[OpenPower-Firmware] Building OpenPOWER firmware on NixOS

[Hugo Landau]

The following documents what it took to build OpenPOWER firmware on
NixOS, mainly for posterity.

**Building natively in nix-shell:** Initially I tried to build the
firmware natively on NixOS. This had a lot of problems because of fixed
paths and shebang lines like #!/usr/bin/perl, since on NixOS the only
binaries with conventional paths are /bin/sh and /usr/bin/env. These
need to be patched to appropriate values before building. There were a
number of other miscellaneous small patches which needed to be made, as
well. Eventually I abandoned this route after some peculiar interactions
with include paths and an #include_next statement in gcc's system header
files when trying to build gcc-ppe42.

**Building using FHS emulation:** This worked, and took much less
effort. Save the following file as e.g. fhs.nix:

  { pkgs ? import <nixpkgs> {} }:
  (pkgs.buildFHSUserEnv {
    name = "fhs-env";

    targetPkgs = pkgs: (with pkgs; [
      # Build tools.
      gcc
      binutils
      gnumake
      flex
      bison
      git
      file
      cscope
      ctags
      python
      unzip
      wget
      bc
      texinfo
      which
      ccache
      perl
      rsync
      cpio
      patch
      m4
      getopt

      # Libraries.
      zlib
      expat
      libxslt
      openssl.dev
      perlPackages.XMLLibXMLSimple
      perlPackages.XMLSimple
      perlPackages.XMLSAX
      libxml2.dev

      # Needed for make menuconfig.
      ncurses.dev
      pkgconfig
    ]);

    profile = ''
      # There are some warnings during the build which get turned into
      # errors if this isn't set.
      export hardeningDisable=all

      # This is fixed by nixpkgs 2a036ca1a5e but it isn't in stable yet.
      export NIX_CC_WRAPPER_x86_64_unknown_linux_gnu_TARGET_HOST=1

      # Perl include path doesn't seem to get set properly.
      export PERL5LIB="/usr/lib/perl5/site_perl"
    '';
  }).env

After cloning op-build and updating the submodules, enter the shell
using `nix-shell fhs.nix`. In the shell, source op-build-env; op-build a
configuration (I used zaius_defconfig); optionally op-build menuconfig
to make adjustments (the only change I made was to enable ccache). Run
op-build -j... to run parallel make.

The following subsidiary repositories need some small patches. These
repositories only get downloaded when you run op-build. It's rather
manual, but I just ran op-build first, patched them manually and ran
op-build again.

  - buildroot (checked out at buildroot)
    Symptom: Perl can't find XML/Simple.pm.
    Cause: PERL5LIB environment variable gets replaced outright by build
      system, which causes problems if it's actually necessary to find
      Perl packages expected to be present on the system.
    Workaround:
      In buildroot, apply the following patch:

      diff --git a/package/Makefile.in b/package/Makefile.in
      index a1a531605..59173db33 100644
      --- a/package/Makefile.in
      +++ b/package/Makefile.in
      @@ -252,7 +252,7 @@ export PERL=$(shell which perl)
       # host-intltool needs libxml-parser-perl, which Buildroot installs in
       # $(HOST_DIR)/lib/perl, so we must make sure that the system perl
       # finds this perl module by exporting the proper value for PERL5LIB.
      -export PERL5LIB=$(HOST_DIR)/lib/perl
      +export PERL5LIB:=$(HOST_DIR)/lib/perl:$(PERL5LIB)

       TARGET_MAKE_ENV = PATH=$(BR_PATH)

  - openpower-ffs (checked out at output/build/host-openpower-ffs-{hash})
    Symptom: Error in glibc's <bits/fcntl2.h>
      about "invalid use of __builtin_arg_va_pack"
    Cause: Conflicting definition of __inline macro in openpower-ffs.
    Workaround:
      Comment out #define __inline in clib/attribute.h.

      (You may need to delete some .o files if you've already partially
      built openpower-ffs and you get some linker errors about multiple
      definitions of the same function.)

  - fakeroot (checked out at output/build/host-fakeroot-{version})
    Symptom: "chown: invalid argument" errors.
    Cause: See link below.
    Workaround:
      Apply the patch at
        <https://github.com/NixOS/nixpkgs/issues/10496#issuecomment-149893498>
      Run "op-build rebuild-host-fakeroot" to rebuild afterwards; the need
      to rebuild will not be detected automatically.