A few things to help create custom kernel packages for el6 (and some
Debian/Ubuntu).

=====================================
configs
=====================================

This subdirectory contains kernel configurations.

config-xenomai-*:  Partial configs to enable Xenomai

  These are partial configs that can be applied on top of another
  complete config to enable Xenomai.  The files document the source of
  each config variable reasonably well.

config-quantal-3.5.3-*:  Stock kernel 3.5.3 configs from Debian Quantal

  These are complete configs for a vanilla 3.5.3 kernel with the same
  options as Debian Quantal.


======================================
kconfigtool.py
======================================

kconfigtool.py replaces merge.pl in the kernel specfile.  It must be
accompanied by the modified Makefile.config, which calls kconfigtool
with the right arguments instead of merge.pl.

kconfigtool can compare two kernel config files to see what the
differences are.  Just run 'kconfigtool.py -d config1 config2'.
Sometimes this is most useful for config files that have been run
through the kernel 'oldnoconfig' target, which fills in missing
options and removes extra options; see 'Canonicalizing config files'
below.

kconfigtool can merge two or more config files.  That is its purpose
in replacing merge.pl.  Running 'kconfigtool.py -m config1 config2'
merges the two configs together.  In case of conflicts, the first
config file has priority, so in this mode, kconfigtool might be
thought of as applying an 'override' config.

kconfigtool has an option '-v' or '--variant'.  The option's argument
is a file extension to add to merge arguments.  If a file is found with
that extension, it will be merged.  For example, if a file
'config1-debug' exists, running 'kconfigtool.py -m -v -debug config1
config2' is equivalent to running 'kconfigtool.py -m config1-debug
config1 config2'.  This is used in the kernel specfile and
Makefile.config as described below.


======================================
kernel.spec
======================================

This kernel.spec is based on kernel-2.6.32-279.14.1.el6.src.rpm.  It
has been modified to build a xenomai kernel, and some of the annoying
problems have been worked (or hacked) out.

This isn't trivial, since RH now pre-patches their kernel tarballs
before putting them into the kernel source RPM.  (This violates the
RPM philosophy, which they developed, of packaging pristine sources
[1] [2].)  Among those pre-patches are changes related to the specfile
scripts, so simply replacing the RH kernel with a vanilla kernel
results in an RPM that won't build.

The specfile here tries to work around these problems with a light
touch.  A diff of the original specfile will show all the original
code, but where appropriate, disabled or changed with %if..%endif
conditionals.

[1]  http://www.rpm.org/max-rpm/ch-rpm-philosophy.html
[2]  http://docs.fedoraproject.org/en-US/Fedora_Draft_Documentation/0.1/html/RPM_Guide/ch01s02s08.html


======================================
Makefile.config
======================================

The kernel specfile has a macro, %{variant}, in this example, set to
'-xenomai'.  This changes the RPM name to kernel-xenomai.  A
modification to the specfile for convenience is to pass the variant
name to Makefile.config, which then runs 'kconfigtool.py --variant
%{variant}'.

This makes it easy to make incremental changes to the kernel
configuration by adding override config files.  For example, to apply
'CONFIG_XENOMAI=y' to every configuration, put that option in a file
named 'config-generic-xenomai'.  That will be applied to the
'config-generic' file, which is the base of every configuration.

Watch out that your changes are not clobbered by later files.  For
instance, overriding 'CONFIG_FOO' in 'config-generic' by creating
'config-generic-myvariant' may be overridden later by
'config-x86_64-generic-rhel'.


======================================
Canonicalizing config files
======================================

It can be helpful to run config files through the kernel's config file
tools in order to fill in missing values and remove inapplicable
values.  This will help, for example, get two config files from
different sources into a better state for comparing with
kconfigtool.py --diff.

Here is how to do it from a linux kernel tree.

cd $kernelsrc
cp $myconfig .config
# for a vanilla kernel, use 'oldnoconfig'; for RH, use 'nonint_oldconfig'
make ARCH=$myarch oldnoconfig

The resulting config will be placed back into .config.