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+************
+Common Tasks
+************
+
+This chapter presents several common tasks you perform when you work
+with the Yocto Project Linux kernel. These tasks include preparing your
+host development system for kernel development, preparing a layer,
+modifying an existing recipe, patching the kernel, configuring the
+kernel, iterative development, working with your own sources, and
+incorporating out-of-tree modules.
+
+.. note::
+
+   The examples presented in this chapter work with the Yocto Project
+   2.4 Release and forward.
+
+Preparing the Build Host to Work on the Kernel
+==============================================
+
+Before you can do any kernel development, you need to be sure your build
+host is set up to use the Yocto Project. For information on how to get
+set up, see the "`Preparing the Build
+Host <&YOCTO_DOCS_DEV_URL;#dev-preparing-the-build-host>`__" section in
+the Yocto Project Development Tasks Manual. Part of preparing the system
+is creating a local Git repository of the `Source
+Directory <&YOCTO_DOCS_REF_URL;#source-directory>`__ (``poky``) on your
+system. Follow the steps in the "`Cloning the ``poky``
+Repository <&YOCTO_DOCS_DEV_URL;#cloning-the-poky-repository>`__"
+section in the Yocto Project Development Tasks Manual to set up your
+Source Directory.
+
+.. note::
+
+   Be sure you check out the appropriate development branch or you
+   create your local branch by checking out a specific tag to get the
+   desired version of Yocto Project. See the "
+   Checking Out by Branch in Poky
+   " and "
+   Checking Out by Tag in Poky
+   " sections in the Yocto Project Development Tasks Manual for more
+   information.
+
+Kernel development is best accomplished using
+```devtool`` <&YOCTO_DOCS_SDK_URL;#using-devtool-in-your-sdk-workflow>`__
+and not through traditional kernel workflow methods. The remainder of
+this section provides information for both scenarios.
+
+Getting Ready to Develop Using ``devtool``
+------------------------------------------
+
+Follow these steps to prepare to update the kernel image using
+``devtool``. Completing this procedure leaves you with a clean kernel
+image and ready to make modifications as described in the "`Using
+``devtool`` to Patch the Kernel <#using-devtool-to-patch-the-kernel>`__"
+section:
+
+1. *Initialize the BitBake Environment:* Before building an extensible
+   SDK, you need to initialize the BitBake build environment by sourcing
+   the build environment script (i.e.
+   ```oe-init-build-env`` <&YOCTO_DOCS_REF_URL;#structure-core-script>`__):
+   $ cd ~/poky $ source oe-init-build-env
+
+   .. note::
+
+      The previous commands assume the
+      Source Repositories
+      (i.e.
+      poky
+      ) have been cloned using Git and the local repository is named
+      "poky".
+
+2. *Prepare Your ``local.conf`` File:* By default, the
+   ```MACHINE`` <&YOCTO_DOCS_REF_URL;#var-MACHINE>`__ variable is set to
+   "qemux86-64", which is fine if you are building for the QEMU emulator
+   in 64-bit mode. However, if you are not, you need to set the
+   ``MACHINE`` variable appropriately in your ``conf/local.conf`` file
+   found in the `Build
+   Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__ (i.e.
+   ``~/poky/build`` in this example).
+
+   Also, since you are preparing to work on the kernel image, you need
+   to set the
+   ```MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS`` <&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS>`__
+   variable to include kernel modules.
+
+   In this example we wish to build for qemux86 so we must set the
+   ``MACHINE`` variable to "qemux86" and also add the "kernel-modules".
+   As described we do this by appending to ``conf/local.conf``: MACHINE
+   = "qemux86" MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS += "kernel-modules"
+
+3. *Create a Layer for Patches:* You need to create a layer to hold
+   patches created for the kernel image. You can use the
+   ``bitbake-layers create-layer`` command as follows: $ cd ~/poky/build
+   $ bitbake-layers create-layer ../../meta-mylayer NOTE: Starting
+   bitbake server... Add your new layer with 'bitbake-layers add-layer
+   ../../meta-mylayer' $
+
+   .. note::
+
+      For background information on working with common and BSP layers,
+      see the "
+      Understanding and Creating Layers
+      " section in the Yocto Project Development Tasks Manual and the "
+      BSP Layers
+      " section in the Yocto Project Board Support (BSP) Developer's
+      Guide, respectively. For information on how to use the
+      bitbake-layers create-layer
+      command to quickly set up a layer, see the "
+      Creating a General Layer Using the
+      bitbake-layers
+      Script
+      " section in the Yocto Project Development Tasks Manual.
+
+4. *Inform the BitBake Build Environment About Your Layer:* As directed
+   when you created your layer, you need to add the layer to the
+   ```BBLAYERS`` <&YOCTO_DOCS_REF_URL;#var-BBLAYERS>`__ variable in the
+   ``bblayers.conf`` file as follows: $ cd ~/poky/build $ bitbake-layers
+   add-layer ../../meta-mylayer NOTE: Starting bitbake server... $
+
+5. *Build the Extensible SDK:* Use BitBake to build the extensible SDK
+   specifically for use with images to be run using QEMU: $ cd
+   ~/poky/build $ bitbake core-image-minimal -c populate_sdk_ext Once
+   the build finishes, you can find the SDK installer file (i.e.
+   ``*.sh`` file) in the following directory:
+   ~/poky/build/tmp/deploy/sdk For this example, the installer file is
+   named
+   ``poky-glibc-x86_64-core-image-minimal-i586-toolchain-ext-DISTRO.sh``
+
+6. *Install the Extensible SDK:* Use the following command to install
+   the SDK. For this example, install the SDK in the default
+   ``~/poky_sdk`` directory: $ cd ~/poky/build/tmp/deploy/sdk $
+   ./poky-glibc-x86_64-core-image-minimal-i586-toolchain-ext-DISTRO.sh
+   Poky (Yocto Project Reference Distro) Extensible SDK installer
+   version DISTRO
+   ============================================================================
+   Enter target directory for SDK (default: ~/poky_sdk): You are about
+   to install the SDK to "/home/scottrif/poky_sdk". Proceed [Y/n]? Y
+   Extracting SDK......................................done Setting it
+   up... Extracting buildtools... Preparing build system... Parsing
+   recipes: 100%
+   \|#################################################################\|
+   Time: 0:00:52 Initializing tasks: 100% \|##############
+   ###############################################\| Time: 0:00:04
+   Checking sstate mirror object availability: 100%
+   \|######################################\| Time: 0:00:00 Parsing
+   recipes: 100%
+   \|#################################################################\|
+   Time: 0:00:33 Initializing tasks: 100%
+   \|##############################################################\|
+   Time: 0:00:00 done SDK has been successfully set up and is ready to
+   be used. Each time you wish to use the SDK in a new shell session,
+   you need to source the environment setup script e.g. $ .
+   /home/scottrif/poky_sdk/environment-setup-i586-poky-linux
+
+7. *Set Up a New Terminal to Work With the Extensible SDK:* You must set
+   up a new terminal to work with the SDK. You cannot use the same
+   BitBake shell used to build the installer.
+
+   After opening a new shell, run the SDK environment setup script as
+   directed by the output from installing the SDK: $ source
+   ~/poky_sdk/environment-setup-i586-poky-linux "SDK environment now set
+   up; additionally you may now run devtool to perform development
+   tasks. Run devtool --help for further details.
+
+   .. note::
+
+      If you get a warning about attempting to use the extensible SDK in
+      an environment set up to run BitBake, you did not use a new shell.
+
+8. *Build the Clean Image:* The final step in preparing to work on the
+   kernel is to build an initial image using ``devtool`` in the new
+   terminal you just set up and initialized for SDK work: $ devtool
+   build-image Parsing recipes: 100%
+   \|##########################################\| Time: 0:00:05 Parsing
+   of 830 .bb files complete (0 cached, 830 parsed). 1299 targets, 47
+   skipped, 0 masked, 0 errors. WARNING: No packages to add, building
+   image core-image-minimal unmodified Loading cache: 100%
+   \|############################################\| Time: 0:00:00 Loaded
+   1299 entries from dependency cache. NOTE: Resolving any missing task
+   queue dependencies Initializing tasks: 100%
+   \|#######################################\| Time: 0:00:07 Checking
+   sstate mirror object availability: 100% \|###############\| Time:
+   0:00:00 NOTE: Executing SetScene Tasks NOTE: Executing RunQueue Tasks
+   NOTE: Tasks Summary: Attempted 2866 tasks of which 2604 didn't need
+   to be rerun and all succeeded. NOTE: Successfully built
+   core-image-minimal. You can find output files in
+   /home/scottrif/poky_sdk/tmp/deploy/images/qemux86 If you were
+   building for actual hardware and not for emulation, you could flash
+   the image to a USB stick on ``/dev/sdd`` and boot your device. For an
+   example that uses a Minnowboard, see the
+   `TipsAndTricks/KernelDevelopmentWithEsdk <https://wiki.yoctoproject.org/wiki/TipsAndTricks/KernelDevelopmentWithEsdk>`__
+   Wiki page.
+
+At this point you have set up to start making modifications to the
+kernel by using the extensible SDK. For a continued example, see the
+"`Using ``devtool`` to Patch the
+Kernel <#using-devtool-to-patch-the-kernel>`__" section.
+
+Getting Ready for Traditional Kernel Development
+------------------------------------------------
+
+Getting ready for traditional kernel development using the Yocto Project
+involves many of the same steps as described in the previous section.
+However, you need to establish a local copy of the kernel source since
+you will be editing these files.
