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The Debian kernel [[handbook|http://kernel-handbook.alioth.debian.org/]] is a useful reference which works with wheezy.

This page describes how to compile your own modified kernel from the linux-2.6 source package, rather than the debianized source tarball provided in linux-source-2.6.xx. Hopefully it will be integrated into the kernel handbook someday. See also BuildingKernelFromUpstreamSources and HowToRebuildAnOfficialDebianKernelPackage, which, despite its name, seems to address similar problems.

== Purpose ==
My initial purpose was to build two images, very similar to {{{linux-image-2.6.18-4-686}}} and {{{linux-image-2.6.18-4-xen-686}}}, but patched to properly support an Intel-based [MacBookPro].

The first image would be easy to produce by using the method to [[http://kernel-handbook.alioth.debian.org/ch-common-tasks.html#s-common-building|build a custom kernel from the debian kernel source]]. However the second one would be a little more difficult, since there is no xen patch which is easily applyable to a debian kernel source. So I decided to reuse the {{{linux-2.6}}} source package.

== Actions ==
This method mostly follows the instructions detailed in the [[http://kernel-handbook.alioth.debian.org/ch-common-tasks.html#s-common-official|rebuilding an official Debian kernel package]] section of the debian kernel handbook. The only modification is showing how to apply your patch in the build process.

Here's how I did it.

=== Get the source and tools ===
Follow the initial steps detailed in the handbook. Stop before invoking {{{debian/rules}}}. Additionally install package {{{devscripts}}}. In this example I will assume that the version of the downloaded source package was {{{2.6.18.dfsg.1-12etch2}}}

=== Add changes in a new revision ===
It is good to create a new revision when you change the package, to distinguish it from the original. You also get a place to document your changes - in the {{{changelog}}}, which will subsequently be available in {{{/usr/share/doc/linux-image-*/changelog.Debian.gz}}} - for free. No need for long cryptic version strings which try to document all applied changes by themselves.

==== Add a patch ====
First, copy the patch you require into the debian/patches directory - it should be possible to apply it from within the root of the source tree, using {{{patch -p1}}}. In my case the filename was {{{debian/patches/mactel.patch}}}

==== Make the patch apply during build ====
Then, add a new patch series file, which contains the name of the patch, and is named after the revision we are creating. I usually add a suffix, which makes the version larger than the original (this way APT will not try to reinstall the package from the archive), but smaller than the new official package, which will let me notice when I need to update my package.

In this case, I created a file called {{{debian/patches/series/12etch2.0.mactel}}} with the following contents:

+ mactel.patch
that is, the name of the patch to apply, relative to the debian/patches directory

==== Add any config options ====
The kernel config file is created dynamically from a number of snippets scattered around the {{{debian/arch/}}} directory. I needed to add two new config options together with the patch. Since I was going to build only a {{{686}}} and {{{xen-686}}} flavors on the i386 architecture, it was suficient to add the following two lines at the end of the {{{debian/arch/i386/config.686}}} file:

since this file is used by both flavours I needed to build.

==== Create a new revision ====
We need the revision and the name of the series file to match, so invoke the following command when in the {{{debian/}}} directory:

dch -v 2.6.18.dfsg.1-12etch2.0.mactel
And describe the changes in the editor which gets invoked.

=== Prepare for the build ===
First, since we modified some key files in the debian directory, we need to regenerate the control file. Do this using:

fakeroot debian/rules debian/control
The command fails, but that's on purpose, as described in the message which is produced.

Subsequently, as described in the handbook, you need to invoke the following command to prepare the build environment:

fakeroot debian/rules debian/build debian/stamps
=== Build the packages ===
I was then able to build the packages using:

script -c 'DEBIAN_KERNEL_JOBS=2 time fakeroot make -f debian/rules.gen binary-arch_i386_xen_686 binary-arch_i386_none_686 binary-arch_i386_xen_real binary-arch_i386_none_real' ../typescript.2007072001
which is nice, as it saves the typescript of the build for later investigation in case it fails for some reason. The environment variable setting DEBIAN_KERNEL_JOBS=2 causes a parallel build, which is more efficient if you have more than one CPU.

After a (long) while, it produced the following files (grouped with according to targets which produced them):





== Additional notes ==
=== ABI changes ===
The patch I needed to apply was relatively unintrusive, but if your changes modify the kernel's ABI (Binary Interface), by modifying some data structures or function declarations, then the build procedure is going to detect it and fail (possibly in a very verbose manner). You then need to copy the new ABI description file and create a new custom abi-ID.FLAVOR file in your arch directory. FIXME: possibly some more changes will be needed.

 . '' When I ran across this error, "ABI has changed!", 'I did not understand what the original author was talking about in the above paragraph. However, I was able to get it to work by simply changing the abiname value in debian/arch/defines. I changed it from "abiname: 6" to "abiname: 6-custom". This allowed it to continue successfully. ''
 . . This is covered in more detail on [[HowToRebuildAnOfficialDebianKernelPackage]] under the heading ''Add a patch to linux-2.6 and build new kernel packages''

=== Patch clashes ===
In my case, the debian-provided xen patch had a small and luckily easy to fix clash with the mactel patch in the {{{drivers/Makefile}}} file. I was able to fix it by modifying the xen patch by hand to insert the necessary line at the end of the file, instead right next to the line which the mactel patch needed to modify.

CategoryProposedDeletion: Outdated and references an [[DebianEtch|obsolete release]], superseded by the [[http://kernel-handbook.alioth.debian.org/|Debian Linux Kernel Handbook]].