From 4873add6e11c1bd421c83cd08df589f1184aa673 Mon Sep 17 00:00:00 2001 From: Andrew Geissler Date: Mon, 2 Nov 2020 18:44:49 -0600 Subject: Revert "poky: subtree update:b23aa6b753..ad30a6d470" This reverts commit af5e4ef732faedf66c6dc1756432e9de2ac72988. This commit introduced openbmc/openbmc#3720 and no solution has been forthcoming. Revert until we can get to the bottom of this. Change-Id: I2fb0d81eb26cf3dadb2f2abdd1a1bb7a95eaf03c --- .../overview-manual/overview-manual-concepts.xml | 3235 ++++++++++++++++++++ 1 file changed, 3235 insertions(+) create mode 100644 poky/documentation/overview-manual/overview-manual-concepts.xml (limited to 'poky/documentation/overview-manual/overview-manual-concepts.xml') diff --git a/poky/documentation/overview-manual/overview-manual-concepts.xml b/poky/documentation/overview-manual/overview-manual-concepts.xml new file mode 100644 index 000000000..58b64bd26 --- /dev/null +++ b/poky/documentation/overview-manual/overview-manual-concepts.xml @@ -0,0 +1,3235 @@ + %poky; ] > + + + +Yocto Project Concepts + + + This chapter provides explanations for Yocto Project concepts that + go beyond the surface of "how-to" information and reference (or + look-up) material. + Concepts such as components, the + OpenEmbedded build system + workflow, cross-development toolchains, shared state cache, and so + forth are explained. + + +
+ Yocto Project Components + + + The + BitBake + task executor together with various types of configuration files + form the + OpenEmbedded-Core. + This section overviews these components by describing their use and + how they interact. + + + + BitBake handles the parsing and execution of the data files. + The data itself is of various types: + + + Recipes: + Provides details about particular pieces of software. + + + Class Data: + Abstracts common build information (e.g. how to build a + Linux kernel). + + + Configuration Data: + Defines machine-specific settings, policy decisions, and + so forth. + Configuration data acts as the glue to bind everything + together. + + + + + + BitBake knows how to combine multiple data sources together and + refers to each data source as a layer. + For information on layers, see the + "Understanding and Creating Layers" + section of the Yocto Project Development Tasks Manual. + + + + Following are some brief details on these core components. + For additional information on how these components interact during + a build, see the + "OpenEmbedded Build System Concepts" + section. + + +
+ BitBake + + + BitBake is the tool at the heart of the + OpenEmbedded build system + and is responsible for parsing the + Metadata, + generating a list of tasks from it, and then executing those + tasks. + + + + This section briefly introduces BitBake. + If you want more information on BitBake, see the + BitBake User Manual. + + + + To see a list of the options BitBake supports, use either of + the following commands: + + $ bitbake -h + $ bitbake --help + + + + + The most common usage for BitBake is + bitbake packagename, + where packagename is the name of the + package you want to build (referred to as the "target"). + The target often equates to the first part of a recipe's + filename (e.g. "foo" for a recipe named + foo_1.3.0-r0.bb). + So, to process the + matchbox-desktop_1.2.3.bb recipe file, you + might type the following: + + $ bitbake matchbox-desktop + + Several different versions of + matchbox-desktop 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 + "Preferences" + section of the BitBake User Manual. + + + + BitBake also tries to execute any dependent tasks first. + So for example, before building + matchbox-desktop, BitBake would build a + cross compiler and glibc if they had not + already been built. + + + + A useful BitBake option to consider is the + -k or --continue + option. + This option instructs BitBake to try and continue processing + the job as long 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. + +
+ +
+ Recipes + + + Files that have the .bb suffix are + "recipes" files. + In general, a recipe contains information about a single piece + of software. + This information includes the location from which to download + the unaltered source, any source patches to be applied to that + source (if needed), which special configuration options to + apply, how to compile the source files, and how to package the + compiled output. + + + + The term "package" is sometimes used to refer to recipes. + However, since the word "package" is used for the packaged + output from the OpenEmbedded build system (i.e. + .ipk or .deb files), + this document avoids using the term "package" when referring + to recipes. + +
+ +
+ Classes + + + Class files (.bbclass) contain information + that is useful to share between recipes files. + An example is the + autotools + class, which contains common settings for any application that + Autotools uses. + The + "Classes" + chapter in the Yocto Project Reference Manual provides + details about classes and how to use them. + +
+ +
+ Configurations + + + The configuration files (.conf) define + various configuration variables that govern the OpenEmbedded + 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 in + conf/local.conf, which is found in the + Build Directory. + +
+
+ +
+ Layers + + + Layers are repositories that contain related metadata (i.e. + sets of instructions) that tell the OpenEmbedded build system how + to build a target. + Yocto Project's + layer model + facilitates collaboration, sharing, customization, and reuse + within the Yocto Project development environment. + Layers logically separate information for your project. + For example, you can use a layer to hold all the configurations + for a particular piece of hardware. + Isolating hardware-specific configurations allows you to share + other metadata by using a different layer where that metadata + might be common across several pieces of hardware. + + + + Many layers exist that work in the Yocto Project development + environment. + The + Yocto Project Curated Layer Index + and + OpenEmbedded Layer Index + both contain layers from which you can use or leverage. + + + + By convention, layers in the Yocto Project follow a specific form. + Conforming to a known structure allows BitBake to make assumptions + during builds on where to find types of metadata. + You can find procedures and learn about tools (i.e. + bitbake-layers) for creating layers suitable + for the Yocto Project in the + "Understanding and Creating Layers" + section of the Yocto Project Development Tasks Manual. + +
+ +
+ OpenEmbedded Build System Concepts + + + This section takes a more detailed look inside the build + process used by the + OpenEmbedded build system, + which is the build system specific to the Yocto Project. + At the heart of the build system is BitBake, the task executor. + + + + The following diagram represents the high-level workflow of a + build. + The remainder of this section expands on the fundamental input, + output, process, and metadata logical blocks that make up the + workflow. + + + + + + + + In general, the build's workflow consists of several functional + areas: + + + User Configuration: + metadata you can use to control the build process. + + + Metadata Layers: + Various layers that provide software, machine, and + distro metadata. + + + Source Files: + Upstream releases, local projects, and SCMs. + + + Build System: + Processes under the control of + BitBake. + This block expands on how BitBake fetches source, applies + patches, completes compilation, analyzes output for package + generation, creates and tests packages, generates images, + and generates cross-development tools. + + + Package Feeds: + Directories containing output packages (RPM, DEB or IPK), + which are subsequently used in the construction of an + image or Software Development Kit (SDK), produced by the + build system. + These feeds can also be copied and shared using a web + server or other means to facilitate extending or updating + existing images on devices at runtime if runtime package + management is enabled. + + + Images: + Images produced by the workflow. + + + Application Development SDK: + Cross-development tools that are produced along with + an image or separately with BitBake. + + + + +
+ User Configuration + + + User configuration helps define the build. + Through user configuration, you can tell BitBake the + target architecture for which you are building the image, + where to store downloaded source, and other build properties. + + + + The following figure shows an expanded representation of the + "User Configuration" box of the + general workflow figure: + + + + + + + + BitBake needs some basic configuration files in order to + complete a build. + These files are *.conf files. + The minimally necessary ones reside as example files in the + build/conf directory of the + Source Directory. + For simplicity, this section refers to the Source Directory as + the "Poky Directory." + + + + When you clone the + Poky + Git repository or you download and unpack a Yocto Project + release, you can set up the Source Directory to be named + anything you want. + For this discussion, the cloned repository uses the default + name poky. + + The Poky repository is primarily an aggregation of existing + repositories. + It is not a canonical upstream source. + + + + + The meta-poky layer inside Poky contains + a conf directory that has example + configuration files. + These example files are used as a basis for creating actual + configuration files when you source + &OE_INIT_FILE;, + which is the build environment script. + + + + Sourcing the build environment script creates a + Build Directory + if one does not already exist. + BitBake uses the Build Directory for all its work during + builds. + The Build Directory has a conf directory + that contains default versions of your + local.conf and + bblayers.conf configuration files. + These default