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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">
<chapter id='dev-manual-model'>
<title>Common Development Models</title>
<para>
Many development models exist for which you can use the Yocto Project.
However, for the purposes of this manual we are going to focus on two common ones:
System Development and User Application Development.
System Development covers Board Support Package (BSP) development and kernel modification.
User Application Development covers development of applications that you intend to run on some
target hardware.
</para>
<para>
This chapter presents overviews of both system and application models.
If you want to reference specific examples of these development models,
see <xref linkend='dev-manual-bsp-appendix'>BSP Development Example</xref> and
<xref linkend='dev-manual-kernel-appendix'>Kernel Modification Example</xref>.
</para>
<section id='system-development-model'>
<title>System Development</title>
<para>
System development involves modification or creation of an image that you want to run on
a specific hardware target.
Usually when you want to create an image that runs on embedded hardware the image does
not require the same amount of features that a full-fledged Linux distribution provides.
Thus, you can create a much smaller image that is designed to just use the hardware
features for your particular hardware.
</para>
<para>
To help you understand how system development works in the Yocto Project, this section
covers two types of image development: BSP creation and kernel modification.
</para>
<section id='developing-a-board-support-package-bsp'>
<title>Developing a Board Support Package (BSP)</title>
<para>
A BSP is a package of recipes that when applied while building an image results in
an image you can run on a particular board.
Thus, the package, when compiled into the new image, supports the operation of the board.
</para>
<note>
For a brief list of terms used when describing the development process in the Yocto Project,
see <xref linkend='yocto-project-terms'>Yocto Project Terms</xref> in this manual.
</note>
<para>
The remainder of this section presents the basic steps to create a BSP basing it on an
existing BSP that ships with the Yocto Project.
You can reference <xref linkend='dev-manual-bsp-appendix'>BSP Development Case</xref>
for a detailed example that uses the Crown Bay BSP as a base BSP from which to start.
</para>
<para>
Here are the basic steps involved in creating a BSP:
<orderedlist>
<listitem><para><emphasis>Set up your host development system to support
development using the Yocto Project</emphasis>: See
<ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#the-linux-distro'>
The Linux Distributions</ulink> section and
<ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#packages'>
The Packages</ulink> section both
in the Yocto Project Quick Start for requirements.
You will also need a release of Yocto Project installed on the host.</para></listitem>
<listitem><para><emphasis>Establish a local copy of the Yocto Project files on your
system</emphasis>: You need to have the Yocto Project files available on your host system.
Having the Yocto Project files on your system gives you access to the build
process and tools you need.
For information on how to get these files, see the
<xref linkend='getting-setup'>Getting Setup</xref> section in this manual.</para></listitem>
<listitem><para><emphasis>Choose a Yocto Project-supported BSP as your base BSP</emphasis>:
The Yocto Project ships with several BSPs that support various hardware.
It is best to base your new BSP on an existing BSP rather than create all the
recipes and configuration files from scratch.
While it is possible to create everything from scratch, basing your new BSP
on something that is close is much easier.
Or, at a minimum, it gives you some structure with which to start.</para>
<para>At this point you need to understand your target hardware well enough to determine which
existing BSP it most closely matches.
Things to consider are your hardware’s on-board features such as CPU type and graphics support.
You should look at the README files for supported BSPs to get an idea of which one
you could use.
A generic Atom-based BSP to consider is the Crown Bay that does not support
the Intel® Embedded Media Graphics Driver (EMGD).
The remainder of this example uses that base BSP.</para>
<para>To see the supported BSPs, go to the Yocto Project
<ulink url='http://www.yoctoproject.org/download'>download page</ulink> and click
on “BSP Downloads.”</para></listitem>
<listitem><para><emphasis>Establish a local copy of the base BSP files</emphasis>: Having
the BSP files on your system gives you access to the build
process and tools you need.
For information on how to get these files, see
<xref linkend='getting-setup'>Getting Setup</xref> earlier in this manual.</para></listitem>
<listitem><para><emphasis>Create your own BSP layer</emphasis>: Layers are ideal for
isolating and storing work for a given piece of hardware.
A layer is really just a location or area in which you place the recipes for your BSP.
In fact, a BSP is, in itself, a special type of layer.
Consider an application as another example that illustrates a layer.
Suppose you are creating an application that has library or other dependencies in
order for it to compile and run.
The layer, in this case, would be where all the recipes that define those dependencies
are kept. The key point for a layer is that it is an isolated area that contains
all the relevant information for the project that the Yocto Project build
system knows about.</para>
<note>The Yocto Project supports four BSPs that are part of the
Yocto Project release: <filename>atom-pc</filename>, <filename>beagleboard</filename>,
<filename>mpc8315e</filename>, and <filename>routerstationpro</filename>.
The recipes and configurations for these four BSPs are located and dispersed
within local Yocto Project files.
Consequently, they are not totally isolated in the spirit of layers unless you think
of <filename>meta-yocto</filename> as a layer itself.
On the other hand, BSP layers for Crown Bay, Emenlow, Jasper Forest,
N450, and Sugar Bay are isolated.</note>
<para>When you set up a layer for a new BSP you should follow a standard layout.
This layout is described in the
<ulink url='http://www.yoctoproject.org/docs/1.1/bsp-guide/bsp-guide.html#bsp-filelayout'>
Example Filesystem Layout</ulink> section of the Board Support Package (BSP) Development
Guide.
In the standard layout you will notice a suggested structure for recipes and
configuration information.
