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+=========
+Execution
+=========
+
+The primary purpose for running BitBake is to produce some kind of
+output such as a single installable package, a kernel, a software
+development kit, or even a full, board-specific bootable Linux image,
+complete with bootloader, kernel, and root filesystem. Of course, you
+can execute the ``bitbake`` command with options that cause it to
+execute single tasks, compile single recipe files, capture or clear
+data, or simply return information about the execution environment.
+
+This chapter describes BitBake's execution process from start to finish
+when you use it to create an image. The execution process is launched
+using the following command form: $ bitbake target For information on
+the BitBake command and its options, see "`The BitBake
+Command <#bitbake-user-manual-command>`__" section.
+
+.. note::
+
+   Prior to executing BitBake, you should take advantage of available
+   parallel thread execution on your build host by setting the
+   ```BB_NUMBER_THREADS`` <#var-bb-BB_NUMBER_THREADS>`__ variable in
+   your project's ``local.conf`` configuration file.
+
+   A common method to determine this value for your build host is to run
+   the following: $ grep processor /proc/cpuinfo This command returns
+   the number of processors, which takes into account hyper-threading.
+   Thus, a quad-core build host with hyper-threading most likely shows
+   eight processors, which is the value you would then assign to
+   ``BB_NUMBER_THREADS``.
+
+   A possibly simpler solution is that some Linux distributions (e.g.
+   Debian and Ubuntu) provide the ``ncpus`` command.
+
+Parsing the Base Configuration Metadata
+=======================================
+
+The first thing BitBake does is parse base configuration metadata. Base
+configuration metadata consists of your project's ``bblayers.conf`` file
+to determine what layers BitBake needs to recognize, all necessary
+``layer.conf`` files (one from each layer), and ``bitbake.conf``. The
+data itself is of various types:
+
+-  *Recipes:* Details about particular pieces of software.
+
+-  *Class Data:* An abstraction of common build information (e.g. how to
+   build a Linux kernel).
+
+-  *Configuration Data:* Machine-specific settings, policy decisions,
+   and so forth. Configuration data acts as the glue to bind everything
+   together.
+
+The ``layer.conf`` files are used to construct key variables such as
+```BBPATH`` <#var-bb-BBPATH>`__ and ```BBFILES`` <#var-bb-BBFILES>`__.
+``BBPATH`` is used to search for configuration and class files under the
+``conf`` and ``classes`` directories, respectively. ``BBFILES`` is used
+to locate both recipe and recipe append files (``.bb`` and
+``.bbappend``). If there is no ``bblayers.conf`` file, it is assumed the
+user has set the ``BBPATH`` and ``BBFILES`` directly in the environment.
+
+Next, the ``bitbake.conf`` file is located using the ``BBPATH`` variable
+that was just constructed. The ``bitbake.conf`` file may also include
+other configuration files using the ``include`` or ``require``
+directives.
+
+Prior to parsing configuration files, BitBake looks at certain
+variables, including:
+
+-  ```BB_ENV_WHITELIST`` <#var-bb-BB_ENV_WHITELIST>`__
+
+-  ```BB_ENV_EXTRAWHITE`` <#var-bb-BB_ENV_EXTRAWHITE>`__
+
+-  ```BB_PRESERVE_ENV`` <#var-bb-BB_PRESERVE_ENV>`__
+
+-  ```BB_ORIGENV`` <#var-bb-BB_ORIGENV>`__
+
+-  ```BITBAKE_UI`` <#var-bb-BITBAKE_UI>`__
+
+The first four variables in this list relate to how BitBake treats shell
+environment variables during task execution. By default, BitBake cleans
+the environment variables and provides tight control over the shell
+execution environment. However, through the use of these first four
+variables, you can apply your control regarding the environment
+variables allowed to be used by BitBake in the shell during execution of
+tasks. See the "`Passing Information Into the Build Task
+Environment <#passing-information-into-the-build-task-environment>`__"
+section and the information about these variables in the variable
+glossary for more information on how they work and on how to use them.
+
+The base configuration metadata is global and therefore affects all
+recipes and tasks that are executed.
