Yocto Project Linux Kernel Development Manual

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" section:

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" section.

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" section:

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" section.

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 search path by prepending the directory that contains your files to the FILESEXTRAPATHS variable as follows:

     FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
                

The path ${THISDIR}/${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/
                

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 variable and uses the 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 variable to ensure the build process uses the appropriate kernel branch.

Although this particular example does not use it, the KERNEL_FEATURES variable could be used to enable features specific to the kernel. The append file points to specific commits in the 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 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 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.

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 in your .bbappend file as described in the previous section, and then reference them in 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" and "Using Traditional Kernel Development to Patch the Kernel" sections.

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 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 extends the FILESPATH variable (search directories) to include the ${PN} directory you created to hold the configuration changes.

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" section.

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 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 variable description for more information.

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:

The following commands initialize the BitBake environment, run the do_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.

Consider an example that configures the "CONFIG_SMP" setting for the linux-yocto-4.12 kernel.

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
                

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.

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.

To create a defconfig, start with a complete, working Linux kernel .config file. Copy that file to the appropriate ${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 tells the build system how to search for the file, while the FILESEXTRAPATHS extends the FILESPATH variable (search directories) to include the ${PN} directory you created to hold the configuration changes.

For more information on configuring the kernel, see the "Changing the Configuration" section.

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.

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
                

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:

The diffconfig command creates a file that is a list of Linux kernel CONFIG_ assignments. See the "Changing the Configuration" section for additional information on how to use the output as a configuration fragment.

Where do you put your configuration fragment files? You can place these files in an area pointed to by 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"
                

You can use the do_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" 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.
                

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 and do_kernel_configcheck tasks until they produce no warnings.

For more information on how to use the menuconfig tool, see the "Using menuconfig" section.

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:

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:

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.

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.

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 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:

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 variable. The module class sets this variable and the 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 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:

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 and not a RDEPENDS variable, the build will not fail if this module is not available to include in the image.

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.

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
                

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
                

The simplest unit of kernel Metadata is the configuration-only feature. This feature consists of one or more Linux kernel configuration parameters in a configuration fragment file (.cfg) and a .scc file that describes the fragment.

As an example, consider the Symmetric Multi-Processing (SMP) fragment used with the linux-yocto-4.12 kernel as defined outside of the recipe space (i.e. yocto-kernel-cache). This Metadata consists of two files: smp.scc and smp.cfg. You can find these files in the cfg directory of the yocto-4.12 branch in the yocto-kernel-cache Git repository:

     cfg/smp.scc:
        define KFEATURE_DESCRIPTION "Enable SMP for 32 bit builds"
        define KFEATURE_COMPATIBILITY all

        kconf hardware smp.cfg

     cfg/smp.cfg:
        CONFIG_SMP=y
        CONFIG_SCHED_SMT=y
        # Increase default NR_CPUS from 8 to 64 so that platform with
        # more than 8 processors can be all activated at boot time
        CONFIG_NR_CPUS=64
        # The following is needed when setting NR_CPUS to something
        # greater than 8 on x86 architectures, it should be automatically
        # disregarded by Kconfig when using a different arch
        CONFIG_X86_BIGSMP=y
            

You can find general information on configuration fragment files in the "Creating Configuration Fragments" section.

Within the smp.scc file, the KFEATURE_DESCRIPTION statement provides a short description of the fragment. Higher level kernel tools use this description.

Also within the smp.scc file, the kconf command includes the actual configuration fragment in an .scc file, and the "hardware" keyword identifies the fragment as being hardware enabling, as opposed to general policy, which would use the "non-hardware" keyword. The distinction is made for the benefit of the configuration validation tools, which warn you if a hardware fragment overrides a policy set by a non-hardware fragment.

As described in the "Validating Configuration" section, you can use the following BitBake command to audit your configuration:

     $ bitbake linux-yocto -c kernel_configcheck -f
            

Patch descriptions are very similar to configuration fragment descriptions, which are described in the previous section. However, instead of a .cfg file, these descriptions work with source patches (i.e. .patch files).