You can see the standard layout for the Crown Bay BSP in this example by examining the
directory structure of the <filename>meta-crownbay</filename> layer inside the
local Yocto Project files.</para></listitem>
<listitem><para><emphasis>Make configuration and recipe changes to your new BSP
layer</emphasis>: The standard BSP layer structure organizes the files you need to edit in
<filename>conf</filename> and several <filename>recipes-*</filename> within the
BSP layer.</para>
<para>Configuration changes identify where your new layer is on the local system
and identify which kernel you are going to use.
Recipe changes include altering recipes (<filename>.bb</filename> files), removing
recipes you don't use, and adding new recipes that you need to support your hardware.
</para></listitem>
<listitem><para><emphasis>Prepare for the build</emphasis>: Once you have made all the
changes to your BSP layer there remains a few things
you need to do for the Yocto Project build system in order for it to create your image.
You need to get the build environment ready by sourcing an environment setup script
and you need to be sure two key configuration files are configured appropriately.</para>
<para>The entire process for building an image is overviewed in the
<ulink url='http://www.yoctoproject.org/docs/1.1/yocto-project-qs/yocto-project-qs.html#building-image'>
Building an Image</ulink> section of the Yocto Project Quick Start.
You might want to reference this information.</para></listitem>
<listitem><para><emphasis>Build the image</emphasis>: The Yocto Project uses the BitBake
tool to build images based on the type of image
you want to create.
You can find more information on BitBake
<ulink url='http://bitbake.berlios.de/manual/'>here</ulink>.</para>
<para>The build process supports several types of images to satisfy different needs.
When you issue the BitBake command you provide a “top-level” recipe that essentially
starts the process off of building the type of image you want.</para>
<para>[WRITER'S NOTE: Consider moving this to the Poky Reference Manual.]</para>
<para>You can find these recipes in the <filename>meta/recipes-core/images</filename> and
<filename>meta/recipes-sato/images</filename> directories of your local Yocto Project
file structure (Git repository or extracted release tarball).
Although the recipe names are somewhat explanatory, here is a list that describes them:
<itemizedlist>
<listitem><para><emphasis>Base</emphasis> – A foundational basic image without support
for X that can be reasonably used for customization.</para></listitem>
<listitem><para><emphasis>Core</emphasis> – A foundational basic image with support for
X that can be reasonably used for customization.</para></listitem>
<listitem><para><emphasis>Direct Disk</emphasis> – An image that you can copy directory to
the disk of the target device.</para></listitem>
<listitem><para><emphasis>Live</emphasis> – An image you can run from a USB device or from
a CD without having to first install something.</para></listitem>
<listitem><para><emphasis>Minimal</emphasis> – A small image without a GUI.
This image is not much more than a kernel with a shell.</para></listitem>
<listitem><para><emphasis>Minimal Development</emphasis> – A Minimal image suitable for
development work.</para></listitem>
<listitem><para><emphasis>Minimal Direct Disk</emphasis> – A Minimal Direct
Disk image.</para></listitem>
<listitem><para><emphasis>Minimal RAM-based Initial Root Filesystem</emphasis> –
A minimal image
that has the <filename>initramfs</filename> as part of the kernel, which allows the
system to find the first “init” program more efficiently.</para></listitem>
<listitem><para><emphasis>Minimal Live</emphasis> – A Minimal Live image.</para></listitem>
<listitem><para><emphasis>Minimal MTD Utilities</emphasis> – A minimal image that has support
for the MTD utilities, which let the user interact with the MTD subsystem in
the kernel to perform operations on flash devices.</para></listitem>
<listitem><para><emphasis>Sato</emphasis> – An image with Sato support, a mobile environment
and visual style that works well with mobile devices.</para></listitem>
<listitem><para><emphasis>Sato Development</emphasis> – A Sato image suitable for
development work.</para></listitem>
<listitem><para><emphasis>Sato Direct Disk</emphasis> – A Sato Direct
Disk image.</para></listitem>
<listitem><para><emphasis>Sato Live</emphasis> – A Sato Live image.</para></listitem>
<listitem><para><emphasis>Sato SDK</emphasis> – A Sato image that includes the Yocto Project
toolchain and development libraries.</para></listitem>
<listitem><para><emphasis>Sato SDK Direct Disk</emphasis> – A Sato SDK Direct
Disk image.</para></listitem>
<listitem><para><emphasis>Sato SDK Live</emphasis> – A Sato SDK Live
image.</para></listitem>
</itemizedlist>
</para></listitem>
</orderedlist>
</para>
<para>
You can view a video presentation of the BSP creation process
<ulink url='http://free-electrons.com/blog/elc-2011-videos'>here</ulink>.
You can also find supplemental information in the
<ulink url='http://yoctoproject.org/docs/1.1/bsp-guide/bsp-guide.html'>
Board Support Package (BSP) Development Guide</ulink>.
Finally, there is wiki page write up of the example located
<ulink url='https://wiki.yoctoproject.org/wiki/Transcript:_creating_one_generic_Atom_BSP_from_another'>
here</ulink> you might find helpful.
</para>
</section>
<section id='place-holder-section-kernel-development-model'>
<title>Place-Holder Section For Kernel Development Model</title>
<para>
Text needed here.
</para>
</section>
</section>
<section id='place-holder-section-two'>
<title>Place-Holder Section For Application Development</title>
<para>
Text needed here.
</para>
</section>
</chapter>
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