and Pseudo</title>
<para>
Some tasks are easier to implement when allowed to perform certain
operations that are normally reserved for the root user (e.g.
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-install'><filename>do_install</filename></ulink>,
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_deb'><filename>do_package_write*</filename></ulink>,
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-rootfs'><filename>do_rootfs</filename></ulink>,
and
<ulink url='&YOCTO_DOCS_REF_URL;#ref-tasks-image'><filename>do_image*</filename></ulink>).
For example, the <filename>do_install</filename> task benefits
from being able to set the UID and GID of installed files to
arbitrary values.
</para>
<para>
One approach to allowing tasks to perform root-only operations
would be to require BitBake to run as root.
However, this method is cumbersome and has security issues.
The approach that is actually used is to run tasks that benefit
from root privileges in a "fake" root environment.
Within this environment, the task and its child processes believe
that they are running as the root user, and see an internally
consistent view of the filesystem.
As long as generating the final output (e.g. a package or an image)
does not require root privileges, the fact that some earlier
steps ran in a fake root environment does not cause problems.
</para>
<para>
The capability to run tasks in a fake root environment is known as
"<ulink url='http://man.he.net/man1/fakeroot'>fakeroot</ulink>",
which is derived from the BitBake keyword/variable
flag that requests a fake root environment for a task.
</para>
<para>
In the OpenEmbedded build system, the program that implements
fakeroot is known as Pseudo.
Pseudo overrides system calls by using the environment variable
<filename>LD_PRELOAD</filename>, which results in the illusion
of running as root.
To keep track of "fake" file ownership and permissions resulting
from operations that require root permissions, Pseudo uses
an SQLite 3 database.
This database is stored in
<filename>${</filename><ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink><filename>}/pseudo/files.db</filename>
for individual recipes.
Storing the database in a file as opposed to in memory
gives persistence between tasks and builds, which is not
accomplished using fakeroot.
<note><title>Caution</title>
If you add your own task that manipulates the same files or
directories as a fakeroot task, then that task also needs to
run under fakeroot.
Otherwise, the task cannot run root-only operations, and
cannot see the fake file ownership and permissions set by the
other task.
You need to also add a dependency on
<filename>virtual/fakeroot-native:do_populate_sysroot</filename>,
giving the following:
<literallayout class='monospaced'>
fakeroot do_mytask () {
...
}
do_mytask[depends] += "virtual/fakeroot-native:do_populate_sysroot"
</literallayout>
</note>
For more information, see the
<ulink url='&YOCTO_DOCS_BB_URL;#var-FAKEROOT'><filename>FAKEROOT*</filename></ulink>
variables in the BitBake User Manual.
You can also reference the
"<ulink url='http://www.ibm.com/developerworks/opensource/library/os-aapseudo1/index.html'>Pseudo</ulink>"
and
"<ulink url='https://github.com/wrpseudo/pseudo/wiki/WhyNotFakeroot'>Why Not Fakeroot?</ulink>"
articles for background information on Pseudo.
</para>
</section>
<section id="wayland">
<title>Wayland</title>
<para>
<ulink url='http://en.wikipedia.org/wiki/Wayland_(display_server_protocol)'>Wayland</ulink>
is a computer display server protocol that
provides a method for compositing window managers to communicate
directly with applications and video hardware and expects them to
communicate with input hardware using other libraries.
Using Wayland with supporting targets can result in better control
over graphics frame rendering than an application might otherwise
achieve.
</para>
<para>
The Yocto Project provides the Wayland protocol libraries and the
reference
<ulink url='http://en.wikipedia.org/wiki/Wayland_(display_server_protocol)#Weston'>Weston</ulink>
compositor as part of its release.
This section describes what you need to do to implement Wayland and
use the compositor when building an image for a supporting target.
</para>
<section id="wayland-support">
<title>Support</title>
<para>
The Wayland protocol libraries and the reference Weston
compositor ship as integrated packages in the
<filename>meta</filename> layer of the
<ulink url='&YOCTO_DOCS_REF_URL;#source-directory'>Source Directory</ulink>.
Specifically, you can find the recipes that build both Wayland
and Weston at
<filename>meta/recipes-graphics/wayland</filename>.
</para>
<para>
You can build both the Wayland and Weston packages for use only
with targets that accept the
<ulink url='https://en.wikipedia.org/wiki/Mesa_(computer_graphics)'>Mesa 3D and Direct Rendering Infrastructure</ulink>,
which is also known as Mesa DRI.
