[PATCH v3 3/5] kbuild: add read-file macro

[Masahiro Yamada <masahiroy@kernel.org>]

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

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

The added ifeq will break when GNU Make 4.10 or 10.0 is released.
It will take a long time if the current release pace continues.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
---

(no changes since v1)

 Makefile                  |  2 +-
 scripts/Kbuild.include    | 15 +++++++++++++++
 scripts/Makefile.modfinal |  2 +-
 scripts/Makefile.modinst  |  2 +-
 4 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index eb80332f7b51..60ce9dcafc72 100644
--- a/Makefile
+++ b/Makefile
@@ -369,7 +369,7 @@ 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 4b8cf464b53b..55c2243f91c8 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.
@@ -55,6 +59,17 @@ 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
+#
+# This ifeq will break when GNU Make 4.10 is released.
+# Remove this conditional until then.
+ifeq ($(call test-ge, $(MAKE_VERSION), 4.2),y)
+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