tjh.dev
1<section xmlns="http://docbook.org/ns/docbook"
2 xmlns:xlink="http://www.w3.org/1999/xlink"
3 xml:id="sec-linux-kernel">
4 <title>Linux kernel</title>
5
6 <para>
7 The Nix expressions to build the Linux kernel are in <link
8xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/kernel"><filename>pkgs/os-specific/linux/kernel</filename></link>.
9 </para>
10
11 <para>
12 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>).
13 </para>
14
15 <para>
16 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:
17<programlisting>
18modulesTree = [kernel]
19 ++ pkgs.lib.optional (!kernel.features ? iwlwifi) kernelPackages.iwlwifi
20 ++ ...;
21</programlisting>
22 </para>
23
24 <para>
25 How to add a new (major) version of the Linux kernel to Nixpkgs:
26 <orderedlist>
27 <listitem>
28 <para>
29 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.
30 </para>
31 </listitem>
32 <listitem>
33 <para>
34 Add the new kernel to <filename>all-packages.nix</filename> (e.g., create an attribute <varname>kernel_2_6_22</varname>).
35 </para>
36 </listitem>
37 <listitem>
38 <para>
39 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:
40 <orderedlist>
41 <listitem>
42 <para>
43 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>).
44 </para>
45 </listitem>
46 <listitem>
47 <para>
48 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.
49 </para>
50 </listitem>
51 <listitem>
52 <para>
53 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>).
54 </para>
55 </listitem>
56 <listitem>
57 <para>
58 If needed you can also run <literal>make menuconfig</literal>:
59<screen>
60<prompt>$ </prompt>nix-env -i ncurses
61<prompt>$ </prompt>export NIX_CFLAGS_LINK=-lncurses
62<prompt>$ </prompt>make menuconfig ARCH=<replaceable>arch</replaceable></screen>
63 </para>
64 </listitem>
65 <listitem>
66 <para>
67 Copy <filename>.config</filename> over the new config file (e.g. <filename>config-2.6.22-i686-smp</filename>).
68 </para>
69 </listitem>
70 </orderedlist>
71 </para>
72 </listitem>
73 <listitem>
74 <para>
75 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.
76 </para>
77 </listitem>
78 <listitem>
79 <para>
80 It may be that the new kernel requires updating the external kernel modules and kernel-dependent packages listed in the <varname>linuxPackagesFor</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.
81 </para>
82 </listitem>
83 </orderedlist>
84 </para>
85</section>