kbuild: support header-test-pattern-y

In my view, most of headers can be self-contained. So, it would be
tedious to add every header to header-test-y explicitly. We usually
end up with "all headers with some exceptions".

There are two types in exceptions:

[1] headers that are never compiled as standalone units

  For examples, include/linux/compiler-gcc.h is not intended for
  direct inclusion. We should always exclude such ones.

[2] headers that are conditionally compiled as standalone units

  Some headers can be compiled only for particular architectures.
  For example, include/linux/arm-cci.h can be compiled only for
  arm/arm64 because it requires <asm/arm-cci.h> to exist.
  Clang can compile include/soc/nps/mtm.h only for arc because
  it contains an arch-specific register in inline assembler.

So, you can write Makefile like this:

  header-test-                += linux/compiler-gcc.h
  header-test-$(CONFIG_ARM)   += linux/arm-cci.h
  header-test-$(CONFIG_ARM64) += linux/arm-cci.h
  header-test-$(CONFIG_ARC)   += soc/nps/mtm.h

The new syntax header-test-pattern-y will be useful to specify
"the rest".

The typical usage is like this:

  header-test-pattern-y += */*.h

This will add all the headers in sub-directories to the test coverage,
excluding $(header-test-). In this regards, header-test-pattern-y
behaves like a weaker variant of header-test-y.

Caveat:
The patterns in header-test-pattern-y are prefixed with $(srctree)/$(src)/
but not $(objtree)/$(obj)/. Stale generated headers are often left over
when you traverse the git history without cleaning. Wildcard patterns for
$(objtree) may match to stale headers, which could fail to compile.
One pitfall is $(srctree)/$(src)/ and $(objtree)/$(obj)/ point to the
same directory for in-tree building. So, header-test-pattern-y should
be used with care since it can potentially match to stale headers.

Caveat2:
You could use wildcard for header-test-. For example,

  header-test- += asm-generic/%

... will exclude headers in asm-generic directory. Unfortunately, the
wildcard character is '%' instead of '*' here because this is evaluated
by $(filter-out ...) whereas header-test-pattern-y is evaluated by
$(wildcard ...). This is a kludge, but seems useful in some places...
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>

Documentation/kbuild/makefiles.txt

@@ -1025,6 +1025,16 @@ When kbuild executes, the following steps are followed (roughly):
 	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
 
 	Kbuild provides a few macros that are useful when building a

scripts/Makefile.lib

@@ -67,6 +67,17 @@ 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))
 
 # Add subdir path