[Buildroot] [PATCH] Makefile: add a Makefile wrapper in the $(O) dir - closes #2389

[Buildroot] [PATCH] Makefile: add a Makefile wrapper in the $(O) dir - closes #2389

[Yann E. MORIN]

From: Yann E. MORIN <yann.morin.1998@anciens.enib.fr>

If building out-of-tree, add a Makefile wrapper that calls-out
to the real Makefile with proper args.

Avoids having to pass -C and O= every time we call make.

This is highly inspired from how the Linux kernel does it, and
portions of it have been used.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
---
 Makefile           |   18 ++++++++++++++++--
 scripts/mkmakefile |   50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+), 2 deletions(-)
 create mode 100644 scripts/mkmakefile

diff --git a/Makefile b/Makefile
index 1ab446d..1f36fb8 100644
--- a/Makefile
+++ b/Makefile
@@ -45,6 +45,7 @@ space:=$(empty) $(empty)
 ifneq ("$(origin O)", "command line")
 O:=output
 CONFIG_DIR:=$(TOPDIR)
+NEED_WRAPPER=
 else
 # other packages might also support Linux-style out of tree builds
 # with the O=<dir> syntax (E.G. Busybox does). As make automatically
@@ -60,6 +61,7 @@ override O:=$(O)
 CONFIG_DIR:=$(O)
 # we need to pass O= everywhere we call back into the toplevel makefile
 EXTRAMAKEARGS = O=$(O)
+NEED_WRAPPER=y
 endif
 
 # $(shell find . -name *_defconfig |sed 's/.*\///')
@@ -335,7 +337,7 @@ prepare: $(BUILD_DIR)/buildroot-config/auto.conf
 world: prepare dependencies dirs $(BASE_TARGETS) $(TARGETS_ALL)
 
 
-.PHONY: all world dirs clean distclean source \
+.PHONY: all world dirs clean distclean source outputmakefile \
 	$(BASE_TARGETS) $(TARGETS) $(TARGETS_ALL) \
 	$(TARGETS_CLEAN) $(TARGETS_DIRCLEAN) $(TARGETS_SOURCE) \
 	$(DL_DIR) $(TOOLCHAIN_DIR) $(BUILD_DIR) $(STAGING_DIR) $(TARGET_DIR) \
@@ -465,7 +467,7 @@ all: menuconfig
 HOSTCFLAGS=$(CFLAGS_FOR_BUILD)
 export HOSTCFLAGS
 
-$(BUILD_DIR)/buildroot-config/%onf:
+$(BUILD_DIR)/buildroot-config/%onf: |outputmakefile
 	mkdir -p $(@D)/lxdialog
 	$(MAKE) CC="$(HOSTCC)" obj=$(@D) -C $(CONFIG) -f Makefile.br $(@F)
 
@@ -571,6 +573,18 @@ endif # ifeq ($(BR2_HAVE_DOT_CONFIG),y)
 # Cleanup and misc junk
 #
 #############################################################
+
+ifeq ($(NEED_WRAPPER),y)
+# outputmakefile generates a Makefile in the output directory, if using a
+# separate output directory. This allows convenient use of make in the
+# output directory.
+outputmakefile:
+	$(Q)$(SHELL) $(TOPDIR)/scripts/mkmakefile $(CURDIR) $(O)
+else
+outputmakefile:
+	@true
+endif
+
 clean:
 	rm -rf $(STAGING_DIR) $(TARGET_DIR) $(BINARIES_DIR) $(HOST_DIR) \
 		$(STAMP_DIR) $(BUILD_DIR) $(TOOLCHAIN_DIR)
diff --git a/scripts/mkmakefile b/scripts/mkmakefile
new file mode 100644
index 0000000..b2b3555
--- /dev/null
+++ b/scripts/mkmakefile
@@ -0,0 +1,50 @@
+#!/bin/sh
+# Generates a small Makefile used in the root of the output
+# directory, to allow make to be started from there.
+# The Makefile also allow for more convinient build of external modules
+
+# Usage
+# $1 - Kernel src directory
+# $2 - Output directory
+
+
+test ! -r $2/Makefile -o -O $2/Makefile || exit 0
+# Only overwrite automatically generated Makefiles
+# (so we do not overwrite buildroot Makefile)
+if test -e $2/Makefile && ! grep -E "^# Buildroot Makefile wrapper$" $2/Makefile >/dev/null 2>&1
+then
+        exit 0
+fi
+
+cat << EOF > $2/.Makefile
+# Buildroot Makefile wrapper
+# Automatically generated by $0: don't edit
+
+lastword = \$(word \$(words \$(1)),\$(1))
+makedir := \$(dir \$(call lastword,\$(MAKEFILE_LIST)))
+
+MAKEARGS := -C $1
+MAKEARGS += O=\$(if \$(patsubst /%,,\$(makedir)),\$(CURDIR)/)\$(patsubst %/,%,\$(makedir))
+
+MAKEFLAGS += --no-print-directory
+
+.PHONY: all \$(MAKECMDGOALS)
+
+all     := \$(filter-out all Makefile,\$(MAKECMDGOALS))
+
+all:
+	\$(MAKE) \$(MAKEARGS) \$(all)
+
+Makefile:;
+
+\$(all) %/: all
+	@:
+EOF
+
+if ! cmp $2/.Makefile $2/Makefile >/dev/null 2>&1; then
+    echo "  GEN    Makefile"
+    rm -f $2/Makefile
+    mv $2/.Makefile $2/Makefile
+else
+    rm -f $2/.Makefile
+fi
-- 
1.7.1