[PATCH v3 1/2] kbuild: add read-file macro

[Masahiro Yamada]

Since GNU Make 4.2, $(file ...) supports the read operater '<', which
is useful to read a file without forking a new process. No warning is
shown even if the input file is missing.

For older Make versions, it falls back to the cat command.

Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
Reviewed-by: Nicolas Schier <nicolas@xxxxxxxxx>
Reviewed-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
Tested-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
---

Changes in v3:
  - Check the MAKE_VERSION in an ad-hoc way

 Makefile                  |  2 +-
 scripts/Kbuild.include    | 14 ++++++++++++++
 scripts/Makefile.modfinal |  2 +-
 scripts/Makefile.modinst  |  2 +-
 4 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 8801cac4d3d5..2dda1e9a717a 100644
--- a/Makefile
+++ b/Makefile
@@ -376,7 +376,7 @@ else # !mixed-build
 include $(srctree)/scripts/Kbuild.include
 
 # Read KERNELRELEASE from include/config/kernel.release (if it exists)
-KERNELRELEASE = $(shell cat include/config/kernel.release 2> /dev/null)
+KERNELRELEASE = $(call read-file, include/config/kernel.release)
 KERNELVERSION = $(VERSION)$(if $(PATCHLEVEL),.$(PATCHLEVEL)$(if $(SUBLEVEL),.$(SUBLEVEL)))$(EXTRAVERSION)
 export VERSION PATCHLEVEL SUBLEVEL KERNELRELEASE KERNELVERSION
 
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 5019bc1e38e4..abdc269a51da 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -10,6 +10,10 @@ empty   :=
 space   := $(empty) $(empty)
 space_escape := _-_SPACE_-_
 pound := \#
+define newline
+
+
+endef
 
 ###
 # Comparison macros.
@@ -61,6 +65,16 @@ stringify = $(squote)$(quote)$1$(quote)$(squote)
 kbuild-dir = $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
 kbuild-file = $(or $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Makefile)
 
+###
+# Read a file, replacing newlines with spaces
+#
+# Make 4.2 or later can read a file by using its builtin function.
+ifneq ($(filter-out 3.% 4.0 4.1, $(MAKE_VERSION)),)
+read-file = $(subst $(newline),$(space),$(file < $1))
+else
+read-file = $(shell cat $1 2>/dev/null)
+endif
+
 ###
 # Easy method for doing a status message
        kecho := :
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 25bedd83644b..7252f6cf7837 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -13,7 +13,7 @@ include $(srctree)/scripts/Kbuild.include
 include $(srctree)/scripts/Makefile.lib
 
 # find all modules listed in modules.order
-modules := $(sort $(shell cat $(MODORDER)))
+modules := $(sort $(call read-file, $(MODORDER)))
 
 __modfinal: $(modules)
 	@:
diff --git a/scripts/Makefile.modinst b/scripts/Makefile.modinst
index a4c987c23750..509d424dbbd2 100644
--- a/scripts/Makefile.modinst
+++ b/scripts/Makefile.modinst
@@ -9,7 +9,7 @@ __modinst:
 include include/config/auto.conf
 include $(srctree)/scripts/Kbuild.include
 
-modules := $(sort $(shell cat $(MODORDER)))
+modules := $(sort $(call read-file, $(MODORDER)))
 
 ifeq ($(KBUILD_EXTMOD),)
 dst := $(MODLIB)/kernel
-- 
2.34.1