[RFC 1/3] kbuild: add support for ensuring headers are self-contained

From: Jani Nikula
Date: Thu May 16 2019 - 15:50:37 EST


Sometimes it's useful to be able to explicitly ensure certain headers
remain self-contained, i.e. that they are compilable as standalone
units, by including and/or forward declaring everything they depend on.

Add special target header-test-y where individual Makefiles can add
headers to be tested if CONFIG_HEADER_TEST is enabled. This will
generate a dummy C file per header that gets built as part of extra-y.

Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: Michal Marek <michal.lkml@xxxxxxxxxxx>
Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx>
---
Documentation/kbuild/makefiles.txt | 7 +++++++
init/Kconfig | 9 +++++++++
scripts/Makefile.build | 10 ++++++++++
scripts/Makefile.lib | 3 +++
4 files changed, 29 insertions(+)

diff --git a/Documentation/kbuild/makefiles.txt b/Documentation/kbuild/makefiles.txt
index 03c065855eaf..73df58e5ea0c 100644
--- a/Documentation/kbuild/makefiles.txt
+++ b/Documentation/kbuild/makefiles.txt
@@ -1036,6 +1036,13 @@ 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 autogenerates dummy sources to include the headers, and builds them
+ as part of extra-y.

--- 6.7 Commands useful for building a boot image

diff --git a/init/Kconfig b/init/Kconfig
index 4592bf7997c0..d91b157201b1 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -95,6 +95,15 @@ 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 LOCALVERSION
string "Local version - append to kernel release"
help
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 76ca30cc4791..4d4bf698467a 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -291,6 +291,16 @@ quiet_cmd_cc_lst_c = MKLST $@
$(obj)/%.lst: $(src)/%.c FORCE
$(call if_changed_dep,cc_lst_c)

+# Dummy C sources for header test (header-test-y target)
+# ---------------------------------------------------------------------------
+
+quiet_cmd_header_test = HDRTEST $@
+ cmd_header_test = echo "\#include \"$(<F)\"" > $@
+
+# FIXME: would be nice to be able to limit this implicit rule to header-test-y
+$(obj)/%.header_test.c: $(src)/%.h FORCE
+ $(call if_changed,header_test)
+
# Compile assembler sources (.S)
# ---------------------------------------------------------------------------

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 8a1f64f17740..c2839de06485 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -66,6 +66,9 @@ extra-y += $(patsubst %.dtb,%.dt.yaml, $(dtb-y))
extra-$(CONFIG_OF_ALL_DTBS) += $(patsubst %.dtb,%.dt.yaml, $(dtb-))
endif

+# Test self-contained headers
+extra-$(CONFIG_HEADER_TEST) += $(patsubst %.h,%.header_test.o,$(header-test-y))
+
# Add subdir path

extra-y := $(addprefix $(obj)/,$(extra-y))
--
2.20.1