Development Cases

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.

Developing a Board Support Package (BSP) A BSP is a package of recipes that when applied while building an image 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. Packages consist of recipes. Recipes are sets of instructions for building a package. The recipes describe where to get source code and what patches to apply. Recipes also describe dependencies for libraries or for other recipes. They also contain configuration and compilation options. Recipes are logical units of execution. Here are the basic steps involved in creating a BSP: Be sure your host development system is set up 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. Choose a BSP available with Yocto Project that most closely represents your hardware. Get set up with a base BSP. Make a copy of the existing BSP and isolate your work by creating a layer for your recipes. Make configuration and recipe changes to your new BSP layer. Prepare for the build. Select and configure the kernel. (WRITER'S NOTE: Not sure on this step). Identify the machine branch in the Git repository. Build the image. 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.

Setting Up Yocto Project You need to have the Yocto Project source tree available on your host system. You can get that through tarball extraction or by cloning the poky Git repository. Typically, cloning the Git repository is the method to use. This allows you to maintain a complete history of changes and facilitates you contributing back to the Yocto Project. However, if you just want the source you can download the Yocto Project Release tarball from the download page. If you download the tarball you can extract it into any directory you want using the tar command. For example, the following command extracts the Yocto Project 1.0.1 release tarball (bernard 5.0.1) into the current working directory and sets up a Yocto Project source directory named bernard.5.0.1: $ tar xfj poky.bernard.5.0.1.tar.bz2 The following transcript shows how to clone the poky Git repository into the current working directory. The command creates the Git repository in a directory named poky: $ git clone git://git.yoctoproject.org/poky Initialized empty Git repository in /home/scottrif/poky/.git/ remote: Counting objects: 107624, done. remote: Compressing objects: 100% (37128/37128), done. remote: Total 107624 (delta 73393), reused 99851 (delta 67287) Receiving objects: 100% (107624/107624), 69.74 MiB | 483 KiB/s, done. Resolving deltas: 100% (73393/73393), done. Once you have the repository set up you have many development branches from which you can work. From inside the repository you can see the branch names and the tag names used in the Git repository using either of the following two commands: $ git branch -a $ git tag -l For this example we are going to use the Yocto Project 1.1 Release, which maps to the [NEED BRANCH] branch in the repository. These commands create a local branch named [NEED BRANCH] that tracks the remote branch of the same name. $ cd poky $ git checkout -b bernard origin/bernard Switched to a new branch 'bernard' [WRITER'S NOTE: I NEED THE REAL BRANCH NAMES IN THE EXAMPLE.]

Choosing a 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 most closely matches it. 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 with no Intel® Embedded Media Graphics Driver (EMGD) support. That is the BSP that this example is going to use. To see the supported BSPs, go to the Yocto Project download page and click on “BSP Downloads.”

Getting Your Base BSP You need to have the base BSP layer on your development system. Like the Yocto Project source tree you can get the BSP layer one of two ways: download the tarball and extract it, or initialize a Git repository and check out the BSP. You should use the same method that you used for the Yocto Project source tree. If you are using tarball extraction then simply download the tarball for the base BSP you chose in the previous step and then extract it into any directory you choose using the tar command. Upon extraction, the BSP source directory (layer) will be named meta-<BSP_name>. The following command extracts the Crown Bay BSP into the current directory and names it meta-crownbay: $ tar xjf crownbay-noemgd-bernard-5.0.1.tar.bz2 If you cloned a Yocto Project Git repository (poky) then you need to do the same for the BSP, which is located in the meta-intel Git repository. The meta-intel repository contains all the metadata that supports BSP creation. When you set up the meta-intel Git repository you can set it up anywhere you want. We will set up the meta-intel Git repository inside the poky Git repository in this example. The following transcript shows the steps to clone the meta-intel Git repository inside the poky Git repository created earlier in this example. $cd poky $ git clone git://git.yoctoproject.org/meta-intel.git Initialized empty Git repository in /home/scottrif/poky/meta-intel/.git/ remote: Counting objects: 1325, done. remote: Compressing objects: 100% (1078/1078), done. remote: Total 1325 (delta 546), reused 85 (delta 27) Receiving objects: 100% (1325/1325), 1.56 MiB | 330 KiB/s, done. Resolving deltas: 100% (546/546), done. Because meta-intel is its own Git repository you will want to be sure you are in the appropriate branch for your work. For this example we are going to use the [NEED BRANCH] branch. $ cd meta-intel $ git checkout -b bernard origin/bernard Switched to a new branch 'bernard' [WRITER'S NOTE: I NEED THE REAL BRANCH NAMES IN THE EXAMPLE.]