+
+Follow these steps to prepare to update the kernel image using
+traditional kernel development flow with the Yocto Project. Completing
+this procedure leaves you ready to make modifications to the kernel
+source as described in the "`Using Traditional Kernel Development to
+Patch the
+Kernel <#using-traditional-kernel-development-to-patch-the-kernel>`__"
+section:
+
+1. *Initialize the BitBake Environment:* Before you can do anything
+   using BitBake, you need to initialize the BitBake build environment
+   by sourcing the build environment script (i.e.
+   ```oe-init-build-env`` <&YOCTO_DOCS_REF_URL;#structure-core-script>`__).
+   Also, for this example, be sure that the local branch you have
+   checked out for ``poky`` is the Yocto Project DISTRO_NAME branch. If
+   you need to checkout out the DISTRO_NAME branch, see the "`Checking
+   out by Branch in
+   Poky <&YOCTO_DOCS_DEV_URL;#checking-out-by-branch-in-poky>`__"
+   section in the Yocto Project Development Tasks Manual. $ cd ~/poky $
+   git branch master \* DISTRO_NAME $ source oe-init-build-env
+
+   .. note::
+
+      The previous commands assume the
+      Source Repositories
+      (i.e.
+      poky
+      ) have been cloned using Git and the local repository is named
+      "poky".
+
+2. *Prepare Your ``local.conf`` File:* By default, the
+   ```MACHINE`` <&YOCTO_DOCS_REF_URL;#var-MACHINE>`__ variable is set to
+   "qemux86-64", which is fine if you are building for the QEMU emulator
+   in 64-bit mode. However, if you are not, you need to set the
+   ``MACHINE`` variable appropriately in your ``conf/local.conf`` file
+   found in the `Build
+   Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__ (i.e.
+   ``~/poky/build`` in this example).
+
+   Also, since you are preparing to work on the kernel image, you need
+   to set the
+   ```MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS`` <&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS>`__
+   variable to include kernel modules.
+
+   In this example we wish to build for qemux86 so we must set the
+   ``MACHINE`` variable to "qemux86" and also add the "kernel-modules".
+   As described we do this by appending to ``conf/local.conf``: MACHINE
+   = "qemux86" MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS += "kernel-modules"
+
+3. *Create a Layer for Patches:* You need to create a layer to hold
+   patches created for the kernel image. You can use the
+   ``bitbake-layers create-layer`` command as follows: $ cd ~/poky/build
+   $ bitbake-layers create-layer ../../meta-mylayer NOTE: Starting
+   bitbake server... Add your new layer with 'bitbake-layers add-layer
+   ../../meta-mylayer'
+
+   .. note::
+
+      For background information on working with common and BSP layers,
+      see the "
+      Understanding and Creating Layers
+      " section in the Yocto Project Development Tasks Manual and the "
+      BSP Layers
+      " section in the Yocto Project Board Support (BSP) Developer's
+      Guide, respectively. For information on how to use the
+      bitbake-layers create-layer
+      command to quickly set up a layer, see the "
+      Creating a General Layer Using the
+      bitbake-layers
+      Script
+      " section in the Yocto Project Development Tasks Manual.
+
+4. *Inform the BitBake Build Environment About Your Layer:* As directed
+   when you created your layer, you need to add the layer to the
+   ```BBLAYERS`` <&YOCTO_DOCS_REF_URL;#var-BBLAYERS>`__ variable in the
+   ``bblayers.conf`` file as follows: $ cd ~/poky/build $ bitbake-layers
+   add-layer ../../meta-mylayer NOTE: Starting bitbake server ... $
+
+5. *Create a Local Copy of the Kernel Git Repository:* You can find Git
+   repositories of supported Yocto Project kernels organized under
+   "Yocto Linux Kernel" in the Yocto Project Source Repositories at
+   ` <&YOCTO_GIT_URL;>`__.
+
+   For simplicity, it is recommended that you create your copy of the
+   kernel Git repository outside of the `Source
+   Directory <&YOCTO_DOCS_REF_URL;#source-directory>`__, which is
+   usually named ``poky``. Also, be sure you are in the
+   ``standard/base`` branch.
+
+   The following commands show how to create a local copy of the
+   ``linux-yocto-4.12`` kernel and be in the ``standard/base`` branch.
+
+   .. note::
+
+      The
+      linux-yocto-4.12
+      kernel can be used with the Yocto Project 2.4 release and forward.
+      You cannot use the
+      linux-yocto-4.12
+      kernel with releases prior to Yocto Project 2.4:
+
+   $ cd ~ $ git clone git://git.yoctoproject.org/linux-yocto-4.12
+   --branch standard/base Cloning into 'linux-yocto-4.12'... remote:
+   Counting objects: 6097195, done. remote: Compressing objects: 100%
+   (901026/901026), done. remote: Total 6097195 (delta 5152604), reused
+   6096847 (delta 5152256) Receiving objects: 100% (6097195/6097195),
+   1.24 GiB \| 7.81 MiB/s, done. Resolving deltas: 100%
+   (5152604/5152604), done. Checking connectivity... done. Checking out
+   files: 100% (59846/59846), done.
+
+6. *Create a Local Copy of the Kernel Cache Git Repository:* For
+   simplicity, it is recommended that you create your copy of the kernel
+   cache Git repository outside of the `Source
+   Directory <&YOCTO_DOCS_REF_URL;#source-directory>`__, which is
+   usually named ``poky``. Also, for this example, be sure you are in
+   the ``yocto-4.12`` branch.
+
+   The following commands show how to create a local copy of the
+   ``yocto-kernel-cache`` and be in the ``yocto-4.12`` branch: $ cd ~ $
+   git clone git://git.yoctoproject.org/yocto-kernel-cache --branch
+   yocto-4.12 Cloning into 'yocto-kernel-cache'... remote: Counting
+   objects: 22639, done. remote: Compressing objects: 100% (9761/9761),
+   done. remote: Total 22639 (delta 12400), reused 22586 (delta 12347)
+   Receiving objects: 100% (22639/22639), 22.34 MiB \| 6.27 MiB/s, done.
+   Resolving deltas: 100% (12400/12400), done. Checking connectivity...
+   done.
+
+At this point, you are ready to start making modifications to the kernel
+using traditional kernel development steps. For a continued example, see
+the "`Using Traditional Kernel Development to Patch the
+Kernel <#using-traditional-kernel-development-to-patch-the-kernel>`__"
+section.
+
+Creating and Preparing a Layer
+==============================
+
+If you are going to be modifying kernel recipes, it is recommended that
+you create and prepare your own layer in which to do your work. Your
+layer contains its own `BitBake <&YOCTO_DOCS_REF_URL;#bitbake-term>`__
+append files (``.bbappend``) and provides a convenient mechanism to
+create your own recipe files (``.bb``) as well as store and use kernel
+patch files. For background information on working with layers, see the
+"`Understanding and Creating
+Layers <&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers>`__"
+section in the Yocto Project Development Tasks Manual.
+
+.. note::
+
+   The Yocto Project comes with many tools that simplify tasks you need
+   to perform. One such tool is the
+   bitbake-layers create-layer
+   command, which simplifies creating a new layer. See the "
+   Creating a General Layer Using the
+   bitbake-layers
+   Script
+   " section in the Yocto Project Development Tasks Manual for
+   information on how to use this script to quick set up a new layer.
+
+To better understand the layer you create for kernel development, the
+following section describes how to create a layer without the aid of
+tools. These steps assume creation of a layer named ``mylayer`` in your
+home directory:
+
+1. *Create Structure*: Create the layer's structure: $ cd $HOME $ mkdir
+   meta-mylayer $ mkdir meta-mylayer/conf $ mkdir
+   meta-mylayer/recipes-kernel $ mkdir meta-mylayer/recipes-kernel/linux
+   $ mkdir meta-mylayer/recipes-kernel/linux/linux-yocto The ``conf``
+   directory holds your configuration files, while the
+   ``recipes-kernel`` directory holds your append file and eventual
+   patch files.
+
+2. *Create the Layer Configuration File*: Move to the
+   ``meta-mylayer/conf`` directory and create the ``layer.conf`` file as
+   follows: # We have a conf and classes directory, add to BBPATH BBPATH
+   .= ":${LAYERDIR}" # We have recipes-\* directories, add to BBFILES
+   BBFILES += "${LAYERDIR}/recipes-*/*/*.bb \\
+   ${LAYERDIR}/recipes-*/*/*.bbappend" BBFILE_COLLECTIONS += "mylayer"
+   BBFILE_PATTERN_mylayer = "^${LAYERDIR}/" BBFILE_PRIORITY_mylayer =
+   "5" Notice ``mylayer`` as part of the last three statements.
+
+3. *Create the Kernel Recipe Append File*: Move to the
+   ``meta-mylayer/recipes-kernel/linux`` directory and create the
+   kernel's append file. This example uses the ``linux-yocto-4.12``
+   kernel. Thus, the name of the append file is
+   ``linux-yocto_4.12.bbappend``: FILESEXTRAPATHS_prepend :=
+   "${THISDIR}/${PN}:" SRC_URI_append = " file://patch-file-one"
+   SRC_URI_append = " file://patch-file-two" SRC_URI_append = "
+   file://patch-file-three" The
+   ```FILESEXTRAPATHS`` <&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS>`__
+   and ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ statements
+   enable the OpenEmbedded build system to find patch files. For more
+   information on using append files, see the "`Using .bbappend Files in
+   Your Layer <&YOCTO_DOCS_DEV_URL;#using-bbappend-files>`__" section in
+   the Yocto Project Development Tasks Manual.
+
+Modifying an Existing Recipe
+============================
+
+In many cases, you can customize an existing linux-yocto recipe to meet
+the needs of your project. Each release of the Yocto Project provides a
+few Linux kernel recipes from which you can choose. These are located in
+the `Source Directory <&YOCTO_DOCS_REF_URL;#source-directory>`__ in
+``meta/recipes-kernel/linux``.
