Custom Linux Kernel Development
The ability to easily customize and expand any portion of the kernel is a feature of Linux that makes it very well suited for embedded systems development. In the embedded environment, specialized hardware, protocols, and systems may require a look into the kernel internals, custom configuration, feature additions, or driver development. This article aims to provide information on the most common kernel development tasks for EMAC OE Linux systems. In addition, the reader is pointed to additional sources for kernel development resources.
Before continuing, make sure that git tools are installed on your development machine, and review the Building the Linux Kernel document.
Contents
The Kernel Source
Source code for EMAC kernels is provided through our Git server. Refer to the documentation for your system to determine the correct source to use.
For this guide, the 2.6.30-at91 kernel tree will be used, which currently covers many of EMAC's SoM-based ARM products. |
Clone the Git Repository
To clone the git repository over anonymous HTTP, run the following commands:
developer@ldc:~$ git clone http://git.emacinc.com/public/source/linux-2.6.30-at91.git
Once the command has completed, the entire source should be contained in the linux-2.6.30-at91
directory. The master branch will be checked out automatically. Because EMAC uses the master branch for all releases, this is the correct branch to use, but other branches or tags may be checked out if directed or required.
Kernel Source Structure
Within the kernel source tree that was downloaded, you will see several directories. Table 1 gives a brief description of each of these directories.
Directory name | Description |
---|---|
arch |
Architecture-specific kernel code with subdirectories for each architecture |
block |
Block device driver code |
crypto |
Encryption support algorithms |
Documentation |
Text documentation files on the kernel and APIs |
drivers |
Device driver code, with a subdirectory structure for each device type |
firmware |
Code for building firmware required to communicate with devices |
fs |
File systems code |
include |
Include (header) files required to build the kernel code |
init |
Kernel initialization code |
ipc |
Interprocess communications code |
kernel |
Main internal kernel code |
lib |
Library code |
mm |
Memory management code |
net |
Kernel networking code |
samples |
Code examples and drivers that have not been fully developed |
scripts |
Various scripts used for the configuration and build process as well as standalone utility scripts |
security |
Kernel security support |
sound |
Sound and audio driver code |
usr |
Code used during kernel image creation |
virt |
Virtualization support code |
These directories will be referred to as needed in this document.
Configuration
The kernel uses a configuration system that specifies how every aspect of the kernel is built. This configuration information is generated based on selections by the user as well as dependency information built into the Kconfig structure. You will find a file named Kconfig
in most source directories within the kernel. These files follow the language described in Documentation/kbuild/kconfig-language.txt
and control the structure and operation of the kernel configuration utility.
The current kernel configuration is stored in a file named .config
in the top level of the kernel source tree. This file is read and updated by the configuration utility and is used by the make system during the build process. The first step in configuring the kernel often involves initializing this file with a default configuration set up with sane values for the target platform. These default configuration files are stored under the arch
directory. For example, arch/arm/configs/
.
Note that the .config file is a hidden file since its filename begins with a '.' . May tools such as ls will not show this file by default unless options are passed to show hidden files such as -a for ls. |
In this example, copy the arch/arm/configs/som9g45-som210_defconfig
file to the top level of the kernel source as .config
:
developer@ldc:~$ cp arch/arm/configs/som9g45-som210_defconfig .config
This is the default configuration file for the EMAC SoM-9G45 module using an SoM-210ES carrier board.
Refer to configuration section in the kernel building document. Show how to add / remove options in the config.
Adding Support for a new Carrier Board
Show how to add a carrier board in the 2.6.30 "emac-carrier" structure.
Driver Development
Basic info on driver structure here.
Licensing
Where to go for Additional Information
LDD3, mailing list, EMAC support.