From 568a15c821dca324d3e7a1691ad784025a8c8319 Mon Sep 17 00:00:00 2001 From: Scott Rifenbark Date: Wed, 27 Jul 2011 13:03:18 -0700 Subject: documentation/dev-manual/dev-manual-kernel-appendix.xml: Added appendix New file for the kernel example. this will be an appendix. (From yocto-docs rev: fca7e4fbb3d1e738700349d6169d7217c04e4b31) Signed-off-by: Scott Rifenbark Signed-off-by: Richard Purdie --- .../dev-manual/dev-manual-kernel-appendix.xml | 446 +++++++++++++++++++++ 1 file changed, 446 insertions(+) create mode 100644 documentation/dev-manual/dev-manual-kernel-appendix.xml (limited to 'documentation/dev-manual') diff --git a/documentation/dev-manual/dev-manual-kernel-appendix.xml b/documentation/dev-manual/dev-manual-kernel-appendix.xml new file mode 100644 index 0000000000..fc93b53744 --- /dev/null +++ b/documentation/dev-manual/dev-manual-kernel-appendix.xml @@ -0,0 +1,446 @@ + + + + +Development Cases + +
+ 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 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. + + + + 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 + (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. + [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 + + + This section presents a simple example that illustrates kernel modification + based on the linux-yocto-2.6.37 kernel. + The example uses the audio and mixer capabilities supported by the + Advanced Linux + Sound Architecture (ALSA) Project. + As the example progresses you will see how to do the following: + + Iteratively modify a base kernel locally. + Provide a recipe-based solution for your modified kernel. + + Proved an "in-tree" solution for your modified kernel + (i.e. make the modifcations part of the Yocto Project). + + + + + The example flows as follows: + + + + + 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. + Set up your environment for optimal local kernel development. + + Create a layer to isolate your kernel work. + Next item. + Next item. + Next item. + Next item. + + + +
+ Setting Up Yocto Project + + + You need to have the Yocto Project files available on your host system. + The process is identical to that described in the + "Getting Setup" section earlier in this + manual. + Be sure to either set up a local Git repository for poky + or download and unpack the Yocto Project release tarball. + +
+ +
+ Create a Git Repository of <filename>poky-extras</filename> + + + Everytime you change a configuration or add a recipe to the kernel you need to + do a fetch from the Linux Yocto kernel source repositories. + This can get tedious and time consuming if you need to fetch the entire + Linux Yocto 2.6.37 Git repository down from the Internet everytime you make a change + to the kernel. + + + + You can get around this by setting up a meta-kernel-dev + area on your local system. + This area contains "append" files for every kernel recipe, which also include + a KSRC statement that points to the kernel source files. + You can set up the environment so that the KSRC points to the + meta-kernel-dev, thus pulling source from a local area. + This setup can speed up development time. + + + + To get set up you need to do two things: create a local Git repository + of the poky-extras repository, and create a bare clone of the + Linux Yocto 2.6.37 kernel Git repository. + + + + The following transcript shows how to clone the poky-extras + Git repository into the current working directory, which is poky + in this example. + The command creates the repository in a directory named poky-extras: + + $ git clone git://git.yoctoproject.org/poky-extras + Initialized empty Git repository in /home/scottrif/poky/poky-extras/.git/ + remote: Counting objects: 532, done. + remote: Compressing objects: 100% (472/472), done. + remote: Total 532 (delta 138), reused 307 (delta 39) + Receiving objects: 100% (532/532), 534.28 KiB | 362 KiB/s, done. + Resolving deltas: 100% (138/138), done. + + + + + This transcript shows how to clone a bare Git repository of the Linux Yocto + 2.6.37 kernel: + + $ git clone --bare git://git.yoctoproject.org/linux-yocto-2.6.37 + Initialized empty Git repository in /home/scottrif/linux-yocto-2.6.37.git/ + remote: Counting objects: 1886034, done. + remote: Compressing objects: 100% (314326/314326), done. + remote: Total 1886034 (delta 1570202), reused 1870335 (delta 1554798) + Receiving objects: 100% (1886034/1886034), 401.51 MiB | 714 KiB/s, done. + Resolving deltas: 100% (1570202/1570202), done. + + + + + The bare clone of the Linux Yocto 2.6.37 kernel on your local system mirrors + the upstream repository of the kernel. + You can effectively point to this local clone now during development to avoid + having to fetch the entire Linux Yocto 2.6.37 kernel every time you make a + kernel change. + +
+ +