A typical patch includes a description file and the patch itself. As an example, consider the build patches used with the linux-yocto-4.12 kernel as defined outside of the recipe space (i.e. yocto-kernel-cache). This Metadata consists of several files: build.scc and a set of *.patch files. You can find these files in the patches/build directory of the yocto-4.12 branch in the yocto-kernel-cache Git repository.

The following listings show the build.scc file and part of the modpost-mask-trivial-warnings.patch file:

     patches/build/build.scc:
        patch arm-serialize-build-targets.patch
        patch powerpc-serialize-image-targets.patch
        patch kbuild-exclude-meta-directory-from-distclean-processi.patch

        # applied by kgit
        # patch kbuild-add-meta-files-to-the-ignore-li.patch

        patch modpost-mask-trivial-warnings.patch
        patch menuconfig-check-lxdiaglog.sh-Allow-specification-of.patch

     patches/build/modpost-mask-trivial-warnings.patch:
        From bd48931bc142bdd104668f3a062a1f22600aae61 Mon Sep 17 00:00:00 2001
        From: Paul Gortmaker <paul.gortmaker@windriver.com>
        Date: Sun, 25 Jan 2009 17:58:09 -0500
        Subject: [PATCH] modpost: mask trivial warnings

        Newer HOSTCC will complain about various stdio fcns because
                          .
                          .
                          .
 	        char *dump_write = NULL, *files_source = NULL;
 	        int opt;
        --
        2.10.1

        generated by cgit v0.10.2 at 2017-09-28 15:23:23 (GMT)
            

The description file can include multiple patch statements where each statement handles a single patch. In the example build.scc file, five patch statements exist for the five patches in the directory.

You can create a typical .patch file using diff -Nurp or git format-patch commands. For information on how to create patches, see the "Using devtool to Patch the Kernel" and "Using Traditional Kernel Development to Patch the Kernel" sections.

Features are complex kernel Metadata types that consist of configuration fragments, patches, and possibly other feature description files. As an example, consider the following generic listing:

     features/myfeature.scc
        define KFEATURE_DESCRIPTION "Enable myfeature"

        patch 0001-myfeature-core.patch
        patch 0002-myfeature-interface.patch

        include cfg/myfeature_dependency.scc
        kconf non-hardware myfeature.cfg
            

This example shows how the patch and kconf commands are used as well as how an additional feature description file is included with the include command.

Typically, features are less granular than configuration fragments and are more likely than configuration fragments and patches to be the types of things you want to specify in the KERNEL_FEATURES variable of the Linux kernel recipe. See the "Using Kernel Metadata in a Recipe" section earlier in the manual.

A kernel type defines a high-level kernel policy by aggregating non-hardware configuration fragments with patches you want to use when building a Linux kernel of a specific type (e.g. a real-time kernel). Syntactically, kernel types are no different than features as described in the "Features" section. The LINUX_KERNEL_TYPE variable in the kernel recipe selects the kernel type. For example, in the linux-yocto_4.12.bb kernel recipe found in poky/meta/recipes-kernel/linux, a require directive includes the poky/meta/recipes-kernel/linux/linux-yocto.inc file, which has the following statement that defines the default kernel type:

     LINUX_KERNEL_TYPE ??= "standard"
            

Another example would be the real-time kernel (i.e. linux-yocto-rt_4.12.bb). This kernel recipe directly sets the kernel type as follows:

     LINUX_KERNEL_TYPE = "preempt-rt"
            

Three kernel types ("standard", "tiny", and "preempt-rt") are supported for Linux Yocto kernels:

For any given kernel type, the Metadata is defined by the .scc (e.g. standard.scc). Here is a partial listing for the standard.scc file, which is found in the ktypes/standard directory of the yocto-kernel-cache Git repository:

     # Include this kernel type fragment to get the standard features and
     # configuration values.

     # Note: if only the features are desired, but not the configuration
     #       then this should be included as:
     #             include ktypes/standard/standard.scc nocfg
     #       if no chained configuration is desired, include it as:
     #             include ktypes/standard/standard.scc nocfg inherit



     include ktypes/base/base.scc
     branch standard

     kconf non-hardware standard.cfg

     include features/kgdb/kgdb.scc
                .
                .
                .

     include cfg/net/ip6_nf.scc
     include cfg/net/bridge.scc

     include cfg/systemd.scc

     include features/rfkill/rfkill.scc
            

As with any .scc file, a kernel type definition can aggregate other .scc files with include commands. These definitions can also directly pull in configuration fragments and patches with the kconf and patch commands, respectively.

