diff --git a/scripts/bgfx-codegen.lua b/scripts/bgfx-codegen.lua
index 6b253e31f..4069b933f 100644
--- a/scripts/bgfx-codegen.lua
+++ b/scripts/bgfx-codegen.lua
@@ -1,9 +1,8 @@
 -- Copyright 2019 云风 https://github.com/cloudwu . All rights reserved.
 -- License (the same with bgfx) : https://github.com/bkaradzic/bgfx/blob/master/LICENSE
 
-local idl     = require "idl"
 local codegen = require "codegen"
-local doxygen = require "doxygen"
+local idl = codegen.idl "bgfx.idl"
 
 local func_actions = {
 
@@ -31,14 +30,6 @@ local type_actions = {
 	cfuncptrs = "\n",
 }
 
-do
-	local source = doxygen.load "bgfx.idl"
-	local f = assert(load(source, "bgfx.idl" , "t", idl))
-	f()
-end
-
-codegen.nameconversion(idl.types, idl.funcs)
-
 local function cfunc(f)
 	return function(func)
 		if not func.cpponly then
diff --git a/scripts/codegen.lua b/scripts/codegen.lua
index 63949262e..2d90cc223 100644
--- a/scripts/codegen.lua
+++ b/scripts/codegen.lua
@@ -248,7 +248,7 @@ local function calc_flag_values(flag)
 				value = index + base - 1
 			end
 			if value >= cap then
-				error (string.format("Out of range for %s (%d/%d)", name, value, cap))
+				error (string.format("Out of range for %s.%s (%d/%d)", flag.name, item.name, value, cap))
 			end
 			value = value << shift
 		elseif #item == 0 then
@@ -964,4 +964,31 @@ function codegen.gen_handle(handle)
 	return (handle_temp:gsub("$(%u+)", { NAME = handle.name }))
 end
 
+local idl = require "idl"
+local doxygen = require "doxygen"
+local conversion
+local idlfile = {}
+
+function codegen.load(filename)
+	assert(conversion == nil, "Don't call codegen.load() after codegen.idl()")
+	assert(idlfile[filename] == nil, "Duplicate load " .. filename)
+	local source = doxygen.load(filename)
+
+	local f = assert(load(source, filename , "t", idl))
+	f()
+	idlfile[filename] = true
+end
+
+function codegen.idl(filename)
+	if conversion == nil then
+		if filename and not idlfile[filename] then
+			codegen.load(filename)
+		end
+		assert(next(idlfile), "call codegen.load() first")
+		conversion = true
+		codegen.nameconversion(idl.types, idl.funcs)
+	end
+	return idl
+end
+
 return codegen
diff --git a/scripts/csharp.lua b/scripts/csharp.lua
index 7a5388932..f0a1cdc2f 100644
--- a/scripts/csharp.lua
+++ b/scripts/csharp.lua
@@ -1,14 +1,5 @@
-local idl = require "idl"
-
-do
-	local doxygen = require "doxygen"
-	local source = doxygen.load "bgfx.idl"
-	local f = assert(load(source, "bgfx.idl" , "t", idl))
-	f()
-
-	local codegen = require "codegen"
-	codegen.nameconversion(idl.types, idl.funcs)
-end
+local codegen = require "codegen"
+local idl = codegen.idl "bgfx.idl"
 
 local csharp_template = [[
 using System;
diff --git a/scripts/doxygen.lua b/scripts/doxygen.lua
index e2607d2fd..531210e7b 100644
--- a/scripts/doxygen.lua
+++ b/scripts/doxygen.lua
@@ -1,5 +1,3 @@
-local idl = require "idl"
-
 local doxygen = {}
 
 function doxygen.load(filename)