path: root/documentation/bsp-guide/bsp.xml

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<chapter id='bsp'>

        <title>Board Support Packages (BSP) - Developer's Guide</title>

        <para>
            A Board Support Package (BSP) is a collection of information that
            defines how to support a particular hardware device, set of devices, or 
            hardware platform. 
            The BSP includes information about the hardware features 
            present on the device and kernel configuration information along with any 
            additional hardware drivers required.
            The BSP also lists any additional software 
            components required in addition to a generic Linux software stack for both 
            essential and optional platform features.
        </para>

        <para>
            This chapter (or document if you are reading the BSP Developer's Guide) defines
            a structure for these components
            so that BSPs follow a commonly understood layout.
            Providing a common form allows end-users to understand and become familiar 
            with the layout.  
            A common form also encourages standardization 
            of software support of hardware.
        </para>

        <note>
            The information here does not provide an example of how to create a BSP.
            For examples on how to create a BSP, see the
            "<ulink url='&YOCTO_DOCS_DEV_URL;#dev-manual-bsp-appendix'>BSP Development Example</ulink>"
            section in The Yocto Project Development Manual.
            You can also see the 
            <ulink url='&YOCTO_WIKI_URL;/wiki/Transcript:_creating_one_generic_Atom_BSP_from_another'>
            wiki page</ulink>.
        </note>

        <para>
            The proposed format does have elements that are specific to the Yocto Project and 
            OpenEmbedded build systems. 
            It is intended that this information can be 
            used by other systems besides Yocto Project and OpenEmbedded and that it will be simple
            to extract information and convert it to other formats if required.
            Yocto Project, through its standard layers mechanism, can directly accept the format 
            described as a layer.
            The BSP captures all 
            the hardware-specific details in one place in a standard format, which is 
            useful for any person wishing to use the hardware platform regardless of 
            the build system they are using.
        </para>

        <para>
            The BSP specification does not include a build system or other tools -
            it is concerned with the hardware-specific components only. 
            At the end-distribution point, you can ship the BSP combined with a build system
            and other tools. 
            However, it is important to maintain the distinction that these
            are separate components that happen to be combined in certain end products.
        </para>

        <section id='bsp-layers'>
            <title>BSP Layers</title>

            <para>
                The BSP consists of a file structure inside a base directory.
                Collectively, you can think of the base directory and the file structure 
                as a BSP Layer.
                BSP Layers use the following naming convention:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;
                </literallayout>
                "bsp_name" is a placeholder for the machine or platform name.
            </para>

            <para>
                The layer's base directory (<filename>meta-&lt;bsp_name&gt;</filename>) is the root 
                of the BSP Layer.
                This root is what you add to the 
                <ulink url='&YOCTO_DOCS_REF_URL;#var-BBLAYERS'><filename>BBLAYERS</filename></ulink>
                variable in the <filename>conf/bblayers.conf</filename> file found in the 
                <ulink url='&YOCTO_DOCS_DEV_URL;#yocto-project-build-directory'>Yocto Project Build Directory</ulink>.
                Adding the root allows the Yocto Project build system to recognize the BSP 
                definition and from it build an image.
                Here is an example:
                <literallayout class='monospaced'>
     BBLAYERS = " \
        /usr/local/src/yocto/meta \
        /usr/local/src/yocto/meta-yocto \
        /usr/local/src/yocto/meta-&lt;bsp_name&gt; \
        "
                </literallayout>
            </para>

            <para>
                Some BSPs require additional layers on
                top of the BSP's root layer in order to be functional.
                For these cases, you also need to add those layers to the
                <filename>BBLAYERS</filename> variable in order to build the BSP.  
                You must also specify in the "Dependiencies" section of the BSP's 
                <filename>README</filename> file any requirements for additional 
                layers and, preferably, any
                build instructions that might be contained elsewhere
                in the <filename>README</filename> file.
            </para>

            <para>
                For more detailed 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 Manual.
                You can also see the detailed examples in the appendices of  
                <ulink url='&YOCTO_DOCS_DEV_URL;'>The Yocto Project Development Manual</ulink>.
            </para>
        </section>


        <section id="bsp-filelayout">
            <title>Example Filesystem Layout</title>

            <para>
                Below is the common form for the file structure inside a BSP Layer.
                While you can use this basic form for the standard, realize that the actual structures
                for specific BSPs could differ. 