configuration files are created only if versions + do not already exist in the Build Directory at the time you + source the build environment setup script. + + + + Because the Poky repository is fundamentally an aggregation of + existing repositories, some users might be familiar with + running the &OE_INIT_FILE; script + in the context of separate + OpenEmbedded-Core + and BitBake repositories rather than a single Poky repository. + This discussion assumes the script is executed from + within a cloned or unpacked version of Poky. + + + + Depending on where the script is sourced, different + sub-scripts are called to set up the Build Directory + (Yocto or OpenEmbedded). + Specifically, the script + scripts/oe-setup-builddir inside the + poky directory sets up the Build Directory and seeds the + directory (if necessary) with configuration files appropriate + for the Yocto Project development environment. + + The scripts/oe-setup-builddir script + uses the $TEMPLATECONF variable to + determine which sample configuration files to locate. + + + + + The local.conf file provides many + basic variables that define a build environment. + Here is a list of a few. + To see the default configurations in a + local.conf file created by the build + environment script, see the + local.conf.sample + in the meta-poky layer: + + + Target Machine Selection: + Controlled by the + MACHINE + variable. + + + Download Directory: + Controlled by the + DL_DIR + variable. + + + Shared State Directory: + Controlled by the + SSTATE_DIR + variable. + + + Build Output: + Controlled by the + TMPDIR + variable. + + + Distribution Policy: + Controlled by the + DISTRO + variable. + + + Packaging Format: + Controlled by the + PACKAGE_CLASSES + variable. + + + SDK Target Architecture: + Controlled by the + SDKMACHINE + variable. + + + Extra Image Packages: + Controlled by the + EXTRA_IMAGE_FEATURES + variable. + + + + Configurations set in the + conf/local.conf file can also be set + in the conf/site.conf and + conf/auto.conf configuration files. + + + + + The bblayers.conf file tells BitBake what + layers you want considered during the build. + By default, the layers listed in this file include layers + minimally needed by the build system. + However, you must manually add any custom layers you have + created. + You can find more information on working with the + bblayers.conf file in the + "Enabling Your Layer" + section in the Yocto Project Development Tasks Manual. + + + + The files site.conf and + auto.conf are not created by the + environment initialization script. + If you want the site.conf file, you + need to create that yourself. + The auto.conf file is typically created by + an autobuilder: + + + site.conf: + You can use the conf/site.conf + configuration file to configure multiple + build directories. + For example, suppose you had several build environments + and they shared some common features. + You can set these default build properties here. + A good example is perhaps the packaging format to use + through the + PACKAGE_CLASSES + variable. + + One useful scenario for using the + conf/site.conf file is to extend + your + BBPATH + variable to include the path to a + conf/site.conf. + Then, when BitBake looks for Metadata using + BBPATH, it finds the + conf/site.conf file and applies + your common configurations found in the file. + To override configurations in a particular build + directory, alter the similar configurations within + that build directory's + conf/local.conf file. + + + auto.conf: + The file is usually created and written to by + an autobuilder. + The settings put into the file are typically the + same as you would find in the + conf/local.conf or the + conf/site.conf files. + + + + + + You can edit all configuration files to further define + any particular build environment. + This process is represented by the "User Configuration Edits" + box in the figure. + + + + When you launch your build with the + bitbake target + command, BitBake sorts out the configurations to ultimately + define your build environment. + It is important to understand that the + OpenEmbedded build system + reads the configuration files in a specific order: + site.conf, auto.conf, + and local.conf. + And, the build system applies the normal assignment statement + rules as described in the + "Syntax and Operators" + chapter of the BitBake User Manual. + Because the files are parsed in a specific order, variable + assignments for the same variable could be affected. + For example, if the auto.conf file and + the local.conf set + variable1 to different values, + because the build system parses local.conf + after auto.conf, + variable1 is assigned the value from + the local.conf file. + +
+ +
+ Metadata, Machine Configuration, and Policy Configuration + + + The previous section described the user configurations that + define BitBake's global behavior. + This section takes a closer look at the layers the build system + uses to further control the build. + These layers provide Metadata for the software, machine, and + policies. + + + + In general, three types of layer input exists. + You can see them below the "User Configuration" box in the + general workflow figure: + + + Metadata (.bb + Patches): + Software layers containing user-supplied recipe files, + patches, and append files. + A good example of a software layer might be the + meta-qt5 + layer from the + OpenEmbedded Layer Index. + This layer is for version 5.0 of the popular + Qt + cross-platform application development framework for + desktop, embedded and mobile. + + + Machine BSP Configuration: + Board Support Package (BSP) layers (i.e. "BSP Layer" + in the following figure) providing machine-specific + configurations. + This type of information is specific to a particular + target architecture. + A good example of a BSP layer from the + Poky Reference Distribution + is the + meta-yocto-bsp + layer. + + + Policy Configuration: + Distribution Layers (i.e. "Distro Layer" in the + following figure) providing top-level or general + policies for the images or SDKs being built for a + particular distribution. + For example, in the Poky Reference Distribution the + distro layer is the + meta-poky + layer. + Within the distro layer is a + conf/distro directory that + contains distro configuration files (e.g. + poky.conf + that contain many policy configurations for the + Poky distribution. + + + + + + The following figure shows an expanded representation of + these three layers from the + general workflow figure: + + + + + + + + In general, all layers have a similar structure. + They all contain a licensing file + (e.g. COPYING.MIT) if the layer is to be + distributed, a README file as good + practice and especially if the layer is to be distributed, a + configuration directory, and recipe directories. + You can learn about the general structure for layers used with + the Yocto Project in the + "Creating Your Own Layer" + section in the Yocto Project Development Tasks Manual. + For a general discussion on layers and the many layers from + which you can draw, see the + "Layers" and + "The Yocto Project Layer Model" + sections both earlier in this manual. + + + + If you explored the previous links, you discovered some + areas where many layers that work with the Yocto Project + exist. + The + Source Repositories + also shows layers categorized under "Yocto Metadata Layers." + + Layers exist in the Yocto Project Source Repositories that + cannot be found in the OpenEmbedded Layer Index. + These layers are either deprecated or experimental + in nature. + + + + + BitBake uses the conf/bblayers.conf file, + which is part of the user configuration, to find what layers it + should be using as part of the build. + + +
+ Distro Layer + + + The distribution layer provides policy configurations for + your distribution. + Best practices dictate that you isolate these types of + configurations into their own layer. + Settings you provide in + conf/distro/distro.conf override + similar settings that BitBake finds in your + conf/local.conf file in the Build + Directory. + + + + The following list provides some explanation and references + for what you typically find in the distribution layer: + + + classes: + Class files (.bbclass) hold + common functionality that can be shared among + recipes in the distribution. + When your recipes inherit a class, they take on the + settings and functions for that class. + You can read more about class files in the + "Classes" + chapter of the Yocto Reference Manual. + + + conf: + This area holds configuration files for the + layer (conf/layer.conf), + the distribution + (conf/distro/distro.conf), + and any distribution-wide include files. + + + recipes-*: + Recipes and append files that affect common + functionality across the distribution. + This area could include recipes and append files + to add distribution-specific configuration, + initialization scripts, custom image recipes, + and so forth. + Examples of recipes-* + directories are recipes-core + and recipes-extra. + Hierarchy and contents within a + recipes-* directory can vary. + Generally, these directories contain recipe files + (*.bb), recipe append files + (*.bbappend), directories + that are distro-specific for configuration files, + and so forth. + + + +
+ +
+ BSP Layer + + + The BSP Layer provides machine configurations that + target specific hardware. + Everything in this layer is specific to the machine for + which you are building the image or the SDK. + A common structure or form is defined for BSP layers. + You can learn more about this structure in the + Yocto Project Board Support Package (BSP) Developer's Guide. + + In order for a BSP layer to be considered compliant + with the Yocto Project, it must meet some structural + requirements. + + + + + The BSP Layer's configuration directory contains + configuration files for the machine + (conf/machine/machine.conf) + and, of course, the layer + (conf/layer.conf). + + + + The remainder of the layer is dedicated to specific recipes + by function: recipes-bsp, + recipes-core, + recipes-graphics, + recipes-kernel, and so forth. + Metadata can exist for multiple formfactors, graphics + support systems, and so forth. + + While the figure shows several + recipes-* directories, not all + these directories appear in all BSP layers. + + +
