2 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.
2.1 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 “Setting Up to Use the Yocto Project” section in
the Yocto Project Development Tasks Manual. Part of preparing the system
is creating a local Git repository of the
Source Directory (poky) on your system. Follow the steps in the
“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 and not through traditional kernel workflow methods. The remainder of this section provides information for both scenarios.
2.1.1 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”
section:
Initialize the BitBake Environment: you need to initialize the BitBake build environment by sourcing the build environment script (i.e. oe-init-build-env):
$ cd poky $ source oe-init-build-env
Note
The previous commands assume the Yocto Project Source Repositories (i.e.
poky) have been cloned using Git and the local repository is named “poky”.Prepare Your local.conf File: By default, the 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.conffile found in the Build Directory (i.e.poky/buildin this example).Also, since you are preparing to work on the kernel image, you need to set the 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"
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-layercommand 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-layercommand 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.Inform the BitBake Build Environment About Your Layer: As directed when you created your layer, you need to add the layer to the BBLAYERS variable in the
bblayers.conffile as follows:$ cd poky/build $ bitbake-layers add-layer ../../meta-mylayer NOTE: Starting bitbake server... $
Build the Clean Image: The final step in preparing to work on the kernel is to build an initial image using
bitbake:$ bitbake core-image-minimal 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.
If you were building for actual hardware and not for emulation, you could flash the image to a USB stick on
/dev/sddand boot your device. For an example that uses a Minnowboard, see the TipsAndTricks/KernelDevelopmentWithEsdk Wiki page.
At this point you have set up to start making modifications to the kernel. For a continued example, see the “Using devtool to Patch the Kernel” section.
2.1.2 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” section:
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). Also, for this example, be sure that the local branch you have checked out for
pokyis the Yocto Project Nanbield branch. If you need to checkout out the Nanbield branch, see the “Checking Out by Branch in Poky” section in the Yocto Project Development Tasks Manual:$ cd poky $ git branch master * nanbield $ source oe-init-build-env
Note
The previous commands assume the Yocto Project Source Repositories (i.e.
poky) have been cloned using Git and the local repository is named “poky”.Prepare Your local.conf File: By default, the 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.conffile found in the Build Directory (i.e.poky/buildin this example).Also, since you are preparing to work on the kernel image, you need to set the 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"
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-layercommand 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-layercommand 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.Inform the BitBake Build Environment About Your Layer: As directed when you created your layer, you need to add the layer to the BBLAYERS variable in the
bblayers.conffile as follows:$ cd poky/build $ bitbake-layers add-layer ../../meta-mylayer NOTE: Starting bitbake server ... $
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 https://git.yoctoproject.org/.
For simplicity, it is recommended that you create your copy of the kernel Git repository outside of the Source Directory, which is usually named
poky. Also, be sure you are in thestandard/basebranch.The following commands show how to create a local copy of the
linux-yocto-4.12kernel and be in thestandard/basebranch:$ 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.
Note
The
linux-yocto-4.12kernel can be used with the Yocto Project 2.4 release and forward. You cannot use thelinux-yocto-4.12kernel with releases prior to Yocto Project 2.4.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, which is usually named
poky. Also, for this example, be sure you are in theyocto-4.12branch.The following commands show how to create a local copy of the
yocto-kernel-cacheand switch to theyocto-4.12branch:$ 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” section.
2.2 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
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”
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:
Create Structure: Create the layer’s structure:
$ 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
confdirectory holds your configuration files, while therecipes-kerneldirectory holds your append file and eventual patch files.Create the Layer Configuration File: Move to the
meta-mylayer/confdirectory and create thelayer.conffile 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
mylayeras part of the last three statements.Create the Kernel Recipe Append File: Move to the
meta-mylayer/recipes-kernel/linuxdirectory and create the kernel’s append file. This example uses thelinux-yocto-4.12kernel. Thus, the name of the append file islinux-yocto_4.12.bbappend:FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:" SRC_URI += "file://patch-file-one.patch" SRC_URI += "file://patch-file-two.patch" SRC_URI += "file://patch-file-three.patch"
The FILESEXTRAPATHS and SRC_URI statements enable the OpenEmbedded build system to find patch files. For more information on using append files, see the “Appending Other Layers Metadata With Your Layer” section in the Yocto Project Development Tasks Manual.
2.3 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 in
meta/recipes-kernel/linux.
Modifying an existing recipe can consist of the following:
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” section for information.
