Common Development Models

System Development System development involves modification or creation of an image that you want to run on a specific hardware target. Usually when you want to create an image that runs on embedded hardware the image does not require the same amount of features that a full-fledged Linux distribution provides. Thus, you can create a much smaller image that is designed to just use the hardware features for your particular hardware. To help you understand how system development works in the Yocto Project, this section covers two types of image development: BSP creation and kernel modification (see ).

Developing a Board Support Package (BSP) A BSP is a package of recipes that when applied during a build results in an image you can run on a particular board. Thus, the package, when compiled into the new image, supports the operation of the board. For a brief list of terms used when describing the development process in the Yocto Project, see Yocto Project Terms in this manual. The remainder of this section presents the basic steps to create a BSP basing it on an existing BSP that ships with the Yocto Project. You can reference BSP Development Example for a detailed example that uses the Crown Bay BSP as a base BSP from which to start. This illustration and the following list summarizes the BSP creation general workflow. Set up your host development system to support development using the Yocto Project: See The Linux Distributions section and The Packages section both in the Yocto Project Quick Start for requirements. You will also need a release of Yocto Project installed on the host. Establish a local copy of the Yocto Project files on your system: You need to have the Yocto Project files available on your host system. Having the Yocto Project files on your system gives you access to the build process and tools you need. For information on how to get these files, see the Getting Setup section in this manual. Establish a local copy of the base BSP files: Having the BSP files on your system gives you access to the build process and tools you need. For information on how to get these files, see Getting Setup earlier in this manual. Choose a Yocto Project-supported BSP as your base BSP: The Yocto Project ships with several BSPs that support various hardware. It is best to base your new BSP on an existing BSP rather than create all the recipes and configuration files from scratch. While it is possible to create everything from scratch, basing your new BSP on something that is close is much easier. Or, at a minimum, it gives you some structure with which to start. At this point you need to understand your target hardware well enough to determine which existing BSP it most closely matches. Things to consider are your hardware’s on-board features such as CPU type and graphics support. You should look at the README files for supported BSPs to get an idea of which one you could use. A generic Atom-based BSP to consider is the Crown Bay that does not support the Intel® Embedded Media Graphics Driver (EMGD). The remainder of this example uses that base BSP. To see the supported BSPs, go to the Yocto Project download page and click on “BSP Downloads.” Create your own BSP layer: Layers are ideal for isolating and storing work for a given piece of hardware. A layer is really just a location or area in which you place the recipes for your BSP. In fact, a BSP is, in itself, a special type of layer. Consider an application as another example that illustrates a layer. Suppose you are creating an application that has library or other dependencies in order for it to compile and run. The layer, in this case, would be where all the recipes that define those dependencies are kept. The key point for a layer is that it is an isolated area that contains all the relevant information for the project that the Yocto Project build system knows about. The Yocto Project supports four BSPs that are part of the Yocto Project release: atom-pc, beagleboard, mpc8315e, and routerstationpro. The recipes and configurations for these four BSPs are located and dispersed within local Yocto Project files. Consequently, they are not totally isolated in the spirit of layers unless you think of meta-yocto as a layer itself. On the other hand, BSP layers for Crown Bay, Emenlow, Jasper Forest, N450, and Sugar Bay are isolated. When you set up a layer for a new BSP you should follow a standard layout. This layout is described in the Example Filesystem Layout section of the Board Support Package (BSP) Development Guide. In the standard layout you will notice a suggested structure for recipes and configuration information. You can see the standard layout for the Crown Bay BSP in this example by examining the directory structure of the meta-crownbay layer inside the local Yocto Project files. Make configuration changes to your new BSP layer: The standard BSP layer structure organizes the files you need to edit in conf and several recipes-* within the BSP layer. Configuration changes identify where your new layer is on the local system and identify which kernel you are going to use. Make recipe changes to your new BSP layer: Recipe changes include altering recipes (.bb files), removing recipes you don't use, and adding new recipes that you need to support your hardware. Prepare for the build: Once you have made all the changes to your BSP layer there remains a few things you need to do for the Yocto Project build system in order for it to create your image. You need to get the build environment ready by sourcing an environment setup script and you need to be sure two key configuration files are configured appropriately. The entire process for building an image is overviewed in the Building an Image section of the Yocto Project Quick Start. You might want to reference this information. Build the image: The Yocto Project uses the BitBake tool to build images based on the type of image you want to create. You can find more information on BitBake here. The build process supports several types of images to satisfy different needs. See Reference: Images in the Yocto Project Reference Manualfor information on supported images. You can view a video presentation of the BSP creation process here. You can also find supplemental information in the Board Support Package (BSP) Development Guide. Finally, there is wiki page write up of the example located here you might find helpful.

