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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-package-notes">
<title>Package Notes</title>
<para>This chapter contains information about how to use and maintain
the Nix expressions for a number of specific packages, such as the
Linux kernel or X.org.</para>
<!--============================================================-->
<section xml:id="sec-linux-kernel">
<title>Linux kernel</title>
<para>The Nix expressions to build the Linux kernel are in <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/kernel"><filename>pkgs/os-specific/linux/kernel</filename></link>.</para>
<para>The function that builds the kernel has an argument
<varname>kernelPatches</varname> which should be a list of
<literal>{name, patch, extraConfig}</literal> attribute sets, where
<varname>name</varname> is the name of the patch (which is included in
the kernel’s <varname>meta.description</varname> attribute),
<varname>patch</varname> is the patch itself (possibly compressed),
and <varname>extraConfig</varname> (optional) is a string specifying
extra options to be concatenated to the kernel configuration file
(<filename>.config</filename>).</para>
<para>The kernel derivation exports an attribute
<varname>features</varname> specifying whether optional functionality
is or isn’t enabled. This is used in NixOS to implement
kernel-specific behaviour. For instance, if the kernel has the
<varname>iwlwifi</varname> feature (i.e. has built-in support for
Intel wireless chipsets), then NixOS doesn’t have to build the
external <varname>iwlwifi</varname> package:
<programlisting>
modulesTree = [kernel]
++ pkgs.lib.optional (!kernel.features ? iwlwifi) kernelPackages.iwlwifi
++ ...;
</programlisting>
</para>
<para>How to add a new (major) version of the Linux kernel to Nixpkgs:
<orderedlist>
<listitem>
<para>Copy the old Nix expression
(e.g. <filename>linux-2.6.21.nix</filename>) to the new one
(e.g. <filename>linux-2.6.22.nix</filename>) and update it.</para>
</listitem>
<listitem>
<para>Add the new kernel to <filename>all-packages.nix</filename>
(e.g., create an attribute
<varname>kernel_2_6_22</varname>).</para>
</listitem>
<listitem>
<para>Now we’re going to update the kernel configuration. First
unpack the kernel. Then for each supported platform
(<literal>i686</literal>, <literal>x86_64</literal>,
<literal>uml</literal>) do the following:
<orderedlist>
<listitem>
<para>Make an copy from the old
config (e.g. <filename>config-2.6.21-i686-smp</filename>) to
the new one
(e.g. <filename>config-2.6.22-i686-smp</filename>).</para>
</listitem>
<listitem>
<para>Copy the config file for this platform
(e.g. <filename>config-2.6.22-i686-smp</filename>) to
<filename>.config</filename> in the kernel source tree.
</para>
</listitem>
<listitem>
<para>Run <literal>make oldconfig
ARCH=<replaceable>{i386,x86_64,um}</replaceable></literal>
and answer all questions. (For the uml configuration, also
add <literal>SHELL=bash</literal>.) Make sure to keep the
configuration consistent between platforms (i.e. don’t
enable some feature on <literal>i686</literal> and disable
it on <literal>x86_64</literal>).
</para>
</listitem>
<listitem>
<para>If needed you can also run <literal>make
menuconfig</literal>:
<screen>
$ nix-env -i ncurses
$ export NIX_CFLAGS_LINK=-lncurses
$ make menuconfig ARCH=<replaceable>arch</replaceable></screen>
</para>
</listitem>
<listitem>
<para>Copy <filename>.config</filename> over the new config
file (e.g. <filename>config-2.6.22-i686-smp</filename>).</para>
</listitem>
</orderedlist>
</para>
</listitem>
<listitem>
<para>Test building the kernel: <literal>nix-build -A
kernel_2_6_22</literal>. If it compiles, ship it! For extra
credit, try booting NixOS with it.</para>
</listitem>
<listitem>
<para>It may be that the new kernel requires updating the external
kernel modules and kernel-dependent packages listed in the
<varname>kernelPackagesFor</varname> function in
<filename>all-packages.nix</filename> (such as the NVIDIA drivers,
AUFS, etc.). If the updated packages aren’t backwards compatible
with older kernels, you may need to keep the older versions
around.</para>
</listitem>
</orderedlist>
</para>
</section>
<!--============================================================-->
<section>
<title>X.org</title>
<para>The Nix expressions for the X.org packages reside in
<filename>pkgs/servers/x11/xorg/default.nix</filename>. This file is
automatically generated from lists of tarballs in an X.org release.
As such it should not be modified directly; rather, you should modify
the lists, the generator script or the file
<filename>pkgs/servers/x11/xorg/overrides.nix</filename>, in which you
can override or add to the derivations produced by the
generator.</para>
<para>The generator is invoked as follows:
<screen>
$ cd pkgs/servers/x11/xorg
$ cat tarballs-7.5.list extra.list old.list \
| perl ./generate-expr-from-tarballs.pl
</screen>
For each of the tarballs in the <filename>.list</filename> files, the
script downloads it, unpacks it, and searches its
<filename>configure.ac</filename> and <filename>*.pc.in</filename>
files for dependencies. This information is used to generate
<filename>default.nix</filename>. The generator caches downloaded
tarballs between runs. Pay close attention to the <literal>NOT FOUND:
<replaceable>name</replaceable></literal> messages at the end of the
run, since they may indicate missing dependencies. (Some might be
optional dependencies, however.)</para>
<para>A file like <filename>tarballs-7.5.list</filename> contains all
tarballs in a X.org release. It can be generated like this:
<screen>
$ export i="mirror://xorg/X11R7.4/src/everything/"
$ cat $(PRINT_PATH=1 nix-prefetch-url $i | tail -n 1) \
| perl -e 'while (<>) { if (/(href|HREF)="([^"]*.bz2)"/) { print "$ENV{'i'}$2\n"; }; }' \
| sort > tarballs-7.4.list
</screen>
<filename>extra.list</filename> contains libraries that aren’t part of
X.org proper, but are closely related to it, such as
<literal>libxcb</literal>. <filename>old.list</filename> contains
some packages that were removed from X.org, but are still needed by
some people or by other packages (such as
<varname>imake</varname>).</para>
<para>If the expression for a package requires derivation attributes
that the generator cannot figure out automatically (say,
<varname>patches</varname> or a <varname>postInstall</varname> hook),
you should modify
<filename>pkgs/servers/x11/xorg/overrides.nix</filename>.</para>
</section>
<!--============================================================-->
<!--
<section>
<title>Gnome</title>
<para>* Expression is auto-generated</para>
<para>* How to update</para>
</section>
-->
<!--============================================================-->
<!--
<section>
<title>GCC</title>
<para>…</para>
</section>
-->
</chapter>
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