This implies that you cannot build and use the packages if your
target uses, for example, the
<trademark class='registered'>Intel</trademark> Embedded Media
and Graphics Driver
(<trademark class='registered'>Intel</trademark> EMGD) that
overrides Mesa DRI.
<note>
Due to lack of EGL support, Weston 1.0.3 will not run
directly on the emulated QEMU hardware.
However, this version of Weston will run under X emulation
without issues.
</note>
</para>
</section>
<section id="enabling-wayland-in-an-image">
<title>Enabling Wayland in an Image</title>
<para>
To enable Wayland, you need to enable it to be built and enable
it to be included in the image.
</para>
<section id="enable-building">
<title>Building</title>
<para>
To cause Mesa to build the <filename>wayland-egl</filename>
platform and Weston to build Wayland with Kernel Mode
Setting
(<ulink url='https://wiki.archlinux.org/index.php/Kernel_Mode_Setting'>KMS</ulink>)
support, include the "wayland" flag in the
<ulink url="&YOCTO_DOCS_REF_URL;#var-DISTRO_FEATURES"><filename>DISTRO_FEATURES</filename></ulink>
statement in your <filename>local.conf</filename> file:
<literallayout class='monospaced'>
DISTRO_FEATURES_append = " wayland"
</literallayout>
<note>
If X11 has been enabled elsewhere, Weston will build
Wayland with X11 support
</note>
</para>
</section>
<section id="enable-installation-in-an-image">
<title>Installing</title>
<para>
To install the Wayland feature into an image, you must
include the following
<ulink url='&YOCTO_DOCS_REF_URL;#var-CORE_IMAGE_EXTRA_INSTALL'><filename>CORE_IMAGE_EXTRA_INSTALL</filename></ulink>
statement in your <filename>local.conf</filename> file:
<literallayout class='monospaced'>
CORE_IMAGE_EXTRA_INSTALL += "wayland weston"
</literallayout>
</para>
</section>
</section>
<section id="running-weston">
<title>Running Weston</title>
<para>
To run Weston inside X11, enabling it as described earlier and
building a Sato image is sufficient.
If you are running your image under Sato, a Weston Launcher
appears in the "Utility" category.
</para>
<para>
Alternatively, you can run Weston through the command-line
interpretor (CLI), which is better suited for development work.
To run Weston under the CLI, you need to do the following after
your image is built:
<orderedlist>
<listitem><para>
Run these commands to export
<filename>XDG_RUNTIME_DIR</filename>:
<literallayout class='monospaced'>
mkdir -p /tmp/$USER-weston
chmod 0700 /tmp/$USER-weston
export XDG_RUNTIME_DIR=/tmp/$USER-weston
</literallayout>
</para></listitem>
<listitem><para>
Launch Weston in the shell:
<literallayout class='monospaced'>
weston
</literallayout></para></listitem>
</orderedlist>
</para>
</section>
</section>
<section id="overview-licenses">
<title>Licenses</title>
<para>
This section describes the mechanism by which the OpenEmbedded
build system tracks changes to licensing text.
The section also describes how to enable commercially licensed
recipes, which by default are disabled.
</para>
<para>
For information that can help you maintain compliance with
various open source licensing during the lifecycle of the product,
see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#maintaining-open-source-license-compliance-during-your-products-lifecycle'>Maintaining Open Source License Compliance During Your Project's Lifecycle</ulink>"
section in the Yocto Project Development Tasks Manual.
</para>
<section id="usingpoky-configuring-LIC_FILES_CHKSUM">
<title>Tracking License Changes</title>
<para>
The license of an upstream project might change in the future.
In order to prevent these changes going unnoticed, the
<ulink url='&YOCTO_DOCS_REF_URL;#var-LIC_FILES_CHKSUM'><filename>LIC_FILES_CHKSUM</filename></ulink>
variable tracks changes to the license text. The checksums are
validated at the end of the configure step, and if the
checksums do not match, the build will fail.
</para>
<section id="usingpoky-specifying-LIC_FILES_CHKSUM">
<title>Specifying the <filename>LIC_FILES_CHKSUM</filename> Variable</title>
<para>
The <filename>LIC_FILES_CHKSUM</filename>
variable contains checksums of the license text in the
source code for the recipe.
Following is an example of how to specify
<filename>LIC_FILES_CHKSUM</filename>:
<literallayout class='monospaced'>
LIC_FILES_CHKSUM = "file://COPYING;md5=xxxx \
file://licfile1.txt;beginline=5;endline=29;md5=yyyy \
file://licfile2.txt;endline=50;md5=zzzz \
..."
</literallayout>
<note><title>Notes</title>
<itemizedlist>
<listitem><para>
When using "beginline" and "endline", realize
that line numbering begins with one and not
zero.