Making a Copy of the Base BSP to Create Your New BSP Layer Now that you have the Yocto Project and base BSP source you need to create a new layer for your BSP. 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 meta-yocto, which can be found in the Yocto Project source directory. 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, the Yocto Project has isolated BSP layers within meta-intel for the Crown Bay, Emenlow, Jasper Forest, N450, and Sugar Bay. [WRITER'S NOTE: meta-yocto, meta, and meta-intel need some explanation. Not sure about the relationship of meta-yocto as compared to meta-intel.] When you set up a layer for a new BSP you should follow a standard layout. This layout is described in Example Filesystem Layout section of the Board Support Package (BSP) Development Guide. In the standard layout you will notice a suggested hierarchy for BSP kernel recipes, graphics 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 meta-intel Git repository. To create your BSP layer you simply copy the meta-crownbay layer to a new layer. For this example the new layer is named meta-mymachine. The name must follow the BSP layer naming convention, which is meta-<name>. The following example assumes your working directory is meta-intel inside the meta-intel Git repository. If you downloaded and expanded a Crown Bay tarball then you simply copy the resulting meta-crownbay directory structure to a location of your choice. Good practice for a Git repository is to just copy the new layer alongside the existing BSP layers in the meta-intel Git repository: $ cp -a meta-crownbay/ meta-mymachine