                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/                                
     meta-&lt;bsp_name&gt;/&lt;bsp_license_file&gt;
     meta-&lt;bsp_name&gt;/README
     meta-&lt;bsp_name&gt;/binary/&lt;bootable_images&gt;
     meta-&lt;bsp_name&gt;/conf/layer.conf 
     meta-&lt;bsp_name&gt;/conf/machine/*.conf
     meta-&lt;bsp_name&gt;/recipes-bsp/*
     meta-&lt;bsp_name&gt;/recipes-graphics/*            
     meta-&lt;bsp_name&gt;/recipes-kernel/linux/linux-yocto_&lt;kernel_rev&gt;.bbappend
                </literallayout>
            </para>

            <para>
                Below is an example of the Crown Bay BSP:

                <literallayout class='monospaced'>
     meta-crownbay/COPYING.MIT
     meta-crownbay/README
     meta-crownbay/README.sources
     meta-crownbay/binary
     meta-crownbay/conf/
     meta-crownbay/conf/layer.conf
     meta-crownbay/conf/machine/
     meta-crownbay/conf/machine/crownbay.conf
     meta-crownbay/conf/machine/crownbay-noemgd.conf
     meta-crownbay/recipes-bsp/
     meta-crownbay/recipes-bsp/formfactor/
     meta-crownbay/recipes-bsp/formfactor/formfactor_0.0.bbappend
     meta-crownbay/recipes-bsp/formfactor/formfactor/
     meta-crownbay/recipes-bsp/formfactor/formfactor/crownbay/
     meta-crownbay/recipes-bsp/formfactor/formfactor/crownbay/machconfig
     meta-crownbay/recipes-bsp/formfactor/formfactor/crownbay-noemgd/
     meta-crownbay/recipes-bsp/formfactor/formfactor/crownbay-noemgd/machconfig
     meta-crownbay/recipes-core
     meta-crownbay/recipes-core/tasks
     meta-crownbay/recipes-core/tasks/task-core-tools-profile.bbappend
     meta-crownbay/recipes-graphics/
     meta-crownbay/recipes-graphics/xorg-xserver/
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config_0.1.bbappend
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config/
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config/crownbay/
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config/crownbay/xorg.conf
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config/crownbay-noemgd/
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config/crownbay-noemgd/xorg.conf
     meta-crownbay/recipes-kernel/
     meta-crownbay/recipes-kernel/linux/
     meta-crownbay/recipes-kernel/linux/linux-yocto-rt_3.0.bbappend
     meta-crownbay/recipes-kernel/linux/linux-yocto_2.6.37.bbappend
     meta-crownbay/recipes-kernel/linux/linux-yocto_3.0.bbappend
                </literallayout>
            </para>

            <para>
                The following sections describe each part of the proposed BSP format.
            </para>

            <section id="bsp-filelayout-license">
            <title>License Files</title>

            <para>
                You can find these files in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/&lt;bsp_license_file&gt;
                </literallayout>
            </para>

            <para>
                These optional files satisfy licensing requirements for the BSP.
                The type or types of files here can vary depending on the licensing requirements.
                For example, in the Crown Bay BSP all licensing requirements are handled with the 
                <filename>COPYING.MIT</filename> file.  
            </para>

            <para>
                Licensing files can be MIT, BSD, GPLv*, and so forth.
                These files are recommended for the BSP but are optional and totally up to the BSP developer.
            </para>
            </section>

            <section id="bsp-filelayout-readme">
            <title>README File</title>
            <para>
                You can find this file in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/README
                </literallayout>
            </para>

            <para>
                This file provides information on how to boot the live images that are optionally 
                included in the <filename>/binary</filename> directory.
                The <filename>README</filename> file also provides special information needed for 
                building the image.
            </para>

            <para>
                Technically speaking a <filename>README</filename> file is optional but it is highly 
                recommended that every BSP has one.
            </para>
            </section>

            <section id="bsp-filelayout-readme-sources">
            <title>README.sources File</title>
            <para>
                You can find this file in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/README.sources
                </literallayout>
            </para>

            <para>
                This file provides information on where to locate the BSP source files.
                For example, information provides where to find the sources that comprise
                the images shipped with the BSP.
                Information is also included to help you find the metadata used to generate the images
                that ship with the BSP.
            </para>

            <para>
                Technically speaking a <filename>README.sources</filename> file is optional but it is highly 
                recommended that every BSP has one.
            </para>
            </section>

