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 <[email protected]>

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.