# Common Development Models

Many development models exist for which you can use the Yocto Project. However, for the purposes of this manual we are going to focus on two common ones: System Development and User Application Development. System Development covers Board Support Package (BSP) development and kernel modification. User Application Development covers development of applications that you intend to run on some target hardware.

This chapter presents overviews of both system and application models. If you want to reference specific examples of these development models, see BSP Development Example and Kernel Modification Example.

## 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.

1. 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.

2. 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.

3. 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.

4. 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.”

5. 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.

6. 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.

7. 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.

8. 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.

9. 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 on "Building Custom Embedded Images with Yocto" at Free Electrons. 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.

### 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.

You can find a web interface to the Linux Yocto kernel source repositories at . If you look at the interface, you will see to the left a grouping of Git repositories titled "Yocto Linux Kernel." Within this group, you will find the four different kernels supported by the Yocto Project:

- `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 stable Linux Yocto kernel that is based on the Linux 2.6.37 release.

- `linux-yocto-3.0` - The current Linux Yocto kernel that is based on the Linux 3.0 release.

- `linux-yocto-dev` - A development kernel based on the latest upstream release candidate available.

The kernels are maintained using the Git application that, in a sense, structures them in a "tree" complete with branches and leaves. Branches represent diversions from general code to more specific code, while leaves represent the end-points for a complete and unique kernel whose source files when gathered from the root of the tree to the leaf accumulate to create the files necessary for a specific piece of hardware and its features. The following figure displays this concept:

Within the figure, the "Kernel.org Branch Point" represents the point in the tree where a supported base kernel diverges from the Linux kernel. For example, this could be the branch point for the `linux-yocto-3.0` kernel. Thus, everything further to the right in the structure is based on the `linux-yocto-3.0` kernel. Branch points to right in the figure represent where the `linux-yocto-3.0` kernel is modified for specific hardware or types of kernels, such as real-time kernels. Each leaf thus represents the end-point for a kernel designed to run on a specific targeted device.

The overall result is a Git-maintained repository from which all the supported Yocto Project kernels can be derived for all the supported Yocto Project devices. A big advantage to this scheme is the sharing of common features by keeping them in "larger" branches within the tree. This practice eliminates redundant storage of similar features shared among kernels.

Keep in mind the figure does not take into account all four supported Linux Yocto kernel types, but rather shows a single generic kernel just for conceptual purposes. Also keep in mind that this structure represents the Yocto Project source repositories that are either pulled from during the build or established on the host development system prior to the build by either cloning a particular kernel's Git repository or by downloading and unpacking a tarball.

Storage of all the available kernel source code is one thing, while representing the code on your host development system is another. Conceptually, you can think of the Yocto Project kernel source repositories as all the source files necessary for all the supported kernels. As a developer, you are just interested in the source files for the kernel on on which you are working. And, furthermore, you need them available on your host system.

You make kernel source code available on your host development system by using Git to create a bare clone of the Linux Yocto kernel Git repository in which you are interested. Then, you use Git again to clone a copy of that bare clone. This copy represents the directory structure on your host system that is particular to the kernel you want. These are the files you actually modify to change the kernel. See the Linux Yocto Kernel item earlier in this manual for an example of how to set up the kernel source directory structure on your host system.

This next figure illustrates how the kernel source files might be arranged on your host system.

In the previous figure, the file structure on the left represents the bare clone set up to track the Yocto Project kernel Git repository. The structure on the right represents the copy of the bare clone. When you make modifcations to the kernel source code, this is the area in which you work. Once you make corrections, you must use Git to push the committed changes to the bare clone. The example in Modifying the Kernel Source Code provides a detailed example.

What happens during the build? 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.

The following figure shows the temporary file structure created on your host system when the build occurs. This build directory contains all the source files used during the build.

Again, 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. Also, you can reference Modifying the Kernel Source Code for a detailed example that modifies the kernel.

#### Kernel Modification Workflow

This illustration and the following list summarizes the kernel modification general workflow.

1. 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.

2. 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.

3. Set up the `poky-extras` Git repository: This repository is the area for your configuration fragments, new kernel recipes, and the kernel `.bbappend` file used during the build. It is good practice to set this repository up inside the local Yocto Project files Git repository. For information on how to get these files, see the bulleted item The `poky-extras` Git Repository in Getting Setup earlier in this manual.

4. Establish a local copy of the Linux Yocto kernel files on your system: In order to make modifications to the kernel you need two things: a bare clone of the Linux Yocto kernel you are modifying and a copy of that bare clone. The bare clone is required by the build process and is the area to which you push your kernel source changes. The copy of the bare clone is a local Git repository that contains all the kernel's source files. You make your changes to the files in this copy of the bare clone. For information on how to set these two items up, see the bulleted item Linux Yocto Kernel in Getting Setup earlier in this manual.

5. Make changes to the kernel source code if applicable: Modifying the kernel does not always mean directly changing source files. However, if you have to do this then you make the changes in the local Git repository you set up to hold the source files (i.e. the copy of the bare clone). Once the changes are made you need to use Git commands to commit the changes and then push them to the bare clone.

6. Make kernel configuration changes to your local kernel layer if applicable: If your situation calls for changing the kernel's configuration you can use `menuconfig` to enable and disable kernel configurations. Using `menuconfig` allows you to develop and test the configuration changes you are making to the kernel.

7. Add new kernel recipes if applicable: The standard layer structure organizes recipe files inside the `meta-kernel-dev` layer that is within the `poky-extras` Git repository. If you need to add new kernel recipes you add them within this layer. Also within this area you will find the `.bbappend` file that appends information to the kernel's recipe file used during the build.

8. Prepare for the build: Once you have made all the changes to your kernel (configurations, source code changes, recipe additions, or recipe changes) 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 (`local.conf` and `bblayers.conf`) 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. Also, you should look at the detailed examples found in the appendices at end of this manual.

9. 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.

10. 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. [WRITER'S NOTE: Not sure if the layer is meta-kernel-dev or if it would be a new layer copied from the work done there.]

11. 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.

## Place-Holder Section For Application Development

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