BSP descriptions (i.e. *.scc files) combine kernel types with hardware-specific features. The hardware-specific Metadata is typically defined independently in the BSP layer, and then aggregated with each supported kernel type.

This section overviews the BSP description structure, the aggregation concepts, and presents a detailed example using a BSP supported by the Yocto Project (i.e. BeagleBone Board). For complete information on BSP layer file hierarchy, see the Yocto Project Board Support Package (BSP) Developer's Guide.

     bsp_root_name-kernel_type.scc
                

     beaglebone-standard.scc
     beaglebone-preempt-rt.scc
                

     define KMACHINE beaglebone
     define KTYPE standard
     define KARCH arm

     include ktypes/standard/standard.scc
     branch beaglebone

     include beaglebone.scc

     # default policy for standard kernels
     include features/latencytop/latencytop.scc
     include features/profiling/profiling.scc
                

     include ktypes/standard/standard.scc
                

     include mybsp.scc
                

     include beaglebone.scc
                

     kconf hardware mybsp-extra.cfg
                

     define KMACHINE mti-malta32-le
     define KMACHINE qemumipsel
     define KTYPE standard
     define KARCH mips

     include ktypes/standard/standard.scc
     branch mti-malta32

     include mti-malta32.scc
     kconf hardware mti-malta32-le.cfg
                

         include cfg/x86.scc
         include features/eg20t/eg20t.scc
         include cfg/dmaengine.scc
         include features/power/intel.scc
         include cfg/efi.scc
         include features/usb/ehci-hcd.scc
         include features/usb/ohci-hcd.scc
         include features/usb/usb-gadgets.scc
         include features/usb/touchscreen-composite.scc
         include cfg/timer/hpet.scc
         include features/leds/leds.scc
         include features/spi/spidev.scc
         include features/i2c/i2cdev.scc
         include features/mei/mei-txe.scc

         # Earlyprintk and port debug requires 8250
         kconf hardware cfg/8250.cfg

         kconf hardware minnow.cfg
         kconf hardware minnow-dev.cfg
                

         define KMACHINE minnow
         define KTYPE standard
         define KARCH i386

         include ktypes/standard

         include minnow.scc

         # Extra minnow configs above the minimal defined in minnow.scc
         include cfg/efi-ext.scc
         include features/media/media-all.scc
         include features/sound/snd_hda_intel.scc

         # The following should really be in standard.scc
         # USB live-image support
         include cfg/usb-mass-storage.scc
         include cfg/boot-live.scc

         # Basic profiling
         include features/latencytop/latencytop.scc
         include features/profiling/profiling.scc

         # Requested drivers that don't have an existing scc
         kconf hardware minnow-drivers-extra.cfg
                

        define KMACHINE minnow
        define KTYPE tiny
        define KARCH i386

        include ktypes/tiny

        include minnow.scc
                

When stored in recipe-space, the kernel Metadata files reside in a directory hierarchy below FILESEXTRAPATHS. For a linux-yocto recipe or for a Linux kernel recipe derived by copying and modifying oe-core/meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb to a recipe in your layer, FILESEXTRAPATHS is typically set to ${THISDIR}/${PN}. See the "Modifying an Existing Recipe" section for more information.

Here is an example that shows a trivial tree of kernel Metadata stored in recipe-space within a BSP layer:

     meta-my_bsp_layer/
     `-- recipes-kernel
         `-- linux
             `-- linux-yocto
                 |-- bsp-standard.scc
                 |-- bsp.cfg
                 `-- standard.cfg
            

When the Metadata is stored in recipe-space, you must take steps to ensure BitBake has the necessary information to decide what files to fetch and when they need to be fetched again. It is only necessary to specify the .scc files on the SRC_URI. BitBake parses them and fetches any files referenced in the .scc files by the include, patch, or kconf commands. Because of this, it is necessary to bump the recipe PR value when changing the content of files not explicitly listed in the SRC_URI.