+
+BitBake first searches the current working directory for an optional
+``conf/bblayers.conf`` configuration file. This file is expected to
+contain a ```BBLAYERS`` <#var-bb-BBLAYERS>`__ variable that is a
+space-delimited list of 'layer' directories. Recall that if BitBake
+cannot find a ``bblayers.conf`` file, then it is assumed the user has
+set the ``BBPATH`` and ``BBFILES`` variables directly in the
+environment.
+
+For each directory (layer) in this list, a ``conf/layer.conf`` file is
+located and parsed with the ```LAYERDIR`` <#var-bb-LAYERDIR>`__ variable
+being set to the directory where the layer was found. The idea is these
+files automatically set up ```BBPATH`` <#var-bb-BBPATH>`__ and other
+variables correctly for a given build directory.
+
+BitBake then expects to find the ``conf/bitbake.conf`` file somewhere in
+the user-specified ``BBPATH``. That configuration file generally has
+include directives to pull in any other metadata such as files specific
+to the architecture, the machine, the local environment, and so forth.
+
+Only variable definitions and include directives are allowed in BitBake
+``.conf`` files. Some variables directly influence BitBake's behavior.
+These variables might have been set from the environment depending on
+the environment variables previously mentioned or set in the
+configuration files. The "`Variables
+Glossary <#ref-bb-variables-glos>`__" chapter presents a full list of
+variables.
+
+After parsing configuration files, BitBake uses its rudimentary
+inheritance mechanism, which is through class files, to inherit some
+standard classes. BitBake parses a class when the inherit directive
+responsible for getting that class is encountered.
+
+The ``base.bbclass`` file is always included. Other classes that are
+specified in the configuration using the
+```INHERIT`` <#var-bb-INHERIT>`__ variable are also included. BitBake
+searches for class files in a ``classes`` subdirectory under the paths
+in ``BBPATH`` in the same way as configuration files.
+
+A good way to get an idea of the configuration files and the class files
+used in your execution environment is to run the following BitBake
+command: $ bitbake -e > mybb.log Examining the top of the ``mybb.log``
+shows you the many configuration files and class files used in your
+execution environment.
+
+.. note::
+
+   You need to be aware of how BitBake parses curly braces. If a recipe
+   uses a closing curly brace within the function and the character has
+   no leading spaces, BitBake produces a parsing error. If you use a
+   pair of curly braces in a shell function, the closing curly brace
+   must not be located at the start of the line without leading spaces.
+
+   Here is an example that causes BitBake to produce a parsing error:
+   fakeroot create_shar() { cat << "EOF" >
+   ${SDK_DEPLOY}/${TOOLCHAIN_OUTPUTNAME}.sh usage() { echo "test" ######
+   The following "}" at the start of the line causes a parsing error
+   ###### } EOF } Writing the recipe this way avoids the error: fakeroot
+   create_shar() { cat << "EOF" >
+   ${SDK_DEPLOY}/${TOOLCHAIN_OUTPUTNAME}.sh usage() { echo "test"
+   ######The following "}" with a leading space at the start of the line
+   avoids the error ###### } EOF }
+
+Locating and Parsing Recipes
+============================
+
+During the configuration phase, BitBake will have set
+```BBFILES`` <#var-bb-BBFILES>`__. BitBake now uses it to construct a
+list of recipes to parse, along with any append files (``.bbappend``) to
+apply. ``BBFILES`` is a space-separated list of available files and
+supports wildcards. An example would be: BBFILES =
+"/path/to/bbfiles/*.bb /path/to/appends/*.bbappend" BitBake parses each
+recipe and append file located with ``BBFILES`` and stores the values of
+various variables into the datastore.
+
+.. note::
+
+   Append files are applied in the order they are encountered in
+   BBFILES
+   .
+
+For each file, a fresh copy of the base configuration is made, then the
+recipe is parsed line by line. Any inherit statements cause BitBake to
+find and then parse class files (``.bbclass``) using
+```BBPATH`` <#var-bb-BBPATH>`__ as the search path. Finally, BitBake
+parses in order any append files found in ``BBFILES``.