            <section id="bsp-filelayout-binary">
            <title>Pre-built User Binaries</title>
            <para>
                You can find these files in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/binary/&lt;bootable_images&gt;
                </literallayout>
            </para>

            <para>
                This optional area contains useful pre-built kernels and user-space filesystem 
                images appropriate to the target system.
                This directory typically contains graphical (e.g. sato) and minimal live images 
                when the BSP tarball has been created and made available in the 
                <ulink url='&YOCTO_HOME_URL;'>Yocto Project</ulink> website. 
                You can use these kernels and images to get a system running and quickly get started 
                on development tasks.
            </para>

            <para> 
                The exact types of binaries present are highly hardware-dependent.
                However, a README file should be present in the BSP Layer that explains how to use 
                the kernels and images with the target hardware. 
                If pre-built binaries are present, source code to meet licensing requirements must also 
                exist in some form.
            </para>
            </section>

            <section id='bsp-filelayout-layer'>
            <title>Layer Configuration File</title>
            <para>
                You can find this file in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/conf/layer.conf
                </literallayout>
            </para>

            <para>
                The <filename>conf/layer.conf</filename> file identifies the file structure as a Yocto 
                Project layer, identifies the  
                contents of the layer, and contains information about how Yocto Project should use it. 
                Generally, a standard boilerplate file such as the following works.
                In the following example, you would replace "<filename>bsp</filename>" and 
                "<filename>_bsp</filename>" with the actual name
                of the BSP (i.e. <filename>&lt;bsp_name&gt;</filename> from the example template).
            </para>

            <para>
               <literallayout class='monospaced'>
     # We have a conf and classes directory, add to BBPATH
     BBPATH := "${BBPATH}:${LAYERDIR}"

     # We have a recipes directory, add to BBFILES
     BBFILES := "${BBFILES} ${LAYERDIR}/recipes/*/*.bb \ 
                 ${LAYERDIR}/recipes/*/*.bbappend"

     BBFILE_COLLECTIONS += "bsp"
     BBFILE_PATTERN_bsp := "^${LAYERDIR}/"
     BBFILE_PRIORITY_bsp = "6"
                </literallayout>
            </para>

            <para>
                To illustrate the string substitutions, here are the last three statements from the Crown 
                Bay <filename>conf/layer.conf</filename> file:
               <literallayout class='monospaced'>
     BBFILE_COLLECTIONS += "crownbay"
     BBFILE_PATTERN_crownbay := "^${LAYERDIR}/"
     BBFILE_PRIORITY_crownbay = "6"
                </literallayout>
            </para>

            <para>
                This file simply makes BitBake aware of the recipes and configuration directories.
                The file must exist so that the Yocto Project build system can recognize the BSP.
            </para>
            </section>

            <section id="bsp-filelayout-machine">
            <title>Hardware Configuration Options</title>
            <para>
                You can find these files in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/conf/machine/*.conf
                </literallayout>
            </para>

            <para>
                The machine files bind together all the information contained elsewhere
                in the BSP into a format that the Yocto Project build system can understand. 
                If the BSP supports multiple machines, multiple machine configuration files
                can be present. 
                These filenames correspond to the values to which users have set the 
                <ulink url='&YOCTO_DOCS_REF_URL;#var-MACHINE'><filename>MACHINE</filename></ulink> variable.
            </para>

            <para>
                These files define things such as the kernel package to use
                (<ulink url='&YOCTO_DOCS_REF_URL;#var-PREFERRED_PROVIDER'><filename>PREFERRED_PROVIDER</filename></ulink>
                of virtual/kernel), the hardware drivers to
                include in different types of images, any special software components
                that are needed, any bootloader information, and also any special image
                format requirements.
            </para>

            <para>
                Each BSP Layer requires at least one machine file.
                However, you can supply more than one file.
                For example, in the Crown Bay BSP shown earlier in this section, the 
                <filename>conf/machine</filename> directory contains two configuration files:
                <filename>crownbay.conf</filename> and <filename>crownbay-noemgd.conf</filename>.
                The <filename>crownbay.conf</filename> file is used for the Crown Bay BSP
                that supports the <trademark class='registered'>Intel</trademark> Embedded
                Media and Graphics Driver (<trademark class='registered'>Intel</trademark>
                EMGD), while the <filename>crownbay-noemgd.conf</filename> file is used for the 
                Crown Bay BSP that does not support the <trademark class='registered'>Intel</trademark>
                EMGD.
            </para>