+
+Modifying an existing recipe can consist of the following:
+
+-  Creating the append file
+
+-  Applying patches
+
+-  Changing the configuration
+
+Before modifying an existing recipe, be sure that you have created a
+minimal, custom layer from which you can work. See the "`Creating and
+Preparing a Layer <#creating-and-preparing-a-layer>`__" section for
+information.
+
+Creating the Append File
+------------------------
+
+You create this file in your custom layer. You also name it accordingly
+based on the linux-yocto recipe you are using. For example, if you are
+modifying the ``meta/recipes-kernel/linux/linux-yocto_4.12.bb`` recipe,
+the append file will typically be located as follows within your custom
+layer: your-layer/recipes-kernel/linux/linux-yocto_4.12.bbappend The
+append file should initially extend the
+```FILESPATH`` <&YOCTO_DOCS_REF_URL;#var-FILESPATH>`__ search path by
+prepending the directory that contains your files to the
+```FILESEXTRAPATHS`` <&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS>`__
+variable as follows: FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:" The
+path
+``${``\ ```THISDIR`` <&YOCTO_DOCS_REF_URL;#var-THISDIR>`__\ ``}/${``\ ```PN`` <&YOCTO_DOCS_REF_URL;#var-PN>`__\ ``}``
+expands to "linux-yocto" in the current directory for this example. If
+you add any new files that modify the kernel recipe and you have
+extended ``FILESPATH`` as described above, you must place the files in
+your layer in the following area:
+your-layer/recipes-kernel/linux/linux-yocto/
+
+.. note::
+
+   If you are working on a new machine Board Support Package (BSP), be
+   sure to refer to the
+   Yocto Project Board Support Package (BSP) Developer's Guide
+   .
+
+As an example, consider the following append file used by the BSPs in
+``meta-yocto-bsp``:
+meta-yocto-bsp/recipes-kernel/linux/linux-yocto_4.12.bbappend The
+following listing shows the file. Be aware that the actual commit ID
+strings in this example listing might be different than the actual
+strings in the file from the ``meta-yocto-bsp`` layer upstream.
+KBRANCH_genericx86 = "standard/base" KBRANCH_genericx86-64 =
+"standard/base" KMACHINE_genericx86 ?= "common-pc"
+KMACHINE_genericx86-64 ?= "common-pc-64" KBRANCH_edgerouter =
+"standard/edgerouter" KBRANCH_beaglebone = "standard/beaglebone"
+SRCREV_machine_genericx86 ?= "d09f2ce584d60ecb7890550c22a80c48b83c2e19"
+SRCREV_machine_genericx86-64 ?=
+"d09f2ce584d60ecb7890550c22a80c48b83c2e19" SRCREV_machine_edgerouter ?=
+"b5c8cfda2dfe296410d51e131289fb09c69e1e7d" SRCREV_machine_beaglebone ?=
+"b5c8cfda2dfe296410d51e131289fb09c69e1e7d" COMPATIBLE_MACHINE_genericx86
+= "genericx86" COMPATIBLE_MACHINE_genericx86-64 = "genericx86-64"
+COMPATIBLE_MACHINE_edgerouter = "edgerouter"
+COMPATIBLE_MACHINE_beaglebone = "beaglebone" LINUX_VERSION_genericx86 =
+"4.12.7" LINUX_VERSION_genericx86-64 = "4.12.7" LINUX_VERSION_edgerouter
+= "4.12.10" LINUX_VERSION_beaglebone = "4.12.10" This append file
+contains statements used to support several BSPs that ship with the
+Yocto Project. The file defines machines using the
+```COMPATIBLE_MACHINE`` <&YOCTO_DOCS_REF_URL;#var-COMPATIBLE_MACHINE>`__
+variable and uses the
+```KMACHINE`` <&YOCTO_DOCS_REF_URL;#var-KMACHINE>`__ variable to ensure
+the machine name used by the OpenEmbedded build system maps to the
+machine name used by the Linux Yocto kernel. The file also uses the
+optional ```KBRANCH`` <&YOCTO_DOCS_REF_URL;#var-KBRANCH>`__ variable to
+ensure the build process uses the appropriate kernel branch.
+
+Although this particular example does not use it, the
+```KERNEL_FEATURES`` <&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES>`__
+variable could be used to enable features specific to the kernel. The
+append file points to specific commits in the `Source
+Directory <&YOCTO_DOCS_REF_URL;#source-directory>`__ Git repository and
+the ``meta`` Git repository branches to identify the exact kernel needed
+to build the BSP.
+
+One thing missing in this particular BSP, which you will typically need
+when developing a BSP, is the kernel configuration file (``.config``)
+for your BSP. When developing a BSP, you probably have a kernel
+configuration file or a set of kernel configuration files that, when
+taken together, define the kernel configuration for your BSP. You can
+accomplish this definition by putting the configurations in a file or a
+set of files inside a directory located at the same level as your
+kernel's append file and having the same name as the kernel's main
+recipe file. With all these conditions met, simply reference those files
+in the ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ statement in
+the append file.
+
+For example, suppose you had some configuration options in a file called
+``network_configs.cfg``. You can place that file inside a directory
+named ``linux-yocto`` and then add a ``SRC_URI`` statement such as the
+following to the append file. When the OpenEmbedded build system builds
+the kernel, the configuration options are picked up and applied. SRC_URI
++= "file://network_configs.cfg"
+
+To group related configurations into multiple files, you perform a
+similar procedure. Here is an example that groups separate
+configurations specifically for Ethernet and graphics into their own
+files and adds the configurations by using a ``SRC_URI`` statement like
+the following in your append file: SRC_URI += "file://myconfig.cfg \\
+file://eth.cfg \\ file://gfx.cfg"
+
+Another variable you can use in your kernel recipe append file is the
+```FILESEXTRAPATHS`` <&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS>`__
+variable. When you use this statement, you are extending the locations
+used by the OpenEmbedded system to look for files and patches as the
+recipe is processed.
+
+.. note::
+
+   Other methods exist to accomplish grouping and defining configuration
+   options. For example, if you are working with a local clone of the
+   kernel repository, you could checkout the kernel's ``meta`` branch,
+   make your changes, and then push the changes to the local bare clone
+   of the kernel. The result is that you directly add configuration
+   options to the ``meta`` branch for your BSP. The configuration
+   options will likely end up in that location anyway if the BSP gets
+   added to the Yocto Project.
+
+   In general, however, the Yocto Project maintainers take care of
+   moving the ``SRC_URI``-specified configuration options to the
+   kernel's ``meta`` branch. Not only is it easier for BSP developers to
+   not have to worry about putting those configurations in the branch,
+   but having the maintainers do it allows them to apply 'global'
+   knowledge about the kinds of common configuration options multiple
+   BSPs in the tree are typically using. This allows for promotion of
+   common configurations into common features.
+
+Applying Patches
+----------------
+
+If you have a single patch or a small series of patches that you want to
+apply to the Linux kernel source, you can do so just as you would with
+any other recipe. You first copy the patches to the path added to
+```FILESEXTRAPATHS`` <&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS>`__ in
+your ``.bbappend`` file as described in the previous section, and then
+reference them in ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__
+statements.
+
+For example, you can apply a three-patch series by adding the following
+lines to your linux-yocto ``.bbappend`` file in your layer: SRC_URI +=
+"file://0001-first-change.patch" SRC_URI +=
+"file://0002-second-change.patch" SRC_URI +=
+"file://0003-third-change.patch" The next time you run BitBake to build
+the Linux kernel, BitBake detects the change in the recipe and fetches
+and applies the patches before building the kernel.
+
+For a detailed example showing how to patch the kernel using
+``devtool``, see the "`Using ``devtool`` to Patch the
+Kernel <#using-devtool-to-patch-the-kernel>`__" and "`Using Traditional
+Kernel Development to Patch the
+Kernel <#using-traditional-kernel-development-to-patch-the-kernel>`__"
+sections.
+
+Changing the Configuration
+--------------------------
+
+You can make wholesale or incremental changes to the final ``.config``
+file used for the eventual Linux kernel configuration by including a
+``defconfig`` file and by specifying configuration fragments in the
+```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ to be applied to that
+file.
+
+If you have a complete, working Linux kernel ``.config`` file you want
+to use for the configuration, as before, copy that file to the
+appropriate ``${PN}`` directory in your layer's ``recipes-kernel/linux``
+directory, and rename the copied file to "defconfig". Then, add the
+following lines to the linux-yocto ``.bbappend`` file in your layer:
+FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:" SRC_URI +=
+"file://defconfig" The ``SRC_URI`` tells the build system how to search
+for the file, while the
+```FILESEXTRAPATHS`` <&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS>`__
+extends the ```FILESPATH`` <&YOCTO_DOCS_REF_URL;#var-FILESPATH>`__
+variable (search directories) to include the ``${PN}`` directory you
+created to hold the configuration changes.
+
+.. note::
+
+   The build system applies the configurations from the
+   defconfig
+   file before applying any subsequent configuration fragments. The
+   final kernel configuration is a combination of the configurations in
+   the
+   defconfig
+   file and any configuration fragments you provide. You need to realize
+   that if you have any configuration fragments, the build system
+   applies these on top of and after applying the existing
+   defconfig
+   file configurations.
+
+Generally speaking, the preferred approach is to determine the
+incremental change you want to make and add that as a configuration
+fragment. For example, if you want to add support for a basic serial
+console, create a file named ``8250.cfg`` in the ``${PN}`` directory
+with the following content (without indentation): CONFIG_SERIAL_8250=y
+CONFIG_SERIAL_8250_CONSOLE=y CONFIG_SERIAL_8250_PCI=y
+CONFIG_SERIAL_8250_NR_UARTS=4 CONFIG_SERIAL_8250_RUNTIME_UARTS=4
+CONFIG_SERIAL_CORE=y CONFIG_SERIAL_CORE_CONSOLE=y Next, include this
+configuration fragment and extend the ``FILESPATH`` variable in your
+``.bbappend`` file: FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
+SRC_URI += "file://8250.cfg" The next time you run BitBake to build the
+Linux kernel, BitBake detects the change in the recipe and fetches and
+applies the new configuration before building the kernel.