+
+One common convention is to use the recipe filename to define pieces of
+metadata. For example, in ``bitbake.conf`` the recipe name and version
+are used to set the variables ```PN`` <#var-bb-PN>`__ and
+```PV`` <#var-bb-PV>`__: PN =
+"${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE', False),d)[0] or
+'defaultpkgname'}" PV =
+"${@bb.parse.BBHandler.vars_from_file(d.getVar('FILE', False),d)[1] or
+'1.0'}" In this example, a recipe called "something_1.2.3.bb" would set
+``PN`` to "something" and ``PV`` to "1.2.3".
+
+By the time parsing is complete for a recipe, BitBake has a list of
+tasks that the recipe defines and a set of data consisting of keys and
+values as well as dependency information about the tasks.
+
+BitBake does not need all of this information. It only needs a small
+subset of the information to make decisions about the recipe.
+Consequently, BitBake caches the values in which it is interested and
+does not store the rest of the information. Experience has shown it is
+faster to re-parse the metadata than to try and write it out to the disk
+and then reload it.
+
+Where possible, subsequent BitBake commands reuse this cache of recipe
+information. The validity of this cache is determined by first computing
+a checksum of the base configuration data (see
+```BB_HASHCONFIG_WHITELIST`` <#var-bb-BB_HASHCONFIG_WHITELIST>`__) and
+then checking if the checksum matches. If that checksum matches what is
+in the cache and the recipe and class files have not changed, BitBake is
+able to use the cache. BitBake then reloads the cached information about
+the recipe instead of reparsing it from scratch.
+
+Recipe file collections exist to allow the user to have multiple
+repositories of ``.bb`` files that contain the same exact package. For
+example, one could easily use them to make one's own local copy of an
+upstream repository, but with custom modifications that one does not
+want upstream. Here is an example: BBFILES = "/stuff/openembedded/*/*.bb
+/stuff/openembedded.modified/*/*.bb" BBFILE_COLLECTIONS = "upstream
+local" BBFILE_PATTERN_upstream = "^/stuff/openembedded/"
+BBFILE_PATTERN_local = "^/stuff/openembedded.modified/"
+BBFILE_PRIORITY_upstream = "5" BBFILE_PRIORITY_local = "10"
+
+.. note::
+
+   The layers mechanism is now the preferred method of collecting code.
+   While the collections code remains, its main use is to set layer
+   priorities and to deal with overlap (conflicts) between layers.
+
+.. _bb-bitbake-providers:
+
+Providers
+=========
+
+Assuming BitBake has been instructed to execute a target and that all
+the recipe files have been parsed, BitBake starts to figure out how to
+build the target. BitBake looks through the ``PROVIDES`` list for each
+of the recipes. A ``PROVIDES`` list is the list of names by which the
+recipe can be known. Each recipe's ``PROVIDES`` list is created
+implicitly through the recipe's ```PN`` <#var-bb-PN>`__ variable and
+explicitly through the recipe's ```PROVIDES`` <#var-bb-PROVIDES>`__
+variable, which is optional.
+
+When a recipe uses ``PROVIDES``, that recipe's functionality can be
+found under an alternative name or names other than the implicit ``PN``
+name. As an example, suppose a recipe named ``keyboard_1.0.bb``
+contained the following: PROVIDES += "fullkeyboard" The ``PROVIDES``
+list for this recipe becomes "keyboard", which is implicit, and
+"fullkeyboard", which is explicit. Consequently, the functionality found
+in ``keyboard_1.0.bb`` can be found under two different names.
+
+.. _bb-bitbake-preferences:
+
+Preferences
+===========
+
+The ``PROVIDES`` list is only part of the solution for figuring out a
+target's recipes. Because targets might have multiple providers, BitBake
+needs to prioritize providers by determining provider preferences.
+
+A common example in which a target has multiple providers is
+"virtual/kernel", which is on the ``PROVIDES`` list for each kernel
+recipe. Each machine often selects the best kernel provider by using a
+line similar to the following in the machine configuration file:
+PREFERRED_PROVIDER_virtual/kernel = "linux-yocto" The default
+```PREFERRED_PROVIDER`` <#var-bb-PREFERRED_PROVIDER>`__ is the provider
+with the same name as the target. BitBake iterates through each target
+it needs to build and resolves them and their dependencies using this
+process.