            <para>
                This <filename>crownbay.conf</filename> file could also include
                a hardware "tuning" file that is commonly used to
                define the package architecture and specify 
                optimization flags, which are carefully chosen to give best
                performance on a given processor.
            </para>

            <para>
                Tuning files are found in the <filename>meta/conf/machine/include</filename>
                directory of the 
                <ulink url='&YOCTO_DOCS_DEV_URL;#yocto-project-files'>Yocto Project Files</ulink>.
                Tuning files can also reside in the BSP Layer itself.  
                For example, the <filename>ia32-base.inc</filename> file resides in the 
                <filename>meta-intel</filename> BSP Layer in <filename>conf/machine/include</filename>.
            </para>

            <para>
                To use an include file, you simply include them in the machine configuration file. 
                For example, the Crown Bay BSP <filename>crownbay.conf</filename> has the 
                following statements:
                <literallayout class='monospaced'>
     include conf/machine/include/tune-atom.inc
     include conf/machine/include/ia32-base.inc
                </literallayout>
            </para>
            </section>

            <section id='bsp-filelayout-misc-recipes'>
            <title>Miscellaneous Recipe Files</title>
            <para>
                You can find these files in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/recipes-bsp/*
                </literallayout> 
            </para>

            <para>
                This optional directory contains miscellaneous recipe files for the BSP.
                Most notably would be the formfactor files.
                For example, in the Crown Bay BSP there is the 
                <filename>formfactor_0.0.bbappend</filename> file, which is an append file used 
                to augment the recipe that starts the build.  
                Furthermore, there are machine-specific settings used during the build that are
                defined by the <filename>machconfig</filename> files.
                In the Crown Bay example, two <filename>machconfig</filename> files exist:
                one that supports the 
                <trademark class='registered'>Intel</trademark> Embedded
                Media and Graphics Driver (<trademark class='registered'>Intel</trademark>
                EMGD) and one that does not:
                <literallayout class='monospaced'>
     meta-crownbay/recipes-bsp/formfactor/formfactor/crownbay/machconfig
     meta-crownbay/recipes-bsp/formfactor/formfactor/crownbay-noemgd/machconfig
     meta-crownbay/recipes-bsp/formfactor/formfactor_0.0.bbappend
                </literallayout>
            </para>

            <note><para>
                If a BSP does not have a formfactor entry, defaults are established according to
                the formfactor configuration file that is installed by the main 
                formfactor recipe 
                <filename>meta/recipes-bsp/formfactor/formfactor_0.0.bb</filename>, 
                which is found in the 
                <ulink url='&YOCTO_DOCS_DEV_URL;#yocto-project-files'>Yocto Project Files</ulink>.
            </para></note>
            </section>

            <section id='bsp-filelayout-core-recipes'>
            <title>Core Recipe Files</title>
            <para>
                You can find these files in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/recipes-core/*
                </literallayout> 
            </para>

            <para>
                This directory contains recipe files that are almost always necessary to build a 
                useful, working Linux image.
                Thus, the term "core" is used to group these recipes.
                For example, in the Crown Bay BSP there is the 
                <filename>task-core-tools-profile.bbappend</filename> file, which is an append file used 
                to recommend that the 
                <ulink url='http://sourceware.org/systemtap/wiki'>SystemTap</ulink>
                package be included as a package when the image is built.
            </para>
            </section>

            <section id='bsp-filelayout-recipes-graphics'>
            <title>Display Support Files</title>
            <para>
                You can find these files in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/recipes-graphics/*            
                </literallayout>
            </para>

            <para>
                This optional directory contains recipes for the BSP if it has 
                special requirements for graphics support.
                All files that are needed for the BSP to support a display are kept here. 
                For example, the Crown Bay BSP contains two versions of the 
                <filename>xorg.conf</filename> file.
                The version in <filename>crownbay</filename> builds a BSP that supports the 
                <trademark class='registered'>Intel</trademark> Embedded Media Graphics Driver (EMGD),
                while the version in <filename>crownbay-noemgd</filename> builds 
                a BSP that supports Video Electronics Standards Association (VESA) graphics only:
                <literallayout class='monospaced'>
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config_0.1.bbappend
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config/crownbay/xorg.conf
     meta-crownbay/recipes-graphics/xorg-xserver/xserver-xf86-config/crownbay-noemgd/xorg.conf
                </literallayout>
            </para>
            </section>