+
+For a detailed example showing how to configure the kernel, see the
+"`Configuring the Kernel <#configuring-the-kernel>`__" section.
+
+Using an "In-Tree"  ``defconfig`` File
+--------------------------------------
+
+It might be desirable to have kernel configuration fragment support
+through a ``defconfig`` file that is pulled from the kernel source tree
+for the configured machine. By default, the OpenEmbedded build system
+looks for ``defconfig`` files in the layer used for Metadata, which is
+"out-of-tree", and then configures them using the following: SRC_URI +=
+"file://defconfig" If you do not want to maintain copies of
+``defconfig`` files in your layer but would rather allow users to use
+the default configuration from the kernel tree and still be able to add
+configuration fragments to the
+```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ through, for example,
+append files, you can direct the OpenEmbedded build system to use a
+``defconfig`` file that is "in-tree".
+
+To specify an "in-tree" ``defconfig`` file, use the following statement
+form: KBUILD_DEFCONFIG_KMACHINE ?= defconfig_file Here is an example
+that assigns the ``KBUILD_DEFCONFIG`` variable based on "raspberrypi2"
+and provides the path to the "in-tree" ``defconfig`` file to be used for
+a Raspberry Pi 2, which is based on the Broadcom 2708/2709 chipset:
+KBUILD_DEFCONFIG_raspberrypi2 ?= "bcm2709_defconfig"
+
+Aside from modifying your kernel recipe and providing your own
+``defconfig`` file, you need to be sure no files or statements set
+``SRC_URI`` to use a ``defconfig`` other than your "in-tree" file (e.g.
+a kernel's ``linux-``\ machine\ ``.inc`` file). In other words, if the
+build system detects a statement that identifies an "out-of-tree"
+``defconfig`` file, that statement will override your
+``KBUILD_DEFCONFIG`` variable.
+
+See the
+```KBUILD_DEFCONFIG`` <&YOCTO_DOCS_REF_URL;#var-KBUILD_DEFCONFIG>`__
+variable description for more information.
+
+Using ``devtool`` to Patch the Kernel
+=====================================
+
+The steps in this procedure show you how you can patch the kernel using
+the extensible SDK and ``devtool``.
+
+.. note::
+
+   Before attempting this procedure, be sure you have performed the
+   steps to get ready for updating the kernel as described in the "
+   Getting Ready to Develop Using
+   devtool
+   " section.
+
+Patching the kernel involves changing or adding configurations to an
+existing kernel, changing or adding recipes to the kernel that are
+needed to support specific hardware features, or even altering the
+source code itself.
+
+This example creates a simple patch by adding some QEMU emulator console
+output at boot time through ``printk`` statements in the kernel's
+``calibrate.c`` source code file. Applying the patch and booting the
+modified image causes the added messages to appear on the emulator's
+console. The example is a continuation of the setup procedure found in
+the "`Getting Ready to Develop Using
+``devtool`` <#getting-ready-to-develop-using-devtool>`__" Section.
+
+1. *Check Out the Kernel Source Files:* First you must use ``devtool``
+   to checkout the kernel source code in its workspace. Be sure you are
+   in the terminal set up to do work with the extensible SDK.
+
+   .. note::
+
+      See this
+      step
+      in the "
+      Getting Ready to Develop Using
+      devtool
+      " section for more information.
+
+   Use the following ``devtool`` command to check out the code: $
+   devtool modify linux-yocto
+
+   .. note::
+
+      During the checkout operation, a bug exists that could cause
+      errors such as the following to appear:
+      ::
+
+              ERROR: Taskhash mismatch 2c793438c2d9f8c3681fd5f7bc819efa versus
+                     be3a89ce7c47178880ba7bf6293d7404 for
+                     /path/to/esdk/layers/poky/meta/recipes-kernel/linux/linux-yocto_4.10.bb.do_unpack
+                                 
+
+      You can safely ignore these messages. The source code is correctly
+      checked out.
+
+2. *Edit the Source Files* Follow these steps to make some simple
+   changes to the source files:
+
+   1. *Change the working directory*: In the previous step, the output
+      noted where you can find the source files (e.g.
+      ``~/poky_sdk/workspace/sources/linux-yocto``). Change to where the
+      kernel source code is before making your edits to the
+      ``calibrate.c`` file: $ cd
+      ~/poky_sdk/workspace/sources/linux-yocto
+
+   2. *Edit the source file*: Edit the ``init/calibrate.c`` file to have
+      the following changes: void calibrate_delay(void) { unsigned long
+      lpj; static bool printed; int this_cpu = smp_processor_id();
+      printk("*************************************\n"); printk("\*
+      \*\n"); printk("\* HELLO YOCTO KERNEL \*\n"); printk("\* \*\n");
+      printk("*************************************\n"); if
+      (per_cpu(cpu_loops_per_jiffy, this_cpu)) { . . .
+
+3. *Build the Updated Kernel Source:* To build the updated kernel
+   source, use ``devtool``: $ devtool build linux-yocto
+
+4. *Create the Image With the New Kernel:* Use the
+   ``devtool build-image`` command to create a new image that has the
+   new kernel.
+
+   .. note::
+
+      If the image you originally created resulted in a Wic file, you
+      can use an alternate method to create the new image with the
+      updated kernel. For an example, see the steps in the
+      TipsAndTricks/KernelDevelopmentWithEsdk
+      Wiki Page.
+
+   $ cd ~ $ devtool build-image core-image-minimal
+
+5. *Test the New Image:* For this example, you can run the new image
+   using QEMU to verify your changes:
+
+   1. *Boot the image*: Boot the modified image in the QEMU emulator
+      using this command: $ runqemu qemux86
+
+   2. *Verify the changes*: Log into the machine using ``root`` with no
+      password and then use the following shell command to scroll
+      through the console's boot output. # dmesg \| less You should see
+      the results of your ``printk`` statements as part of the output
+      when you scroll down the console window.
+
+6. *Stage and commit your changes*: Within your eSDK terminal, change
+   your working directory to where you modified the ``calibrate.c`` file
+   and use these Git commands to stage and commit your changes: $ cd
+   ~/poky_sdk/workspace/sources/linux-yocto $ git status $ git add
+   init/calibrate.c $ git commit -m "calibrate: Add printk example"
+
+7. *Export the Patches and Create an Append File:* To export your
+   commits as patches and create a ``.bbappend`` file, use the following
+   command in the terminal used to work with the extensible SDK. This
+   example uses the previously established layer named ``meta-mylayer``.
+
+   .. note::
+
+      See Step 3 of the "
+      Getting Ready to Develop Using devtool
+      " section for information on setting up this layer.
+
+   $ devtool finish linux-yocto ~/meta-mylayer Once the command
+   finishes, the patches and the ``.bbappend`` file are located in the
+   ``~/meta-mylayer/recipes-kernel/linux`` directory.
+
+8. *Build the Image With Your Modified Kernel:* You can now build an
+   image that includes your kernel patches. Execute the following
+   command from your `Build
+   Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__ in the terminal
+   set up to run BitBake: $ cd ~/poky/build $ bitbake core-image-minimal
+
+Using Traditional Kernel Development to Patch the Kernel
+========================================================
+
+The steps in this procedure show you how you can patch the kernel using
+traditional kernel development (i.e. not using ``devtool`` and the
+extensible SDK as described in the "`Using ``devtool`` to Patch the
+Kernel <#using-devtool-to-patch-the-kernel>`__" section).
+
+.. note::
+
+   Before attempting this procedure, be sure you have performed the
+   steps to get ready for updating the kernel as described in the "
+   Getting Ready for Traditional Kernel Development
+   " section.
+
+Patching the kernel involves changing or adding configurations to an
+existing kernel, changing or adding recipes to the kernel that are
+needed to support specific hardware features, or even altering the
+source code itself.
+
+The example in this section creates a simple patch by adding some QEMU
+emulator console output at boot time through ``printk`` statements in
+the kernel's ``calibrate.c`` source code file. Applying the patch and
+booting the modified image causes the added messages to appear on the
+emulator's console. The example is a continuation of the setup procedure
+found in the "`Getting Ready for Traditional Kernel
+Development <#getting-ready-for-traditional-kernel-development>`__"
+Section.
+
+1. *Edit the Source Files* Prior to this step, you should have used Git
+   to create a local copy of the repository for your kernel. Assuming
+   you created the repository as directed in the "`Getting Ready for
+   Traditional Kernel
+   Development <#getting-ready-for-traditional-kernel-development>`__"
+   section, use the following commands to edit the ``calibrate.c`` file:
+
+   1. *Change the working directory*: You need to locate the source
+      files in the local copy of the kernel Git repository: Change to
+      where the kernel source code is before making your edits to the
+      ``calibrate.c`` file: $ cd ~/linux-yocto-4.12/init
+
+   2. *Edit the source file*: Edit the ``calibrate.c`` file to have the
+      following changes: void calibrate_delay(void) { unsigned long lpj;
+      static bool printed; int this_cpu = smp_processor_id();
+      printk("*************************************\n"); printk("\*
+      \*\n"); printk("\* HELLO YOCTO KERNEL \*\n"); printk("\* \*\n");
+      printk("*************************************\n"); if
+      (per_cpu(cpu_loops_per_jiffy, this_cpu)) { . . .
