Add some comments

luatexconfig.tex

@@ -6,6 +6,8 @@
   \catcode`\{=1 %
   \catcode`\}=2 %
   \catcode`\#=6 %
+  % Settings that were primitives
+  % accessed in newer LuaTeX releases via \pdfvariable
   \def\list{%
       {compresslevel}%
       {decimaldigits}%
@@ -21,9 +23,13 @@
       \expandafter\do
     \fi
   }%
+  % Enable all \pdf... primitives in one go
   \expandafter\do\list{output}{pageheight}{pagewidth}{variable}\relax
+  % Other required primitives that are not named \pdf...
   \directlua{tex.enableprimitives("",
     {"pageheight", "pagewidth", "outputmode"})}%
+  % Newer LuaTeX releases don't have \pdfoutput, etc.:
+  % emulate names where appropriate
   \ifx\pdfoutput\undefined
     \global\let\pdfoutput\outputmode
     \global\let\pdfpageheight\pageheight
@@ -36,13 +42,19 @@
     }%
     \expandafter\do\list\relax
   \fi
+  % The file pdftexconfig.tex contains only <primitive> = <value> lines
+  % so can now be read using the (emulated) primitives
+  % This needs to be global so set \globaldefs for this step
   \globaldefs=1 %
   \input{pdftexconfig}%
   \globaldefs=0 %
+  % Pick up on a request for DVI mode and apply it whilst \pdfoutput is
+  % still defined
   \ifx\dvimode\relax
     \global\pdfoutput=0 %
   \fi
   \global\let\dvimode\undefined
+  % Clean up all of the globally-allocated names
   \def\do#1{%
     \ifx\relax#1\else
       \global\expandafter\let\csname pdf#1\endcsname\undefined