Also, the included lines are inclusive (i.e.
lines five through and including 29 in the
previous example for
<filename>licfile1.txt</filename>).
</para></listitem>
<listitem><para>
When a license check fails, the selected license
text is included as part of the QA message.
Using this output, you can determine the exact
start and finish for the needed license text.
</para></listitem>
</itemizedlist>
</note>
</para>
<para>
The build system uses the
<ulink url='&YOCTO_DOCS_REF_URL;#var-S'><filename>S</filename></ulink>
variable as the default directory when searching files
listed in <filename>LIC_FILES_CHKSUM</filename>.
The previous example employs the default directory.
</para>
<para>
Consider this next example:
<literallayout class='monospaced'>
LIC_FILES_CHKSUM = "file://src/ls.c;beginline=5;endline=16;\
md5=bb14ed3c4cda583abc85401304b5cd4e"
LIC_FILES_CHKSUM = "file://${WORKDIR}/license.html;md5=5c94767cedb5d6987c902ac850ded2c6"
</literallayout>
</para>
<para>
The first line locates a file in
<filename>${S}/src/ls.c</filename> and isolates lines five
through 16 as license text.
The second line refers to a file in
<ulink url='&YOCTO_DOCS_REF_URL;#var-WORKDIR'><filename>WORKDIR</filename></ulink>.
</para>
<para>
Note that <filename>LIC_FILES_CHKSUM</filename> variable is
mandatory for all recipes, unless the
<filename>LICENSE</filename> variable is set to "CLOSED".
</para>
</section>
<section id="usingpoky-LIC_FILES_CHKSUM-explanation-of-syntax">
<title>Explanation of Syntax</title>
<para>
As mentioned in the previous section, the
<filename>LIC_FILES_CHKSUM</filename> variable lists all
the important files that contain the license text for the
source code.
It is possible to specify a checksum for an entire file,
or a specific section of a file (specified by beginning and
ending line numbers with the "beginline" and "endline"
parameters, respectively).
The latter is useful for source files with a license
notice header, README documents, and so forth.
If you do not use the "beginline" parameter, then it is
assumed that the text begins on the first line of the file.
Similarly, if you do not use the "endline" parameter,
it is assumed that the license text ends with the last
line of the file.
</para>
<para>
The "md5" parameter stores the md5 checksum of the license
text.
If the license text changes in any way as compared to
this parameter then a mismatch occurs.
This mismatch triggers a build failure and notifies
the developer.
Notification allows the developer to review and address
the license text changes.
Also note that if a mismatch occurs during the build,
the correct md5 checksum is placed in the build log and
can be easily copied to the recipe.
</para>
<para>
There is no limit to how many files you can specify using
the <filename>LIC_FILES_CHKSUM</filename> variable.
Generally, however, every project requires a few
specifications for license tracking.
Many projects have a "COPYING" file that stores the
license information for all the source code files.
This practice allows you to just track the "COPYING"
file as long as it is kept up to date.
<note><title>Tips</title>
<itemizedlist>
<listitem><para>
If you specify an empty or invalid "md5"
parameter, BitBake returns an md5 mis-match
error and displays the correct "md5" parameter
value during the build.
The correct parameter is also captured in
the build log.
</para></listitem>
<listitem><para>
If the whole file contains only license text,
you do not need to use the "beginline" and
"endline" parameters.
</para></listitem>
</itemizedlist>
</note>
</para>
</section>
</section>
<section id="enabling-commercially-licensed-recipes">
<title>Enabling Commercially Licensed Recipes</title>
<para>
By default, the OpenEmbedded build system disables
components that have commercial or other special licensing
requirements.
Such requirements are defined on a
recipe-by-recipe basis through the
<ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE_FLAGS'><filename>LICENSE_FLAGS</filename></ulink>
variable definition in the affected recipe.
For instance, the
<filename>poky/meta/recipes-multimedia/gstreamer/gst-plugins-ugly</filename>
recipe contains the following statement:
<literallayout class='monospaced'>
LICENSE_FLAGS = "commercial"
</literallayout>
Here is a slightly more complicated example that contains both
an explicit recipe name and version (after variable expansion):
<literallayout class='monospaced'>
LICENSE_FLAGS = "license_${PN}_${PV}"
</literallayout>
In order for a component restricted by a
<filename>LICENSE_FLAGS</filename> definition to be enabled and
included in an image, it needs to have a matching entry in the
global
<ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE_FLAGS_WHITELIST'><filename>LICENSE_FLAGS_WHITELIST</filename></ulink>
variable, which is a variable typically defined in your
<filename>local.conf</filename> file.