<anchor id='kernel-spot' />Modifying the Kernel Kernel modification involves changing the Linux Yocto kernel, which could involve changing configuration variables as well as adding new kernel recipes. Configuration changes can be added in the form of configuration fragments, while recipe modification comes through the kernel's recipes-kernel area in a kernel layer you create. The remainder of this section presents a high-level overview of the Linux Yocto kernel architecture and the steps to modify the Linux Yocto kernel. For a complete discussion of the kernel, see the Yocto Project Kernel Architecture and Use Manual. You can reference Kernel Modification Example for a detailed example that changes the configuration of a kernel.

Kernel Overview When one thinks of the source files for a kernel they usually think of a fixed structure of files that contain kernel patches. The Yocto Project, however, employs mechanisims that in a sense result in a kernel source generator. The Yocto Project uses the source code management (SCM) tool Git to manage and track Yocto Project files. Git employs branching strategies that effectively produce a tree-like structure whose branches represent diversions from more general code. For example, suppose two kernels are basically identical with the exception of a couple different features in each. In the Yocto Project source repositories managed by Git a main branch can contain the common or shared parts of the kernel source and two branches that diverge from that common branch can each contain the features specific to the respective kernel. The result is a managed tree whose "leaves" represent the end of a specific path that yields a set of kernel source files necessary for a specific piece of hardware and its features. A big advantage to this scheme is the sharing of common features by keeping them in "larger" branches that are further up the tree. This practice eliminates redundant storage of similar features shared among kernels. When you build the kernel on your development system all files needed for the build are taken from the Yocto Project source repositories pointed to by the SRC_URI variable and gathered in a temporary work area where they are subsequently used to create the unique kernel. Thus, in a sense, the process constructs a local source tree specific to your kernel to generate the new kernel image - a source generator if you will. You can find a web interface to the Yocto Project source repository at . Within the interface you will see groups of related source code, each of which can be cloned using Git to result in a working Git repository on your local system (referred to as the "local Yocto Project files" in this manual). The Yocto Project supports four types of kernels in its source repositories at : linux-yocto-2.6.34 - The stable Linux Yocto kernel that is based on the Linux 2.6.34 release. linux-yocto-2.6.37 - The current Linux Yocto kernel that is based on the Linux 2.6.37 release. linux-yocto-dev - A development kernel based on the Linux 2.6.39-rc1 release. linux-2.6 - A kernel based on minimal Linux mainline tracking. For a complete discussion of the Yocto Project kernel's architcture and its branching strategy, see the The Yocto Project Kernel Architecture and Use Manual.

Kernel Modification Workflow This illustration and the following list summarizes the kernel modification general workflow. Set up your host development system to support development using the Yocto Project: See The Linux Distributions section and The Packages section both in the Yocto Project Quick Start for requirements. You will also need a release of Yocto Project installed on the host. Establish a local copy of the Yocto Project files on your system: Having the Yocto Project files on your system gives you access to the build process and tools you need. For information on how to get these files, see the bulleted item Yocto Project Release in Getting Setup earlier in this manual. Establish a local kernel layer by copying the meta-skeleton layer: When you set up a kernel layer for your changes you should follow a standard layout. For kernel layers you can start with meta-skeleton, which is a minimal, base kernel layer in the local Yocto Project files. You can examine meta-skeleton in the poky Git repository. A layer is really just a location or area in which you place configuration fragments and recipes that modify your kernel. The layer, in this case, would be where all the recipes that define those dependencies are kept. The key point for a layer is that it is an isolated area that contains all the relevant information for the project that the Yocto Project build system knows about. Prepare to use the menuconfig tool: The tool menuconfig used within Bitbake provides an interactive method with which you can configure the kernel. In order to use menuconfig you need to first source an environment setup script found in the local poky Git repository. The script is named oe-init-build-env. Make kernel configuration changes to your local kernel layer: Use menuconfig to enable and disable the configurations to the Linux Yocto kernel. Using menuconfig allows you to develop and test the configuration changes you are making to the kernel. Make kernel recipe changes to your new kernel layer: The standard layer structure organizes recipe files you need to edit in several recipes-* directories within the kernel layer. Recipe changes include altering recipes (.bb files), removing recipes you don't use, and adding new recipes that you need to support your hardware. Prepare for the build: Once you have made all the changes to your kernel layer there remains a few things you need to do for the Yocto Project build system in order for it to create your image. If you have not done so you need to get the build environment ready by sourcing the environment setup script described earlier. You also need to be sure two key configuration files are configured appropriately. The entire process for building an image is overviewed in the Building an Image section of the Yocto Project Quick Start. You might want to reference this information. Build the image: The Yocto Project uses the BitBake tool to build images based on the type of image you want to create. You can find more information on BitBake here. The build process supports several types of images to satisfy different needs. See Reference: Images in the Yocto Project Reference Manual for information on supported images. Make your configuration changes available in the kernel layer: Up to this point all the configuration changes to the kernel have been done and tested iteratively. Once they are tested and ready to go you can move them into the kernel layer, which allows you to distribute the layer. Push your configuration and recipe changes upstream to the linux Yocto Git repository (in-tree changes): If the changes you made are suited for all Linux Yocto users you might want to push the changes up into the Linux Yocto Git repository so that they become part of the kernel tree and available to everyone using the kernel.