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+.. SPDX-License-Identifier: CC-BY-2.0-UK
+
+******************************
+Customizing the Extensible SDK
+******************************
+
+This appendix describes customizations you can apply to the extensible
+SDK.
+
+Configuring the Extensible SDK
+==============================
+
+The extensible SDK primarily consists of a pre-configured copy of the
+OpenEmbedded build system from which it was produced. Thus, the SDK's
+configuration is derived using that build system and the filters shown
+in the following list. When these filters are present, the OpenEmbedded
+build system applies them against ``local.conf`` and ``auto.conf``:
+
+-  Variables whose values start with "/" are excluded since the
+   assumption is that those values are paths that are likely to be
+   specific to the :term:`Build Host`.
+
+-  Variables listed in
+   :term:`SDK_LOCAL_CONF_BLACKLIST`
+   are excluded. These variables are not allowed through from the
+   OpenEmbedded build system configuration into the extensible SDK
+   configuration. Typically, these variables are specific to the machine
+   on which the build system is running and could be problematic as part
+   of the extensible SDK configuration.
+
+   For a list of the variables excluded by default, see the
+   :term:`SDK_LOCAL_CONF_BLACKLIST`
+   in the glossary of the Yocto Project Reference Manual.
+
+-  Variables listed in
+   :term:`SDK_LOCAL_CONF_WHITELIST`
+   are included. Including a variable in the value of
+   ``SDK_LOCAL_CONF_WHITELIST`` overrides either of the previous two
+   filters. The default value is blank.
+
+-  Classes inherited globally with
+   :term:`INHERIT` that are listed in
+   :term:`SDK_INHERIT_BLACKLIST`
+   are disabled. Using ``SDK_INHERIT_BLACKLIST`` to disable these
+   classes is the typical method to disable classes that are problematic
+   or unnecessary in the SDK context. The default value blacklists the
+   :ref:`buildhistory <ref-classes-buildhistory>`
+   and :ref:`icecc <ref-classes-icecc>` classes.
+
+Additionally, the contents of ``conf/sdk-extra.conf``, when present, are
+appended to the end of ``conf/local.conf`` within the produced SDK,
+without any filtering. The ``sdk-extra.conf`` file is particularly
+useful if you want to set a variable value just for the SDK and not the
+OpenEmbedded build system used to create the SDK.
+
+Adjusting the Extensible SDK to Suit Your Build Host's Setup
+============================================================
+
+In most cases, the extensible SDK defaults should work with your :term:`Build
+Host`'s setup.
+However, some cases exist for which you might consider making
+adjustments:
+
+-  If your SDK configuration inherits additional classes using the
+   :term:`INHERIT` variable and you
+   do not need or want those classes enabled in the SDK, you can
+   blacklist them by adding them to the
+   :term:`SDK_INHERIT_BLACKLIST`
+   variable as described in the fourth bullet of the previous section.
+
+   .. note::
+
+      The default value of
+      SDK_INHERIT_BLACKLIST
+      is set using the "?=" operator. Consequently, you will need to
+      either define the entire list by using the "=" operator, or you
+      will need to append a value using either "_append" or the "+="
+      operator. You can learn more about these operators in the "
+      Basic Syntax
+      " section of the BitBake User Manual.
+
+   .
+
+-  If you have classes or recipes that add additional tasks to the
+   standard build flow (i.e. the tasks execute as the recipe builds as
+   opposed to being called explicitly), then you need to do one of the
+   following:
+
+   -  After ensuring the tasks are :ref:`shared
+      state <overview-manual/overview-manual-concepts:shared state cache>` tasks (i.e. the
+      output of the task is saved to and can be restored from the shared
+      state cache) or ensuring the tasks are able to be produced quickly
+      from a task that is a shared state task, add the task name to the
+      value of
+      :term:`SDK_RECRDEP_TASKS`.
+
+   -  Disable the tasks if they are added by a class and you do not need
+      the functionality the class provides in the extensible SDK. To
+      disable the tasks, add the class to the ``SDK_INHERIT_BLACKLIST``
+      variable as described in the previous section.
+
+-  Generally, you want to have a shared state mirror set up so users of
+   the SDK can add additional items to the SDK after installation
+   without needing to build the items from source. See the "`Providing
+   Additional Installable Extensible SDK
+   Content <#sdk-providing-additional-installable-extensible-sdk-content>`__"
+   section for information.
+
+-  If you want users of the SDK to be able to easily update the SDK, you
+   need to set the
+   :term:`SDK_UPDATE_URL`
+   variable. For more information, see the "`Providing Updates to the
+   Extensible SDK After
+   Installation <#sdk-providing-updates-to-the-extensible-sdk-after-installation>`__"
+   section.
+
+-  If you have adjusted the list of files and directories that appear in
+   :term:`COREBASE` (other than
+   layers that are enabled through ``bblayers.conf``), then you must
+   list these files in
+   :term:`COREBASE_FILES` so
+   that the files are copied into the SDK.