+
+2. *Stage and Commit Your Changes:* Use standard Git commands to stage
+   and commit the changes you just made: $ git add calibrate.c $ git
+   commit -m "calibrate.c - Added some printk statements" If you do not
+   stage and commit your changes, the OpenEmbedded Build System will not
+   pick up the changes.
+
+3. *Update Your ``local.conf`` File to Point to Your Source Files:* In
+   addition to your ``local.conf`` file specifying to use
+   "kernel-modules" and the "qemux86" machine, it must also point to the
+   updated kernel source files. Add
+   ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ and
+   ```SRCREV`` <&YOCTO_DOCS_REF_URL;#var-SRCREV>`__ statements similar
+   to the following to your ``local.conf``: $ cd ~/poky/build/conf Add
+   the following to the ``local.conf``: SRC_URI_pn-linux-yocto =
+   "git:///path-to/linux-yocto-4.12;protocol=file;name=machine;branch=standard/base;
+   \\
+   git:///path-to/yocto-kernel-cache;protocol=file;type=kmeta;name=meta;branch=yocto-4.12;destsuffix=${KMETA}"
+   SRCREV_meta_qemux86 = "${AUTOREV}" SRCREV_machine_qemux86 =
+   "${AUTOREV}"
+
+   .. note::
+
+      Be sure to replace
+      path-to
+      with the pathname to your local Git repositories. Also, you must
+      be sure to specify the correct branch and machine types. For this
+      example, the branch is
+      standard/base
+      and the machine is "qemux86".
+
+4. *Build the Image:* With the source modified, your changes staged and
+   committed, and the ``local.conf`` file pointing to the kernel files,
+   you can now use BitBake to build the image: $ cd ~/poky/build $
+   bitbake core-image-minimal
+
+5. *Boot the image*: Boot the modified image in the QEMU emulator using
+   this command. When prompted to login to the QEMU console, use "root"
+   with no password: $ cd ~/poky/build $ runqemu qemux86
+
+6. *Look for Your Changes:* As QEMU booted, you might have seen your
+   changes rapidly scroll by. If not, use these commands to see your
+   changes: # dmesg \| less You should see the results of your
+   ``printk`` statements as part of the output when you scroll down the
+   console window.
+
+7. *Generate the Patch File:* Once you are sure that your patch works
+   correctly, you can generate a ``*.patch`` file in the kernel source
+   repository: $ cd ~/linux-yocto-4.12/init $ git format-patch -1
+   0001-calibrate.c-Added-some-printk-statements.patch
+
+8. *Move the Patch File to Your Layer:* In order for subsequent builds
+   to pick up patches, you need to move the patch file you created in
+   the previous step to your layer ``meta-mylayer``. For this example,
+   the layer created earlier is located in your home directory as
+   ``meta-mylayer``. When the layer was created using the
+   ``yocto-create`` script, no additional hierarchy was created to
+   support patches. Before moving the patch file, you need to add
+   additional structure to your layer using the following commands: $ cd
+   ~/meta-mylayer $ mkdir recipes-kernel $ mkdir recipes-kernel/linux $
+   mkdir recipes-kernel/linux/linux-yocto Once you have created this
+   hierarchy in your layer, you can move the patch file using the
+   following command: $ mv
+   ~/linux-yocto-4.12/init/0001-calibrate.c-Added-some-printk-statements.patch
+   ~/meta-mylayer/recipes-kernel/linux/linux-yocto
+
+9. *Create the Append File:* Finally, you need to create the
+   ``linux-yocto_4.12.bbappend`` file and insert statements that allow
+   the OpenEmbedded build system to find the patch. The append file
+   needs to be in your layer's ``recipes-kernel/linux`` directory and it
+   must be named ``linux-yocto_4.12.bbappend`` and have the following
+   contents: FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
+   SRC_URI_append = "
+   file://0001-calibrate.c-Added-some-printk-statements.patch" The
+   ```FILESEXTRAPATHS`` <&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS>`__
+   and ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ statements
+   enable the OpenEmbedded build system to find the patch file.
+
+   For more information on append files and patches, see the "`Creating
+   the Append File <#creating-the-append-file>`__" and "`Applying
+   Patches <#applying-patches>`__" sections. You can also see the
+   "`Using .bbappend Files in Your
+   Layer" <&YOCTO_DOCS_DEV_URL;#using-bbappend-files>`__" section in the
+   Yocto Project Development Tasks Manual.
+
+   .. note::
+
+      To build
+      core-image-minimal
+      again and see the effects of your patch, you can essentially
+      eliminate the temporary source files saved in
+      poky/build/tmp/work/...
+      and residual effects of the build by entering the following
+      sequence of commands:
+      ::
+
+              $ cd ~/poky/build
+              $ bitbake -c cleanall yocto-linux
+              $ bitbake core-image-minimal -c cleanall
+              $ bitbake core-image-minimal
+              $ runqemu qemux86
+                                 
+
+Configuring the Kernel
+======================
+
+Configuring the Yocto Project kernel consists of making sure the
+``.config`` file has all the right information in it for the image you
+are building. You can use the ``menuconfig`` tool and configuration
+fragments to make sure your ``.config`` file is just how you need it.
+You can also save known configurations in a ``defconfig`` file that the
+build system can use for kernel configuration.
+
+This section describes how to use ``menuconfig``, create and use
+configuration fragments, and how to interactively modify your
+``.config`` file to create the leanest kernel configuration file
+possible.
+
+For more information on kernel configuration, see the "`Changing the
+Configuration <#changing-the-configuration>`__" section.
+
+Using  ``menuconfig``
+---------------------
+
+The easiest way to define kernel configurations is to set them through
+the ``menuconfig`` tool. This tool provides an interactive method with
+which to set kernel configurations. For general information on
+``menuconfig``, see ` <http://en.wikipedia.org/wiki/Menuconfig>`__.
+
+To use the ``menuconfig`` tool in the Yocto Project development
+environment, you must do the following:
+
+-  Because you launch ``menuconfig`` using BitBake, you must be sure to
+   set up your environment by running the
+   ````` <&YOCTO_DOCS_REF_URL;#structure-core-script>`__ script found in
+   the `Build Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__.
+
+-  You must be sure of the state of your build's configuration in the
+   `Source Directory <&YOCTO_DOCS_REF_URL;#source-directory>`__.
+
+-  Your build host must have the following two packages installed:
+   libncurses5-dev libtinfo-dev
+
+The following commands initialize the BitBake environment, run the
+```do_kernel_configme`` <&YOCTO_DOCS_REF_URL;#ref-tasks-kernel_configme>`__
+task, and launch ``menuconfig``. These commands assume the Source
+Directory's top-level folder is ``~/poky``: $ cd poky $ source
+oe-init-build-env $ bitbake linux-yocto -c kernel_configme -f $ bitbake
+linux-yocto -c menuconfig Once ``menuconfig`` comes up, its standard
+interface allows you to interactively examine and configure all the
+kernel configuration parameters. After making your changes, simply exit
+the tool and save your changes to create an updated version of the
+``.config`` configuration file.
+
+.. note::
+
+   You can use the entire
+   .config
+   file as the
+   defconfig
+   file. For information on
+   defconfig
+   files, see the "
+   Changing the Configuration
+   ", "
+   Using an In-Tree
+   defconfig
+   File
+   , and "
+   Creating a
+   defconfig
+   File
+   " sections.
+
+Consider an example that configures the "CONFIG_SMP" setting for the
+``linux-yocto-4.12`` kernel.
+
+.. note::
+
+   The OpenEmbedded build system recognizes this kernel as
+   linux-yocto
+   through Metadata (e.g.
+   PREFERRED_VERSION
+   \_linux-yocto ?= "12.4%"
+   ).
+
+Once ``menuconfig`` launches, use the interface to navigate through the
+selections to find the configuration settings in which you are
+interested. For this example, you deselect "CONFIG_SMP" by clearing the
+"Symmetric Multi-Processing Support" option. Using the interface, you
+can find the option under "Processor Type and Features". To deselect
+"CONFIG_SMP", use the arrow keys to highlight "Symmetric
+Multi-Processing Support" and enter "N" to clear the asterisk. When you
+are finished, exit out and save the change.
+
+Saving the selections updates the ``.config`` configuration file. This
+is the file that the OpenEmbedded build system uses to configure the
+kernel during the build. You can find and examine this file in the Build
+Directory in ``tmp/work/``. The actual ``.config`` is located in the
+area where the specific kernel is built. For example, if you were
+building a Linux Yocto kernel based on the ``linux-yocto-4.12`` kernel
+and you were building a QEMU image targeted for ``x86`` architecture,
+the ``.config`` file would be:
+poky/build/tmp/work/qemux86-poky-linux/linux-yocto/4.12.12+gitAUTOINC+eda4d18...
+...967-r0/linux-qemux86-standard-build/.config
+
+.. note::
+
+   The previous example directory is artificially split and many of the
+   characters in the actual filename are omitted in order to make it
+   more readable. Also, depending on the kernel you are using, the exact
+   pathname might differ.
+
+Within the ``.config`` file, you can see the kernel settings. For
+example, the following entry shows that symmetric multi-processor
+support is not set: # CONFIG_SMP is not set
+
+A good method to isolate changed configurations is to use a combination
+of the ``menuconfig`` tool and simple shell commands. Before changing
+configurations with ``menuconfig``, copy the existing ``.config`` and
+rename it to something else, use ``menuconfig`` to make as many changes
+as you want and save them, then compare the renamed configuration file
+against the newly created file. You can use the resulting differences as
+your base to create configuration fragments to permanently save in your
+kernel layer.
+
+.. note::
+
+   Be sure to make a copy of the
+   .config
+   file and do not just rename it. The build system needs an existing
+   .config
+   file from which to work.
