%poky; ] > This chapter describes common tasks you need to do when working on the Yocto Project Linux kernel.

Customizing recipes is best done in a layer with bbappend files. Layers also provide a convenient mechanism to create your own recipes. This guide assumes you will be working from within a layer independent from those released with the Yocto Project. For details on how to create and work with layers, refer to section 5.1 Understanding and Creating Layers in the Yocto Project Development Manual. (Kernel specific directions in 5.7.4)

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 to choose from. To get started, prepare a layer and a bbappend file corresponding to the recipe you wish to modify. See [New example in Dev Manual] for instructions to create a minimal layer. The bbappend will typical be located at the following location relative to the layer (if modifying the linux-yocto_3.4.bb recipe): recipes-kernel/linux/linux-yocto_3.4.bbappend And should contain the following text initially: FILESEXTRAPATHS := "${THISDIR}/${PN}" The ${PN} will expand to "linux-yocto" in this example. Any new files added to modify the recipe should then be added to the following path within the layer: recipes-kernel/linux/linux-yocto/ NOTE: If you are working on a new machine BSP, be sure to refer to the Yocto Project Board Support Package Developer's Guide.

If you have a patch, or a small series of patches, 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 the bbappend file as described in 2.2 and then reference them in the SRC_URI. 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-first-change.patch" SRC_URI += "file://0003-first-change.patch" At the next build, bitbake will detect the change in the recipe and fetch and apply the patches before rebuilding the Linux kernel.

Making wholesale or incremental changes to the Linux kernel config can be made by including a defconfig or configuration fragments in the SRC_URI. If you have a complete Linux kernel .config file you want to use, copy it as "defconfig" to the ${FILES} directory and add the following line to your linux-yocto bbappend file in your layer: SRC_URI += "file://defconfig" Generally speaking, the preferred approach is to determine the incremental change you want to make and add that as a fragment. For example, if you wanted to add support for a basic serial console, create a file named "8250.cfg" in the ${FILES} 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 Then include this configuration fragment in the SRC_URI: SRC_URI += "file://8250.cfg" At the next build, bitbake will detect the change in the recipe and fetch and apply the new configuration before rebuilding the Linux kernel.

If you do not have existing patches or configuration files, you can easily generate them from within the bitbake build environment, as will be described below. As you do, running previously completed bitbake tasks will cause bitbake to invalidate the tasks that follow them in the build sequence, causing them to rebuild at the next invocation of the build. Throughout this section, be sure to substitute "linux-yocto" with the name of the Linux kernel recipe you are working with.

You can manipulate the config used to build a linux-yocto recipe with the "menuconfig" command. $ bitbake linux-yocto -c menuconfig This will start the Linux kernel configuration tool, allowing you to prepare a new .config for the build. When you exit, be sure to save the changes when prompted. The resulting .config file will be located in the ${WORKDIR} under the linux-${MACHINE}-${KTYPE}-build directory. You can use this in its entirety as the defconfig file described in 2.2.2. Better still, would be to use the "menuconfig" command and take the difference of the new .config file with the previous one to generate a configuration fragment. To do this, be sure to complete a build at least through the kernel configuration task: $ bitbake linux-yocto -c kernel_configme -f Then make a copy of the .config file, calling it "config.orig", for example. Run the "menuconfig" command as described above. Finally, prepare a configuration fragment from the difference between the files. Ultimately you want a list of Linux kernel CONFIG_ assignments, and not something in diff format. Something like the following will do that automatically (but plan on reviewing the output as you can usually remove some of the defaults): $ diff -Nurp config.orig .config | sed -n "s/^\+//p" > frag.cfg You can use the output as a configuration fragment described in 2.2.2. This method can also be used to define a BSP configuration fragment (See 3.3.5). The Yocto Project kernel tools provide some configuration validation tools which will warn when a configuration you requested does not appear in the final config, or when you override a policy configuration in a hardware configuration fragment. You can run these tools with the following command: $ bitbake linux-yocto -c kernel_configcheck -f ... NOTE: validating kernel configuration This BSP sets 3 invalid/obsolete kernel options. These config options are not offered anywhere within this kernel. The full list can be found in your kernel src dir at: meta/cfg/standard/mybsp/invalid.cfg This BSP sets 21 kernel options that are possibly non-hardware related. The full list can be found in your kernel src dir at: meta/cfg/standard/mybsp/specified_non_hdw.cfg WARNING: There were 2 hardware options requested that do not have a corresponding value present in the final ".config" file. This probably means you aren't getting the config you wanted. The full list can be found in your kernel src dir at: meta/cfg/standard/mybsp/mismatch.cfg The various problems that you can encounter are described in the output along with where to find the offending configuration items. You can use these logs to adjust your configuration files and repeat the "kernel_configme" and "kernel_configcheck" commands until no warnings are produced.

