From 30a7729771731971839cc969d2a321e6ea7a144b Mon Sep 17 00:00:00 2001
From: Masahiro Yamada <masahiroy@kernel.org>
Date: Thu, 23 Apr 2020 23:23:53 +0900
Subject: [PATCH] kbuild: use -MMD instead of -MD to exclude system headers
 from dependency

This omits system headers from the generated header dependency.

System headers are not updated unless you upgrade the compiler. Nor do
they contain CONFIG options, so fixdep does not need to parse them.

Having said that, the effect of this optimization will be quite small
because the kernel code generally does not include system headers
except <stdarg.h>. Host programs include a lot of system headers,
but there are not so many in the kernel tree.

At first, keeping system headers in .*.cmd files might be useful to
detect the compiler update, but there is no guarantee that <stdarg.h>
is included from every file. So, I implemented a more reliable way in
the previous commit.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---
 scripts/Kbuild.include | 2 +-
 scripts/Makefile.host  | 4 ++--
 scripts/Makefile.lib   | 8 ++++----
 usr/include/Makefile   | 2 +-
 4 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 6cabf20ce66a3..0c3dc983439bc 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -16,7 +16,7 @@ pound := \#
 dot-target = $(dir $@).$(notdir $@)
 
 ###
-# The temporary file to save gcc -MD generated dependencies must not
+# The temporary file to save gcc -MMD generated dependencies must not
 # contain a comma
 depfile = $(subst $(comma),_,$(dot-target).d)
 
diff --git a/scripts/Makefile.host b/scripts/Makefile.host
index 2045855d0b759..c8a4a033dc3e9 100644
--- a/scripts/Makefile.host
+++ b/scripts/Makefile.host
@@ -88,8 +88,8 @@ _hostcxx_flags += -I $(objtree)/$(obj)
 endif
 endif
 
-hostc_flags    = -Wp,-MD,$(depfile) $(_hostc_flags)
-hostcxx_flags  = -Wp,-MD,$(depfile) $(_hostcxx_flags)
+hostc_flags    = -Wp,-MMD,$(depfile) $(_hostc_flags)
+hostcxx_flags  = -Wp,-MMD,$(depfile) $(_hostcxx_flags)
 
 #####
 # Compile programs on the host
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 4b799737722c8..12f6a331a8f33 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -171,22 +171,22 @@ modkern_aflags = $(if $(part-of-module),				\
 			$(KBUILD_AFLAGS_MODULE) $(AFLAGS_MODULE),	\
 			$(KBUILD_AFLAGS_KERNEL) $(AFLAGS_KERNEL))
 
-c_flags        = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
+c_flags        = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
 		 -include $(srctree)/include/linux/compiler_types.h       \
 		 $(_c_flags) $(modkern_cflags)                           \
 		 $(basename_flags) $(modname_flags)
 
-a_flags        = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
+a_flags        = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
 		 $(_a_flags) $(modkern_aflags)
 
-cpp_flags      = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
+cpp_flags      = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE)     \
 		 $(_cpp_flags)
 
 ld_flags       = $(KBUILD_LDFLAGS) $(ldflags-y) $(LDFLAGS_$(@F))
 
 DTC_INCLUDE    := $(srctree)/scripts/dtc/include-prefixes
 
-dtc_cpp_flags  = -Wp,-MD,$(depfile).pre.tmp -nostdinc                    \
+dtc_cpp_flags  = -Wp,-MMD,$(depfile).pre.tmp -nostdinc                    \
 		 $(addprefix -I,$(DTC_INCLUDE))                          \
 		 -undef -D__DTS__
 
diff --git a/usr/include/Makefile b/usr/include/Makefile
index b568a95d1f62e..5a7ee3e5ed869 100644
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -8,7 +8,7 @@
 # We cannot go as far as adding -Wpedantic since it emits too many warnings.
 UAPI_CFLAGS := -std=c90 -Wall -Werror=implicit-function-declaration
 
-override c_flags = $(UAPI_CFLAGS) -Wp,-MD,$(depfile) -I$(objtree)/usr/include
+override c_flags = $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I$(objtree)/usr/include
 
 # The following are excluded for now because they fail to build.
 #
-- 
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