kbuild: remove header compile test

There are both positive and negative options about this feature.
At first, I thought it was a good idea, but actually Linus stated a
negative opinion (https://lkml.org/lkml/2019/9/29/227). I admit it
is ugly and annoying.

The baseline I'd like to keep is the compile-test of uapi headers.
(Otherwise, kernel developers have no way to ensure the correctness
of the exported headers.)

I will maintain a small build rule in usr/include/Makefile.
Remove the other header test functionality.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/makefiles.rst

@@ -1115,23 +1115,6 @@ When kbuild executes, the following steps are followed (roughly):
 	In this example, extra-y is used to list object files that
 	shall be built, but shall not be linked as part of built-in.a.
 
-    header-test-y
-
-	header-test-y specifies headers (`*.h`) in the current directory that
-	should be compile tested to ensure they are self-contained,
-	i.e. compilable as standalone units. If CONFIG_HEADER_TEST is enabled,
-	this builds them as part of extra-y.
-
-    header-test-pattern-y
-
-	This works as a weaker version of header-test-y, and accepts wildcard
-	patterns. The typical usage is::
-
-		header-test-pattern-y += *.h
-
-	This specifies all the files that matches to `*.h` in the current
-	directory, but the files in 'header-test-' are excluded.
-
 6.7 Commands useful for building a boot image
 ---------------------------------------------
 

Makefile

@@ -618,7 +618,6 @@ ifeq ($(KBUILD_EXTMOD),)
 init-y		:= init/
 drivers-y	:= drivers/ sound/
 drivers-$(CONFIG_SAMPLES) += samples/
-drivers-$(CONFIG_KERNEL_HEADER_TEST) += include/
 net-y		:= net/
 libs-y		:= lib/
 core-y		:= usr/

drivers/gpu/drm/i915/Kconfig.debug

@@ -7,7 +7,6 @@ config DRM_I915_WERROR
         # We use the dependency on !COMPILE_TEST to not be enabled in
         # allmodconfig or allyesconfig configurations
         depends on !COMPILE_TEST
-	select HEADER_TEST
         default n
         help
           Add -Werror to the build flags for (and only for) i915.ko.

init/Kconfig

@@ -104,29 +104,9 @@ config COMPILE_TEST
 	  here. If you are a user/distributor, say N here to exclude useless
 	  drivers to be distributed.
 
-config HEADER_TEST
-	bool "Compile test headers that should be standalone compilable"
-	help
-	  Compile test headers listed in header-test-y target to ensure they are
-	  self-contained, i.e. compilable as standalone units.
-
-	  If you are a developer or tester and want to ensure the requested
-	  headers are self-contained, say Y here. Otherwise, choose N.
-
-config KERNEL_HEADER_TEST
-	bool "Compile test kernel headers"
-	depends on HEADER_TEST
-	help
-	  Headers in include/ are used to build external moduls.
-	  Compile test them to ensure they are self-contained, i.e.
-	  compilable as standalone units.
-
-	  If you are a developer or tester and want to ensure the headers
-	  in include/ are self-contained, say Y here. Otherwise, choose N.
-
 config UAPI_HEADER_TEST
 	bool "Compile test UAPI headers"
-	depends on HEADER_TEST && HEADERS_INSTALL && CC_CAN_LINK
+	depends on HEADERS_INSTALL && CC_CAN_LINK
 	help
 	  Compile test headers exported to user-space to ensure they are
 	  self-contained, i.e. compilable as standalone units.

scripts/Makefile.build

@@ -283,15 +283,6 @@ quiet_cmd_cc_lst_c = MKLST   $@
 $(obj)/%.lst: $(src)/%.c FORCE
 	$(call if_changed_dep,cc_lst_c)
 
-# header test (header-test-y, header-test-m target)
-# ---------------------------------------------------------------------------
-
-quiet_cmd_cc_s_h = CC      $@
-      cmd_cc_s_h = $(CC) $(c_flags) -S -o $@ -x c /dev/null -include $<
-
-$(obj)/%.h.s: $(src)/%.h FORCE
-	$(call if_changed_dep,cc_s_h)
-
 # Compile assembler sources (.S)
 # ---------------------------------------------------------------------------
 

scripts/Makefile.lib

@@ -65,20 +65,6 @@ extra-y += $(patsubst %.dtb,%.dt.yaml, $(dtb-y))
 extra-$(CONFIG_OF_ALL_DTBS) += $(patsubst %.dtb,%.dt.yaml, $(dtb-))
 endif
 
-# Test self-contained headers
-
-# Wildcard searches in $(srctree)/$(src)/, but not in $(objtree)/$(obj)/.
-# Stale generated headers are often left over, so pattern matching should
-# be avoided. Please notice $(srctree)/$(src)/ and $(objtree)/$(obj) point
-# to the same location for in-tree building. So, header-test-pattern-y should
-# be used with care.
-header-test-y	+= $(filter-out $(header-test-), \
-		$(patsubst $(srctree)/$(src)/%, %, \
-		$(wildcard $(addprefix $(srctree)/$(src)/, \
-		$(header-test-pattern-y)))))
-
-extra-$(CONFIG_HEADER_TEST) += $(addsuffix .s, $(header-test-y) $(header-test-m))
-
 # Add subdir path
 
 extra-y		:= $(addprefix $(obj)/,$(extra-y))

usr/include/Makefile

@@ -95,9 +95,13 @@ endif
 # asm-generic/*.h is used by asm/*.h, and should not be included directly
 header-test- += asm-generic/%
 
-# The rest are compile-tested
-header-test-y += $(filter-out $(header-test-), \
-			$(patsubst $(obj)/%,%, $(wildcard \
-			$(addprefix $(obj)/, *.h */*.h */*/*.h */*/*/*.h))))
+extra-y := $(patsubst %.h,%.hdrtest, $(filter-out $(header-test-), \
+		$(patsubst $(obj)/%,%, $(shell find $(obj) -name '*.h'))))
+
+quiet_cmd_hdrtest = HDRTEST $<
+      cmd_hdrtest = $(CC) $(c_flags) -S -o /dev/null -x c /dev/null -include $<; touch $@
+
+$(obj)/%.hdrtest: $(obj)/%.h FORCE
+	$(call if_changed_dep,hdrtest)
 
 clean-files += $(filter-out Makefile, $(notdir $(wildcard $(obj)/*)))