+ +
+ Software Layer + + + The software layer provides the Metadata for additional + software packages used during the build. + This layer does not include Metadata that is specific to + the distribution or the machine, which are found in their + respective layers. + + + + This layer contains any recipes, append files, and + patches, that your project needs. + +
+
+ +
+ Sources + + + In order for the OpenEmbedded build system to create an + image or any target, it must be able to access source files. + The + general workflow figure + represents source files using the "Upstream Project Releases", + "Local Projects", and "SCMs (optional)" boxes. + The figure represents mirrors, which also play a role in + locating source files, with the "Source Materials" box. + + + + The method by which source files are ultimately organized is + a function of the project. + For example, for released software, projects tend to use + tarballs or other archived files that can capture the + state of a release guaranteeing that it is statically + represented. + On the other hand, for a project that is more dynamic or + experimental in nature, a project might keep source files in a + repository controlled by a Source Control Manager (SCM) such as + Git. + Pulling source from a repository allows you to control + the point in the repository (the revision) from which you + want to build software. + Finally, a combination of the two might exist, which would + give the consumer a choice when deciding where to get + source files. + + + + BitBake uses the + SRC_URI + variable to point to source files regardless of their location. + Each recipe must have a SRC_URI variable + that points to the source. + + + + Another area that plays a significant role in where source + files come from is pointed to by the + DL_DIR + variable. + This area is a cache that can hold previously downloaded + source. + You can also instruct the OpenEmbedded build system to create + tarballs from Git repositories, which is not the default + behavior, and store them in the DL_DIR + by using the + BB_GENERATE_MIRROR_TARBALLS + variable. + + + + Judicious use of a DL_DIR directory can + save the build system a trip across the Internet when looking + for files. + A good method for using a download directory is to have + DL_DIR point to an area outside of your + Build Directory. + Doing so allows you to safely delete the Build Directory + if needed without fear of removing any downloaded source file. + + + + The remainder of this section provides a deeper look into the + source files and the mirrors. + Here is a more detailed look at the source file area of the + general workflow figure: + + + + + + +
+ Upstream Project Releases + + + Upstream project releases exist anywhere in the form of an + archived file (e.g. tarball or zip file). + These files correspond to individual recipes. + For example, the figure uses specific releases each for + BusyBox, Qt, and Dbus. + An archive file can be for any released product that can be + built using a recipe. + +
+ +
+ Local Projects + + + Local projects are custom bits of software the user + provides. + These bits reside somewhere local to a project - perhaps + a directory into which the user checks in items (e.g. + a local directory containing a development source tree + used by the group). + + + + The canonical method through which to include a local + project is to use the + externalsrc + class to include that local project. + You use either the local.conf or a + recipe's append file to override or set the + recipe to point to the local directory on your disk to pull + in the whole source tree. + +
+ +
+ Source Control Managers (Optional) + + + Another place from which the build system can get source + files is with + fetchers + employing various Source Control Managers (SCMs) such as + Git or Subversion. + In such cases, a repository is cloned or checked out. + The + do_fetch + task inside BitBake uses + the SRC_URI + variable and the argument's prefix to determine the correct + fetcher module. + + For information on how to have the OpenEmbedded build + system generate tarballs for Git repositories and place + them in the + DL_DIR + directory, see the + BB_GENERATE_MIRROR_TARBALLS + variable in the Yocto Project Reference Manual. + + + + + When fetching a repository, BitBake uses the + SRCREV + variable to determine the specific revision from which to + build. + +
+ +
+ Source Mirror(s) + + + Two kinds of mirrors exist: pre-mirrors and regular + mirrors. + The + PREMIRRORS + and + MIRRORS + variables point to these, respectively. + BitBake checks pre-mirrors before looking upstream for any + source files. + Pre-mirrors are appropriate when you have a shared + directory that is not a directory defined by the + DL_DIR + variable. + A Pre-mirror typically points to a shared directory that is + local to your organization. + + + + Regular mirrors can be any site across the Internet + that is used as an alternative location for source + code should the primary site not be functioning for + some reason or another. + +
+
+ +
+ Package Feeds + + + When the OpenEmbedded build system generates an image or an + SDK, it gets the packages from a package feed area located + in the + Build Directory. + The + general workflow figure + shows this package feeds area in the upper-right corner. + + + + This section looks a little closer into the package feeds + area used by the build system. + Here is a more detailed look at the area: + + + + + Package feeds are an intermediary step in the build process. + The OpenEmbedded build system provides classes to generate + different package types, and you specify which classes to + enable through the + PACKAGE_CLASSES + variable. + Before placing the packages into package feeds, + the build process validates them with generated output quality + assurance checks through the + insane + class. + + + + The package feed area resides in the Build Directory. + The directory the build system uses to temporarily store + packages is determined by a combination of variables and the + particular package manager in use. + See the "Package Feeds" box in the illustration and note the + information to the right of that area. + In particular, the following defines where package files are + kept: + + + DEPLOY_DIR: + Defined as tmp/deploy in the Build + Directory. + + + DEPLOY_DIR_*: + Depending on the package manager used, the package type + sub-folder. + Given RPM, IPK, or DEB packaging and tarball creation, + the + DEPLOY_DIR_RPM, + DEPLOY_DIR_IPK, + DEPLOY_DIR_DEB, + or + DEPLOY_DIR_TAR, + variables are used, respectively. + + + PACKAGE_ARCH: + Defines architecture-specific sub-folders. + For example, packages could exist for the i586 or + qemux86 architectures. + + + + + + BitBake uses the + do_package_write_* + tasks to generate packages and place them into the package + holding area (e.g. do_package_write_ipk + for IPK packages). + See the + "do_package_write_deb", + "do_package_write_ipk", + "do_package_write_rpm", + and + "do_package_write_tar" + sections in the Yocto Project Reference Manual + for additional information. + As an example, consider a scenario where an IPK packaging + manager is being used and package architecture support for + both i586 and qemux86 exist. + Packages for the i586 architecture are placed in + build/tmp/deploy/ipk/i586, while packages + for the qemux86 architecture are placed in + build/tmp/deploy/ipk/qemux86. + +
+ +
+ BitBake + + + The OpenEmbedded build system uses + BitBake + to produce images and Software Development Kits (SDKs). + You can see from the + general workflow figure, + the BitBake area consists of several functional areas. + This section takes a closer look at each of those areas. + + Separate documentation exists for the BitBake tool. + See the + BitBake User Manual + for reference material on BitBake. + + + +
+ Source Fetching + + + The first stages of building a recipe are to fetch and + unpack the source code: + + + + + The + do_fetch + and + do_unpack + tasks fetch the source files and unpack them into the + Build Directory. + + For every local file (e.g. file://) + that is part of a recipe's + SRC_URI + statement, the OpenEmbedded build system takes a + checksum of the file for the recipe and inserts the + checksum into the signature for the + do_fetch task. + If any local file has been modified, the + do_fetch task and all tasks that + depend on it are re-executed. + + By default, everything is accomplished in the Build + Directory, which has a defined structure. + For additional general information on the Build Directory, + see the + "build/" + section in the Yocto Project Reference Manual. + + + + Each recipe has an area in the Build Directory where the + unpacked source code resides. + The + S + variable points to this area for a recipe's unpacked source + code. + The name of that directory for any given recipe is defined + from several different variables. + The preceding figure and the following list describe + the Build Directory's hierarchy: + + + TMPDIR: + The base directory where the OpenEmbedded build + system performs all its work during the build. + The default base directory is the + tmp directory. + + + PACKAGE_ARCH: + The architecture of the built package or packages. + Depending on the eventual destination of the + package or packages (i.e. machine architecture, + build host, + SDK, or specific machine), + PACKAGE_ARCH varies. + See the variable's description for details. + + + TARGET_OS: + The operating system of the target device. + A typical value would be "linux" (e.g. + "qemux86-poky-linux"). + + + PN: + The name of the recipe used to build the package. + This variable can have multiple meanings. + However, when used in the context of input files, + PN represents the the name + of the recipe. + + + WORKDIR: + The location where the OpenEmbedded build system + builds a recipe (i.e. does the work to create the + package). + + + PV: + The version of the recipe used to build the + package. + + + PR: + The revision of the recipe used to build the + package. + + + + + S: + Contains the unpacked source files for a given + recipe. + + + BPN: + The name of the recipe used to build the + package. + The BPN variable is + a version of the PN + variable but with common prefixes and + suffixes removed. + + + PV: + The version of the recipe used to build the + package. + + + + + + In the previous figure, notice that two sample + hierarchies exist: one based on package architecture (i.e. + PACKAGE_ARCH) and one based on a + machine (i.e. MACHINE). + The underlying structures are identical. + The differentiator being what the OpenEmbedded build + system is using as a build target (e.g. general + architecture, a build host, an SDK, or a specific + machine). + + +