For example, to enable the
<filename>poky/meta/recipes-multimedia/gstreamer/gst-plugins-ugly</filename>
package, you could add either the string
"commercial_gst-plugins-ugly" or the more general string
"commercial" to <filename>LICENSE_FLAGS_WHITELIST</filename>.
See the
"<link linkend='license-flag-matching'>License Flag Matching</link>"
section for a full
explanation of how <filename>LICENSE_FLAGS</filename> matching
works.
Here is the example:
<literallayout class='monospaced'>
LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly"
</literallayout>
Likewise, to additionally enable the package built from the
recipe containing
<filename>LICENSE_FLAGS = "license_${PN}_${PV}"</filename>,
and assuming that the actual recipe name was
<filename>emgd_1.10.bb</filename>, the following string would
enable that package as well as the original
<filename>gst-plugins-ugly</filename> package:
<literallayout class='monospaced'>
LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly license_emgd_1.10"
</literallayout>
As a convenience, you do not need to specify the complete
license string in the whitelist for every package.
You can use an abbreviated form, which consists
of just the first portion or portions of the license
string before the initial underscore character or characters.
A partial string will match any license that contains the
given string as the first portion of its license.
For example, the following whitelist string will also match
both of the packages previously mentioned as well as any other
packages that have licenses starting with "commercial" or
"license".
<literallayout class='monospaced'>
LICENSE_FLAGS_WHITELIST = "commercial license"
</literallayout>
</para>
<section id="license-flag-matching">
<title>License Flag Matching</title>
<para>
License flag matching allows you to control what recipes
the OpenEmbedded build system includes in the build.
Fundamentally, the build system attempts to match
<ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE_FLAGS'><filename>LICENSE_FLAGS</filename></ulink>
strings found in recipes against
<ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE_FLAGS_WHITELIST'><filename>LICENSE_FLAGS_WHITELIST</filename></ulink>
strings found in the whitelist.
A match causes the build system to include a recipe in the
build, while failure to find a match causes the build
system to exclude a recipe.
</para>
<para>
In general, license flag matching is simple.
However, understanding some concepts will help you
correctly and effectively use matching.
</para>
<para>
Before a flag
defined by a particular recipe is tested against the
contents of the whitelist, the expanded string
<filename>_${PN}</filename> is appended to the flag.
This expansion makes each
<filename>LICENSE_FLAGS</filename> value recipe-specific.
After expansion, the string is then matched against the
whitelist.
Thus, specifying
<filename>LICENSE_FLAGS = "commercial"</filename>
in recipe "foo", for example, results in the string
<filename>"commercial_foo"</filename>.
And, to create a match, that string must appear in the
whitelist.
</para>
<para>
Judicious use of the <filename>LICENSE_FLAGS</filename>
strings and the contents of the
<filename>LICENSE_FLAGS_WHITELIST</filename> variable
allows you a lot of flexibility for including or excluding
recipes based on licensing.
For example, you can broaden the matching capabilities by
using license flags string subsets in the whitelist.
<note>
When using a string subset, be sure to use the part of
the expanded string that precedes the appended
underscore character (e.g.
<filename>usethispart_1.3</filename>,
<filename>usethispart_1.4</filename>, and so forth).
</note>
For example, simply specifying the string "commercial" in
the whitelist matches any expanded
<filename>LICENSE_FLAGS</filename> definition that starts
with the string "commercial" such as "commercial_foo" and
"commercial_bar", which are the strings the build system
automatically generates for hypothetical recipes named
"foo" and "bar" assuming those recipes simply specify the
following:
<literallayout class='monospaced'>
LICENSE_FLAGS = "commercial"
</literallayout>
Thus, you can choose to exhaustively
enumerate each license flag in the whitelist and
allow only specific recipes into the image, or
you can use a string subset that causes a broader range of
matches to allow a range of recipes into the image.
</para>
<para>
This scheme works even if the
<filename>LICENSE_FLAGS</filename> string already
has <filename>_${PN}</filename> appended.
For example, the build system turns the license flag
"commercial_1.2_foo" into "commercial_1.2_foo_foo" and
would match both the general "commercial" and the specific
"commercial_1.2_foo" strings found in the whitelist, as
expected.
</para>
<para>
Here are some other scenarios:
<itemizedlist>
<listitem><para>
You can specify a versioned string in the recipe
such as "commercial_foo_1.2" in a "foo" recipe.
The build system expands this string to
"commercial_foo_1.2_foo".
Combine this license flag with a whitelist that has
the string "commercial" and you match the flag
along with any other flag that starts with the
string "commercial".
