kbuild: generate asm-generic wrappers if mandatory headers are missing

Some time ago, Sam pointed out a certain degree of overwrap between
generic-y and mandatory-y. (https://lkml.org/lkml/2017/7/10/121)

I tweaked the meaning of mandatory-y a little bit; now it defines the
minimum set of ASM headers that all architectures must have.

If arch does not have specific implementation of a mandatory header,
Kbuild will let it fallback to the asm-generic one by automatically
generating a wrapper. This will allow to drop lots of redundant
generic-y defines.

Previously, "mandatory" was used in the context of UAPI, but I guess
this can be extended to kernel space ASM headers.
Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/makefiles.txt

@@ -1296,9 +1296,12 @@ See subsequent chapter for the syntax of the Kbuild file.
 
 --- 7.4 mandatory-y
 
-	mandatory-y is essentially used by include/uapi/asm-generic/Kbuild.asm
-	to define the minimum set of headers that must be exported in
-	include/asm.
+	mandatory-y is essentially used by include/(uapi/)asm-generic/Kbuild.asm
+	to define the minimum set of ASM headers that all architectures must have.
+
+	This works like optional generic-y. If a mandatory header is missing
+	in arch/$(ARCH)/include/(uapi/)/asm, Kbuild will automatically generate
+	a wrapper of the asm-generic one.
 
 	The convention is to list one subdir per line and
 	preferably in alphabetic order.

scripts/Makefile.asm-generic

@@ -14,6 +14,10 @@ src := $(subst /generated,,$(obj))
 
 include scripts/Kbuild.include
 
+# If arch does not implement mandatory headers, fallback to asm-generic ones.
+mandatory-y := $(filter-out $(generated-y), $(mandatory-y))
+generic-y   += $(foreach f, $(mandatory-y), $(if $(wildcard $(srctree)/$(src)/$(f)),,$(f)))
+
 generic-y   := $(addprefix $(obj)/, $(generic-y))
 generated-y := $(addprefix $(obj)/, $(generated-y))
 

scripts/Makefile.headersinst

@@ -56,13 +56,6 @@ check-file    := $(installdir)/.check
 all-files     := $(header-files) $(genhdr-files)
 output-files  := $(addprefix $(installdir)/, $(all-files))
 
-ifneq ($(mandatory-y),)
-missing       := $(filter-out $(all-files),$(mandatory-y))
-ifneq ($(missing),)
-$(error Some mandatory headers ($(missing)) are missing in $(obj))
-endif
-endif
-
 # Work out what needs to be removed
 oldheaders    := $(patsubst $(installdir)/%,%,$(wildcard $(installdir)/*.h))
 unwanted      := $(filter-out $(all-files),$(oldheaders))