If the BSP description is in recipe space, you cannot simply list the *.scc in the SRC_URI statement. You need to use the following form from your kernel append file:

     SRC_URI_append_myplatform = " \
        file://myplatform;type=kmeta;destsuffix=myplatform \
        "
            

When stored outside of the recipe-space, the kernel Metadata files reside in a separate repository. The OpenEmbedded build system adds the Metadata to the build as a "type=kmeta" repository through the SRC_URI variable. As an example, consider the following SRC_URI statement from the linux-yocto_4.12.bb kernel recipe:

     SRC_URI = "git://git.yoctoproject.org/linux-yocto-4.12.git;name=machine;branch=${KBRANCH}; \
                git://git.yoctoproject.org/yocto-kernel-cache;type=kmeta;name=meta;branch=yocto-4.12;destsuffix=${KMETA}"
            

${KMETA}, in this context, is simply used to name the directory into which the Git fetcher places the Metadata. This behavior is no different than any multi-repository SRC_URI statement used in a recipe (e.g. see the previous section).

You can keep kernel Metadata in a "kernel-cache", which is a directory containing configuration fragments. As with any Metadata kept outside the recipe-space, you simply need to use the SRC_URI statement with the "type=kmeta" attribute. Doing so makes the kernel Metadata available during the configuration phase.

If you modify the Metadata, you must not forget to update the SRCREV statements in the kernel's recipe. In particular, you need to update the SRCREV_meta variable to match the commit in the KMETA branch you wish to use. Changing the data in these branches and not updating the SRCREV statements to match will cause the build to fetch an older commit.

if you are reusing patches from an external tree and are not working on the patches, you might find the encapsulated feature to be appropriate. Given this scenario, you do not need to create any branches in the source repository. Rather, you just take the static patches you need and encapsulate them within a feature description. Once you have the feature description, you simply include that into the BSP description as described in the "BSP Descriptions" section.

You can find information on how to create patches and BSP descriptions in the "Patches" and "BSP Descriptions" sections.

When you have multiple machines and architectures to support, or you are actively working on board support, it is more efficient to create branches in the repository based on individual machines. Having machine branches allows common source to remain in the "master" branch with any features specific to a machine stored in the appropriate machine branch. This organization method frees you from continually reintegrating your patches into a feature.

Once you have a new branch, you can set up your kernel Metadata to use the branch a couple different ways. In the recipe, you can specify the new branch as the KBRANCH to use for the board as follows:

     KBRANCH = "mynewbranch"
            

Another method is to use the branch command in the BSP description:

     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386
        include standard.scc

        branch mynewbranch

        include mybsp-hw.scc
            

If you find yourself with numerous branches, you might consider using a hierarchical branching system similar to what the Yocto Linux Kernel Git repositories use:

     common/kernel_type/machine
            

If you had two kernel types, "standard" and "small" for instance, three machines, and common as mydir, the branches in your Git repository might look like this:

     mydir/base
     mydir/standard/base
     mydir/standard/machine_a
     mydir/standard/machine_b
     mydir/standard/machine_c
     mydir/small/base
     mydir/small/machine_a
            

This organization can help clarify the branch relationships. In this case, mydir/standard/machine_a includes everything in mydir/base and mydir/standard/base. The "standard" and "small" branches add sources specific to those kernel types that for whatever reason are not appropriate for the other branches.

When you are actively developing new features, it can be more efficient to work with that feature as a branch, rather than as a set of patches that have to be regularly updated. The Yocto Project Linux kernel tools provide for this with the git merge command.

To merge a feature branch into a BSP, insert the git merge command after any branch commands:

     mybsp.scc:
        define KMACHINE mybsp
        define KTYPE standard
        define KARCH i386
        include standard.scc

        branch mynewbranch
        git merge myfeature

        include mybsp-hw.scc