View source for Building the Linux Kernel

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{{#seo:
|title=Building the Linux Kernel 
|titlemode=append
|keywords=Building Linux Kernel,Configuring Kernel,Loading Kernel Modules
|description=Process of configuring and compiling the Linux kernel using the EMAC kernel build script.
}}
This page covers the process of configuring and compiling the Linux kernel using the EMAC kernel build script. This process assumes that you have already acquired the following software: 

# EMAC Software Development Kit [[ Installing_EMAC_OE_4.0_SDK | OE 4 ]] or [[ Install_the_EMAC_SDK | OE 5 ]]
# Linux kernel source for target hardware (provided via EMAC public [http://git.emacinc.com/ GIT server] )
# [ftp://ftp.emacinc.com/EMAC_Linux/EMAC_OE_Linux/Common_Items/kernel-build-cross.tar.gz Kernel build script]

The example below will assume that a kernel image for the SoM-9x25 module will be created, although the instructions apply to other hardware as well assuming that the correct SDK, kernel tree, and build script is used.

==Setup==

The steps below assume that the <code>kernel-build-cross.sh</code> script is
located in the same directory as the kernel tree. Be sure to modify the <code>environment.cfg.sh</code> script for the correct 
architecture and EMAC OE version.

==Configuring the Kernel==

The first step for building the kernel is to configure it as desired. It is recommended to start with the kernel configuration file used by EMAC to build the kernel for the target device. Starting with EMAC OE 5, the kernel configuration can be 
obtained on a running board from /proc/config.gz. Please contact [http://www.emacinc.com/support EMAC support] 
for earlier EMAC OE versions. 
<br >
The following are steps to configure the kernel:
<cl>
1. Copy the default configuration file to the same directory as the kernel source tree and kernel-build-cross.sh and rename it defconfig. 

* The kernel-build-cross script accepts the SOURCE_TREE as the first argument and either config or build as the second argument. Optionally, a third argument, BUILD_SUFFIX may be supplied as a suffix to add to the build directory. BUILD_SUFFIX is commonly used to add a date tag or machine name to a build.

{{cli | username=developer | hostname=ldc |./kernel-build-cross.sh linux-at91 config som9x25}}

* The kernel menu-driven configuration utility will be displayed. Features can be selected/deselected to be built into the kernel. Some features can be built as a loadable module, denoted by < >, and not built directly into the kernel. 
{{ warning | Disabling  or modularizing some kernel features may prevent the kernel from starting correctly or at all. }}

Use the space bar to select an option or the 'm' key to configure the selected option as a module. Select <code>Exit</code> to close the kernel configuration menu and save the configuration to the newly created build directory. When the same build-suffix is used for subsequent builds, this configuration will be used. 
</cl>

==Building the Kernel==

<cl>
1. Run the kernel-build-cross script again with the build option, this time using the same build-suffix used in the configuration step. 

{{cli | username=developer | hostname=ldc |./kernel-build-cross.sh linux-at91 build som9x25}}

* The kernel will begin compiling now. This will take several minutes to complete depending on the kernel configuration and the speed of the development machine. Only move on to the next step if the build completes with no errors.

* The new kernel image will be in the <code>build-3.10.0-som9x25/Install/boot</code> directory called zImage. For the 3.10 and later device tree enabled kernels, the desired device tree blob needs to be appended to the kernel. 

{{clo}}
{{clio | username=developer | hostname=ldc |cd build-3.10.0-som9x25/Install/boot}}
{{clio | username=developer | hostname=ldc |cat zImage-3.10.0 som-9x25-150es.dtb > zImage-boot}}
{{clos}}

This is the image that will get loaded onto the board and executed by the bootloader. To load the new kernel onto the target machine, see the [[Loading Linux Kernels Onto a Board]] page.

The build script will also create an archive of all of the modules created during the build process and place it in the <code>build-3.10.0-som9x25/Install/</code> directory. The archive will be called <code>modules.tar.gz</code>.
</cl>

==Loading Kernel Modules==

If the kernel is recompiled without changing the configuration or source code for any modules, it is not necessary to reload the modules archive. Alternatively, if a module was modified or added, it is only necessary to reload the modules archive.
To reload the modules:
<cl>
1. Copy the archive to the root of the filesystem of the target machine

{{cli | username=developer | hostname=ldc |scp build-3.10.0-som9x25/Install/modules.tar.gz root@1IP_ADDRESS:/ }}

* Log onto the target machine

{{cli | username=developer | hostname=ldc |ssh root@IP_ADDRESS}}

* Extract the kernel modules archive and force the kernel to reload the modules. Make sure that the root flash is mounted read/write before extracting.

{{clo}}
{{clio |cd /}}
{{clio |tar xzvf modules.tar.gz}}
{{clio |depmod -a}}
{{clio |reboot}}
{{clos}}
</cl>

{{:Templateimpl:conclusion | initials=MG | title=System Logging | desc=The page describes how to build the linux kernel. | project=OE 5.0 }}
Using EMAC's kernel build script along with the EMAC SDK allows one to simply configure and build a customized Linux kernel.
{{:Templateimpl:moreinfo | initials=MG | title=System Logging | desc=The page describes how to build the linux kernel. | project=OE 5.0 }}
* [[Loading_Linux_Kernels_Onto_a_Board | Loading Linux Kernels Onto a Board]]

{{:Templateimpl:whatnext | initials=MG | title=Installing the EMAC SDK | desc=The page describes how to build the linux kernel. | project=OE 5.0 }}
* [[Install_the_EMAC_SDK | Install the EMAC SDK]]

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