path: root/documentation/poky-ref-manual/technical-details.xml

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<chapter id='technical-details'>
<title>Technical Details</title>

    <para>
        This chapter provides technical details for various parts of the Yocto Project. 
        Currently, topics include Yocto Project components and shared state (sstate) cache.
    </para>

<section id='usingpoky-components'>
    <title>Yocto Project Components</title>

    <para>
        The BitBake task executor together with various types of configuration files form the 
        Yocto Project core.
        This section overviews the BitBake task executor and the
        configuration files by describing what they are used for 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>  An abstraction of 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, etc.
            Configuration data acts as the glue to bind everything together.</para></listitem>
    </itemizedlist>
        For more information on data, see the
        <ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html#yocto-project-terms'>
        Yocto Project Terms</ulink> section in 
        <ulink url='http://www.yoctoproject.org/docs/latest/dev-manual/dev-manual.html'>
        The Yocto Project Development Manual</ulink>.
    </para>

    <para> 
        BitBake knows how to combine multiple data sources together and refers to each data source
        as a "<link linkend='usingpoky-changes-layers'>layer</link>".
    </para>

    <para>
        Following are some brief details on these core components.
        For more detailed information on these components see the 
        <link linkend='ref-structure'>'Reference: Directory Structure'</link>
        appendix.
    </para>

    <section id='usingpoky-components-bitbake'>
        <title>BitBake</title>

        <para>
            BitBake is the tool at the heart of the Yocto Project and is responsible
            for parsing the metadata, generating a list of tasks from it,
            and then executing those tasks. 
            To see a list of the options BitBake supports, use the following help command:
            <literallayout class='monospaced'>
     $ bitbake --help
            </literallayout>
        </para>

        <para>
            The most common usage for BitBake is <filename>bitbake &lt;packagename&gt;</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 <filename>.bb</filename> filename.
            So, to run the <filename>matchbox-desktop_1.2.3.bb</filename> 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 
            <link linkend='ref-bitbake-providers'>Preferences and Providers</link> section.
        </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>eglibc</filename> if they had not already 
            been built.
            <note>This release of the Yocto Project does not support the <filename>glibc</filename>
                GNU version of the Unix standard C library.  By default, the Yocto Project builds with
                <filename>eglibc</filename>.</note>
        </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 much 
            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>
            The <filename>.bb</filename> files are usually referred to as "recipes." 
            In general, a recipe contains information about a single piece of software.
            The information includes the location from which to download the source patches 
            (if any are 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" can also be used to describe recipes.
            However, since the same word is used for the packaged output from the Yocto 
            Project (i.e. <filename>.ipk</filename> or <filename>.deb</filename> files), 
            this document avoids using the term "package" when refering to recipes.
        </para>
    </section>

    <section id='usingpoky-components-classes'>
        <title>Classes</title>

        <para>
            Class files (<filename>.bbclass</filename>) contain information that is useful to share
            between metadata files. 
            An example is the Autotools class, which contains
            common settings for any application that Autotools uses.
            The <link linkend='ref-classes'>Reference: Classes</link> appendix provides details
            about common 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 Yocto Project 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 (<filename>local.conf</filename>, which is found
            in the Yocto Project files build directory).
        </para>
    </section>
</section>