+
+-  If your OpenEmbedded build system setup uses a different environment
+   setup script other than
+   :ref:`structure-core-script`, then you must
+   set
+   :term:`OE_INIT_ENV_SCRIPT`
+   to point to the environment setup script you use.
+
+   .. note::
+
+      You must also reflect this change in the value used for the
+      COREBASE_FILES
+      variable as previously described.
+
+Changing the Extensible SDK Installer Title
+===========================================
+
+You can change the displayed title for the SDK installer by setting the
+:term:`SDK_TITLE` variable and then
+rebuilding the the SDK installer. For information on how to build an SDK
+installer, see the "`Building an SDK
+Installer <#sdk-building-an-sdk-installer>`__" section.
+
+By default, this title is derived from
+:term:`DISTRO_NAME` when it is
+set. If the ``DISTRO_NAME`` variable is not set, the title is derived
+from the :term:`DISTRO` variable.
+
+The
+:ref:`populate_sdk_base <ref-classes-populate-sdk-*>`
+class defines the default value of the ``SDK_TITLE`` variable as
+follows:
+::
+
+   SDK_TITLE ??= "${@d.getVar('DISTRO_NAME') or d.getVar('DISTRO')} SDK"
+
+While several ways exist to change this variable, an efficient method is
+to set the variable in your distribution's configuration file. Doing so
+creates an SDK installer title that applies across your distribution. As
+an example, assume you have your own layer for your distribution named
+"meta-mydistro" and you are using the same type of file hierarchy as
+does the default "poky" distribution. If so, you could update the
+``SDK_TITLE`` variable in the
+``~/meta-mydistro/conf/distro/mydistro.conf`` file using the following
+form:
+::
+
+   SDK_TITLE = "your_title"
+
+Providing Updates to the Extensible SDK After Installation
+==========================================================
+
+When you make changes to your configuration or to the metadata and if
+you want those changes to be reflected in installed SDKs, you need to
+perform additional steps. These steps make it possible for anyone using
+the installed SDKs to update the installed SDKs by using the
+``devtool sdk-update`` command:
+
+1. Create a directory that can be shared over HTTP or HTTPS. You can do
+   this by setting up a web server such as an `Apache HTTP
+   Server <https://en.wikipedia.org/wiki/Apache_HTTP_Server>`__ or
+   `Nginx <https://en.wikipedia.org/wiki/Nginx>`__ server in the cloud
+   to host the directory. This directory must contain the published SDK.
+
+2. Set the
+   :term:`SDK_UPDATE_URL`
+   variable to point to the corresponding HTTP or HTTPS URL. Setting
+   this variable causes any SDK built to default to that URL and thus,
+   the user does not have to pass the URL to the ``devtool sdk-update``
+   command as described in the "`Applying Updates to an Installed
+   Extensible
+   SDK <#sdk-applying-updates-to-an-installed-extensible-sdk>`__"
+   section.
+
+3. Build the extensible SDK normally (i.e., use the
+   ``bitbake -c populate_sdk_ext`` imagename command).
+
+4. Publish the SDK using the following command:
+   ::
+
+      $ oe-publish-sdk some_path/sdk-installer.sh path_to_shared_http_directory
+
+   You must
+   repeat this step each time you rebuild the SDK with changes that you
+   want to make available through the update mechanism.
+
+Completing the above steps allows users of the existing installed SDKs
+to simply run ``devtool sdk-update`` to retrieve and apply the latest
+updates. See the "`Applying Updates to an Installed Extensible
+SDK <#sdk-applying-updates-to-an-installed-extensible-sdk>`__" section
+for further information.
+
+Changing the Default SDK Installation Directory
+===============================================
+
+When you build the installer for the Extensible SDK, the default
+installation directory for the SDK is based on the
+:term:`DISTRO` and
+:term:`SDKEXTPATH` variables from
+within the
+:ref:`populate_sdk_base <ref-classes-populate-sdk-*>`
+class as follows:
+::
+
+   SDKEXTPATH ??= "~/${@d.getVar('DISTRO')}_sdk"
+
+You can
+change this default installation directory by specifically setting the
+``SDKEXTPATH`` variable.
+
+While a number of ways exist through which you can set this variable,
+the method that makes the most sense is to set the variable in your
+distribution's configuration file. Doing so creates an SDK installer
+default directory that applies across your distribution. As an example,
+assume you have your own layer for your distribution named
+"meta-mydistro" and you are using the same type of file hierarchy as
+does the default "poky" distribution. If so, you could update the
+``SDKEXTPATH`` variable in the
+``~/meta-mydistro/conf/distro/mydistro.conf`` file using the following
+form:
+::
+
+   SDKEXTPATH = "some_path_for_your_installed_sdk"
+
+After building your installer, running it prompts the user for
+acceptance of the some_path_for_your_installed_sdk directory as the
+default location to install the Extensible SDK.