</para></listitem>
<listitem><para>
Under the same circumstances, you can use
"commercial_foo" in the whitelist and the build
system not only matches "commercial_foo_1.2" but
also matches any license flag with the string
"commercial_foo", regardless of the version.
</para></listitem>
<listitem><para>
You can be very specific and use both the
package and version parts in the whitelist (e.g.
"commercial_foo_1.2") to specifically match a
versioned recipe.
</para></listitem>
</itemizedlist>
</para>
</section>
<section id="other-variables-related-to-commercial-licenses">
<title>Other Variables Related to Commercial Licenses</title>
<para>
Other helpful variables related to commercial
license handling exist and are defined in the
<filename>poky/meta/conf/distro/include/default-distrovars.inc</filename> file:
<literallayout class='monospaced'>
COMMERCIAL_AUDIO_PLUGINS ?= ""
COMMERCIAL_VIDEO_PLUGINS ?= ""
</literallayout>
If you want to enable these components, you can do so by
making sure you have statements similar to the following
in your <filename>local.conf</filename> configuration file:
<literallayout class='monospaced'>
COMMERCIAL_AUDIO_PLUGINS = "gst-plugins-ugly-mad \
gst-plugins-ugly-mpegaudioparse"
COMMERCIAL_VIDEO_PLUGINS = "gst-plugins-ugly-mpeg2dec \
gst-plugins-ugly-mpegstream gst-plugins-bad-mpegvideoparse"
LICENSE_FLAGS_WHITELIST = "commercial_gst-plugins-ugly commercial_gst-plugins-bad commercial_qmmp"
</literallayout>
Of course, you could also create a matching whitelist
for those components using the more general "commercial"
in the whitelist, but that would also enable all the
other packages with
<ulink url='&YOCTO_DOCS_REF_URL;#var-LICENSE_FLAGS'><filename>LICENSE_FLAGS</filename></ulink>
containing "commercial", which you may or may not want:
<literallayout class='monospaced'>
LICENSE_FLAGS_WHITELIST = "commercial"
</literallayout>
</para>
<para>
Specifying audio and video plug-ins as part of the
<filename>COMMERCIAL_AUDIO_PLUGINS</filename> and
<filename>COMMERCIAL_VIDEO_PLUGINS</filename> statements
(along with the enabling
<filename>LICENSE_FLAGS_WHITELIST</filename>) includes the
plug-ins or components into built images, thus adding
support for media formats or components.
</para>
</section>
</section>
</section>
<section id='x32'>
<title>x32 psABI</title>
<para>
x32 processor-specific Application Binary Interface
(<ulink url='https://software.intel.com/en-us/node/628948'>x32 psABI</ulink>)
is a native 32-bit processor-specific ABI for
<trademark class='registered'>Intel</trademark> 64 (x86-64)
architectures.
An ABI defines the calling conventions between functions in a
processing environment.
The interface determines what registers are used and what the sizes are
for various C data types.
</para>
<para>
Some processing environments prefer using 32-bit applications even
when running on Intel 64-bit platforms.
Consider the i386 psABI, which is a very old 32-bit ABI for Intel
64-bit platforms.
The i386 psABI does not provide efficient use and access of the
Intel 64-bit processor resources, leaving the system underutilized.
Now consider the x86_64 psABI.
This ABI is newer and uses 64-bits for data sizes and program
pointers.
The extra bits increase the footprint size of the programs,
libraries, and also increases the memory and file system size
requirements.
Executing under the x32 psABI enables user programs to utilize CPU
and system resources more efficiently while keeping the memory
footprint of the applications low.
Extra bits are used for registers but not for addressing mechanisms.
</para>
<para>
The Yocto Project supports the final specifications of x32 psABI
as follows:
<itemizedlist>
<listitem><para>
You can create packages and images in x32 psABI format on
x86_64 architecture targets.
</para></listitem>
<listitem><para>
You can successfully build recipes with the x32 toolchain.
</para></listitem>
<listitem><para>
You can create and boot
<filename>core-image-minimal</filename> and
<filename>core-image-sato</filename> images.
</para></listitem>
<listitem><para>
RPM Package Manager (RPM) support exists for x32 binaries.
</para></listitem>
<listitem><para>
Support for large images exists.
</para></listitem>
</itemizedlist>
</para>
<para>
For steps on how to use x32 psABI, see the
"<ulink url='&YOCTO_DOCS_DEV_URL;#using-x32-psabi'>Using x32 psABI</ulink>"
section in the Yocto Project Development Tasks Manual.
</para>
</section>
</chapter>
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