            <section id='bsp-filelayout-kernel'>
            <title>Linux Kernel Configuration</title>
            <para>
                You can find these files in the BSP Layer at:
                <literallayout class='monospaced'>
     meta-&lt;bsp_name&gt;/recipes-kernel/linux/linux-yocto_*.bbappend
                </literallayout>
            </para>

            <para>
                These files append your specific changes to the kernel you are using.
            </para>
            <para>
                For your BSP, you typically want to use an existing Yocto Project kernel found in the 
                <ulink url='&YOCTO_DOCS_DEV_URL;#yocto-project-files'>Yocto 
                Project Files</ulink> at <filename>meta/recipes-kernel/linux</filename>.
                You can append your specific changes to the kernel recipe by using a
                similarly named append file, which is located in the BSP Layer (e.g. 
                the <filename>meta-&lt;bsp_name&gt;/recipes-kernel/linux</filename> directory).
            </para>
            <para>
                Suppose the BSP uses the <filename>linux-yocto_3.0.bb</filename> kernel, 
                which is the preferred kernel to use for developing a new BSP using the Yocto Project.
                In other words, you have selected the kernel in your 
                <filename>&lt;bsp_name&gt;.conf</filename> file by adding the following statements:
                <literallayout class='monospaced'>
     PREFERRED_PROVIDER_virtual/kernel ?= "linux-yocto"
     PREFERRED_VERSION_linux-yocto = "3.0%"
                </literallayout>
                You would use the <filename>linux-yocto_3.0.bbappend</filename> file to append 
                specific BSP settings to the kernel, thus configuring the kernel for your particular BSP.
            </para>
            <para>
                As an example, look at the existing Crown Bay BSP.
                The append file used is:
                <literallayout class='monospaced'>
     meta-crownbay/recipes-kernel/linux/linux-yocto_3.0.bbappend
                </literallayout>
                The following listing shows the file.
                Be aware that the actual commit ID strings in this example listing might be different
                than the actual strings in the file from the <filename>meta-intel</filename>
                Git source repository.
                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"

     COMPATIBLE_MACHINE_crownbay = "crownbay"
     KMACHINE_crownbay  = "yocto/standard/crownbay"
     KERNEL_FEATURES_append_crownbay += " cfg/smp.scc"

     COMPATIBLE_MACHINE_crownbay-noemgd = "crownbay-noemgd"
     KMACHINE_crownbay-noemgd  = "yocto/standard/crownbay"
     KERNEL_FEATURES_append_crownbay-noemgd += " cfg/smp.scc"

     SRCREV_machine_pn-linux-yocto_crownbay ?= "63c65842a3a74e4bd3128004ac29b5639f16433f"
     SRCREV_meta_pn-linux-yocto_crownbay ?= "59314a3523e360796419d76d78c6f7d8c5ef2593"

     SRCREV_machine_pn-linux-yocto_crownbay-noemgd ?= "63c65842a3a74e4bd3128004ac29b5639f16433f"
     SRCREV_meta_pn-linux-yocto_crownbay-noemgd ?= "59314a3523e360796419d76d78c6f7d8c5ef2593"
                </literallayout>
                This append file contains statements used to support the Crown Bay BSP for both 
                <trademark class='registered'>Intel</trademark> EMGD and the VESA graphics.
                The build process, in this case, recognizes and uses only the statements that 
                apply to the defined machine name - <filename>crownbay</filename> in this case.
                So, the applicable statements in the <filename>linux-yocto_3.0.bbappend</filename> 
                file are follows:
                <literallayout class='monospaced'>
     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"

     COMPATIBLE_MACHINE_crownbay = "crownbay"
     KMACHINE_crownbay  = "yocto/standard/crownbay"
     KERNEL_FEATURES_append_crownbay += " cfg/smp.scc"