+
+Providing Additional Installable Extensible SDK Content
+=======================================================
+
+If you want the users of an extensible SDK you build to be able to add
+items to the SDK without requiring the users to build the items from
+source, you need to do a number of things:
+
+1. Ensure the additional items you want the user to be able to install
+   are already built:
+
+   -  Build the items explicitly. You could use one or more "meta"
+      recipes that depend on lists of other recipes.
+
+   -  Build the "world" target and set
+      ``EXCLUDE_FROM_WORLD_pn-``\ recipename for the recipes you do not
+      want built. See the
+      :term:`EXCLUDE_FROM_WORLD`
+      variable for additional information.
+
+2. Expose the ``sstate-cache`` directory produced by the build.
+   Typically, you expose this directory by making it available through
+   an `Apache HTTP
+   Server <https://en.wikipedia.org/wiki/Apache_HTTP_Server>`__ or
+   `Nginx <https://en.wikipedia.org/wiki/Nginx>`__ server.
+
+3. Set the appropriate configuration so that the produced SDK knows how
+   to find the configuration. The variable you need to set is
+   :term:`SSTATE_MIRRORS`:
+   ::
+
+      SSTATE_MIRRORS = "file://.* http://example.com/some_path/sstate-cache/PATH"
+
+   You can set the
+   ``SSTATE_MIRRORS`` variable in two different places:
+
+   -  If the mirror value you are setting is appropriate to be set for
+      both the OpenEmbedded build system that is actually building the
+      SDK and the SDK itself (i.e. the mirror is accessible in both
+      places or it will fail quickly on the OpenEmbedded build system
+      side, and its contents will not interfere with the build), then
+      you can set the variable in your ``local.conf`` or custom distro
+      configuration file. You can then "whitelist" the variable through
+      to the SDK by adding the following:
+      ::
+
+         SDK_LOCAL_CONF_WHITELIST = "SSTATE_MIRRORS"
+
+   -  Alternatively, if you just want to set the ``SSTATE_MIRRORS``
+      variable's value for the SDK alone, create a
+      ``conf/sdk-extra.conf`` file either in your
+      :term:`Build Directory` or within any
+      layer and put your ``SSTATE_MIRRORS`` setting within that file.
+
+      .. note::
+
+         This second option is the safest option should you have any
+         doubts as to which method to use when setting
+         SSTATE_MIRRORS
+         .
+
+Minimizing the Size of the Extensible SDK Installer Download
+============================================================
+
+By default, the extensible SDK bundles the shared state artifacts for
+everything needed to reconstruct the image for which the SDK was built.
+This bundling can lead to an SDK installer file that is a Gigabyte or
+more in size. If the size of this file causes a problem, you can build
+an SDK that has just enough in it to install and provide access to the
+``devtool command`` by setting the following in your configuration:
+::
+
+   SDK_EXT_TYPE = "minimal"
+
+Setting
+:term:`SDK_EXT_TYPE` to
+"minimal" produces an SDK installer that is around 35 Mbytes in size,
+which downloads and installs quickly. You need to realize, though, that
+the minimal installer does not install any libraries or tools out of the
+box. These libraries and tools must be installed either "on the fly" or
+through actions you perform using ``devtool`` or explicitly with the
+``devtool sdk-install`` command.
+
+In most cases, when building a minimal SDK you need to also enable
+bringing in the information on a wider range of packages produced by the
+system. Requiring this wider range of information is particularly true
+so that ``devtool add`` is able to effectively map dependencies it
+discovers in a source tree to the appropriate recipes. Additionally, the
+information enables the ``devtool search`` command to return useful
+results.
+
+To facilitate this wider range of information, you would need to set the
+following:
+::
+
+   SDK_INCLUDE_PKGDATA = "1"
+
+See the :term:`SDK_INCLUDE_PKGDATA` variable for additional information.
+
+Setting the ``SDK_INCLUDE_PKGDATA`` variable as shown causes the "world"
+target to be built so that information for all of the recipes included
+within it are available. Having these recipes available increases build
+time significantly and increases the size of the SDK installer by 30-80
+Mbytes depending on how many recipes are included in your configuration.
+
+You can use ``EXCLUDE_FROM_WORLD_pn-``\ recipename for recipes you want
+to exclude. However, it is assumed that you would need to be building
+the "world" target if you want to provide additional items to the SDK.
+Consequently, building for "world" should not represent undue overhead
+in most cases.
+
+.. note::
+
+   If you set
+   SDK_EXT_TYPE
+   to "minimal", then providing a shared state mirror is mandatory so
+   that items can be installed as needed. See the "
+   Providing Additional Installable Extensible SDK Content
+   " section for more information.
+
+You can explicitly control whether or not to include the toolchain when
+you build an SDK by setting the
+:term:`SDK_INCLUDE_TOOLCHAIN`
+variable to "1". In particular, it is useful to include the toolchain
+when you have set ``SDK_EXT_TYPE`` to "minimal", which by default,
+excludes the toolchain. Also, it is helpful if you are building a small
+SDK for use with an IDE or some other tool where you do not want to take
+extra steps to install a toolchain.