+
+Creating a  ``defconfig`` File
+------------------------------
+
+A ``defconfig`` file in the context of the Yocto Project is often a
+``.config`` file that is copied from a build or a ``defconfig`` taken
+from the kernel tree and moved into recipe space. You can use a
+``defconfig`` file to retain a known set of kernel configurations from
+which the OpenEmbedded build system can draw to create the final
+``.config`` file.
+
+.. note::
+
+   Out-of-the-box, the Yocto Project never ships a
+   defconfig
+   or
+   .config
+   file. The OpenEmbedded build system creates the final
+   .config
+   file used to configure the kernel.
+
+To create a ``defconfig``, start with a complete, working Linux kernel
+``.config`` file. Copy that file to the appropriate
+``${``\ ```PN`` <&YOCTO_DOCS_REF_URL;#var-PN>`__\ ``}`` directory in
+your layer's ``recipes-kernel/linux`` directory, and rename the copied
+file to "defconfig" (e.g.
+``~/meta-mylayer/recipes-kernel/linux/linux-yocto/defconfig``). Then,
+add the following lines to the linux-yocto ``.bbappend`` file in your
+layer: FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:" SRC_URI +=
+"file://defconfig" The
+```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ tells the build
+system how to search for the file, while the
+```FILESEXTRAPATHS`` <&YOCTO_DOCS_REF_URL;#var-FILESEXTRAPATHS>`__
+extends the ```FILESPATH`` <&YOCTO_DOCS_REF_URL;#var-FILESPATH>`__
+variable (search directories) to include the ``${PN}`` directory you
+created to hold the configuration changes.
+
+.. note::
+
+   The build system applies the configurations from the
+   defconfig
+   file before applying any subsequent configuration fragments. The
+   final kernel configuration is a combination of the configurations in
+   the
+   defconfig
+   file and any configuration fragments you provide. You need to realize
+   that if you have any configuration fragments, the build system
+   applies these on top of and after applying the existing defconfig
+   file configurations.
+
+For more information on configuring the kernel, see the "`Changing the
+Configuration <#changing-the-configuration>`__" section.
+
+.. _creating-config-fragments:
+
+Creating Configuration Fragments
+--------------------------------
+
+Configuration fragments are simply kernel options that appear in a file
+placed where the OpenEmbedded build system can find and apply them. The
+build system applies configuration fragments after applying
+configurations from a ``defconfig`` file. Thus, the final kernel
+configuration is a combination of the configurations in the
+``defconfig`` file and then any configuration fragments you provide. The
+build system applies fragments on top of and after applying the existing
+defconfig file configurations.
+
+Syntactically, the configuration statement is identical to what would
+appear in the ``.config`` file, which is in the `Build
+Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__.
+
+.. note::
+
+   For more information about where the
+   .config
+   file is located, see the example in the "
+   Using
+   menuconfig
+   " section.
+
+It is simple to create a configuration fragment. One method is to use
+shell commands. For example, issuing the following from the shell
+creates a configuration fragment file named ``my_smp.cfg`` that enables
+multi-processor support within the kernel: $ echo "CONFIG_SMP=y" >>
+my_smp.cfg
+
+.. note::
+
+   All configuration fragment files must use the
+   .cfg
+   extension in order for the OpenEmbedded build system to recognize
+   them as a configuration fragment.
+
+Another method is to create a configuration fragment using the
+differences between two configuration files: one previously created and
+saved, and one freshly created using the ``menuconfig`` tool.
+
+To create a configuration fragment using this method, follow these
+steps:
+
+1. *Complete a Build Through Kernel Configuration:* Complete a build at
+   least through the kernel configuration task as follows: $ bitbake
+   linux-yocto -c kernel_configme -f This step ensures that you create a
+   ``.config`` file from a known state. Because situations exist where
+   your build state might become unknown, it is best to run this task
+   prior to starting ``menuconfig``.
+
+2. *Launch ``menuconfig``:* Run the ``menuconfig`` command: $ bitbake
+   linux-yocto -c menuconfig
+
+3. *Create the Configuration Fragment:* Run the ``diffconfig`` command
+   to prepare a configuration fragment. The resulting file
+   ``fragment.cfg`` is placed in the
+   ``${``\ ```WORKDIR`` <&YOCTO_DOCS_REF_URL;#var-WORKDIR>`__\ ``}``
+   directory: $ bitbake linux-yocto -c diffconfig
+
+The ``diffconfig`` command creates a file that is a list of Linux kernel
+``CONFIG_`` assignments. See the "`Changing the
+Configuration <#changing-the-configuration>`__" section for additional
+information on how to use the output as a configuration fragment.
+
+.. note::
+
+   You can also use this method to create configuration fragments for a
+   BSP. See the "
+   BSP Descriptions
+   " section for more information.
+
+Where do you put your configuration fragment files? You can place these
+files in an area pointed to by
+```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ as directed by your
+``bblayers.conf`` file, which is located in your layer. The OpenEmbedded
+build system picks up the configuration and adds it to the kernel's
+configuration. For example, suppose you had a set of configuration
+options in a file called ``myconfig.cfg``. If you put that file inside a
+directory named ``linux-yocto`` that resides in the same directory as
+the kernel's append file within your layer and then add the following
+statements to the kernel's append file, those configuration options will
+be picked up and applied when the kernel is built:
+FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:" SRC_URI +=
+"file://myconfig.cfg"
+
+As mentioned earlier, you can group related configurations into multiple
+files and name them all in the ``SRC_URI`` statement as well. For
+example, you could group separate configurations specifically for
+Ethernet and graphics into their own files and add those by using a
+``SRC_URI`` statement like the following in your append file: SRC_URI +=
+"file://myconfig.cfg \\ file://eth.cfg \\ file://gfx.cfg"
+
+Validating Configuration
+------------------------
+
+You can use the
+```do_kernel_configcheck`` <&YOCTO_DOCS_REF_URL;#ref-tasks-kernel_configcheck>`__
+task to provide configuration validation: $ bitbake linux-yocto -c
+kernel_configcheck -f Running this task produces warnings for when a
+requested configuration does not appear in the final ``.config`` file or
+when you override a policy configuration in a hardware configuration
+fragment.
+
+In order to run this task, you must have an existing ``.config`` file.
+See the "`Using ``menuconfig`` <#using-menuconfig>`__" section for
+information on how to create a configuration file.
+
+Following is sample output from the ``do_kernel_configcheck`` task:
+Loading cache: 100%
+\|########################################################\| Time:
+0:00:00 Loaded 1275 entries from dependency cache. NOTE: Resolving any
+missing task queue dependencies Build Configuration: . . . NOTE:
+Executing SetScene Tasks NOTE: Executing RunQueue Tasks WARNING:
+linux-yocto-4.12.12+gitAUTOINC+eda4d18ce4_16de014967-r0
+do_kernel_configcheck: [kernel config]: specified values did not make it
+into the kernel's final configuration: ---------- CONFIG_X86_TSC
+----------------- Config: CONFIG_X86_TSC From:
+/home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/bsp/common-pc/common-pc-cpu.cfg
+Requested value: CONFIG_X86_TSC=y Actual value: ----------
+CONFIG_X86_BIGSMP ----------------- Config: CONFIG_X86_BIGSMP From:
+/home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/cfg/smp.cfg
+/home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/defconfig
+Requested value: # CONFIG_X86_BIGSMP is not set Actual value: ----------
+CONFIG_NR_CPUS ----------------- Config: CONFIG_NR_CPUS From:
+/home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/cfg/smp.cfg
+/home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/bsp/common-pc/common-pc.cfg
+/home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/defconfig
+Requested value: CONFIG_NR_CPUS=8 Actual value: CONFIG_NR_CPUS=1
+---------- CONFIG_SCHED_SMT ----------------- Config: CONFIG_SCHED_SMT
+From:
+/home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/cfg/smp.cfg
+/home/scottrif/poky/build/tmp/work-shared/qemux86/kernel-source/.kernel-meta/configs/standard/defconfig
+Requested value: CONFIG_SCHED_SMT=y Actual value: NOTE: Tasks Summary:
+Attempted 288 tasks of which 285 didn't need to be rerun and all
+succeeded. Summary: There were 3 WARNING messages shown.
+
+.. note::
+
+   The previous output example has artificial line breaks to make it
+   more readable.
+
+The output describes the various problems that you can encounter along
+with where to find the offending configuration items. You can use the
+information in the logs to adjust your configuration files and then
+repeat the
+```do_kernel_configme`` <&YOCTO_DOCS_REF_URL;#ref-tasks-kernel_configme>`__
+and
+```do_kernel_configcheck`` <&YOCTO_DOCS_REF_URL;#ref-tasks-kernel_configcheck>`__
+tasks until they produce no warnings.
+
+For more information on how to use the ``menuconfig`` tool, see the
+"`Using ``menuconfig`` <#using-menuconfig>`__" section.
+
+Fine-Tuning the Kernel Configuration File
+-----------------------------------------
+
+You can make sure the ``.config`` file is as lean or efficient as
+possible by reading the output of the kernel configuration fragment
+audit, noting any issues, making changes to correct the issues, and then
+repeating.
+
+As part of the kernel build process, the ``do_kernel_configcheck`` task
+runs. This task validates the kernel configuration by checking the final
+``.config`` file against the input files. During the check, the task
+produces warning messages for the following issues:
+
+-  Requested options that did not make the final ``.config`` file.
+
+-  Configuration items that appear twice in the same configuration
+   fragment.
+
+-  Configuration items tagged as "required" that were overridden.
+
+-  A board overrides a non-board specific option.
+
+-  Listed options not valid for the kernel being processed. In other
+   words, the option does not appear anywhere.
+
+.. note::
+
+   The
+   do_kernel_configcheck
+   task can also optionally report if an option is overridden during
+   processing.