+
+Understanding how providers are chosen is made complicated by the fact
+that multiple versions might exist for a given provider. BitBake
+defaults to the highest version of a provider. Version comparisons are
+made using the same method as Debian. You can use the
+```PREFERRED_VERSION`` <#var-bb-PREFERRED_VERSION>`__ variable to
+specify a particular version. You can influence the order by using the
+```DEFAULT_PREFERENCE`` <#var-bb-DEFAULT_PREFERENCE>`__ variable.
+
+By default, files have a preference of "0". Setting
+``DEFAULT_PREFERENCE`` to "-1" makes the recipe unlikely to be used
+unless it is explicitly referenced. Setting ``DEFAULT_PREFERENCE`` to
+"1" makes it likely the recipe is used. ``PREFERRED_VERSION`` overrides
+any ``DEFAULT_PREFERENCE`` setting. ``DEFAULT_PREFERENCE`` is often used
+to mark newer and more experimental recipe versions until they have
+undergone sufficient testing to be considered stable.
+
+When there are multiple “versions” of a given recipe, BitBake defaults
+to selecting the most recent version, unless otherwise specified. If the
+recipe in question has a
+```DEFAULT_PREFERENCE`` <#var-bb-DEFAULT_PREFERENCE>`__ set lower than
+the other recipes (default is 0), then it will not be selected. This
+allows the person or persons maintaining the repository of recipe files
+to specify their preference for the default selected version.
+Additionally, the user can specify their preferred version.
+
+If the first recipe is named ``a_1.1.bb``, then the
+```PN`` <#var-bb-PN>`__ variable will be set to “a”, and the
+```PV`` <#var-bb-PV>`__ variable will be set to 1.1.
+
+Thus, if a recipe named ``a_1.2.bb`` exists, BitBake will choose 1.2 by
+default. However, if you define the following variable in a ``.conf``
+file that BitBake parses, you can change that preference:
+PREFERRED_VERSION_a = "1.1"
+
+.. note::
+
+   It is common for a recipe to provide two versions -- a stable,
+   numbered (and preferred) version, and a version that is automatically
+   checked out from a source code repository that is considered more
+   "bleeding edge" but can be selected only explicitly.
+
+   For example, in the OpenEmbedded codebase, there is a standard,
+   versioned recipe file for BusyBox, ``busybox_1.22.1.bb``, but there
+   is also a Git-based version, ``busybox_git.bb``, which explicitly
+   contains the line DEFAULT_PREFERENCE = "-1" to ensure that the
+   numbered, stable version is always preferred unless the developer
+   selects otherwise.
+
+.. _bb-bitbake-dependencies:
+
+Dependencies
+============
+
+Each target BitBake builds consists of multiple tasks such as ``fetch``,
+``unpack``, ``patch``, ``configure``, and ``compile``. For best
+performance on multi-core systems, BitBake considers each task as an
+independent entity with its own set of dependencies.
+
+Dependencies are defined through several variables. You can find
+information about variables BitBake uses in the `Variables
+Glossary <#ref-bb-variables-glos>`__ near the end of this manual. At a
+basic level, it is sufficient to know that BitBake uses the
+```DEPENDS`` <#var-bb-DEPENDS>`__ and
+```RDEPENDS`` <#var-bb-RDEPENDS>`__ variables when calculating
+dependencies.
+
+For more information on how BitBake handles dependencies, see the
+"`Dependencies <#dependencies>`__" section.
+
+.. _ref-bitbake-tasklist:
+
+The Task List
+=============
+
+Based on the generated list of providers and the dependency information,
+BitBake can now calculate exactly what tasks it needs to run and in what
+order it needs to run them. The "`Executing Tasks <#executing-tasks>`__"
+section has more information on how BitBake chooses which task to
+execute next.
+
+The build now starts with BitBake forking off threads up to the limit
+set in the ```BB_NUMBER_THREADS`` <#var-bb-BB_NUMBER_THREADS>`__
+variable. BitBake continues to fork threads as long as there are tasks
+ready to run, those tasks have all their dependencies met, and the
+thread threshold has not been exceeded.
+
+It is worth noting that you can greatly speed up the build time by
+properly setting the ``BB_NUMBER_THREADS`` variable.