Making Changes to Your BSP Right now you have two identical BSP layers with different names: meta-crownbay and meta-mymachine. You need to change your configurations so that they work for your new BSP and your particular hardware. We will look first at the configurations, which are all done in the layer’s conf directory. First, since in this example the new BSP will not support EMGD we will get rid of the crownbay.conf file and then rename the crownbay-noemgd.conf file to mymachine.conf. Much of what we do in the configuration directory is designed to help the Yocto Project build system work with the new layer and to be able to find and use the right software. The following two commands result in a single machine configuration file named mymachine.conf. $ rm meta-mymachine/conf/machine/crownbay.conf $ mv meta-mymachine/conf/machine/crownbay-noemgd.conf \ meta-mymachine/conf/machine/mymachine.conf The next step makes changes to mymachine.conf itself. The only changes needed for this example are changes to the comment lines. Here we simply substitute the Crown Bay name with an appropriate name. Note that inside the mymachine.conf is the PREFERRED_PROVIDER_virtual/kernel statement. This statement identifies the kernel that the BSP is going to use. In this case the BSP is using linux-yocto, which is the current Linux Yocto kernel based on the Linux 2.6.37 release. The next configuration file in the new BSP layer we need to edit is layer.conf. This file identifies build information needed for the new layer. You can see the Layer Configuration File section in the Board Support Packages (BSP) Development Guide for more information on this configuration file. Basically, we are removing statements that support EMGD and changing the ones that support no EMGD. The file contains these statements that reference the Crown Bay BSP: BBFILE_COLLECTIONS += "crownbay" BBFILE_PATTERN_crownbay := "^${LAYERDIR}/" BBFILE_PRIORITY_crownbay = "6" Simply substitute the machine string name crownbay with the new machine name mymachine to get the following: BBFILE_COLLECTIONS_mymachine += "mymachine" BBFILE_PATTERN_mymachine := "^${LAYERDIR}/" BBFILE_PRIORITY_mymachine = "6" Now we will take a look at the recipes in your new layer. The standard BSP structure has areas for BSP, graphics, core, and kernel recipes. When you create a BSP you use these areas for appropriate recipes and append files. Recipes take the form of .bb files. If you want to leverage off of existing recipes elsewhere in the Yocto Project source tree but change them you can use .bbappend files. All new recipes and append files for your layer go in the layer’s recipes-bsp, recipes-kernel, recipes-core, and recipes-graphics directories. First, let us look at recipes-bsp. For this example we are not adding any new BSP recipes. And, we only need to remove the formfactor we do not want and change the name of the remaining one that supports no EMGD. These commands take care of the new layer’s BSP recipes: $ rm ‐rf meta-mymachine/recipes-graphics/xorg-xserver/*emgd* $ mv meta-mymachine/recipes-bsp/formfactor/formfactor/crownbay-noemgd/ \ meta-mymachine/recipes-bsp/formfactor/formfactor/mymachine Now let us look at recipes-graphics. For this example we want to remove anything that supports EMGD and be sure to rename remaining directories appropriately. The following commands clean up the recipes-graphics directory: $ rm ‐rf meta-mymachine/recipes-graphics/xorg-xserver/xserver-xf86-emgd* $ rm ‐rf meta-mymachine/recipes-graphics/xorg-xserver/xserver-xf86-config/crownbay $ mv meta-mymachine/recipes-graphics/xorg-xserver/xserver-xf86-config/crownbay-noemgd \ meta-mymachine/recipes-graphics/xorg-xserver/xserver-xf86-config/mymachine At this point the recipes-graphics directory just has files that support Video Electronics Standards Association (VESA) graphics modes. Now let us look at changes in recipes-core. The file task-core-tools.bbappend in recipes-core/tasks appends the similarly named recipe located in the Yocto Project Git repository at meta/recipes-core/tasks. The "append" file in our layer right now is Crown Bay-specific and supports EMGD and non-EMGD. Here are the contents of the file: RRECOMMENDS_task-core-tools-profile_append_crownbay = " systemtap" RRECOMMENDS_task-core-tools-profile_append_crownbay-noemgd = " systemtap" The RRECOMMENDS statements list packages that extend usability. The first RRECOMMENDS statement can be removed, while the second one can be changed to reflect meta-mymachine: RRECOMMENDS_task-core-tools-profile_append_mymachine = " systemtap" Finally, let us look at recipes-kernel changes. Recall that the BSP uses the linux-yocto kernel as determined earlier in the mymachine.conf. The recipe for that kernel is located is not located in the BSP layer but rather in Yocto Project Git repository at meta/recipes-kernel/linux and is named linux-yocto-2.6.37.bb. The SRCREV_machine and SRCREV_meta statements point to the exact commits in the Yocto Project Git repository from which the source for the kernel can be found. However, in the meta-mymachine layer in recipes-kernel/linux resides a .bbappend file named linux-yocto-2.6.37.bbappend that is appended to the recipe of the same name in meta/recipes-kernel/link. Thus, the SRCREV statements in the "append" file override the more general statements found in meta. The SRCREV statements in the "append" file currently point to the kernel that supports the Crown Bay BSP with and without EMGD support. Here are the statements: SRCREV_machine_pn-linux-yocto_crownbay ?= \ "372c0ab135978bd8ca3a77c88816a25c5ed8f303" SRCREV_meta_pn-linux-yocto_crownbay ?= \ "d5d3c6480d61f83503ccef7fbcd765f7aca8b71b" SRCREV_machine_pn-linux-yocto_crownbay-noemgd ?= \ "372c0ab135978bd8ca3a77c88816a25c5ed8f303" SRCREV_meta_pn-linux-yocto_crownbay-noemgd ?= \ "d5d3c6480d61f83503ccef7fbcd765f7aca8b71b" You will notice that there are two pairs of SRCREV statements. The top pair points to a kernel that supports EMGD, which we don’t care about in this example. The bottom pair points to the kernel that we will use: linux-yocto. At this point though, the unique commit strings all are still associated with Crown Bay. So the next changes we make modify this linux-yocto-2.6.37.bbappend file. First, we delete the two SRCREV statements that support EMGD (the top pair). Next, we change the remaining pair to specify mymachine and then we insert the commit identifier to point to the kernel in which we are interested, which will be based on the atom-pc-standard kernel. Here are the final SRCREV statements: SRCREV_machine_pn-linux-yocto-_mymachine ?= \ "fce17f046d3756045e4dfb49221d1cf60fcae329" SRCREV_meta_pn-linux-yocto-stable_mymachine ?= \ "84f1a422d7e21fbc23a687035bdf9d42471f19e0" If you are familiar with Git repositories you probably won’t have trouble locating the exact commit strings you need to change the SRCREV statements. You can find all the machine and meta branch points (commits) for the linux-yocto-2.6.37 kernel here. If you need a little more assistance after going to the link then do the following: Expand the list of branches by clicking […] Click on the yocto/standard/common-pc/atom-pc branch Click on the commit column header to view the top commit Copy the commit string for use in the linux-yocto-2.6.37.bbappend file For the SRCREV statement that points to the meta branch use the same procedure except expand the meta branch in step 2 above. Also in the linux-yocto-2.6.47.bbappend file are COMPATIBLE_MACHINE, KMACHINE, and KERNEL_FEATURES statements. Again, there are two sets of these: one for EMGD support and one for non-EMGD support. Because we are not interested in supporting EMGD those three can be deleted. The remaining three must be changed so that mymachine replaces crownbay-noemgd and crownbay. Here is the final linux-yocto-2.6.37.bbappend file after all the edits: FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:" COMPATIBLE_MACHINE_mymachine = "mymachine" KMACHINE_mymachine = "yocto/standard/mymachine" KERNEL_FEATURES_append_mymachine += " cfg/smp.scc" SRCREV_machine_pn-linux-yocto_mymachine ?= \ "fce17f046d3756045e4dfb49221d1cf60fcae329" SRCREV_meta_pn-linux-yocto_mymachine ?= \ "84f1a422d7e21fbc23a687035bdf9d42471f19e0" In summary, the edits to the layer’s recipe files result in removal of any files and statements that do not support your targeted hardware in addition to the inclusion of any new recipes you might need. In this example, it was simply a matter of ridding the new layer meta-machine of any code that supported the EMGD features and making sure we were pointing to the kernel that supports our example, which is the atom-pc-standard kernel. We did not introduce any new recipes to the layer. Finally, it is also important to update the layer’s README file so that the information in it reflects your BSP.