<section id="shared-state-cache">
    <title>Shared State Cache</title>

    <para>
        By design, the Yocto Project build system builds everything from scratch unless 
        BitBake can determine that parts don't 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 
        rebuiding things that don't necessarily need 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 Yocto Project incremental build support system:
        <itemizedlist>
            <listitem>What pieces of the system have changed and what pieces have not changed?</listitem>
            <listitem>How are changed pieces of software removed and replaced?</listitem>
            <listitem>How are pre-built components that don't need to be rebuilt from scratch
                used when they are available?</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.
    </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 
            uses a per-task basis and does not use 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, <filename>do_install</filename> and <filename>do_package</filename>
            output 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 situation.
            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='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 <filename>WORKDIR</filename>. 
            It does not matter if the working directory changes because it should not 
            affect the output for target packages.
            Also, the build process has the objective of making native/cross packages relocatable. 
            The checksum therefore needs to exclude <filename>WORKDIR</filename>.
            The simplistic approach for excluding the worknig 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 <filename>PACKAGE_ARCHS</filename> variable does not 
            depend on the value of <filename>MACHINE</filename>, 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 inline 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 build directory. 
            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>
            While figuring out the dependencies and creating these checksums is good,
            what does the Yocto Project build system do with the checksum information? 
            The build system uses a signature handler that is responsible for 
            processing the checksum information.
            By default, there is a dummy "noop" signature handler enabled in BitBake.
            This means that behaviour is unchanged from previous versions. 
            OECore uses the "basic" signature handler through this setting in the
            <filename>bitbake.conf</filename> file:
            <literallayout class='monospaced'>
     BB_SIGNATURE_HANDLER ?= "basic"
            </literallayout>
            Also within the BitBake configuration file, we can give BitBake
            some extra information to help it handle this information.
            The following statements effectively result 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"
     BB_HASHBASE_WHITELIST += "DL_DIR SSTATE_DIR THISDIR FILESEXTRAPATHS"
     BB_HASHBASE_WHITELIST += "FILE_DIRNAME HOME LOGNAME SHELL TERM USER"
     BB_HASHBASE_WHITELIST += "FILESPATH USERNAME STAGING_DIR_HOST STAGING_DIR_TARGET"
     BB_HASHTASK_WHITELIST += "(.*-cross$|.*-native$|.*-cross-initial$| \
         .*-cross-intermediate$|^virtual:native:.*|^virtual:nativesdk:.*)"
            </literallayout>
            This example is actually where <filename>WORKDIR</filename>
            is excluded since <filename>WORKDIR</filename> is constructed as a
            path within <filename>TMPDIR</filename>, which is on the whitelist.
        </para>

        <para>
            The <filename>BB_HASHTASK_WHITELIST</filename> covers dependent tasks and 
            excludes certain kinds of tasks from the dependency chains. 
            The effect of the previous example is to isolate the native, target,
            and cross-components.
            So, for example, toolchain changes do not force a rebuild of the whole system.
        </para>

        <para>
            The end result of the "basic" handler is to make some dependency and
            hash information available to the build. 
            This includes:
            <literallayout class='monospaced'>
     BB_BASEHASH_task-&lt;taskname&gt; - the base hashes for each task in the recipe
     BB_BASEHASH_&lt;filename:taskname&gt; - the base hashes for each dependent task
     BBHASHDEPS_&lt;filename:taskname&gt; - The task dependencies for each task
     BB_TASKHASH - the hash of the currently running task
            </literallayout>
            There is also a "basichash" <filename>BB_SIGNATURE_HANDLER</filename>,
            which is the same as the basic version but adds the task hash to the stamp files. 
            This results in any metadata change that changes the task hash,
            automatically causing the task to be run again. 
            This removes the need to bump <filename>PR</filename>
            values and changes to metadata automatically ripple across the build.
            Currently, this behavior is not the default behavior.
            However, it is likely that the Yocto Project team will go forward with this 
            behavior in the future since all the functionality exists. 
            The reason for the delay is the potential impact to the distribution feed 
            creation as they need increasing <filename>PR</filename> fields
            and the Yocto Project currently lacks a mechanism to automate incrementing 
            this field.
        </para>
    </section>

    <section id='shared-state'>
        <title>Shared State</title>

        <para>
            Checksums and dependencies, as discussed in the previous section, solve half the 
            problem.
            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 shared state class (<filename>sstate.bbclass</filename>) 
            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 doesn't need to worry about its source.
        </para>

        <para>
            There are two types of output, one is just about creating a directory
            in <filename>WORKDIR</filename>.
            A good example is the output of either <filename>do_install</filename> or 
            <filename>do_package</filename>. 
            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.bbclass</filename>. 
            From a user's perspective, adding shared state wrapping to a task
            is as simple as this <filename>do_deploy</filename> example taken from 
            <filename>do_deploy.bbclass</filename>:
            <literallayout class='monospaced'>
     DEPLOYDIR = "${WORKDIR}/deploy-${PN}"
     SSTATETASKS += "do_deploy"
     do_deploy[sstate-name] = "deploy"
     do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"
     do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"

     python do_deploy_setscene () {
         sstate_setscene(d)
     }
     addtask do_deploy_setscene
            </literallayout>
            In the example, we add some extra flags to the task, a name field ("deploy"), an
            input directory where the task sends data, and the output
            directory where the data from the task should eventually be copied. 
            We also add a <filename>_setscene</filename> variant of the task and add the task
            name to the <filename>SSTATETASKS</filename> list.
        </para>

