# Package Notes

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.

## Linux kernel

The Nix expressions to build the Linux kernel are in `pkgs/os-specific/linux/kernel`.

The function that builds the kernel has an argument `kernelPatches` which should be a list of `{name, patch, extraConfig}` attribute sets, where `name` is the name of the patch (which is included in the kernel’s `meta.description` attribute), `patch` is the patch itself (possibly compressed), and `extraConfig` (optional) is a string specifying extra options to be concatenated to the kernel configuration file (`.config`).

The kernel derivation exports an attribute `features` 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 `iwlwifi` feature (i.e. has built-in support for Intel wireless chipsets), then NixOS doesn’t have to build the external `iwlwifi` package:

    modulesTree = [kernel]
      ++ pkgs.lib.optional (!kernel.features ? iwlwifi) kernelPackages.iwlwifi
      ++ ...;

How to add a new (major) version of the Linux kernel to Nixpkgs:

1. Copy (`svn cp`) the old Nix expression (e.g. `linux-2.6.21.nix`) to the new one (e.g. `linux-2.6.22.nix`) and update it.

2. Add the new kernel to `all-packages.nix` (e.g., create an attribute `kernel_2_6_22`).

3. Now we’re going to update the kernel configuration. First unpack the kernel. Then for each supported platform (`i686`, `x86_64`, `uml`) do the following:

   1. Make an `svn copy` from the old config (e.g. `config-2.6.21-i686-smp`) to the new one (e.g. `config-2.6.22-i686-smp`).

   2. Copy the config file for this platform (e.g. `config-2.6.22-i686-smp`) to `.config` in the kernel source tree.

   3. Run `make oldconfig ARCH=`{i386,x86_64,um}`` and answer all questions. (For the uml configuration, also add `SHELL=bash`.) Make sure to keep the configuration consistent between platforms (i.e. don’t enable some feature on `i686` and disable it on `x86_64`).

   4. If needed you can also run `make menuconfig`:

          $ nix-env -i ncurses
          $ export NIX_CFLAGS_LINK=-lncurses
          $ make menuconfig ARCH=arch

   5. Make sure that `CONFIG_FB_TILEBLITTING` is not set (otherwise `fbsplash` won't work). This option has a tendency to be enabled as a side-effect of other options. If it is, investigate why (there's probably another option that forces it to be on) and fix it.

   6. Copy `.config` over the new config file (e.g. `config-2.6.22-i686-smp`).

4. Test building the kernel: `nix-build -A kernel_2_6_22`. If it compiles, ship it! For extra credit, try booting NixOS with it.

5. It may be that the new kernel requires updating the external kernel modules and kernel-dependent packages listed in the `kernelPackagesFor` function in `all-packages.nix` (such as the NVIDIA drivers, AUFS, splashutils, etc.). If the updated packages aren’t backwards compatible with older kernels, you need to keep the older versions and use some conditionals. For example, new kernels require splashutils 1.5 while old kernel require 1.3, so `kernelPackagesFor` says:

       splashutils =
         if kernel.features ? fbSplash then splashutils_13 else
         if kernel.features ? fbConDecor then splashutils_15 else
         null;

       splashutils_13 = ...;
       splashutils_15 = ...;

## X.org

The Nix expressions for the X.org packages reside in `pkgs/servers/x11/xorg/default.nix`. 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 `pkgs/servers/x11/xorg/overrides.nix`, in which you can override or add to the derivations produced by the generator.

The generator is invoked as follows:

    $ cd pkgs/servers/x11/xorg
    $ cat tarballs-7.5.list extra.list old.list \
      | perl ./generate-expr-from-tarballs.pl

For each of the tarballs in the `.list` files, the script downloads it, unpacks it, and searches its `configure.ac` and `*.pc.in` files for dependencies. This information is used to generate `default.nix`. The generator caches downloaded tarballs between runs. Pay close attention to the `NOT FOUND: `name`` messages at the end of the run, since they may indicate missing dependencies. (Some might be optional dependencies, however.)

A file like `tarballs-7.5.list` contains all tarballs in a X.org release. It can be generated like this:

    $ 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

`extra.list` contains libraries that aren’t part of X.org proper, but are closely related to it, such as `libxcb`. `old.list` contains some packages that were removed from X.org, but are still needed by some people or by other packages (such as `imake`).

If the expression for a package requires derivation attributes that the generator cannot figure out automatically (say, `patches` or a `postInstall` hook), you should modify `pkgs/servers/x11/xorg/overrides.nix`.