diff --git a/nixos/modules/system/boot/loader/grub/install-grub.pl b/nixos/modules/system/boot/loader/grub/install-grub.pl
index 4fa157641a4..4110335a9eb 100644
--- a/nixos/modules/system/boot/loader/grub/install-grub.pl
+++ b/nixos/modules/system/boot/loader/grub/install-grub.pl
@@ -12,8 +12,10 @@ require List::Compare;
 use POSIX;
 use Cwd;
 
+# system.build.toplevel path
 my $defaultConfig = $ARGV[1] or die;
 
+# Grub config XML generated by grubConfig function in grub.nix
 my $dom = XML::LibXML->load_xml(location => $ARGV[0]);
 
 sub get { my ($name) = @_; return $dom->findvalue("/expr/attrs/attr[\@name = '$name']/*/\@value"); }
@@ -97,6 +99,8 @@ sub PathInMount {
     }
     return 1;
 }
+
+# Figure out what filesystem is used for the directory with init/initrd/kernel files
 sub GetFs {
     my ($dir) = @_;
     my $bestFs = Fs->new(device => "", type => "", mount => "");