        <para>
            If you have a directory whose contents you need to preserve, you can do this with 
            a line like the following:
            <literallayout class='monospaced'>
     do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}"
            </literallayout>
            This method, as well as the following example, also works for mutliple directories.
            <literallayout class='monospaced'>
     do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}"
     do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}"
     do_package[sstate-lockfile] = "${PACKAGELOCK}"
            </literallayout>
            These methods also include the ability to take a lockfile when manipulating
            shared state directory structures since some cases are sensitive to file
            additions or removals.
        </para>

        <para>
            Behind the scenes, the shared state code works by looking in 
            <filename>SSTATE_DIR</filename> and  
            <filename>SSTATE_MIRRORS</filename> for shared state files. 
            Here is an example:
            <literallayout class='monospaced'>
     SSTATE_MIRRORS ?= "\
     file://.* http://someserver.tld/share/sstate/ \n \
     file://.* file:///some/local/dir/sstate/"
            </literallayout>
        </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 uses 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 <filename>do_package_write_ipk</filename> 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 Yocto Project 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='debugging'>
            <title>Debugging</title>

            <para>
                When things go wrong, debugging needs to be straightforward. 
                Because of this, the Yocto Project team included strong debugging
                tools:
                <itemizedlist>
                    <listitem><para>Whenever a shared state package is written out, so is a
                        corresponding <filename>.siginfo</filename> file. 
                        This practice results in a pickled python database of all
                        the metadata that went into creating the hash for a given shared state
                        package.</para></listitem>
                    <listitem><para>If BitBake is run with the <filename>--dump-signatures</filename>
                        (or <filename>-S</filename>) option, BitBake dumps out 
                        <filename>.siginfo</filename> files in
                        the stamp directory for every task it would have executed instead of
                        building the specified target package.</para></listitem>
                    <listitem><para>There is a <filename>bitbake-diffsigs</filename> command that
                        can process these <filename>.siginfo</filename> files. 
                        If one file is specified, it will dump out the dependency
                        information in the file. 
                        If two files are specified, it will compare the two files and dump out 
                        the differences between the two.
                        This allows the question of "What changed between X and Y?" to be
                        answered easily.</para></listitem>
                </itemizedlist>
            </para>
        </section>

        <section id='invalidating-shared-state'>
            <title>Invalidating Shared State</title>

            <para>
                The shared state code uses checksums and shared state memory 
                cache to avoid unnecessarily rebuilding tasks.
                As with all schemes, this one has some drawbacks.
                It is possible that you could make implicit changes that are not factored 
                into the checksum calculation, but do affect a task's output. 
                A good example is perhaps when a tool changes its output.
                Let's say that the output of <filename>rpmdeps</filename> needed to change.
                The result of the change should be that all the "package", "package_write_rpm",
                and "package_deploy-rpm" shared state cache items would become invalid.
                But, because this is a change that is external to the code and therefore implicit,
                the associated shared state cache items do not become invalidated.
                In this case, the build process would use 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
                change you make.
                Note that any changes you make directly to a function automatically are factored into 
                the checksum calculation and thus, will invalidate the associated area of sstate cache.
                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. 
                Once you are aware of such a change, you can take steps to invalidate the cache 
                and force the task to run. 
                The step to take is as simple as changing a function's comments in the source code. 
                For example, to invalidate package shared state files, change the comment statments
                of <filename>do_package</filename> or the comments of one of the functions it calls.
                The change is purely cosmetic, but it causes the checksum to be recalculated and  
                forces the task to be run again.
            </para>

            <note>
                For an example of a commit that makes a cosmetic change to invalidate 
                a shared state, see this
                <ulink url='http://git.yoctoproject.org/cgit.cgi/poky/commit/meta/classes/package.bbclass?id=737f8bbb4f27b4837047cb9b4fbfe01dfde36d54'>commit</ulink>.
            </note>
        </section>                
    </section>
</section>

</chapter>
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