Prerequisites

sudo apt install gcc g++ \
  python3-distutils python-is-python3

Images

Images are recipes that ultimately inherit from meta/classes/image.bbclass and set the global variable IMAGE_INSTALL. Most images, however, will inherit from meta/classes/core-image.bbclass which inherits directly from image.bbclass, provides a mapping from IMAGE_FEATURES to package groups, and adds the two package groups packagegroup-core-boot and packagegroup-base-extended to IMAGE_INSTALL.

Images are defined in recipe files with the file extension 'bb'. In order to separate them from package recipes, there is a convention to put image recipes into a folder named images. Making use of this, we can list all images defined by all layers via

cd poky
ls meta*/recipes*/images/*.bb

IMAGE_FEATURES

The IMAGE_FEATURES variable is defined in meta/classes/image.bbclass with an empty value. It accepts only a predefined set of features (e.g., x11, debug-tweaks, dev-pkgs, dbg-pkgs). This affects how recipes will execute their tasks and which packages will be selected to go into the image. The image recipe meta/recipes-core/images/core-image-base.bb, for example, adds the splash feature.

IMAGE_INSTALL

Holds a list of package groups whose packages (from the package feed area) will go into the filesystem image.

Reference images

The following reference images are defined in the meta layer:


	core-image-minimal	small image that just boots the target device
	core-image-minimal-dev	core-image-minimal with headers and libraries allowing development work
	core-image-minimal-initramfs	core-image-minimal with kernel support for in-RAM filesystem
	core-image-base	console-only system fully supporting the target hardware
	core-image-full-cmdline	console-only image that includes many system tools
	core-image-x11	image with basic X11 and a terminal

These image definitions are provided as a starting point for creating project-specific, custom image definitions.

Creating a Custom Image

In your custom layer, create the images subdirectory in one of your recipes-...' directory.
In the images directory, create two image recipe files that inherit from core-image:

SUMMARY = "My deployment image"
LICENSE = "MIT"

inherit core-image
IMAGE_FEATURES += "splash"

and

SUMMARY = "My development image"

inherit core-image
require my-image.bb

IMAGE_FEATURES += "ssh-server-dropbear tools-debug debug-tweaks"

CORE_IMAGE_EXTRA_INSTALL += "i2c-tools "

where the first image recipe is for deployment and the second is for development. Note that the development version includes the deployment version with the require statement, and adds to IMAGE_FEATURES and CORE_IMAGE_EXTRA_INSTALL.

Image Filesystem Packaging

The system image generated by Bitbake can be packaged in the form of an image file, whose contents can be transferred on a boot medium (SD card or eMMC) such that the primary boot loader of the target device can discover the system and boot it.

Unless you restrict the type of packaging used for the image file, Yocto will generate multiple rootfs packages, each with a large footprint. To restrict the generated image files to a desired type, edit conf/local.conf of your project build directory and set the type.

#IMAGE_FSTYPES = "tar.gz"
#IMAGE_FSTYPES = "tar.bz2"
IMAGE_FSTYPES = "wic.bz2 wic.bmap"

The wic (kickstart) format with accompanying bmap is recommended.

Installing the Rootfs Image

Using bmaptool we can copy the generated wic image (with matching bmap file) to a mounted SD card or eMMC storage.

sudo bmaptool copy tmp/deploy/images/raspberrypi4-64/<wic.bz2 file> <drive>

where <wic.bz2 file> is the root filesystem image wic.bz2 file, and <drive> is the drive at which the storage is mounted (for example, /dev/sdb).

Package Groups

Package groups are recipes that inherit from meta/classes/packagegroup.bbclass and set the content of the global variable PACKAGES. Package groups are defined in recipe files with the file extension 'bb'.

Note that the packagegroup must be inherited before PACKAGES is defined.

inherit packagegroup

PACKAGES = '...'

Useful Image Features

Distro Features

Distro features are system features that affect multiple recipes. Large parts of the Poky image may need to be rebaked after making changes to the distro features!

systemd

In the conf/local.conf file of your project build directory (or layer.conf file of your layer) add

DISTRO_FEATURES:append = " systemd"

NFS

In Embedded Linux projects, NFS is often included in development images to enable faster code-deploy turnaround times.
In the conf/local.conf file of your project build directory (or layer.conf file of your layer) add

DISTRO_FEATURES:append = " nfs"

Vulkan and OpenGL

DISTRO_FEATURES:append = " vulkan opengl x11 wayland"

Bluetooth and NFC

DISTRO_FEATURES:append = " bluetooth nfc"

devtool

Use devtool to make changes to an existing recipe and automatically generate an append recipe from those changes.

Initialization Manager

After the kernel is done with its basic booting tasks, it passes control to the initialization manager. To use the systemd initialization manager (used by the Linux distributions Debian and Ubuntu) instead of SysVinit, make the following entries in ${TOPDIR}/conf/distro/distro.conf

DISTRO_FEATURES_append = " systemd"
DISTRO_FEATURES_BACKFILL_CONSIDERED = "sysvinit"
VIRTUAL-RUNTIME_init_manager = "systemd"
VIRTUAL-RUNTIME_initscripts = ""