+
+As each task completes, a timestamp is written to the directory
+specified by the ```STAMP`` <#var-bb-STAMP>`__ variable. On subsequent
+runs, BitBake looks in the build directory within ``tmp/stamps`` and
+does not rerun tasks that are already completed unless a timestamp is
+found to be invalid. Currently, invalid timestamps are only considered
+on a per recipe file basis. So, for example, if the configure stamp has
+a timestamp greater than the compile timestamp for a given target, then
+the compile task would rerun. Running the compile task again, however,
+has no effect on other providers that depend on that target.
+
+The exact format of the stamps is partly configurable. In modern
+versions of BitBake, a hash is appended to the stamp so that if the
+configuration changes, the stamp becomes invalid and the task is
+automatically rerun. This hash, or signature used, is governed by the
+signature policy that is configured (see the "`Checksums
+(Signatures) <#checksums>`__" section for information). It is also
+possible to append extra metadata to the stamp using the
+``[stamp-extra-info]`` task flag. For example, OpenEmbedded uses this
+flag to make some tasks machine-specific.
+
+.. note::
+
+   Some tasks are marked as "nostamp" tasks. No timestamp file is
+   created when these tasks are run. Consequently, "nostamp" tasks are
+   always rerun.
+
+For more information on tasks, see the "`Tasks <#tasks>`__" section.
+
+Executing Tasks
+===============
+
+Tasks can be either a shell task or a Python task. For shell tasks,
+BitBake writes a shell script to
+``${``\ ```T`` <#var-bb-T>`__\ ``}/run.do_taskname.pid`` and then
+executes the script. The generated shell script contains all the
+exported variables, and the shell functions with all variables expanded.
+Output from the shell script goes to the file
+``${T}/log.do_taskname.pid``. Looking at the expanded shell functions in
+the run file and the output in the log files is a useful debugging
+technique.
+
+For Python tasks, BitBake executes the task internally and logs
+information to the controlling terminal. Future versions of BitBake will
+write the functions to files similar to the way shell tasks are handled.
+Logging will be handled in a way similar to shell tasks as well.
+
+The order in which BitBake runs the tasks is controlled by its task
+scheduler. It is possible to configure the scheduler and define custom
+implementations for specific use cases. For more information, see these
+variables that control the behavior:
+
+-  ```BB_SCHEDULER`` <#var-bb-BB_SCHEDULER>`__
+
+-  ```BB_SCHEDULERS`` <#var-bb-BB_SCHEDULERS>`__
+
+It is possible to have functions run before and after a task's main
+function. This is done using the ``[prefuncs]`` and ``[postfuncs]``
+flags of the task that lists the functions to run.
+
+.. _checksums:
+
+Checksums (Signatures)
+======================
+
+A checksum is a unique signature of a task's inputs. The signature of a
+task can be used to determine if a task needs to be run. Because it is a
+change in a task's inputs that triggers running the task, BitBake needs
+to detect all the inputs to a given task. For shell tasks, this turns
+out to be fairly easy because BitBake 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.
+
+To complicate the problem, some things should not be included in the
+checksum. First, there is the actual specific build path of a given task
+- the working directory. It does not matter if the working directory
+changes because it should not affect the output for target packages. The
+simplistic approach for excluding the working directory is to set it to
+some fixed value and create the checksum for the "run" script. BitBake
+goes one step better and uses the
+```BB_HASHBASE_WHITELIST`` <#var-bb-BB_HASHBASE_WHITELIST>`__ variable
+to define a list of variables that should never be included when
+generating the signatures.
+
+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.
+
+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.
+
+Like the working directory 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:
+PACKAGE_ARCHS[vardepsexclude] = "MACHINE" This example ensures that the
+``PACKAGE_ARCHS`` variable does not depend on the value of ``MACHINE``,
+even if it does reference it.
+
+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: PACKAGE_ARCHS[vardeps] = "MACHINE" This example explicitly
+adds the ``MACHINE`` variable as a dependency for ``PACKAGE_ARCHS``.
+
+Consider a case with in-line Python, for example, where BitBake is not
+able to figure out dependencies. When running in debug mode (i.e. using
+``-DDD``), BitBake produces output when it discovers something for which
+it cannot figure out dependencies.
+
+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.