+
+For each output warning, a message points to the file that contains a
+list of the options and a pointer to the configuration fragment that
+defines them. Collectively, the files are the key to streamlining the
+configuration.
+
+To streamline the configuration, do the following:
+
+1. *Use a Working Configuration:* Start with a full configuration that
+   you know works. Be sure the configuration builds and boots
+   successfully. Use this configuration file as your baseline.
+
+2. *Run Configure and Check Tasks:* Separately run the
+   ``do_kernel_configme`` and ``do_kernel_configcheck`` tasks: $ bitbake
+   linux-yocto -c kernel_configme -f $ bitbake linux-yocto -c
+   kernel_configcheck -f
+
+3. *Process the Results:* Take the resulting list of files from the
+   ``do_kernel_configcheck`` task warnings and do the following:
+
+   -  Drop values that are redefined in the fragment but do not change
+      the final ``.config`` file.
+
+   -  Analyze and potentially drop values from the ``.config`` file that
+      override required configurations.
+
+   -  Analyze and potentially remove non-board specific options.
+
+   -  Remove repeated and invalid options.
+
+4. *Re-Run Configure and Check Tasks:* After you have worked through the
+   output of the kernel configuration audit, you can re-run the
+   ``do_kernel_configme`` and ``do_kernel_configcheck`` tasks to see the
+   results of your changes. If you have more issues, you can deal with
+   them as described in the previous step.
+
+Iteratively working through steps two through four eventually yields a
+minimal, streamlined configuration file. Once you have the best
+``.config``, you can build the Linux Yocto kernel.
+
+Expanding Variables
+===================
+
+Sometimes it is helpful to determine what a variable expands to during a
+build. You can do examine the values of variables by examining the
+output of the ``bitbake -e`` command. The output is long and is more
+easily managed in a text file, which allows for easy searches: $ bitbake
+-e virtual/kernel > some_text_file Within the text file, you can see
+exactly how each variable is expanded and used by the OpenEmbedded build
+system.
+
+Working with a "Dirty" Kernel Version String
+============================================
+
+If you build a kernel image and the version string has a "+" or a
+"-dirty" at the end, uncommitted modifications exist in the kernel's
+source directory. Follow these steps to clean up the version string:
+
+1. *Discover the Uncommitted Changes:* Go to the kernel's locally cloned
+   Git repository (source directory) and use the following Git command
+   to list the files that have been changed, added, or removed: $ git
+   status
+
+2. *Commit the Changes:* You should commit those changes to the kernel
+   source tree regardless of whether or not you will save, export, or
+   use the changes: $ git add $ git commit -s -a -m "getting rid of
+   -dirty"
+
+3. *Rebuild the Kernel Image:* Once you commit the changes, rebuild the
+   kernel.
+
+   Depending on your particular kernel development workflow, the
+   commands you use to rebuild the kernel might differ. For information
+   on building the kernel image when using ``devtool``, see the "`Using
+   ``devtool`` to Patch the
+   Kernel <#using-devtool-to-patch-the-kernel>`__" section. For
+   information on building the kernel image when using Bitbake, see the
+   "`Using Traditional Kernel Development to Patch the
+   Kernel <#using-traditional-kernel-development-to-patch-the-kernel>`__"
+   section.
+
+Working With Your Own Sources
+=============================
+
+If you cannot work with one of the Linux kernel versions supported by
+existing linux-yocto recipes, you can still make use of the Yocto
+Project Linux kernel tooling by working with your own sources. When you
+use your own sources, you will not be able to leverage the existing
+kernel `Metadata <&YOCTO_DOCS_REF_URL;#metadata>`__ and stabilization
+work of the linux-yocto sources. However, you will be able to manage
+your own Metadata in the same format as the linux-yocto sources.
+Maintaining format compatibility facilitates converging with linux-yocto
+on a future, mutually-supported kernel version.
+
+To help you use your own sources, the Yocto Project provides a
+linux-yocto custom recipe (``linux-yocto-custom.bb``) that uses
+``kernel.org`` sources and the Yocto Project Linux kernel tools for
+managing kernel Metadata. You can find this recipe in the ``poky`` Git
+repository of the Yocto Project `Source Repository <&YOCTO_GIT_URL;>`__
+at: poky/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb
+
+Here are some basic steps you can use to work with your own sources:
+
+1. *Create a Copy of the Kernel Recipe:* Copy the
+   ``linux-yocto-custom.bb`` recipe to your layer and give it a
+   meaningful name. The name should include the version of the Yocto
+   Linux kernel you are using (e.g. ``linux-yocto-myproject_4.12.bb``,
+   where "4.12" is the base version of the Linux kernel with which you
+   would be working).
+
+2. *Create a Directory for Your Patches:* In the same directory inside
+   your layer, create a matching directory to store your patches and
+   configuration files (e.g. ``linux-yocto-myproject``).
+
+3. *Ensure You Have Configurations:* Make sure you have either a
+   ``defconfig`` file or configuration fragment files in your layer.
+   When you use the ``linux-yocto-custom.bb`` recipe, you must specify a
+   configuration. If you do not have a ``defconfig`` file, you can run
+   the following: $ make defconfig After running the command, copy the
+   resulting ``.config`` file to the ``files`` directory in your layer
+   as "defconfig" and then add it to the
+   ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ variable in the
+   recipe.
+
+   Running the ``make defconfig`` command results in the default
+   configuration for your architecture as defined by your kernel.
+   However, no guarantee exists that this configuration is valid for
+   your use case, or that your board will even boot. This is
+   particularly true for non-x86 architectures.
+
+   To use non-x86 ``defconfig`` files, you need to be more specific and
+   find one that matches your board (i.e. for arm, you look in
+   ``arch/arm/configs`` and use the one that is the best starting point
+   for your board).
+
+4. *Edit the Recipe:* Edit the following variables in your recipe as
+   appropriate for your project:
+
+   -  ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__: The
+      ``SRC_URI`` should specify a Git repository that uses one of the
+      supported Git fetcher protocols (i.e. ``file``, ``git``, ``http``,
+      and so forth). The ``SRC_URI`` variable should also specify either
+      a ``defconfig`` file or some configuration fragment files. The
+      skeleton recipe provides an example ``SRC_URI`` as a syntax
+      reference.
+
+   -  ```LINUX_VERSION`` <&YOCTO_DOCS_REF_URL;#var-LINUX_VERSION>`__:
+      The Linux kernel version you are using (e.g. "4.12").
+
+   -  ```LINUX_VERSION_EXTENSION`` <&YOCTO_DOCS_REF_URL;#var-LINUX_VERSION_EXTENSION>`__:
+      The Linux kernel ``CONFIG_LOCALVERSION`` that is compiled into the
+      resulting kernel and visible through the ``uname`` command.
+
+   -  ```SRCREV`` <&YOCTO_DOCS_REF_URL;#var-SRCREV>`__: The commit ID
+      from which you want to build.
+
+   -  ```PR`` <&YOCTO_DOCS_REF_URL;#var-PR>`__: Treat this variable the
+      same as you would in any other recipe. Increment the variable to
+      indicate to the OpenEmbedded build system that the recipe has
+      changed.
+
+   -  ```PV`` <&YOCTO_DOCS_REF_URL;#var-PV>`__: The default ``PV``
+      assignment is typically adequate. It combines the
+      ``LINUX_VERSION`` with the Source Control Manager (SCM) revision
+      as derived from the ```SRCPV`` <&YOCTO_DOCS_REF_URL;#var-SRCPV>`__
+      variable. The combined results are a string with the following
+      form:
+      3.19.11+git1+68a635bf8dfb64b02263c1ac80c948647cc76d5f_1+218bd8d2022b9852c60d32f0d770931e3cf343e2
+      While lengthy, the extra verbosity in ``PV`` helps ensure you are
+      using the exact sources from which you intend to build.
+
+   -  ```COMPATIBLE_MACHINE`` <&YOCTO_DOCS_REF_URL;#var-COMPATIBLE_MACHINE>`__:
+      A list of the machines supported by your new recipe. This variable
+      in the example recipe is set by default to a regular expression
+      that matches only the empty string, "(^$)". This default setting
+      triggers an explicit build failure. You must change it to match a
+      list of the machines that your new recipe supports. For example,
+      to support the ``qemux86`` and ``qemux86-64`` machines, use the
+      following form: COMPATIBLE_MACHINE = "qemux86|qemux86-64"
+
+5. *Customize Your Recipe as Needed:* Provide further customizations to
+   your recipe as needed just as you would customize an existing
+   linux-yocto recipe. See the "`Modifying an Existing
+   Recipe <#modifying-an-existing-recipe>`__" section for information.
+
+Working with Out-of-Tree Modules
+================================
+
+This section describes steps to build out-of-tree modules on your target
+and describes how to incorporate out-of-tree modules in the build.
+
+Building Out-of-Tree Modules on the Target
+------------------------------------------
+
+While the traditional Yocto Project development model would be to
+include kernel modules as part of the normal build process, you might
+find it useful to build modules on the target. This could be the case if
+your target system is capable and powerful enough to handle the
+necessary compilation. Before deciding to build on your target, however,
+you should consider the benefits of using a proper cross-development
+environment from your build host.
+
+If you want to be able to build out-of-tree modules on the target, there
+are some steps you need to take on the target that is running your SDK
+image. Briefly, the ``kernel-dev`` package is installed by default on
+all ``*.sdk`` images and the ``kernel-devsrc`` package is installed on
+many of the ``*.sdk`` images. However, you need to create some scripts
+prior to attempting to build the out-of-tree modules on the target that
+is running that image.
