[Buildroot] [PATCH] Makefile: generate a Makefile wrapper in $(O)

[Yann E. MORIN]

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. We can't use exactly the same implementation as the
kernel does, because:

 - the kernel always overwrites the wrapper at each call: doing so in
   buildroot makes the kconfig stuff be rebuilt every time;

 - the script writing the wrapper has been expunged of the few lines
   that were too kernel-related: in buildroot we do not need the version
   string in the wrapper, and we do not have a patchlevel version;

 - "in-tree build" does not have the same meaning for the kernel and for
   buildroot: for the kernel, $(O) point to the $(TOPDIR), while for
   buildroot $(O) points to $(TOPDIR)/output.

For more complete explnanations, see:
  http://lists.busybox.net/pipermail/buildroot/2010-September/037815.html

Signed-off-by: "Yann E. MORIN" <yann.morin.1998 at anciens.enib.fr>
---
 Makefile           |   18 ++++++++++++++-
 scripts/mkmakefile |   53 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index faa802b..ec425c6 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/.*\///')
@@ -324,7 +326,7 @@ 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) $(TARGET_DIR) \
@@ -453,7 +455,7 @@ all: menuconfig
 HOSTCFLAGS=$(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)
 
@@ -559,6 +561,18 @@ 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..39dd292
--- /dev/null
+++ b/scripts/mkmakefile
@@ -0,0 +1,53 @@
+#!/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
+	@:
+
+%/: 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