+ +
+ Patching + + + Once source code is fetched and unpacked, BitBake locates + patch files and applies them to the source files: + + + + + The + do_patch + task uses a recipe's + SRC_URI + statements and the + FILESPATH + variable to locate applicable patch files. + + + + Default processing for patch files assumes the files have + either *.patch or + *.diff file types. + You can use SRC_URI parameters to + change the way the build system recognizes patch files. + See the + do_patch + task for more information. + + + + BitBake finds and applies multiple patches for a single + recipe in the order in which it locates the patches. + The FILESPATH variable defines the + default set of directories that the build system uses to + search for patch files. + Once found, patches are applied to the recipe's source + files, which are located in the + S + directory. + + + + For more information on how the source directories are + created, see the + "Source Fetching" + section. + For more information on how to create patches and how the + build system processes patches, see the + "Patching Code" + section in the Yocto Project Development Tasks Manual. + You can also see the + "Use devtool modify to Modify the Source of an Existing Component" + section in the Yocto Project Application Development and + the Extensible Software Development Kit (SDK) manual and + the + "Using Traditional Kernel Development to Patch the Kernel" + section in the Yocto Project Linux Kernel Development + Manual. + +
+ +
+ Configuration, Compilation, and Staging + + + After source code is patched, BitBake executes tasks that + configure and compile the source code. + Once compilation occurs, the files are copied to a holding + area (staged) in preparation for packaging: + + + + + This step in the build process consists of the following + tasks: + + + do_prepare_recipe_sysroot: + This task sets up the two sysroots in + ${WORKDIR} + (i.e. recipe-sysroot and + recipe-sysroot-native) so that + during the packaging phase the sysroots can contain + the contents of the + do_populate_sysroot + tasks of the recipes on which the recipe + containing the tasks depends. + A sysroot exists for both the target and for the + native binaries, which run on the host system. + + + do_configure: + This task configures the source by enabling and + disabling any build-time and configuration options + for the software being built. + Configurations can come from the recipe itself as + well as from an inherited class. + Additionally, the software itself might configure + itself depending on the target for which it is + being built. + + The configurations handled by the + do_configure + task are specific to configurations for the source + code being built by the recipe. + + If you are using the + autotools + class, you can add additional configuration options + by using the + EXTRA_OECONF + or + PACKAGECONFIG_CONFARGS + variables. + For information on how this variable works within + that class, see the + autotools + class + here. + + + do_compile: + Once a configuration task has been satisfied, + BitBake compiles the source using the + do_compile + task. + Compilation occurs in the directory pointed to by + the + B + variable. + Realize that the B directory + is, by default, the same as the + S + directory. + + + do_install: + After compilation completes, BitBake executes the + do_install + task. + This task copies files from the + B directory and places them + in a holding area pointed to by the + D + variable. + Packaging occurs later using files from this + holding directory. + + + +
+ +
+ Package Splitting + + + After source code is configured, compiled, and staged, the + build system analyzes the results and splits the output + into packages: + + + + + The + do_package + and + do_packagedata + tasks combine to analyze the files found in the + D + directory and split them into subsets based on available + packages and files. + Analysis involves the following as well as other items: + splitting out debugging symbols, looking at shared library + dependencies between packages, and looking at package + relationships. + + + + The do_packagedata task creates + package metadata based on the analysis such that the + build system can generate the final packages. + The + do_populate_sysroot + task stages (copies) a subset of the files installed by + the + do_install + task into the appropriate sysroot. + Working, staged, and intermediate results of the analysis + and package splitting process use several areas: + + + PKGD: + The destination directory + (i.e. package) for packages + before they are split into individual packages. + + + PKGDESTWORK: + A temporary work area (i.e. + pkgdata) used by the + do_package task to save + package metadata. + + + PKGDEST: + The parent directory (i.e. + packages-split) for packages + after they have been split. + + + PKGDATA_DIR: + A shared, global-state directory that holds + packaging metadata generated during the packaging + process. + The packaging process copies metadata from + PKGDESTWORK to the + PKGDATA_DIR area where it + becomes globally available. + + + STAGING_DIR_HOST: + The path for the sysroot for the system on which + a component is built to run (i.e. + recipe-sysroot). + + + STAGING_DIR_NATIVE: + The path for the sysroot used when building + components for the build host (i.e. + recipe-sysroot-native). + + + STAGING_DIR_TARGET: + The path for the sysroot used when a component that + is built to execute on a system and it generates + code for yet another machine (e.g. cross-canadian + recipes). + + + The + FILES + variable defines the files that go into each package in + PACKAGES. + If you want details on how this is accomplished, you can + look at + package.bbclass. + + + + Depending on the type of packages being created (RPM, DEB, + or IPK), the + do_package_write_* + task creates the actual packages and places them in the + Package Feed area, which is + ${TMPDIR}/deploy. + You can see the + "Package Feeds" + section for more detail on that part of the build process. + + Support for creating feeds directly from the + deploy/* directories does not + exist. + Creating such feeds usually requires some kind of feed + maintenance mechanism that would upload the new + packages into an official package feed (e.g. the + Ångström distribution). + This functionality is highly distribution-specific + and thus is not provided out of the box. + + +
+ +
+ Image Generation + + + Once packages are split and stored in the Package Feeds + area, the build system uses BitBake to generate the root + filesystem image: + + + + + The image generation process consists of several stages and + depends on several tasks and variables. + The + do_rootfs + task creates the root filesystem (file and directory + structure) for an image. + This task uses several key variables to help create the + list of packages to actually install: + + + IMAGE_INSTALL: + Lists out the base set of packages from which to + install from the Package Feeds area. + + + PACKAGE_EXCLUDE: + Specifies packages that should not be installed + into the image. + + + IMAGE_FEATURES: + Specifies features to include in the image. + Most of these features map to additional packages + for installation. + + + PACKAGE_CLASSES: + Specifies the package backend (e.g. RPM, DEB, or + IPK) to use and consequently helps determine where + to locate packages within the Package Feeds area. + + + IMAGE_LINGUAS: + Determines the language(s) for which additional + language support packages are installed. + + + PACKAGE_INSTALL: + The final list of packages passed to the package + manager for installation into the image. + + + + + + With + IMAGE_ROOTFS + pointing to the location of the filesystem under + construction and the PACKAGE_INSTALL + variable providing the final list of packages to install, + the root file system is created. + + + + Package installation is under control of the package + manager (e.g. dnf/rpm, opkg, or apt/dpkg) regardless of + whether or not package management is enabled for the + target. + At the end of the process, if package management is not + enabled for the target, the package manager's data files + are deleted from the root filesystem. + As part of the final stage of package installation, + post installation scripts that are part of the packages + are run. + Any scripts that fail to run on the build host are run on + the target when the target system is first booted. + If you are using a + read-only root filesystem, + all the post installation scripts must succeed on the + build host during the package installation phase since the + root filesystem on the target is read-only. + + + + The final stages of the do_rootfs task + handle post