<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>


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<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>


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<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 $i | tail -n 1) \
  | perl -e 'while (&lt;>) { if (/(href|HREF)="([^"]*.bz2)"/) { print "$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>



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<section>
  <title>Gnome</title>
  <para>* Expression is auto-generated</para>
  <para>* How to update</para>
</section>
-->


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<section>
  <title>GCC</title>
  <para>…</para>
</section>
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</chapter>