2.3.1 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_6.1.bb recipe,
the append file will typically be located as follows within your custom
layer:
your-layer/recipes-kernel/linux/linux-yocto_6.1.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/
Note
If you are working on a new machine Board Support Package (BSP), be sure to refer to the Yocto Project Board Support Package 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_6.1.bbappend
Here are the contents of this 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 = "v6.1/standard/base"
KBRANCH:genericx86-64 = "v6.1/standard/base"
KBRANCH:beaglebone-yocto = "v6.1/standard/beaglebone"
KMACHINE:genericx86 ?= "common-pc"
KMACHINE:genericx86-64 ?= "common-pc-64"
KMACHINE:beaglebone-yocto ?= "beaglebone"
SRCREV_machine:genericx86 ?= "6ec439b4b456ce929c4c07fe457b5d6a4b468e86"
SRCREV_machine:genericx86-64 ?= "6ec439b4b456ce929c4c07fe457b5d6a4b468e86"
SRCREV_machine:beaglebone-yocto ?= "423e1996694b61fbfc8ec3bf062fc6461d64fde1"
COMPATIBLE_MACHINE:genericx86 = "genericx86"
COMPATIBLE_MACHINE:genericx86-64 = "genericx86-64"
COMPATIBLE_MACHINE:beaglebone-yocto = "beaglebone-yocto"
LINUX_VERSION:genericx86 = "6.1.30"
LINUX_VERSION:genericx86-64 = "6.1.30"
LINUX_VERSION:beaglebone-yocto = "6.1.20"
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.
Note
There are other ways of 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
not to have to put 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.
2.3.2 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 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.
2.3.3 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 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.
You can also use a regular defconfig file, as generated by the
do_savedefconfig
task instead of a complete .config file. This only specifies the
non-default configuration values. You need to additionally set
KCONFIG_MODE
in the linux-yocto .bbappend file in your layer:
KCONFIG_MODE = "alldefconfig"
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” section.
2.3.4 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 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:<machine> ?= "defconfig_file"
Here is an example that assigns the KBUILD_DEFCONFIG variable utilizing
an override for the “raspberrypi2” MACHINE 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.
2.4 Using devtool to Patch the Kernel
The steps in this procedure show you how you can patch the kernel using
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” Section.
Check Out the Kernel Source Files: First you must use
devtoolto checkout the kernel source code in its workspace.Note
See this step in the “Getting Ready to Develop Using devtool” section for more information.
Use the following
devtoolcommand to check out the code:$ devtool modify linux-yocto
Note
During the checkout operation, there is a bug that could cause errors such as the following:
ERROR: Taskhash mismatch 2c793438c2d9f8c3681fd5f7bc819efa versus be3a89ce7c47178880ba7bf6293d7404 for /path/to/esdk/layers/poky/meta/recipes-kernel/linux/linux-yocto_4.10.bb.do_unpackYou can safely ignore these messages. The source code is correctly checked out.
Edit the Source Files Follow these steps to make some simple changes to the source files:
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 thecalibrate.cfile:$ cd poky_sdk/workspace/sources/linux-yocto
Edit the source file: Edit the
init/calibrate.cfile 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)) { . . .
Build the Updated Kernel Source: To build the updated kernel source, use
devtool:$ devtool build linux-yocto
Create the Image With the New Kernel: Use the
devtool build-imagecommand to create a new image that has the new kernel:$ cd ~ $ devtool build-image core-image-minimal
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.
Test the New Image: For this example, you can run the new image using QEMU to verify your changes:
Boot the image: Boot the modified image in the QEMU emulator using this command:
$ runqemu qemux86
Verify the changes: Log into the machine using
rootwith 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
printkstatements as part of the output when you scroll down the console window.
Stage and commit your changes: Change your working directory to where you modified the
calibrate.cfile 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"
Export the Patches and Create an Append File: To export your commits as patches and create a
.bbappendfile, use the following command. This example uses the previously established layer namedmeta-mylayer:$ devtool finish linux-yocto ~/meta-mylayer
Note
See Step 3 of the “Getting Ready to Develop Using devtool” section for information on setting up this layer.
Once the command finishes, the patches and the
.bbappendfile are located in the~/meta-mylayer/recipes-kernel/linuxdirectory.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 in the terminal set up to run BitBake:
$ cd poky/build $ bitbake core-image-minimal
2.5 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
as described in the
“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”
Section.
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” section, use the following commands to edit the
calibrate.cfile: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.cfile:$ cd ~/linux-yocto-4.12/init
Edit the source file: Edit the
calibrate.cfile 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)) { . . .
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.
Update Your local.conf File to Point to Your Source Files: In addition to your
local.conffile specifying to use “kernel-modules” and the “qemux86” machine, it must also point to the updated kernel source files. Add SRC_URI and SRCREV statements similar to the following to yourlocal.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 = "${AUTOREV}"
Note
Be sure to replace path-to with the pathname to your local Git repositories. Also, you must be sure to specify the correct branch and machine types. For this example, the branch is
standard/baseand the machine isqemux86.Build the Image: With the source modified, your changes staged and committed, and the
local.conffile pointing to the kernel files, you can now use BitBake to build the image:$ cd poky/build $ bitbake core-image-minimal
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
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
printkstatements as part of the output when you scroll down the console window.Generate the Patch File: Once you are sure that your patch works correctly, you can generate a
*.patchfile in the kernel source repository:$ cd ~/linux-yocto-4.12/init $ git format-patch -1 0001-calibrate.c-Added-some-printk-statements.patch
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 asmeta-mylayer. When the layer was created using theyocto-createscript, 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
Create the Append File: Finally, you need to create the
linux-yocto_4.12.bbappendfile and insert statements that allow the OpenEmbedded build system to find the patch. The append file needs to be in your layer’srecipes-kernel/linuxdirectory and it must be namedlinux-yocto_4.12.bbappendand have the following contents:FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:" SRC_URI += "file://0001-calibrate.c-Added-some-printk-statements.patch"
The FILESEXTRAPATHS and 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” and “Applying Patches” sections. You can also see the “Appending Other Layers Metadata With Your Layer” section in the Yocto Project Development Tasks Manual.