     SRCREV_machine_pn-linux-yocto_crownbay ?= "63c65842a3a74e4bd3128004ac29b5639f16433f"
     SRCREV_meta_pn-linux-yocto_crownbay ?= "59314a3523e360796419d76d78c6f7d8c5ef2593"
                </literallayout>
                The append file defines <filename>crownbay</filename> as the compatible machine and
                defines the <filename>KMACHINE</filename>.
                The file also points to some configuration fragments to use by setting the 
                <filename>KERNEL_FEATURES</filename> variable.
                The location for the configuration fragments is the kernel tree itself in the 
                <ulink url='&YOCTO_DOCS_DEV_URL;#yocto-project-build-directory'>Yocto Project Build
                Directory</ulink> under <filename>linux/meta</filename>.
                Finally, the append file points to the specific commits in the 
                <ulink url='&YOCTO_DOCS_DEV_URL;#yocto-project-files'>Yocto Project Files</ulink> Git 
                repository and the <filename>meta</filename> Git repository branches to identify the 
                exact kernel needed to build the Crown Bay BSP.
            </para>
            <para>
                One thing missing in this particular BSP, which you will typically need when 
                developing a BSP, is the kernel configuration file (<filename>.config</filename>) for your BSP.
                When developing a BSP, you probably have a kernel configuration file or a set of kernel
                configuration files that, when taken together, define the kernel configuration for your BSP.
                You can accomplish this definition by putting the configurations in a file or a set of files 
                inside a directory located at the same level as your append file and having the same name 
                as the kernel.
                With all these conditions met simply reference those files in a 
                <filename>SRC_URI</filename> statement in the append file.
            </para>
            <para>
                For example, suppose you had a set of configuration options in a file called 
                <filename>myconfig</filename>.  
                If you put that file inside a directory named 
                <filename>/linux-yocto</filename> and then added 
                a <filename>SRC_URI</filename> statement such as the following to the append file, 
                those configuration
                options will be picked up and applied when the kernel is built.
                <literallayout class='monospaced'>
     SRC_URI += "file://myconfig"
                </literallayout>
            </para>
            <para>
                As mentioned earlier, you can group related configurations into multiple files and 
                name them all in the <filename>SRC_URI</filename> statement as well.
                For example, you could group separate configurations specifically for Ethernet and graphics
                into their own files and add those by using a <filename>SRC_URI</filename> statement like the 
                following in your append file:
                <literallayout class='monospaced'>
     SRC_URI += "file://myconfig \
            file://eth.cfg \
            file://gfx.cfg"
                </literallayout>
            </para>
            <para>
                The <filename>FILESEXTRAPATHS</filename> variable is in boilerplate form in the 
                previous example in order to make it easy to do that.
                This variable must be in your layer or BitBake will not find the patches or 
                configurations even if you have them in your <filename>SRC_URI</filename>.
                The <filename>FILESEXTRAPATHS</filename> variable enables the build process to
                find those configuration files.
            </para>
            <note>
                <para>
                Other methods exist to accomplish grouping and defining configuration options.
                For example, if you are working with a local clone of the kernel repository, 
                you could checkout the kernel's <filename>meta</filename> branch, make your changes, 
                and then push the changes to the local bare clone of the kernel.
                The result is that you directly add configuration options to the Yocto kernel 
                <filename>meta</filename> branch for your BSP.
                The configuration options will likely end up in that location anyway if the BSP gets 
                added to the Yocto Project. 
                For an example showing how to change the BSP configuration, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#changing-the-bsp-configuration'>Changing the BSP Configuration</ulink>" 
                section in the Yocto Project Development Manual.
                For a better understanding of working with a local clone of the kernel repository
                and a local bare clone of the kernel, see the
                "<ulink url='&YOCTO_DOCS_DEV_URL;#modifying-the-kernel-source-code'>Modifying the Kernel
                Source Code</ulink>" section also in the Yocto Project Development Manual.</para>
                <para>
                In general, however, the Yocto Project maintainers take care of moving the 
                <filename>SRC_URI</filename>-specified 
                configuration options to the kernel's <filename>meta</filename> branch.
                Not only is it easier for BSP developers to not have to worry about putting those 
                configurations in the branch, but having the maintainers do it allows them to apply 
                'global' knowledge about the kinds of common configuration options multiple BSPs in 
                the tree are typically using.  
                This allows for promotion of common configurations into common features.</para>
            </note>
            </section>
        </section>

        <section id='bsp-licensing-considerations'>
            <title>BSP Licensing Considerations</title>

            <para>
                In some cases, a BSP contains separately licensed Intellectual Property (IP)
                for a component or components.
                For these cases, you are required to accept the terms of a commercial or other 
                type of license that requires some kind of explicit End User License Agreement (EULA).  
                Once the license is accepted, the Yocto Project build system can then build and 
                include the corresponding component in the final BSP image.
                If the BSP is available as a pre-built image, you can download the image after
                agreeing to the license or EULA.
            </para>