processing. + Post processing includes creation of a manifest file and + optimizations. + + + + The manifest file (.manifest) resides + in the same directory as the root filesystem image. + This file lists out, line-by-line, the installed packages. + The manifest file is useful for the + testimage + class, for example, to determine whether or not to run + specific tests. + See the + IMAGE_MANIFEST + variable for additional information. + + + + Optimizing processes that are run across the image include + mklibs, prelink, + and any other post-processing commands as defined by the + ROOTFS_POSTPROCESS_COMMAND + variable. + The mklibs process optimizes the size + of the libraries, while the prelink + process optimizes the dynamic linking of shared libraries + to reduce start up time of executables. + + + + After the root filesystem is built, processing begins on + the image through the + do_image + task. + The build system runs any pre-processing commands as + defined by the + IMAGE_PREPROCESS_COMMAND + variable. + This variable specifies a list of functions to call before + the build system creates the final image output files. + + + + The build system dynamically creates + do_image_* tasks as needed, based + on the image types specified in the + IMAGE_FSTYPES + variable. + The process turns everything into an image file or a set of + image files and can compress the root filesystem image to + reduce the overall size of the image. + The formats used for the root filesystem depend on the + IMAGE_FSTYPES variable. + Compression depends on whether the formats support + compression. + + + + As an example, a dynamically created task when creating a + particular image type would + take the following form: + + do_image_type + + So, if the type as specified by + the IMAGE_FSTYPES were + ext4, the dynamically generated task + would be as follows: + + do_image_ext4 + + + + + The final task involved in image creation is the + do_image_complete + task. + This task completes the image by applying any image + post processing as defined through the + IMAGE_POSTPROCESS_COMMAND + variable. + The variable specifies a list of functions to call once the + build system has created the final image output files. + + The entire image generation process is run under + Pseudo. + Running under Pseudo ensures that the files in the + root filesystem have correct ownership. + + +
+ +
+ SDK Generation + + + The OpenEmbedded build system uses BitBake to generate the + Software Development Kit (SDK) installer scripts for both + the standard SDK and the extensible SDK (eSDK): + + + + + + For more information on the cross-development toolchain + generation, see the + "Cross-Development Toolchain Generation" + section. + For information on advantages gained when building a + cross-development toolchain using the + do_populate_sdk + task, see the + "Building an SDK Installer" + section in the Yocto Project Application Development + and the Extensible Software Development Kit (eSDK) + manual. + + + + + Like image generation, the SDK script process consists of + several stages and depends on many variables. + The + do_populate_sdk + and + do_populate_sdk_ext + tasks use these key variables to help create the list of + packages to actually install. + For information on the variables listed in the figure, + see the + "Application Development SDK" + section. + + + + The do_populate_sdk task helps create + the standard SDK and handles two parts: a target part and a + host part. + The target part is the part built for the target hardware + and includes libraries and headers. + The host part is the part of the SDK that runs on the + SDKMACHINE. + + + + The do_populate_sdk_ext task helps + create the extensible SDK and handles host and target parts + differently than its counter part does for the standard SDK. + For the extensible SDK, the task encapsulates the build + system, which includes everything needed (host and target) + for the SDK. + + + + Regardless of the type of SDK being constructed, the + tasks perform some cleanup after which a cross-development + environment setup script and any needed configuration files + are created. + The final output is the Cross-development + toolchain installation script (.sh + file), which includes the environment setup script. + +
+ +
+ Stamp Files and the Rerunning of Tasks + + + For each task that completes successfully, BitBake writes a + stamp file into the + STAMPS_DIR + directory. + The beginning of the stamp file's filename is determined + by the + STAMP + variable, and the end of the name consists of the task's + name and current + input checksum. + + This naming scheme assumes that + BB_SIGNATURE_HANDLER + is "OEBasicHash", which is almost always the case in + current OpenEmbedded. + + To determine if a task needs to be rerun, BitBake checks + if a stamp file with a matching input checksum exists + for the task. + If such a stamp file exists, the task's output is + assumed to exist and still be valid. + If the file does not exist, the task is rerun. + + The stamp mechanism is more general than the + shared state (sstate) cache mechanism described in the + "Setscene Tasks and Shared State" + section. + BitBake avoids rerunning any task that has a valid + stamp file, not just tasks that can be accelerated + through the sstate cache. + + However, you should realize that stamp files only + serve as a marker that some work has been done and that + these files do not record task output. + The actual task output would usually be somewhere in + TMPDIR + (e.g. in some recipe's + WORKDIR.) + What the sstate cache mechanism adds is a way to cache + task output that can then be shared between build + machines. + + Since STAMPS_DIR is usually a + subdirectory of TMPDIR, removing + TMPDIR will also remove + STAMPS_DIR, which means tasks will + properly be rerun to repopulate + TMPDIR. + + + + If you want some task to always be considered "out of + date", you can mark it with the + nostamp + varflag. + If some other task depends on such a task, then that + task will also always be considered out of date, which + might not be what you want. + + + + For details on how to view information about a task's + signature, see the + "Viewing Task Variable Dependencies" + section in the Yocto Project Development Tasks Manual. + +
+ +
+ Setscene Tasks and Shared State + + + The description of tasks so far assumes that BitBake needs + to build everything and no available prebuilt objects + exist. + BitBake does support skipping tasks if prebuilt objects are + available. + These objects are usually made available in the form of a + shared state (sstate) cache. + + For information on variables affecting sstate, see the + SSTATE_DIR + and + SSTATE_MIRRORS + variables. + + + + + The idea of a setscene task (i.e + do_taskname_setscene) + is a version of the task where + instead of building something, BitBake can skip to the end + result and simply place a set of files into specific + locations as needed. + In some cases, it makes sense to have a setscene task + variant (e.g. generating package files in the + do_package_write_* + task). + In other cases, it does not make sense (e.g. a + do_patch + task or a + do_unpack + task) since the work involved would be equal to or greater + than the underlying task. + + + + In the build system, the common tasks that have setscene + variants are + do_package, + do_package_write_*, + do_deploy, + do_packagedata, + and + do_populate_sysroot. + Notice that these tasks represent most of the tasks whose + output is an end result. + + + + The build system has knowledge of the relationship between + these tasks and other preceding tasks. + For example, if BitBake runs + do_populate_sysroot_setscene for + something, it does not make sense to run any of the + do_fetch, + do_unpack, + do_patch, + do_configure, + do_compile, and + do_install tasks. + However, if do_package needs to be + run, BitBake needs to run those other tasks. + + + + It becomes more complicated if everything can come + from an sstate cache because some objects are simply + not required at all. + For example, you do not need a compiler or native tools, + such as quilt, if nothing exists to compile or patch. + If the do_package_write_* packages + are available from sstate, BitBake does not need the + do_package task data. + + + + To handle all these complexities, BitBake runs in two + phases. + The first is the "setscene" stage. + During this stage, BitBake first checks the sstate cache + for any targets it is planning to build. + BitBake does a fast check to see if the object exists + rather than a complete download. + If nothing exists, the second phase, which is the setscene + stage, completes and the main build proceeds. + + + + If objects are found in the sstate cache, the build system + works backwards from the end targets specified by the user. + For example, if an image is being built, the build system + first looks for the packages needed for that image and the + tools needed to construct an image. + If those are available, the compiler is not needed. + Thus, the compiler is not even downloaded. + If something was found to be unavailable, or the + download or setscene task fails, the build system then + tries to install dependencies, such as the compiler, from + the cache. + + + + The availability of objects in the sstate cache is + handled by the function specified by the + BB_HASHCHECK_FUNCTION + variable and returns a list of available objects. + The function specified by the + BB_SETSCENE_DEPVALID + variable is the function that determines whether a given + dependency needs to be followed, and whether for any given + relationship the function needs to be passed. + The function returns a True or False value. + +
+
+ +
