path: root/handbook/bsp.xml

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<chapter id='bsp'>

        <title>Board Support Packages (BSP) - Developers Guide</title>

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

        <para>
            The intend of this document is to define a structure for these components
            so that BSPs follow a commonly understood layout allowing them to be 
            provided in a common way that everyone understands. It also allows end 
            users to become familiar with one common format and encourages standardisation 
            of software support of hardware.
        </para>

        <para>
            The proposed format does have elements that are specific to the Poky and 
            OpenEmbedded build systems. It is intended that this information can be 
            used by other systems besides Poky/OpenEmbedded and that it will be simple
            to extract information and convert to other formats if required. The format 
            described can be directly accepted as a layer by Poky using its standard 
            layers mechanism, but it is important to recognise that 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 in use.
        </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 the BSP may be shipped combined with a build system
            and other tools, but it is important to maintain the distinction that these
            are separate components which may just be combined in certain end products.
        </para>

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

            <para>
                The BSP consists of a file structure inside a base directory, meta-bsp in this example, where "bsp" is a placeholder for the machine or platform name. Examples of some files that it could contain are:
            </para>

            <para>
                <programlisting>
meta-bsp/                                
meta-bsp/binary/zImage
meta-bsp/binary/poky-image-minimal.directdisk
meta-bsp/conf/layer.conf 
meta-bsp/conf/machine/*.conf             
meta-bsp/conf/machine/include/tune-*.inc
meta-bsp/packages/bootloader/bootloader_0.1.bb
meta-bsp/packages/linux/linux-bsp-2.6.50/*.patch
meta-bsp/packages/linux/linux-bsp-2.6.50/defconfig-bsp
meta-bsp/packages/linux/linux-bsp_2.6.50.bb
meta-bsp/packages/modem/modem-driver_0.1.bb
meta-bsp/packages/modem/modem-daemon_0.1.bb
meta-bsp/packages/image-creator/image-creator-native_0.1.bb
meta-bsp/prebuilds/

                </programlisting>
            </para>

            <para>
                The following sections detail what these files and directories could contain.
            </para>

        </section>

        <section id='bsp-filelayout-binary'>
            <title>Prebuilt User Binaries (meta-bsp/binary/*)</title>

            <para>
                This optional area cotains useful prebuilt kernels and userspace filesystem 
                images appropriate to the target system. Users could use these to get a system 
                running and quickly get started on development tasks. The exact types of binaries
                present will be highly hardware dependent but a README file should be present 
                explaining how to use them with the target hardware. If prebuilt binaries are 
                present, source code to meet licensing requirements must also be provided in 
                some form.
            </para>
       
        </section>

        <section id='bsp-filelayout-layer'>
            <title>Layer Configuration (meta-bsp/conf/layer.conf)</title>

            <para>
                This file identifies the structure as a Poky layer. This file identifies the 
                contents of the layer and how contains information about how Poky should use 
                it. In general it will most likely be a standard boilerplate file consisting of:
            </para>

            <para>
               <programlisting>
# We have a conf directory, add to BBPATH
BBPATH := "${BBPATH}${LAYERDIR}"

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

BBFILE_COLLECTIONS += "bsp"
BBFILE_PATTERN_bsp := "^${LAYERDIR}/"
BBFILE_PRIORITY_bsp = "5"
                </programlisting>
            </para>

            <para>
                which simply makes bitbake aware of the packages and conf directories.
            </para>

            <para>
                This file is required for recognition of the BSP by Poky.
            </para>

        </section>

        <section id='bsp-filelayout-machine'>
            <title>Hardware Configuration Options (meta-bsp/conf/machine/*.conf)</title>

            <para>
                The machine files bind together all the information contained elsewhere
                in the BSP into a format that Poky/OpenEmbedded can understand. If
                the BSP supports multiple machines, multiple machine configuration files
                can be present. These filenames correspond to the values users set the
                MACHINE variable to.
            </para>

            <para>
                These files would define things like which kernel package to use
                (PREFERRED_PROVIDER of virtual/kernel), which 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>
                At least one machine file is required for a Poky BSP layer but more than one may be present.
            </para>

        </section>

        <section id='bsp-filelayout-tune'>
            <title>Hardware Optimisation Options (meta-bsp/conf/machine/include/tune-*.inc)</title>

            <para>
                These are shared hardware "tuning" definitions and are commonly used to
                pass specific optimisation flags to the compiler. An example is
                tune-atom.inc:
            </para>
            <para>
               <programlisting>
BASE_PACKAGE_ARCH = "core2"
TARGET_CC_ARCH = "-m32 -march=core2 -msse3 -mtune=generic -mfpmath=sse"
               </programlisting>
            </para>
            <para>
                which defines a new package architecture called "core2" and uses the
                optimization flags specified, which are carefully chosen to give best
                performance on atom cpus.
            </para>
            <para>
                The tune file would be included by the machine definition and can be
                contained in the BSP or reference one from the standard core set of
                files included with Poky itself.
            </para>
            <para>
                These files are optional for a Poky BSP layer.
            </para>
        </section>
        <section id='bsp-filelayout-kernel'>
            <title>Linux Kernel Configuration (meta-bsp/packages/linux/*)</title>

