nixos-hardware/raspberry-pi/4/apply-overlays-dtmerge.nix

# modification of nixpkgs deviceTree.applyOverlays to resolve https://github.com/NixOS/nixpkgs/issues/125354
# derived from https://github.com/NixOS/nixpkgs/blob/916ca8f2b0c208def051f8ea9760c534a40309db/pkgs/os-specific/linux/device-tree/default.nix
{ lib, pkgs, stdenvNoCC, dtc, libraspberrypi }:

with lib; (base: overlays': stdenvNoCC.mkDerivation {
  name = "device-tree-overlays";
  nativeBuildInputs = [ dtc libraspberrypi ];
  buildCommand = let
    overlays = toList overlays';
  in ''
    mkdir -p $out
    cd "${base}"
    find . -type f -name '*.dtb' -print0 \
      | xargs -0 cp -v --no-preserve=mode --target-directory "$out" --parents

    for dtb in $(find "$out" -type f -name '*.dtb'); do
      dtbCompat=$(fdtget -t s "$dtb" / compatible 2>/dev/null || true)
      # skip files without `compatible` string
      test -z "$dtbCompat" && continue

      ${flip (concatMapStringsSep "\n") overlays (o: ''
      overlayCompat="$(fdtget -t s "${o.dtboFile}" / compatible)"

      # skip incompatible and non-matching overlays
      if [[ ! "$dtbCompat" =~ "$overlayCompat" ]]; then
        echo "Skipping overlay ${o.name}: incompatible with $(basename "$dtb")"
        continue
      fi
      ${optionalString ((o.filter or null) != null) ''
      if [[ "''${dtb//${o.filter}/}" ==  "$dtb" ]]; then
        echo "Skipping overlay ${o.name}: filter does not match $(basename "$dtb")"
        continue
      fi
      ''}

      echo -n "Applying overlay ${o.name} to $(basename "$dtb")... "
      mv "$dtb"{,.in}

      # dtmerge requires a .dtbo ext for dtbo files, otherwise it adds it to the given file implicitly
      dtboWithExt="$TMPDIR/$(basename "${o.dtboFile}").dtbo"
      cp -r ${o.dtboFile} "$dtboWithExt"

      dtmerge "$dtb.in" "$dtb" "$dtboWithExt"

      echo "ok"
      rm "$dtb.in" "$dtboWithExt"
      '')}

    done'';
})