+
+At the code level, there are a variety of ways both the basehash and the
+dependent task hashes can be influenced. Within the BitBake
+configuration file, we can give BitBake some extra information to help
+it construct the basehash. The following statement effectively results
+in a list of global variable dependency excludes - variables never
+included in any checksum. This example uses variables from OpenEmbedded
+to help illustrate the concept: BB_HASHBASE_WHITELIST ?= "TMPDIR FILE
+PATH PWD BB_TASKHASH BBPATH DL_DIR \\ SSTATE_DIR THISDIR FILESEXTRAPATHS
+FILE_DIRNAME HOME LOGNAME SHELL \\ USER FILESPATH STAGING_DIR_HOST
+STAGING_DIR_TARGET COREBASE PRSERV_HOST \\ PRSERV_DUMPDIR
+PRSERV_DUMPFILE PRSERV_LOCKDOWN PARALLEL_MAKE \\ CCACHE_DIR
+EXTERNAL_TOOLCHAIN CCACHE CCACHE_DISABLE LICENSE_PATH SDKPKGSUFFIX" The
+previous example excludes the work directory, which is part of
+``TMPDIR``.
+
+The rules for deciding which hashes of dependent tasks to include
+through dependency chains are more complex and are generally
+accomplished with a Python function. The code in
+``meta/lib/oe/sstatesig.py`` shows two examples of this and also
+illustrates how you can insert your own policy into the system if so
+desired. This file defines the two basic signature generators
+OpenEmbedded-Core uses: "OEBasic" and "OEBasicHash". By default, there
+is a dummy "noop" signature handler enabled in BitBake. This means that
+behavior is unchanged from previous versions. ``OE-Core`` uses the
+"OEBasicHash" signature handler by default through this setting in the
+``bitbake.conf`` file: BB_SIGNATURE_HANDLER ?= "OEBasicHash" The
+"OEBasicHash" ``BB_SIGNATURE_HANDLER`` is the same as the "OEBasic"
+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
+```PR`` <#var-bb-PR>`__ values, and changes to metadata automatically
+ripple across the build.
+
+It is also worth noting that the end result of these signature
+generators is to make some dependency and hash information available to
+the build. This information includes:
+
+-  ``BB_BASEHASH_task-``\ taskname: The base hashes for each task in the
+   recipe.
+
+-  ``BB_BASEHASH_``\ filename\ ``:``\ taskname: The base hashes for each
+   dependent task.
+
+-  ``BBHASHDEPS_``\ filename\ ``:``\ taskname: The task dependencies for
+   each task.
+
+-  ``BB_TASKHASH``: The hash of the currently running task.
+
+It is worth noting that BitBake's "-S" option lets you debug BitBake's
+processing of signatures. The options passed to -S allow different
+debugging modes to be used, either using BitBake's own debug functions
+or possibly those defined in the metadata/signature handler itself. The
+simplest parameter to pass is "none", which causes a set of signature
+information to be written out into ``STAMPS_DIR`` corresponding to the
+targets specified. The other currently available parameter is
+"printdiff", which causes BitBake to try to establish the closest
+signature match it can (e.g. in the sstate cache) and then run
+``bitbake-diffsigs`` over the matches to determine the stamps and delta
+where these two stamp trees diverge.
+
+.. note::
+
+   It is likely that future versions of BitBake will provide other
+   signature handlers triggered through additional "-S" parameters.
+
+You can find more information on checksum metadata in the "`Task
+Checksums and Setscene <#task-checksums-and-setscene>`__" section.
+
+Setscene
+========
+
+The setscene process enables BitBake to handle "pre-built" artifacts.
+The ability to handle and reuse these artifacts allows BitBake the
+luxury of not having to build something from scratch every time.
+Instead, BitBake can use, when possible, existing build artifacts.
+
+BitBake needs to have reliable data indicating whether or not an
+artifact is compatible. Signatures, described in the previous section,
+provide an ideal way of representing whether an artifact is compatible.
+If a signature is the same, an object can be reused.
+
+If an object can be reused, the problem then becomes how to replace a
+given task or set of tasks with the pre-built artifact. BitBake solves
+the problem with the "setscene" process.
+
+When BitBake is asked to build a given target, before building anything,
+it first asks whether cached information is available for any of the
+targets it's building, or any of the intermediate targets. If cached
+information is available, BitBake uses this information instead of
+running the main tasks.