Preparing 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. The remainder of this section will apply to our example of the meta-mymachine layer. To get ready to build your image that uses the new layer you need to do the following: Get the environment ready for the build by sourcing the environment script. The environment script is in the Yocto Project source directory (poky in this example) and has the string init-build-env in the file’s name. For this example, the following command gets the build environment ready: $ source oe-init-build-env yocto-build When you source the script a build directory is created in the current working directory. In our example we were in the Yocto Project source directory. Thus, entering the previous command created the yocto-build directory. If you do not provide a name for the build directory it defaults to build. The yocot-build directory contains a conf directory that has two configuration files you will need to check: bblayers.conf and local.conf. Check and edit the resulting local.conf file. This file minimally identifies the machine for which to build the image by configuring the MACHINE variable. For this example you must set the variable to mymachine as follows: MACHINE ??= “mymachine” You should also be sure any other variables in which you are interested are set. Some variables to consider are BB_NUMBER_THREADS and PARALLEL_MAKE, both of which can greatly reduce your build time if you are using a multi-threaded development system (e.g. values of 8 and j 6, respectively are optimal for a development machine that has four available cores). Update the bblayers.conf file so that it includes the path to your new BSP layer. In this example you need to include the pathname to meta-mymachine. For this example the BBLAYERS variable in the file would need to include the following path: $HOME/poky/meta-intel/meta-mymachine The appendix Reference: Variables Glossary in the Yocto Project Reference Manual has more information on configuration variables.

