After some testing with an ultrasparc cross-compiler:

Updating the cross-build expressions, adding some flexibility.
Updated the linux headers used cross building, as 2.6.28 had bugs on endianness in
sparc64.
There were, as usual some bugs in gcc. Maybe not many make a cross compiler to
ultrasparc.

For the record, I could build an ultrasparc kernel with this base nix:
import /etc/nixos/nixpkgs/default.nix   # The root nixpkgs default.nix
{
    crossSystem = {
        config = "sparc64-unknown-linux";
        bigEndian = true;
        arch = "sparc64";
        float = "soft";
        withTLS = true;
        cpu = "ultrasparc";
    };

    config = pkgs: {
      packageOverrides = pkgs : {
        platform = {
            name = "sparc64";
            kernelHeadersBaseConfig = "sparc64_defconfig";
            kernelBaseConfig = "sparc64_defconfig";
            kernelArch = "sparc";
            kernelAutoModules = false;
            kernelTarget = "zImage";
            uboot = null;
        };
      };
    };
}

Although it did not boot directly in qemu-system-sparc64:
[sparc64] Kernel already loaded
Unhandled Exception 0x0000000000000020
PC = 0x0000000000404000 NPC = 0x0000000000404004

svn path=/nixpkgs/trunk/; revision=20269

1 files changed, 6 insertions, 5 deletions

diff --git a/pkgs/os-specific/linux/kernel-headers/2.6.32.nix b/pkgs/os-specific/linux/kernel-headers/2.6.32.nix
index a09ad98c85c..dbc6797934d 100644
--- a/pkgs/os-specific/linux/kernel-headers/2.6.32.nix
+++ b/pkgs/os-specific/linux/kernel-headers/2.6.32.nix
@@ -1,21 +1,21 @@
-{stdenv, fetchurl, perl, cross ? null}:
+{stdenv, fetchurl, perl, cross ? null, platform}:
 
-assert stdenv.isLinux;
+assert cross == null -> stdenv.isLinux;
 
-let version = "2.6.32.4"; in
+let version = "2.6.32.9"; in
 
 stdenv.mkDerivation {
   name = "linux-headers-${version}";
 
   src = fetchurl {
     url = "mirror://kernel/linux/kernel/v2.6/linux-${version}.tar.bz2";
-    sha256 = "1n8pj05sazxv1dgi68q61lrvrnzvvx61qqw6kx80vqizqanz97z1";
+    sha256 = "1g6hs7j5kmifb3phbnckdmrnxd0cpqrijnnbry86z26npsh9my7l";
   };
 
   targetConfig = if (cross != null) then cross.config else null;
 
   platform =
-    if cross != null then cross.arch else
+    if cross != null then platform.kernelArch else
     if stdenv.system == "i686-linux" then "i386" else
     if stdenv.system == "x86_64-linux" then "x86_64" else
     if stdenv.system == "powerpc-linux" then "powerpc" else
@@ -33,6 +33,7 @@ stdenv.mkDerivation {
     if test -n "$targetConfig"; then
        export ARCH=$platform
     fi
+    make ${platform.kernelHeadersBaseConfig}
     make mrproper headers_check
   '';