+
+BitBake first calls the function defined by the
+```BB_HASHCHECK_FUNCTION`` <#var-bb-BB_HASHCHECK_FUNCTION>`__ variable
+with a list of tasks and corresponding hashes it wants to build. This
+function is designed to be fast and returns a list of the tasks for
+which it believes in can obtain artifacts.
+
+Next, for each of the tasks that were returned as possibilities, BitBake
+executes a setscene version of the task that the possible artifact
+covers. Setscene versions of a task have the string "_setscene" appended
+to the task name. So, for example, the task with the name ``xxx`` has a
+setscene task named ``xxx_setscene``. The setscene version of the task
+executes and provides the necessary artifacts returning either success
+or failure.
+
+As previously mentioned, an artifact can cover more than one task. For
+example, it is pointless to obtain a compiler if you already have the
+compiled binary. To handle this, BitBake calls the
+```BB_SETSCENE_DEPVALID`` <#var-bb-BB_SETSCENE_DEPVALID>`__ function for
+each successful setscene task to know whether or not it needs to obtain
+the dependencies of that task.
+
+Finally, after all the setscene tasks have executed, BitBake calls the
+function listed in
+```BB_SETSCENE_VERIFY_FUNCTION2`` <#var-bb-BB_SETSCENE_VERIFY_FUNCTION2>`__
+with the list of tasks BitBake thinks has been "covered". The metadata
+can then ensure that this list is correct and can inform BitBake that it
+wants specific tasks to be run regardless of the setscene result.
+
+You can find more information on setscene metadata in the "`Task
+Checksums and Setscene <#task-checksums-and-setscene>`__" section.
+
+Logging
+=======
+
+In addition to the standard command line option to control how verbose
+builds are when execute, bitbake also supports user defined
+configuration of the `Python
+logging <https://docs.python.org/3/library/logging.html>`__ facilities
+through the ```BB_LOGCONFIG`` <#var-bb-BB_LOGCONFIG>`__ variable. This
+variable defines a json or yaml `logging
+configuration <https://docs.python.org/3/library/logging.config.html>`__
+that will be intelligently merged into the default configuration. The
+logging configuration is merged using the following rules:
+
+-  The user defined configuration will completely replace the default
+   configuration if top level key ``bitbake_merge`` is set to the value
+   ``False``. In this case, all other rules are ignored.
+
+-  The user configuration must have a top level ``version`` which must
+   match the value of the default configuration.
+
+-  Any keys defined in the ``handlers``, ``formatters``, or ``filters``,
+   will be merged into the same section in the default configuration,
+   with the user specified keys taking replacing a default one if there
+   is a conflict. In practice, this means that if both the default
+   configuration and user configuration specify a handler named
+   ``myhandler``, the user defined one will replace the default. To
+   prevent the user from inadvertently replacing a default handler,
+   formatter, or filter, all of the default ones are named with a prefix
+   of "``BitBake.``"
+
+-  If a logger is defined by the user with the key ``bitbake_merge`` set
+   to ``False``, that logger will be completely replaced by user
+   configuration. In this case, no other rules will apply to that
+   logger.
+
+-  All user defined ``filter`` and ``handlers`` properties for a given
+   logger will be merged with corresponding properties from the default
+   logger. For example, if the user configuration adds a filter called
+   ``myFilter`` to the ``BitBake.SigGen``, and the default configuration
+   adds a filter called ``BitBake.defaultFilter``, both filters will be
+   applied to the logger
+
+As an example, consider the following user logging configuration file
+which logs all Hash Equivalence related messages of VERBOSE or higher to
+a file called ``hashequiv.log`` { "version": 1, "handlers": {
+"autobuilderlog": { "class": "logging.FileHandler", "formatter":
+"logfileFormatter", "level": "DEBUG", "filename": "hashequiv.log",
+"mode": "w" } }, "formatters": { "logfileFormatter": { "format":
+"%(name)s: %(levelname)s: %(message)s" } }, "loggers": {
+"BitBake.SigGen.HashEquiv": { "level": "VERBOSE", "handlers":
+["autobuilderlog"] }, "BitBake.RunQueue.HashEquiv": { "level":
+"VERBOSE", "handlers": ["autobuilderlog"] } } }