+ Create a Layer for Your Kernel Work + + + It is always good to isolate your work using your own layer. + Doing so allows you to experiment and easily start over should things go wrong. + This example uses a layer named meta-amixer. + + + + When you set up a layer for kernel work you should follow the general layout + guidelines as described for BSP layers. + 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. + [WRITER'S NOTE: The meta-elc example uses an + images directory. + Currently, images is not part of the standard BSP layout. + I need to find out from Darren if this directory is required for kernel work.] + + + + [WRITER'S NOTE: I need a paragraph here describing how to set up the layer. + I am not sure if you should copy an existing BSP layer and modify from there. + Or, if you should just look at a BSP layer and then create your own files. + Email to Darren on this but no answer yet.] + +
+ +
+ Making Changes to Your Kernel Layer + + + In the standard layer structure you have several areas that you need to examine or + modify. + For this example the layer contains four areas: + + conf - Contains the + layer.conf that identifies the location of the recipe files. + + images - Contains the + image recipe file. + This recipe includes the base image you will be using and specifies other + packages the image might need. + recipes-bsp - Contains + recipes specific to the hardware for which you are developing the kernel. + + recipes-kernel - Contains the + "append" files that add information to the main recipe kernel. + + + + + + Let's take a look at the layer.conf in the + conf directory first. + This configuration file enables the Yocto Project build system to locate and + use the information in your new layer. + + + + The variable BBPATH needs to include the path to your layer + as follows: + + BBPATH := "${BBPATH}:${LAYERDIR}" + + And, the variable BBFILES needs to be modified to include your + recipe and append files: + + BBFILES := "${BBFILES} ${LAYERDIR}/images/*.bb \ + ${LAYERDIR}/images/*.bbappend \ + ${LAYERDIR}/recipes-*/*/*.bb \ + ${LAYERDIR}/recipes-*/*/*.bbappend" + + Finally, you need to be sure to use your layer name in these variables at the + end of the file: + + BBFILE_COLLECTIONS += "elc" + BBFILE_PATTERN_elc := "^${LAYERDIR}/" + BBFILE_PRIORITY_elc = "9" + + + + + The images directory contains an append file that helps + further define the image. + In our example, the base image is core-image-minimal. + The image does, however, need some additional modules that we are using + for this example. + These modules support the amixer functionality. + Here is the append file: + + require recipes-core/images/poky-image-minimal.bb + + IMAGE_INSTALL += "dropbear alsa-utils-aplay alsa-utils-alsamixer" + IMAGE_INSTALL_append_qemux86 += " kernel-module-snd-ens1370 \ + kernel-module-snd-rawmidi kernel-module-loop kernel-module-nls-cp437 \ + kernel-module-nls-iso8859-1 qemux86-audio alsa-utils-amixer" + + LICENSE = "MIT" + + + + + While the focus of this example is not on the BSP, it is worth mentioning that the + recipes-bsp directory has the recipes and append files for + features that the hardware requires. + In this example, there is a script and a recipe to support the + amixer functionality in QEMU. + It is beyond the scope of this manual to go too deeply into the script. + Suffice it to say that the script tests for the presence of the mixer, sets up + default mixer values, enables the mixer, unmutes master and then + sets the volume to 100. + + + + The recipe qemu86-audio.bb installs and runs the + amixer when the system boots. + Here is the recipe: + + SUMMARY = "Provide a basic init script to enable audio" + DESCRIPTION = "Set the volume and unmute the Front mixer setting during boot." + SECTION = "base" + LICENSE = "MIT" + LIC_FILES_CHKSUM = "file://${POKYBASE}/LICENSE;md5=3f40d7994397109285ec7b81fdeb3b58" + + PR = "r4" + + inherit update-rc.d + + RDEPENDS = "alsa-utils-amixer" + + SRC_URI = "file://qemux86-audio" + + INITSCRIPT_NAME = "qemux86-audio" + INITSCRIPT_PARAMS = "defaults 90" + + do_install() { + install -d ${D}${sysconfdir} \ + ${D}${sysconfdir}/init.d + install -m 0755 ${WORKDIR}/qemux86-audio ${D}${sysconfdir}/init.d + cat ${WORKDIR}/${INITSCRIPT_NAME} | \ + sed -e 's,/etc,${sysconfdir},g' \ + -e 's,/usr/sbin,${sbindir},g' \ + -e 's,/var,${localstatedir},g' \ + -e 's,/usr/bin,${bindir},g' \ + -e 's,/usr,${prefix},g' > ${D}${sysconfdir}/init.d/${INITSCRIPT_NAME} + chmod 755 ${D}${sysconfdir}/init.d/${INITSCRIPT_NAME} + } + + + + + The last area to look at is recipes-kernel. + This area holds configuration fragments and kernel append files. + The append file must have the same name as the kernel recipe, which is + linux-yocto-2.6.37 in this example. + The file can SRC_URI statements to point to configuration + fragments you might have in the layer. + The file can also contain KERNEL_FEATURES statements that specify + included kernel configurations that ship with the Yocto Project. + +
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