mesa: Only build OpenCL stuff on platforms where it works, fix build on ppc64

[OPNA2608]

OpenCL support doesn't work when building natively on non-LE platforms: https://catfox.life/2024/11/29/the-complexities-of-enabling-opencl-support/

Eventually, the SPIRV stuff that comes before this might need to be marked broken on non-LE just to communicate this properly. But the first place where I've directly stumbled into this is when building mesa.

OpenCL stuff is only needed when building specific drivers, so we need to exclude those when this option is set to false.

(this is untested beyond "it builds", I'm not super familiar with all this stuff)

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • powerpc64-linux (native)
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[OPNA2608]

(Back on a computer, evals again)

[OPNA2608]

Merge conflicts resolved, and made it build on powerpc64-linux hardware again.

[OPNA2608]

Resolved merge conflict.

[K900]

Wonder if there's a way to DRY this condition. Maybe making it a function argument with a default isn't that bad after all...

[OPNA2608]

Would have to move the default list into at least the let block I think?

{
  # ...
  galliumDrivers ? null,
  vulkanDrivers ? null,
  # ...
}:

let
  platformOpenCLWorks = /* ... */;
  galliumDrivers' =
    if galliumDrivers != null then
      galliumDrivers
    else (
      [
        # ...
      ]
      ++ lib.optionals platformOpenCLWorks [
        # ...
      ]
    );
  vulkanDrivers' =
    /* same pattern as above */;
  # ...
in
stdenv.mkDerivation {
  # ...
}

Don't think this it great… but it would DRY this without introducing a function argument.

[OPNA2608]

Patches to mesa got merged upstream, switched to fetchpatching them instead.

[K900]

Are those patches only in Mesa main or also 26.0.0? Do you have a link to the PRs? I can ask around about getting them backported so we don't have to ship them.

[OPNA2608]

Are those patches only in Mesa main or also 26.0.0?

In main (1, 2), cherry-picked to staging/26.0 (3, 4). You prolly know better than me how their backporting pipeline works, if that's enough etc.

Do you have a link to the PRs?

PR links are in the comments above each fetchpatch call.

[K900]

OK so that means it'll be in 26.0.0 in a few days.

[OPNA2608]

Rebased on master for Mesa 26.0.0, dropped fetchpatches of now-shipped fixes.

With the idea from #473862 (comment), pkgsCross.ppc64-elfv1.mesa builds.