Note
To build
core-image-minimalagain and see the effects of your patch, you can essentially eliminate the temporary source files saved inpoky/build/tmp/work/...and residual effects of the build by entering the following sequence of commands:$ cd poky/build $ bitbake -c cleanall linux-yocto $ bitbake core-image-minimal -c cleanall $ bitbake core-image-minimal $ runqemu qemux86
2.6 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” section.
2.6.2 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
${PN} directory in
your layer’s recipes-kernel/linux directory, and rename the copied
file to “defconfig” (e.g.
~/meta-mylayer/recipes-kernel/linux/linux-yocto/defconfig). Then,
add the following lines to the linux-yocto .bbappend file in your
layer:
FILESEXTRAPATHS:prepend := "${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.
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” section.
2.6.3 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.
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:
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
.configfile from a known state. Because there are situations where your build state might become unknown, it is best to run this task prior to startingmenuconfig.Launch menuconfig: Run the
menuconfigcommand:$ bitbake linux-yocto -c menuconfig
Create the Configuration Fragment: Run the
diffconfigcommand to prepare a configuration fragment. The resulting filefragment.cfgis placed in the${WORKDIR}directory:$ 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” 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 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"
2.6.4 Validating Configuration
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.
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 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.
2.6.5 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 it into the final
.configfile.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:
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.
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
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
.configfile.Analyze and potentially drop values from the
.configfile that override required configurations.Analyze and potentially remove non-board specific options.
Remove repeated and invalid options.
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.
2.7 Expanding Variables
Sometimes it is helpful to determine what a variable expands to during a
build. You can 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.
2.8 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, it means there are uncommitted modifications in the kernel’s source directory. Follow these steps to clean up the version string:
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
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"
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” section. For information on building the kernel image when using BitBake, see the “Using Traditional Kernel Development to Patch the Kernel” section.
2.9 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 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 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:
meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb.
Here are some basic steps you can use to work with your own sources:
Create a Copy of the Kernel Recipe: Copy the
linux-yocto-custom.bbrecipe 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).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).Ensure You Have Configurations: Make sure you have either a
defconfigfile or configuration fragment files in your layer. When you use thelinux-yocto-custom.bbrecipe, you must specify a configuration. If you do not have adefconfigfile, you can run the following:$ make defconfig
After running the command, copy the resulting
.configfile to thefilesdirectory in your layer as “defconfig” and then add it to the SRC_URI variable in the recipe.Running the
make defconfigcommand results in the default configuration for your architecture as defined by your kernel. However, there is no guarantee 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
defconfigfiles, you need to be more specific and find one that matches your board (i.e. for arm, you look inarch/arm/configsand use the one that is the best starting point for your board).Edit the Recipe: Edit the following variables in your recipe as appropriate for your project:
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 adefconfigfile or some configuration fragment files. The skeleton recipe provides an example SRC_URI as a syntax reference.LINUX_VERSION: The Linux kernel version you are using (e.g. “4.12”).
LINUX_VERSION_EXTENSION: The Linux kernel
CONFIG_LOCALVERSIONthat is compiled into the resulting kernel and visible through theunamecommand.SRCREV: The commit ID from which you want to build.
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: The default PV assignment is typically adequate. It combines the LINUX_VERSION with the Source Control Manager (SCM) revision as derived from the 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: 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
qemux86andqemux86-64machines, use the following form:COMPATIBLE_MACHINE = "qemux86|qemux86-64"
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” section for information.
2.10 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.
2.10.1 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.
2.10.2 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:
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.
2.11 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.
2.11.1 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 https://git.yoctoproject.org/.
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 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
2.11.2 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
2.12 Adding Recipe-Space Kernel Features
You can add kernel features in the
recipe-space
by using the KERNEL_FEATURES
variable and by specifying the feature’s .scc file path in the
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” 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.
Create the Feature File: Create a
.sccfile and locate it just as you would any other patch file,.cfgfile, or fetcher item you specify in the SRC_URI statement.Note
You must add the directory of the
.sccfile to the fetcher’s search path in the same manner as you would add a.patchfile.You can create additional
.sccfiles 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.sccfile in the following directory:my_recipe | +-linux-yocto | +-test.cfg +-test.scc
In this example, the
linux-yoctodirectory has both the featuretest.sccfile and a similarly named configuration fragment filetest.cfg.Add the Feature File to SRC_URI: Add the
.sccfile to the recipe’s SRC_URI statement:SRC_URI += "file://test.scc"
The leading space before the path is important as the path is appended to the existing path.
Specify the Feature as a Kernel Feature: Use the KERNEL_FEATURES statement to specify the feature as a kernel feature:
KERNEL_FEATURES += "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.sccfile.