Building 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. When you issue the BitBake command you provide a “top-level” recipe that essentially starts the process off of building the type of image you want. You can find these recipes in the meta/recipes-core/images and meta/recipes-sato/images directories of the Yocto Project source tree (the poky Git repository in this example). Although the recipe names are somewhat explanatory, here is a list that describes them: Base – A foundational basic image without support for X that can be reasonably used for customization. Core – A foundational basic image with support for X that can be reasonably used for customization. Direct Disk – An image that you can copy directory to the disk of the target device. Live – An image you can run from a USB device or from a CD without having to first install something. Minimal – A small image without a GUI. This image is not much more than a kernel with a shell. Minimal Development – A Minimal image suitable for development work. Minimal Direct Disk – A Minimal Direct Disk image. Minimal RAM-based Initial Root Filesystem – A minimal image that has the initramfs as part of the kernel, which allows the system to find the first “init” program more efficiently. Minimal Live – A Minimal Live image. Minimal MTD Utilities – A minimal image that has support for the MTD utilities, which let the user interact with the MTD subsystem in the kernel to perform operations on flash devices. Sato – An image with Sato support, a mobile environment and visual style that works well with mobile devices. Sato Development – A Sato image suitable for development work. Sato Direct Disk – A Sato Direct Disk image. Sato Live – A Sato Live image. Sato SDK – A Sato image that includes the Yocto Project toolchain and development libraries. Sato SDK Direct Disk – A Sato SDK Direct Disk image. Sato SDK Live – A Sato SDK Live image. The remainder of this section applies to our example of the meta-mymachine layer. To build the image for our meta-mymachine BSP enter the following command from the same shell from which you ran the setup script. You should run the bitbake command without any intervening shell commands. For example, moving your working directory around could cause problems. Here is the command for this example: $ bitbake –k core-image-sato-live This command specifies an image that has Sato support and that can be run from a USB device or from a CD without having to first install anything. The build process takes significant time and includes thousands of tasks, which are reported at the console. If the build results in any type of error you should check for misspellings in the files you changed or problems with your host development environment such as missing packages.

Modifying a Kernel Kernel modification involves changing or adding configurations to an existing kernel, or adding recipes to the kernel that are needed to support specific hardware features. The process is similar to creating a Board Support Package (BSP) except that it does not involve a BSP layer. This section presents a brief overview of the kernel structure and then provides a simple example that shows how to modify the kernel.

Yocto Project 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 kernel Git repository 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 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 local tree (the Git repository) and gathered in a temporary work area where they are subsequently used to create the unique kernel. Thus, in a sense, the process uses a local, all-inclusive source tree to generate (or choose) the specific kernel source files you need for the build - a source generator if you will. 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. 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. 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. [WRITER'S NOTE: I don't know which Git repository the user needs to clone to get this repository on their development system.]

Modifying a Kernel Example The remainder of this section presents a simple example that modifies a kernel. For the purpose of this example we are going to base our new kernel on the current Linux Yocto 2.6.37 release. [WRITER'S NOTE: This section is a second example that focuses on just modifying the kernel. I don't have any information on this yet. Here are some points to consider though: Reference Darren's presentation here Reference Getting Started with the Yocto Project section to get set up at minimum. Are there extra steps I need specific to kernel development to get started? What do I do to get set up? Is it a matter of just installing YP and having some pieces together? What are the pieces? Where do I get the base kernel to start with? Do I install the appropriate toolchain? What kernel git repository do I use? What is the conversion script? What does it do? What do I have to do to integrate the kernel layer? What do I use to integrate the kernel layer? HOB? Do I just Bitbake it? Using the System Image Creator.]