+
+Prior to attempting to build the out-of-tree modules, you need to be on
+the target as root and you need to change to the ``/usr/src/kernel``
+directory. Next, ``make`` the scripts: # cd /usr/src/kernel # make
+scripts Because all SDK image recipes include ``dev-pkgs``, the
+``kernel-dev`` packages will be installed as part of the SDK image and
+the ``kernel-devsrc`` packages will be installed as part of applicable
+SDK images. The SDK uses the scripts when building out-of-tree modules.
+Once you have switched to that directory and created the scripts, you
+should be able to build your out-of-tree modules on the target.
+
+Incorporating Out-of-Tree Modules
+---------------------------------
+
+While it is always preferable to work with sources integrated into the
+Linux kernel sources, if you need an external kernel module, the
+``hello-mod.bb`` recipe is available as a template from which you can
+create your own out-of-tree Linux kernel module recipe.
+
+This template recipe is located in the ``poky`` Git repository of the
+Yocto Project `Source Repository <&YOCTO_GIT_URL;>`__ at:
+poky/meta-skeleton/recipes-kernel/hello-mod/hello-mod_0.1.bb
+
+To get started, copy this recipe to your layer and give it a meaningful
+name (e.g. ``mymodule_1.0.bb``). In the same directory, create a new
+directory named ``files`` where you can store any source files, patches,
+or other files necessary for building the module that do not come with
+the sources. Finally, update the recipe as needed for the module.
+Typically, you will need to set the following variables:
+
+-  ```DESCRIPTION`` <&YOCTO_DOCS_REF_URL;#var-DESCRIPTION>`__
+
+-  ```LICENSE*`` <&YOCTO_DOCS_REF_URL;#var-LICENSE>`__
+
+-  ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__
+
+-  ```PV`` <&YOCTO_DOCS_REF_URL;#var-PV>`__
+
+Depending on the build system used by the module sources, you might need
+to make some adjustments. For example, a typical module ``Makefile``
+looks much like the one provided with the ``hello-mod`` template: obj-m
+:= hello.o SRC := $(shell pwd) all: $(MAKE) -C $(KERNEL_SRC) M=$(SRC)
+modules_install: $(MAKE) -C $(KERNEL_SRC) M=$(SRC) modules_install ...
+
+The important point to note here is the
+```KERNEL_SRC`` <&YOCTO_DOCS_REF_URL;#var-KERNEL_SRC>`__ variable. The
+```module`` <&YOCTO_DOCS_REF_URL;#ref-classes-module>`__ class sets this
+variable and the
+```KERNEL_PATH`` <&YOCTO_DOCS_REF_URL;#var-KERNEL_PATH>`__ variable to
+``${STAGING_KERNEL_DIR}`` with the necessary Linux kernel build
+information to build modules. If your module ``Makefile`` uses a
+different variable, you might want to override the
+```do_compile`` <&YOCTO_DOCS_REF_URL;#ref-tasks-compile>`__ step, or
+create a patch to the ``Makefile`` to work with the more typical
+``KERNEL_SRC`` or ``KERNEL_PATH`` variables.
+
+After you have prepared your recipe, you will likely want to include the
+module in your images. To do this, see the documentation for the
+following variables in the Yocto Project Reference Manual and set one of
+them appropriately for your machine configuration file:
+
+-  ```MACHINE_ESSENTIAL_EXTRA_RDEPENDS`` <&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RDEPENDS>`__
+
+-  ```MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS`` <&YOCTO_DOCS_REF_URL;#var-MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS>`__
+
+-  ```MACHINE_EXTRA_RDEPENDS`` <&YOCTO_DOCS_REF_URL;#var-MACHINE_EXTRA_RDEPENDS>`__
+
+-  ```MACHINE_EXTRA_RRECOMMENDS`` <&YOCTO_DOCS_REF_URL;#var-MACHINE_EXTRA_RRECOMMENDS>`__
+
+Modules are often not required for boot and can be excluded from certain
+build configurations. The following allows for the most flexibility:
+MACHINE_EXTRA_RRECOMMENDS += "kernel-module-mymodule" The value is
+derived by appending the module filename without the ``.ko`` extension
+to the string "kernel-module-".
+
+Because the variable is
+```RRECOMMENDS`` <&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS>`__ and not a
+```RDEPENDS`` <&YOCTO_DOCS_REF_URL;#var-RDEPENDS>`__ variable, the build
+will not fail if this module is not available to include in the image.
+
+Inspecting Changes and Commits
+==============================
+
+A common question when working with a kernel is: "What changes have been
+applied to this tree?" Rather than using "grep" across directories to
+see what has changed, you can use Git to inspect or search the kernel
+tree. Using Git is an efficient way to see what has changed in the tree.
+
+What Changed in a Kernel?
+-------------------------
+
+Following are a few examples that show how to use Git commands to
+examine changes. These examples are by no means the only way to see
+changes.
+
+.. note::
+
+   In the following examples, unless you provide a commit range,
+   kernel.org
+   history is blended with Yocto Project kernel changes. You can form
+   ranges by using branch names from the kernel tree as the upper and
+   lower commit markers with the Git commands. You can see the branch
+   names through the web interface to the Yocto Project source
+   repositories at
+   .
+
+To see a full range of the changes, use the ``git whatchanged`` command
+and specify a commit range for the branch (commit\ ``..``\ commit).
+
+Here is an example that looks at what has changed in the ``emenlow``
+branch of the ``linux-yocto-3.19`` kernel. The lower commit range is the
+commit associated with the ``standard/base`` branch, while the upper
+commit range is the commit associated with the ``standard/emenlow``
+branch. $ git whatchanged origin/standard/base..origin/standard/emenlow
+
+To see short, one line summaries of changes use the ``git log`` command:
+$ git log --oneline origin/standard/base..origin/standard/emenlow
+
+Use this command to see code differences for the changes: $ git diff
+origin/standard/base..origin/standard/emenlow
+
+Use this command to see the commit log messages and the text
+differences: $ git show origin/standard/base..origin/standard/emenlow
+
+Use this command to create individual patches for each change. Here is
+an example that that creates patch files for each commit and places them
+in your ``Documents`` directory: $ git format-patch -o $HOME/Documents
+origin/standard/base..origin/standard/emenlow
+
+Showing a Particular Feature or Branch Change
+---------------------------------------------
+
+Tags in the Yocto Project kernel tree divide changes for significant
+features or branches. The ``git show`` tag command shows changes based
+on a tag. Here is an example that shows ``systemtap`` changes: $ git
+show systemtap You can use the ``git branch --contains`` tag command to
+show the branches that contain a particular feature. This command shows
+the branches that contain the ``systemtap`` feature: $ git branch
+--contains systemtap
+
+Adding Recipe-Space Kernel Features
+===================================
+
+You can add kernel features in the
+`recipe-space <#recipe-space-metadata>`__ by using the
+```KERNEL_FEATURES`` <&YOCTO_DOCS_REF_URL;#var-KERNEL_FEATURES>`__
+variable and by specifying the feature's ``.scc`` file path in the
+```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ statement. When you
+add features using this method, the OpenEmbedded build system checks to
+be sure the features are present. If the features are not present, the
+build stops. Kernel features are the last elements processed for
+configuring and patching the kernel. Therefore, adding features in this
+manner is a way to enforce specific features are present and enabled
+without needing to do a full audit of any other layer's additions to the
+``SRC_URI`` statement.
+
+You add a kernel feature by providing the feature as part of the
+``KERNEL_FEATURES`` variable and by providing the path to the feature's
+``.scc`` file, which is relative to the root of the kernel Metadata. The
+OpenEmbedded build system searches all forms of kernel Metadata on the
+``SRC_URI`` statement regardless of whether the Metadata is in the
+"kernel-cache", system kernel Metadata, or a recipe-space Metadata (i.e.
+part of the kernel recipe). See the "`Kernel Metadata
+Location <#kernel-metadata-location>`__" section for additional
+information.
+
+When you specify the feature's ``.scc`` file on the ``SRC_URI``
+statement, the OpenEmbedded build system adds the directory of that
+``.scc`` file along with all its subdirectories to the kernel feature
+search path. Because subdirectories are searched, you can reference a
+single ``.scc`` file in the ``SRC_URI`` statement to reference multiple
+kernel features.
+
+Consider the following example that adds the "test.scc" feature to the
+build.
+
+1. *Create the Feature File:* Create a ``.scc`` file and locate it just
+   as you would any other patch file, ``.cfg`` file, or fetcher item you
+   specify in the ``SRC_URI`` statement.
+
+   .. note::
+
+      -  You must add the directory of the ``.scc`` file to the
+         fetcher's search path in the same manner as you would add a
+         ``.patch`` file.
+
+      -  You can create additional ``.scc`` files beneath the directory
+         that contains the file you are adding. All subdirectories are
+         searched during the build as potential feature directories.
+
+   Continuing with the example, suppose the "test.scc" feature you are
+   adding has a ``test.scc`` file in the following directory: my_recipe
+   \| +-linux-yocto \| +-test.cfg +-test.scc In this example, the
+   ``linux-yocto`` directory has both the feature ``test.scc`` file and
+   a similarly named configuration fragment file ``test.cfg``.
+
+2. *Add the Feature File to ``SRC_URI``:* Add the ``.scc`` file to the
+   recipe's ``SRC_URI`` statement: SRC_URI_append = " file://test.scc"
+   The leading space before the path is important as the path is
+   appended to the existing path.
+
+3. *Specify the Feature as a Kernel Feature:* Use the
+   ``KERNEL_FEATURES`` statement to specify the feature as a kernel
+   feature: KERNEL_FEATURES_append = " test.scc" The OpenEmbedded build
+   system processes the kernel feature when it builds the kernel.
+
+   .. note::
+
+      If other features are contained below "test.scc", then their
+      directories are relative to the directory containing the
+      test.scc
+      file.