[PATCH 3/5] kbuild: generate asm-generic wrappers if mandatory headers are missing

From: Masahiro Yamada
Date: Wed Jan 02 2019 - 20:12:05 EST

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@xxxxxxxxxxxx>
Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Acked-by: Sam Ravnborg <sam@xxxxxxxxxxxx>

Changes in v2:
- Filter-out $(generiated-y) from $(mandatory-y) because
ARCH may generate mandatory headers by other means

Documentation/kbuild/makefiles.txt | 9 ++++++---
scripts/Makefile.asm-generic | 4 ++++
scripts/Makefile.headersinst | 7 -------
3 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 8da26c6..bf28c47 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/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.
diff --git a/scripts/Makefile.asm-generic b/scripts/Makefile.asm-generic
index 760323e..a62d282 100644
--- a/scripts/Makefile.asm-generic
+++ b/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))

diff --git a/scripts/Makefile.headersinst b/scripts/Makefile.headersinst
index 45927fc..3d1ebaa 100644
--- a/scripts/Makefile.headersinst
+++ b/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))
# Work out what needs to be removed
oldheaders := $(patsubst $(installdir)/%,%,$(wildcard $(installdir)/*.h))
unwanted := $(filter-out $(all-files),$(oldheaders))