You can experiment with source code changes and create a simple patch without leaving the bitbake environment. To get started, be sure to complete a build at least through the kernel configuration task: $ bitbake linux-yocto -c kernel_configme -f This step will ensure you have the sources prepared and the configuration completed. You will find the sources in the ${WORKDIR}/linux directory. You can edit the sources as you would any other Linux source tree, but keep in mind that your changes will be lost if you trigger the fetch task for the recipe. Avoid this by not issuing the "cleanall" or "cleansstate", or forcing the "fetch" command. Also be sure not to modify the recipe itself while working with temporary changes or bitbake may run the fetch command (depending on the changes to the recipe). To test your temporary changes, instruct bitbake to run the compile again. The -f option forces the command to run again even though bitbake may think it has already done so: $ bitbake linux-yocto -c compile -f If the compile fails, you can update the sources and repeat the compile command. Once it compiles successfully, you can inspect and test the resulting build (kernel, modules, etc.) from the build directory at ${WORKDIR}/linux-${MACHINE}-${KTYPE}-build. Alternatively, you can run the deploy command to place the kernel image in the tmp/deploy/images directory: $ bitbake linux-yocto -c deploy And of course, you can run through all the remaining installation and packaging steps by issuing: $ bitbake linux-yocto For rapid iterative development, the edit-compile-repeat loop is preferable to rebuilding the entire recipe as the installation and packaging tasks are very time consuming. Once you are happy with your modifications, you can make these permanent by generating patches and applying them to the SRC_URI as described in section 2.2.1 Applying Patches. If you are not familiar with generating patches, refer to the Yocto Project Development Manual, section 5.7.3 Creating the Patch.

If you find yourself unable to 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 while working with your own sources. You will not be able to leverage the existing meta-data and stabilization work of the linux-yocto sources, but you will be able to manage your own meta-data in the same format as the linux-yocto sources which will facilitate converging with linux-yocto on a future mutually-supported kernel version. The linux-yocto-custom recipe, located in the poky repository at: meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb is provided as an example recipe which uses kernel.org sources and the Yocto Project Linux kernel tools for managing meta-data. To get started, copy this recipe to your layer and provide it with a meaningful name, such as linux-yocto-myproject_3.5.bb, where 3.5 is the base version of the Linux kernel you will be working with. In the same directory, create a matching directory, e.g. linux-yocto-myproject to store your patches and configuration files. Edit the following variables in the recipe as appropriate for your project: o SRC_URI o LINUX_VERSION o LINUX_VERSION_EXTENSION o SRCREV o PR o PV o COMPATIBLE_MACHINE The SRC_URI should be a git repository, using one of the supported git fetcher protocols (file, git, http, etc.). The skeleton recipe provides an example SRC_URI as a syntax reference. Set LINUX_VERSION to the Linux kernel version you are using, such as "3.6.3". LINUX_VERSION_EXTENSION is used to define the Linux kernel CONFIG_LOCALVERSION which will be compiled in to the resulting kernel and visible via the uname command. Set SRCREV to the commit ID you wish to build from. Treat the PR as you would the PR of any other recipe. Increment it to indicate to the build system that the recipe has changed. The default PV assignment is typically adequate. It combines the LINUX_VERSION with the SCM revision (from the SRCPV variable) and results in a string something like: "3.4.11+git1+68a635bf8dfb64b02263c1ac80c948647cc76d5f_1+218bd8d2022b9852c60d32f0d770931e3cf343e2" While lengthy, this extra verbosity helps ensure you are building from the exact sources you intend. Finally, the default COMPATIBLE_MACHINE assignment for linux-yocto-custom is set to a regular expression matching only the empty string, "(^$)". This will trigger an explicit build failure. You must change it to match a list of the machines supported by your new recipe, such as "(qemux86|qemux86-64)" With that in place, you can continue to customize this recipe as you would the existing linux-yocto recipes. See Section 2.2 Modifying an Existing Recipe for details.

While it is always preferable to work with sources integrated into the Linux kernel sources, if you have need of an external kernel module, the hello-mod recipe is available as a template to create your own out-of-tree Linux kernel module recipe. It is available in the poky repository at: meta-skeleton/recipes-kernel/hello-mod/hello-mod_0.1.bb To get started, copy this recipe to your layer and provide it with a meaningful name, such as mymodule_1.0.bb. In the same directory, create a directory named "files" where you can store any source files, patches, or other files necessary for building the module which do not come with the sources. Finally, update the recipe as appropriate for the module. Typically you will need to set the following variables: o DESCRIPTION o LICENSE* o SRC_URI o PV Depending on the build system used by the module sources, you may need to make adjustments. For example, a typical module Makefile will look much like that provided with hello-mod: 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 bbclass class sets this, as well as KERNEL_PATH, to ${STAGING_KERNEL_DIR} with the necessary Linux kernel build information to build modules. If your module Makefile uses a different variable, you may 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 the 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 as appropriate in your machine config file: MACHINE_ESSENTIAL_EXTRA_RDEPENDS MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS MACHINE_EXTRA_RDEPENDS MACHINE_EXTRA_RRECOMMENDS As modules are often not required for boot and may be excluded from certain build configurations, the following allows for the most flexibility: MACHINE_EXTRA_RRECOMMENDS += "kernel-module-mymodule" Where the value is derived by appending the module filename without the .ko extension to the string "kernel-module-". As it is an RRECOMMENDS (and not an RDEPENDS) variable, the build will not fail if this module is not available to include in the image.