+ Images + + + The images produced by the build system are compressed forms + of the root filesystem and are ready to boot on a target + device. + You can see from the + general workflow figure + that BitBake output, in part, consists of images. + This section takes a closer look at this output: + + + + + For a list of example images that the Yocto Project provides, + see the + "Images" + chapter in the Yocto Project Reference Manual. + + + + The build process writes images out to the + Build Directory + inside the + tmp/deploy/images/machine/ + folder as shown in the figure. + This folder contains any files expected to be loaded on the + target device. + The + DEPLOY_DIR + variable points to the deploy directory, + while the + DEPLOY_DIR_IMAGE + variable points to the appropriate directory containing images + for the current configuration. + + + kernel-image: + A kernel binary file. + The + KERNEL_IMAGETYPE + variable determines the naming scheme for the + kernel image file. + Depending on this variable, the file could begin with + a variety of naming strings. + The + deploy/images/machine + directory can contain multiple image files for the + machine. + + + root-filesystem-image: + Root filesystems for the target device (e.g. + *.ext3 or + *.bz2 files). + The + IMAGE_FSTYPES + variable determines the root filesystem image type. + The + deploy/images/machine + directory can contain multiple root filesystems for the + machine. + + + kernel-modules: + Tarballs that contain all the modules built for the + kernel. + Kernel module tarballs exist for legacy purposes and + can be suppressed by setting the + MODULE_TARBALL_DEPLOY + variable to "0". + The + deploy/images/machine + directory can contain multiple kernel module tarballs + for the machine. + + + bootloaders: + If applicable to the target machine, bootloaders + supporting the image. + The deploy/images/machine + directory can contain multiple bootloaders for the + machine. + + + symlinks: + The + deploy/images/machine + folder contains a symbolic link that points to the + most recently built file for each machine. + These links might be useful for external scripts that + need to obtain the latest version of each file. + + + +
+ +
+ Application Development SDK + + + In the + general workflow figure, + the output labeled "Application Development SDK" represents an + SDK. + The SDK generation process differs depending on whether you + build an extensible SDK (e.g. + bitbake -c populate_sdk_ext imagename) + or a standard SDK (e.g. + bitbake -c populate_sdk imagename). + This section takes a closer look at this output: + + + + + The specific form of this output is a set of files that + includes a self-extracting SDK installer + (*.sh), host and target manifest files, + and files used for SDK testing. + When the SDK installer file is run, it installs the SDK. + The SDK consists of a cross-development toolchain, a set of + libraries and headers, and an SDK environment setup script. + Running this installer essentially sets up your + cross-development environment. + You can think of the cross-toolchain as the "host" + part because it runs on the SDK machine. + You can think of the libraries and headers as the "target" + part because they are built for the target hardware. + The environment setup script is added so that you can + initialize the environment before using the tools. + + + Notes + + + The Yocto Project supports several methods by which + you can set up this cross-development environment. + These methods include downloading pre-built SDK + installers or building and installing your own SDK + installer. + + + For background information on cross-development + toolchains in the Yocto Project development + environment, see the + "Cross-Development Toolchain Generation" + section. + + + For information on setting up a cross-development + environment, see the + Yocto Project Application Development and the Extensible Software Development Kit (eSDK) + manual. + + + + + + All the output files for an SDK are written to the + deploy/sdk folder inside the + Build Directory + as shown in the previous figure. + Depending on the type of SDK, several variables exist that help + configure these files. + The following list shows the variables associated with an + extensible SDK: + + + DEPLOY_DIR: + Points to the deploy directory. + + + SDK_EXT_TYPE: + Controls whether or not shared state artifacts are + copied into the extensible SDK. + By default, all required shared state artifacts are + copied into the SDK. + + + SDK_INCLUDE_PKGDATA: + Specifies whether or not packagedata is included in the + extensible SDK for all recipes in the "world" target. + + + SDK_INCLUDE_TOOLCHAIN: + Specifies whether or not the toolchain is included + when building the extensible SDK. + + + SDK_LOCAL_CONF_WHITELIST: + A list of variables allowed through from the build + system configuration into the extensible SDK + configuration. + + + SDK_LOCAL_CONF_BLACKLIST: + A list of variables not allowed through from the build + system configuration into the extensible SDK + configuration. + + + SDK_INHERIT_BLACKLIST: + A list of classes to remove from the + INHERIT + value globally within the extensible SDK configuration. + + + This next list, shows the variables associated with a standard + SDK: + + + DEPLOY_DIR: + Points to the deploy directory. + + + SDKMACHINE: + Specifies the architecture of the machine on which the + cross-development tools are run to create packages for + the target hardware. + + + SDKIMAGE_FEATURES: + Lists the features to include in the "target" part + of the SDK. + + + TOOLCHAIN_HOST_TASK: + Lists packages that make up the host part of the SDK + (i.e. the part that runs on the + SDKMACHINE). + When you use + bitbake -c populate_sdk imagename + to create the SDK, a set of default packages apply. + This variable allows you to add more packages. + + + TOOLCHAIN_TARGET_TASK: + Lists packages that make up the target part of the SDK + (i.e. the part built for the target hardware). + + + SDKPATH: + Defines the default SDK installation path offered by + the installation script. + + + SDK_HOST_MANIFEST: + Lists all the installed packages that make up the host + part of the SDK. + This variable also plays a minor role for extensible + SDK development as well. + However, it is mainly used for the standard SDK. + + + SDK_TARGET_MANIFEST: + Lists all the installed packages that make up the + target part of the SDK. + This variable also plays a minor role for extensible + SDK development as well. + However, it is mainly used for the standard SDK. + + + +
+
+ +
+ Cross-Development Toolchain Generation + + + The Yocto Project does most of the work for you when it comes to + creating + cross-development toolchains. + This section provides some technical background on how + cross-development toolchains are created and used. + For more information on toolchains, you can also see the + Yocto Project Application Development and the Extensible Software Development Kit (eSDK) + manual. + + + + In the Yocto Project development environment, cross-development + toolchains are used to build images and applications that run + on the target hardware. + With just a few commands, the OpenEmbedded build system creates + these necessary toolchains for you. + + + + The following figure shows a high-level build environment regarding + toolchain construction and use. + + + + + + + + Most of the work occurs on the Build Host. + This is the machine used to build images and generally work within + the the Yocto Project environment. + When you run + BitBake + to create an image, the OpenEmbedded build system + uses the host gcc compiler to bootstrap a + cross-compiler named gcc-cross. + The gcc-cross compiler is what BitBake uses to + compile source files when creating the target image. + You can think of gcc-cross simply as an + automatically generated cross-compiler that is used internally + within BitBake only. + + The extensible SDK does not use + gcc-cross-canadian since this SDK + ships a copy of the OpenEmbedded build system and the sysroot + within it contains gcc-cross. + + + + + The chain of events that occurs when gcc-cross is + bootstrapped is as follows: + + gcc -> binutils-cross -> gcc-cross-initial -> linux-libc-headers -> glibc-initial -> glibc -> gcc-cross -> gcc-runtime + + + + gcc: + The build host's GNU Compiler Collection (GCC). + + + binutils-cross: + The bare minimum binary utilities needed in order to run + the gcc-cross-initial phase of the + bootstrap operation. + + + gcc-cross-initial: + An early stage of the bootstrap process for creating + the cross-compiler. + This stage builds enough of the gcc-cross, + the C library, and other pieces needed to finish building the + final cross-compiler in later stages. + This tool is a "native" package (i.e. it is designed to run on + the build host). + + + linux-libc-headers: + Headers needed for the cross-compiler. + + + glibc-initial: + An initial version of the Embedded GNU C Library + (GLIBC) needed to bootstrap glibc. + + + glibc: + The GNU C Library. + + + gcc-cross: + The final stage of the bootstrap process for the + cross-compiler. + This stage results in the actual cross-compiler that + BitBake uses when it builds an image for a targeted + device. + + If you are replacing this cross compiler toolchain + with a custom version, you must replace + gcc-cross. + + This tool is also a "native" package (i.e. it is + designed to run on the build host). + + + gcc-runtime: + Runtime libraries resulting from the toolchain bootstrapping + process. + This tool produces a binary that consists of the + runtime libraries need for the targeted device. + + + + + + You can use the OpenEmbedded build system to build an installer for + the relocatable SDK used to develop applications. + When you run the installer, it installs the toolchain, which + contains the development tools (e.g., + gcc-cross-canadian, + binutils-cross-canadian, and other + nativesdk-* tools), + which are tools native to the SDK (i.e. native to + SDK_ARCH), + you need to cross-compile and test your software. + The figure shows the commands you use to easily build