            <para>
                These files make up the definition of a kernel to use with this
                hardware. In this case it is a complete self contained kernel with its own
                configuration and patches but kernels can be shared between many
                machines as well. Taking some specific example files:
            </para>
            <para>
               <programlisting>
meta-bsp/packages/linux/linux-bsp_2.6.50.bb
               </programlisting>
            </para>
            <para>
                which is the core kernel recipe which firstly details where to get the kernel
                source from. All standard source code locations are supported so this could 
                be a release tarball, some git repository or source included in
                the directory within the BSP itself. It then contains information about which 
                patches to apply and how to configure and build it. It can reuse the main
                Poky kernel build class, so the definitions here can remain very simple.
            </para>
            <para>
               <programlisting>
linux-bsp-2.6.50/*.patch
               </programlisting>
            </para>
            <para>
                which are patches which may be applied against the base kernel, wherever
                that may have been obtained from.
            </para>
            <para>
               <programlisting>
meta-bsp/packages/linux/linux-bsp-2.6.50/defconfig-bsp
               </programlisting>
            </para>
            <para>
                which is the configuration information to use to configure the kernel.
            </para>
            <para>
                Examples of kernel recipes are available in Poky itself. These files are
                optional since a kernel from Poky itself could be selected although it
                would be unusual not to have a kernel configuration.
            </para>
        </section>

        <section id='bsp-filelayout-packages'>
            <title>Other Software (meta-bsp/packages/*)</title>

            <para>
                This area includes other pieces of software which the hardware may need for best
                operation. These are just examples of the kind of things that may be
                encountered. The are standard .bb file recipes in the usual Poky format,
                so for examples, see standard Poky recipes. The source can be included directly, 
                referred to in source control systems or release tarballs of external software projects.
            </para>
            <para>
               <programlisting>
meta-bsp/packages/bootloader/bootloader_0.1.bb
               </programlisting>
            </para>
            <para>
                Some kind of bootloader recipe which may be used to generate a new
                bootloader binary. Sometimes these are included in the final image
                format and needed to reflash hardware.
            </para>
            <para>
               <programlisting>
meta-bsp/packages/modem/modem-driver_0.1.bb
meta-bsp/packages/modem/modem-daemon_0.1.bb
               </programlisting>
            </para>
            <para>
                These are examples of a hardware driver and also a hardware daemon which
                may need to be included in images to make the hardware useful. "modem"
                is one example but there may be other components needed like firmware.
            </para>
            <para>
               <programlisting>
meta-bsp/packages/image-creator/image-creator-native_0.1.bb
               </programlisting>
            </para>
            <para>
                Sometimes the device will need an image in a very specific format for
                its update mechanism to accept and reflash with it. Recipes to build the
                tools needed to do this can be included with the BSP.
            </para>
            <para>
                These files only need be provided if the platform requires them.
            </para>
        </section>

        <section id='bs-filelayout-bbappend'>
            <title>Append BSP specific information to existing recipes</title>

            <para>
            Say you have a recipe like pointercal which has machine specific information in it,
            and then you have your new bsp code in a layer. Before .bbappend extension is
            introduced, you have to copy the whole pointercal recipe and files into your layer,
            and then add the single file for your machine which is ugly.

            .bbappend makes above work much easier, to allow bsp specific information merged
            with original recipe easily. When bitbake finds any X.bbappend files, they will be
            included after bitbake loads X.bb but before finalise and any anonymous methods run.
            This allows bsp layer to poke around and do whatever it might want to customise
            the original recipe.

            .bbappend is expected to include below two lines in the head (which may be changed
            in the future):
            </para>

            <programlisting>
THISDIR := "${@os.path.dirname(bb.data.getVar('FILE', d, True))}"
FILESPATH =. "${@base_set_filespath(["${THISDIR}/${PN}"], d)}:"
            </programlisting>

            <para>
            Then bsp could add machine specific config files in layer directory, which will be
            added by bitbake. You could look at meta-emenlow/packages/formfactor as example
            </para>
        </section>

        <section id='bsp-filelayout-prebuilds'>
            <title>Prebuild Data (meta-bsp/prebuilds/*)</title>

            <para>
                The location can contains a precompiled representation of the source code 
                contained elsewhere in the BSP layer. It can be processed and used by
                Poky to provide much faster build times assuming a compatible configuration is used.
            </para>

            <para>
                These files are optional.
            </para>

        </section>

</chapter>