[PATCH] tools build: Fix fixdep dependencies

[Josh Poimboeuf]

The tools version of fixdep has broken dependencies.  It doesn't get
rebuilt if the host compiler or headers change.

Build fixdep with the tools kbuild infrastructure, so fixdep runs on
itself.  Due to the recursive dependency, its dependency file is
incomplete the very first time it gets built.  In that case build it a
second time to achieve fixdep inception.

Reported-by: Arthur Marsh <arthur.marsh@xxxxxxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 tools/build/Build    |  2 ++
 tools/build/Makefile | 23 +++++++++++++++++++++--
 2 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 tools/build/Build

diff --git a/tools/build/Build b/tools/build/Build
new file mode 100644
index 000000000000..1c7e598e9f59
--- /dev/null
+++ b/tools/build/Build
@@ -0,0 +1,2 @@
+hostprogs	:= fixdep
+fixdep-y	:= fixdep.o
diff --git a/tools/build/Makefile b/tools/build/Makefile
index 63ef21878761..a5b3c29b8d70 100644
--- a/tools/build/Makefile
+++ b/tools/build/Makefile
@@ -37,5 +37,24 @@ ifneq ($(wildcard $(TMP_O)),)
 	$(Q)$(MAKE) -C feature OUTPUT=$(TMP_O) clean >/dev/null
 endif
 
-$(OUTPUT)fixdep: $(srctree)/tools/build/fixdep.c
-	$(QUIET_CC)$(HOSTCC) $(KBUILD_HOSTCFLAGS) $(KBUILD_HOSTLDFLAGS) -o $@ $<
+include $(srctree)/tools/build/Makefile.include
+
+FIXDEP		:= $(OUTPUT)fixdep
+FIXDEP_IN	:= $(OUTPUT)fixdep-in.o
+
+# To track fixdep's dependencies properly, fixdep needs to run on itself.
+# Build it twice the first time.
+$(FIXDEP_IN): FORCE
+	$(Q)if [ ! -f $(FIXDEP) ]; then						\
+		$(MAKE) $(build)=fixdep HOSTCFLAGS="$(KBUILD_HOSTCFLAGS)";	\
+		rm -f $(FIXDEP).o;						\
+	fi
+	$(Q)$(MAKE) $(build)=fixdep HOSTCFLAGS="$(KBUILD_HOSTCFLAGS)"
+
+
+$(FIXDEP): $(FIXDEP_IN)
+	$(QUIET_LINK)$(HOSTCC) $(FIXDEP_IN) $(KBUILD_HOSTLDFLAGS) -o $@
+
+FORCE:
+
+.PHONY: FORCE
-- 
2.48.1