out this + toolchain. + This cross-development toolchain is built to execute on the + SDKMACHINE, + which might or might not be the same + machine as the Build Host. + + If your target architecture is supported by the Yocto Project, + you can take advantage of pre-built images that ship with the + Yocto Project and already contain cross-development toolchain + installers. + + + + + Here is the bootstrap process for the relocatable toolchain: + + gcc -> binutils-crosssdk -> gcc-crosssdk-initial -> linux-libc-headers -> + glibc-initial -> nativesdk-glibc -> gcc-crosssdk -> gcc-cross-canadian + + + + gcc: + The build host's GNU Compiler Collection (GCC). + + + binutils-crosssdk: + The bare minimum binary utilities needed in order to run + the gcc-crosssdk-initial phase of the + bootstrap operation. + + + gcc-crosssdk-initial: + An early stage of the bootstrap process for creating + the cross-compiler. + This stage builds enough of the + gcc-crosssdk and supporting pieces so that + the final stage of the bootstrap process can produce the + finished cross-compiler. + This tool is a "native" binary that runs on the build host. + + + linux-libc-headers: + Headers needed for the cross-compiler. + + + glibc-initial: + An initial version of the Embedded GLIBC needed to bootstrap + nativesdk-glibc. + + + nativesdk-glibc: + The Embedded GLIBC needed to bootstrap the + gcc-crosssdk. + + + gcc-crosssdk: + The final stage of the bootstrap process for the + relocatable cross-compiler. + The gcc-crosssdk is a transitory + compiler and never leaves the build host. + Its purpose is to help in the bootstrap process to create + the eventual gcc-cross-canadian + compiler, which is relocatable. + This tool is also a "native" package (i.e. it is + designed to run on the build host). + + + gcc-cross-canadian: + The final relocatable cross-compiler. + When run on the + SDKMACHINE, + this tool + produces executable code that runs on the target device. + Only one cross-canadian compiler is produced per architecture + since they can be targeted at different processor optimizations + using configurations passed to the compiler through the + compile commands. + This circumvents the need for multiple compilers and thus + reduces the size of the toolchains. + + + + + + For information on advantages gained when building a + cross-development toolchain installer, see the + "Building an SDK Installer" + appendix in the Yocto Project Application Development and the + Extensible Software Development Kit (eSDK) manual. + +
+ +
+ Shared State Cache + + + By design, the OpenEmbedded build system builds everything from + scratch unless + BitBake + can determine that parts do not need to be rebuilt. + Fundamentally, building from scratch is attractive as it means all + parts are built fresh and no possibility of stale data exists that + can cause problems. + When developers hit problems, they typically default back to + building from scratch so they have a know state from the + start. + + + + 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 rebuilding things that do not necessarily need + to be rebuilt. + + + + 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 OpenEmbedded + incremental build support system: + + + What pieces of the system have changed and what pieces have + not changed? + + + How are changed pieces of software removed and replaced? + + + How are pre-built components that do not need to be rebuilt + from scratch used when they are available? + + + + + + 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. + Notes + + + The build system does not maintain + PR + information as part of the shared state packages. + Consequently, considerations exist that affect + maintaining shared state feeds. + For information on how the build system works with + packages and can track incrementing + PR information, see the + "Automatically Incrementing a Binary Package Revision Number" + section in the Yocto Project Development Tasks Manual. + + + The code in the build system that supports incremental + builds is not simple code. + For techniques that help you work around issues related + to shared state code, see the + "Viewing Metadata Used to Create the Input Signature of a Shared State Task" + and + "Invalidating Shared State to Force a Task to Run" + sections both in the Yocto Project Development Tasks + Manual. + + + + + + + The rest of this section goes into detail about the overall + incremental build architecture, the checksums (signatures), and + shared state. + + +
+ Overall Architecture + + + When determining what parts of the system need to be built, + BitBake works on a per-task basis rather than 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, the + do_install + and + do_package + task outputs are still valid. + However, with a per-recipe approach, the build would not + include the .deb files. + Consequently, you would have to invalidate the whole build and + rerun it. + Rerunning everything is not the best solution. + 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. + +
+ +
+ Checksums (Signatures) + + + 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. + + + + 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 + WORKDIR. + It does not matter if the work directory changes because it + should not affect the output for target packages. + Also, the build process has the objective of making native + or cross packages relocatable. + + Both native and cross packages run on the + build host. + However, cross packages generate output for the target + architecture. + + The checksum therefore needs to exclude + WORKDIR. + The simplistic approach for excluding the work directory is to + set WORKDIR to some fixed value and + create the checksum for the "run" script. + + + + 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, solutions for shell scripts exist. + 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 WORKDIR case, situations exist + where dependencies should be ignored. + For these situations, 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 you 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. + + + + As an example, consider a case with in-line Python 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. + The Yocto Project team has currently not managed to cover + those dependencies in detail and is aware of the need to fix + this situation. + + + + 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, the question of a task's indirect inputs still + exits - items 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, a variety of ways exist by which both the + basehash and the dependent task hashes can be influenced. + Within the BitBake configuration file, you 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 (i.e. variables never + included in any checksum): + + BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH DL_DIR \ + SSTATE_DIR THISDIR FILESEXTRAPATHS FILE_DIRNAME HOME LOGNAME SHELL TERM \ + 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 + WORKDIR + since that variable is actually constructed as a path within + TMPDIR, + which is on the whitelist. + + + + 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 + OE-Core + uses: "OEBasic" and "OEBasicHash". + By default, a dummy "noop" signature handler is 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 + 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. + + + +
+ +
+ Shared State + + + Checksums and dependencies, as discussed in the previous + section, solve half the problem of supporting a shared state. + The other half of the problem is being able to use checksum + information during the build and being able to reuse or rebuild + specific components. + + + + The + sstate + class 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. + In other words, the build process does not need to worry about + its origin. + + + + Two types of output exist. + One type is just about creating a directory in + WORKDIR. + A good example is the output of either + do_install + or + do_package. + The other type of output occurs when a set of data is merged + into a shared directory tree such as the sysroot. + + + + The Yocto Project team has tried to keep the details of the + implementation hidden in sstate class. + From a user's perspective, adding shared state wrapping to a + task is as simple as this + do_deploy + example taken from the + deploy + class: + + DEPLOYDIR = "${WORKDIR}/deploy-${PN}" + SSTATETASKS += "do_deploy" + do_deploy[sstate-inputdirs] = "${DEPLOYDIR}" + do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}" + + python do_deploy_setscene () { + sstate_setscene(d) + } + addtask do_deploy_setscene + do_deploy[dirs] = "${DEPLOYDIR} ${B}" + do_deploy[stamp-extra-info] = "${MACHINE_ARCH}" + + The following list explains the previous example: + + + Adding "do_deploy" to SSTATETASKS + adds some required sstate-related processing, which is + implemented in the + sstate + class, to before and after the + do_deploy + task. + + + The + do_deploy[sstate-inputdirs] = "${DEPLOYDIR}" + declares that do_deploy places its + output in ${DEPLOYDIR} when run + normally (i.e. when not using the sstate cache). + This output becomes the input to the shared state cache. + + + The + do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}" + line causes the contents of the shared state cache to be + copied to ${DEPLOY_DIR_IMAGE}. + + If do_deploy is not already in + the shared state cache or if its input checksum + (signature) has changed from when the output was + cached, the task runs to populate the shared + state cache, after which the contents of the shared + state cache is copied to + ${DEPLOY_DIR_IMAGE}. + If do_deploy is in the shared + state cache and its signature indicates that the + cached output is still valid (i.e. if no + relevant task inputs have changed), then the + contents of the shared state cache copies + directly to + ${DEPLOY_DIR_IMAGE} by the + do_deploy_setscene task + instead, skipping