            <para>
                You could find that some separately licensed components that are essential 
                for normal operation of the system might not have an unencumbered (or free)
                substitute.
                Without these essential components, the system would be non-functional.
                Then again, you might find that other licensed components that are simply 
                'good-to-have' or purely elective do have an unencumbered, free replacement 
                component that you can use rather than agreeing to the separately licensed component.
                Even for components essential to the system, you might find an unencumbered component 
                that is not identical but will work as a less-capable version of the 
                licensed version in the BSP recipe.
            </para>

            <para>
                For cases where you can substitute a free component and still
                maintain the system's functionality, the Yocto Project website's
                <ulink url='&YOCTO_HOME_URL;/download/all?keys=&amp;download_type=1&amp;download_version='>BSP
                Download Page</ulink> makes available de-featured BSPs
                that are completely free of any IP encumbrances. 
                For these cases, you can use the substitution directly and
                without any further licensing requirements.  
                If present, these fully de-featured BSPs are named appropriately
                different as compared to the names of the respective
                encumbered BSPs.  
                If available, these substitutions are your
                simplest and most preferred options.  
                Use of these substitutions of course assumes the resulting functionality meets
                system requirements.
            </para>

            <para>
                If however, a non-encumbered version is unavailable or
                it provides unsuitable functionality or quality, you can use an encumbered
                version.
            </para>

            <para> 
                A couple different methods exist within the Yocto
                Project build system to satisfy the licensing
                requirements for an encumbered BSP.  
                The following list describes them in order of preference:
	        <orderedlist>
                    <listitem><para><emphasis>Use the <filename>LICENSE_FLAGS</filename> variable
                        to define the Yocto Project recipes that have commercial or other types of
                        specially-licensed packages:</emphasis>  
                        For each of those recipes, you can 
                        specify a matching license string in a
                        <filename>local.conf</filename> variable named 
                        <filename>LICENSE_FLAGS_WHITELIST</filename>.
                        Specifying the matching license string signifies that you agree to the license.
                        Thus, the build system can build the corresponding recipe and include 
                        the component in the image.
                        See the 
                        "<ulink url='&YOCTO_DOCS_REF_URL;#enabling-commercially-licensed-recipes'>Enabling 
                        Commercially Licensed Recipes</ulink>" section in the Yocto Project Reference
                        Manual for details on how to use these variables.</para>
                        <para>If you build as you normally would, without
		        specifying any recipes in the
		        <filename>LICENSE_FLAGS_WHITELIST</filename>, the build stops and
		        provides you with the list of recipes that you have
		        tried to include in the image that need entries in
		        the <filename>LICENSE_FLAGS_WHITELIST</filename>.  
		        Once you enter the appropriate license flags into the whitelist,
		        restart the build to continue where it left off.
		        During the build, the prompt will not appear again
		        since you have satisfied the requirement.</para>
                        <para>Once the appropriate license flags are whitelisted
		        in the <filename>LICENSE_FLAGS_WHITELIST</filename> variable, you 
		        can build the encumbered image with no change at all
		        to the normal build process.</para></listitem> 
                    <listitem><para><emphasis>Get a pre-built version of the BSP:</emphasis>
                        You can get this type of BSP by visiting the Yocto Project website's
                        <ulink url='&YOCTO_HOME_URL;/download'>Download</ulink>
                        page and clicking on "BSP Downloads".
                        You can download BSP tarballs that contain proprietary components
                        after agreeing to the licensing
                        requirements of each of the individually encumbered
                        packages as part of the download process.  
                        Obtaining the BSP this way allows you to access an encumbered
                        image immediately after agreeing to the
                        click-through license agreements presented by the
                        website.  
                        Note that if you want to build the image
                        yourself using the recipes contained within the BSP
                        tarball, you will still need to create an
                        appropriate <filename>LICENSE_FLAGS_WHITELIST</filename> to match the
                        encumbered recipes in the BSP.</para></listitem>
	        </orderedlist>
            </para>

            <note>
                Pre-compiled images are bundled with
                a time-limited kernel that runs for a
                predetermined amount of time (10 days) before it forces
                the system to reboot.  
                This limitation is meant to discourage direct redistribution
                of the image.
                You must eventually rebuild the image if you want to remove this restriction.
            </note> 
        </section>
</chapter>