the + do_deploy task. + + + + The following task definition is glue logic needed to + make the previous settings effective: + + python do_deploy_setscene () { + sstate_setscene(d) + } + addtask do_deploy_setscene + + sstate_setscene() takes the flags + above as input and accelerates the + do_deploy task through the + shared state cache if possible. + If the task was accelerated, + sstate_setscene() returns True. + Otherwise, it returns False, and the normal + do_deploy task runs. + For more information, see the + "setscene" + section in the BitBake User Manual. + + + The do_deploy[dirs] = "${DEPLOYDIR} ${B}" + line creates ${DEPLOYDIR} and + ${B} before the + do_deploy task runs, and also sets + the current working directory of + do_deploy to + ${B}. + For more information, see the + "Variable Flags" + section in the BitBake User Manual. + + In cases where + sstate-inputdirs and + sstate-outputdirs would be the + same, you can use + sstate-plaindirs. + For example, to preserve the + ${PKGD} and + ${PKGDEST} output from the + do_package + task, use the following: + + do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}" + + + + + The do_deploy[stamp-extra-info] = "${MACHINE_ARCH}" + line appends extra metadata to the + stamp file. + In this case, the metadata makes the task specific + to a machine's architecture. + See + "The Task List" + section in the BitBake User Manual for more + information on the stamp-extra-info + flag. + + + sstate-inputdirs and + sstate-outputdirs can also be used + with multiple directories. + For example, the following declares + PKGDESTWORK and + SHLIBWORK as shared state + input directories, which populates the shared state + cache, and PKGDATA_DIR and + SHLIBSDIR as the corresponding + shared state output directories: + + do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}" + do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}" + + + + These methods also include the ability to take a + lockfile when manipulating shared state directory + structures, for cases where file additions or removals + are sensitive: + + do_package[sstate-lockfile] = "${PACKAGELOCK}" + + + + + + + Behind the scenes, the shared state code works by looking in + SSTATE_DIR + and + SSTATE_MIRRORS + for shared state files. + Here is an example: + + SSTATE_MIRRORS ?= "\ + file://.* http://someserver.tld/share/sstate/PATH;downloadfilename=PATH \n \ + file://.* file:///some/local/dir/sstate/PATH" + + + The shared state directory + (SSTATE_DIR) is organized into + two-character subdirectories, where the subdirectory + names are based on the first two characters of the hash. + If the shared state directory structure for a mirror has the + same structure as SSTATE_DIR, you must + specify "PATH" as part of the URI to enable the build system + to map to the appropriate subdirectory. + + + + + 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. + + + + The build processes use the *_setscene + 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. + + + + As a real world example, the aim is when building an IPK-based + image, only the + do_package_write_ipk + 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. + +
+
+ +
+ Automatically Added Runtime Dependencies + + + The OpenEmbedded build system automatically adds common types of + runtime dependencies between packages, which means that you do not + need to explicitly declare the packages using + RDEPENDS. + Three automatic mechanisms exist (shlibdeps, + pcdeps, and depchains) + that handle shared libraries, package configuration (pkg-config) + modules, and -dev and + -dbg packages, respectively. + For other types of runtime dependencies, you must manually declare + the dependencies. + + + shlibdeps: + During the + do_package + task of each recipe, all shared libraries installed by the + recipe are located. + For each shared library, the package that contains the + shared library is registered as providing the shared + library. + More specifically, the package is registered as providing + the + soname + of the library. + The resulting shared-library-to-package mapping + is saved globally in + PKGDATA_DIR + by the + do_packagedata + task. + + Simultaneously, all executables and shared libraries + installed by the recipe are inspected to see what shared + libraries they link against. + For each shared library dependency that is found, + PKGDATA_DIR is queried to + see if some package (likely from a different recipe) + contains the shared library. + If such a package is found, a runtime dependency is added + from the package that depends on the shared library to the + package that contains the library. + + The automatically added runtime dependency also + includes a version restriction. + This version restriction specifies that at least the + current version of the package that provides the shared + library must be used, as if + "package (>= version)" + had been added to RDEPENDS. + This forces an upgrade of the package containing the shared + library when installing the package that depends on the + library, if needed. + + If you want to avoid a package being registered as + providing a particular shared library (e.g. because the library + is for internal use only), then add the library to + PRIVATE_LIBS + inside the package's recipe. + + + pcdeps: + During the do_package task of each + recipe, all pkg-config modules + (*.pc files) installed by the recipe + are located. + For each module, the package that contains the module is + registered as providing the module. + The resulting module-to-package mapping is saved globally in + PKGDATA_DIR by the + do_packagedata task. + + Simultaneously, all pkg-config modules installed by + the recipe are inspected to see what other pkg-config + modules they depend on. + A module is seen as depending on another module if it + contains a "Requires:" line that specifies the other module. + For each module dependency, + PKGDATA_DIR is queried to see if some + package contains the module. + If such a package is found, a runtime dependency is added + from the package that depends on the module to the package + that contains the module. + + The pcdeps mechanism most often + infers dependencies between -dev + packages. + + + + depchains: + If a package foo depends on a package + bar, then foo-dev + and foo-dbg are also made to depend on + bar-dev and + bar-dbg, respectively. + Taking the -dev packages as an + example, the bar-dev package might + provide headers and shared library symlinks needed by + foo-dev, which shows the need + for a dependency between the packages. + + The dependencies added by + depchains are in the form of + RRECOMMENDS. + + By default, foo-dev also has an + RDEPENDS-style dependency on + foo, because the default value of + RDEPENDS_${PN}-dev (set in + bitbake.conf) includes + "${PN}". + + + To ensure that the dependency chain is never broken, + -dev and -dbg + packages are always generated by default, even if the + packages turn out to be empty. + See the + ALLOW_EMPTY + variable for more information. + + + + + + The do_package task depends on the + do_packagedata task of each recipe in + DEPENDS + through use of a + [deptask] + declaration, which guarantees that the required + shared-library/module-to-package mapping information will be available + when needed as long as DEPENDS has been + correctly set. + +
+ +
+ Fakeroot and Pseudo + + + Some tasks are easier to implement when allowed to perform certain + operations that are normally reserved for the root user (e.g. + do_install, + do_package_write*, + do_rootfs, + and + do_image*). + For example, the do_install task benefits + from being able to set the UID and GID of installed files to + arbitrary values. + + + + One approach to allowing tasks to perform root-only operations + would be to require + BitBake + to run as root. + However, this method is cumbersome and has security issues. + The approach that is actually used is to run tasks that benefit + from root privileges in a "fake" root environment. + Within this environment, the task and its child processes believe + that they are running as the root user, and see an internally + consistent view of the filesystem. + As long as generating the final output (e.g. a package or an image) + does not require root privileges, the fact that some earlier + steps ran in a fake root environment does not cause problems. + + + + The capability to run tasks in a fake root environment is known as + "fakeroot", + which is derived from the BitBake keyword/variable + flag that requests a fake root environment for a task. + + + + In the + OpenEmbedded build system, + the program that implements fakeroot is known as + Pseudo. + Pseudo overrides system calls by using the environment variable + LD_PRELOAD, which results in the illusion + of running as root. + To keep track of "fake" file ownership and permissions resulting + from operations that require root permissions, Pseudo uses + an SQLite 3 database. + This database is stored in + ${WORKDIR}/pseudo/files.db + for individual recipes. + Storing the database in a file as opposed to in memory + gives persistence between tasks and builds, which is not + accomplished using fakeroot. + Caution + If you add your own task that manipulates the same files or + directories as a fakeroot task, then that task also needs to + run under fakeroot. + Otherwise, the task cannot run root-only operations, and + cannot see the fake file ownership and permissions set by the + other task. + You need to also add a dependency on + virtual/fakeroot-native:do_populate_sysroot, + giving the following: + + fakeroot do_mytask () { + ... + } + do_mytask[depends] += "virtual/fakeroot-native:do_populate_sysroot" + + + For more information, see the + FAKEROOT* + variables in the BitBake User Manual. + You can also reference the + "Why Not Fakeroot?" + article for background information on Fakeroot and Pseudo. + +
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