[RFC] xen/arm: build: add missed dependency for head.S

[Wei Chen <wei.chen@linaro.org>]

When we update the header files that had been included in head.S.
The build system would not re-compile the head.S. Because in the
build rules, the dependencies are setting to .*.d (eg. DEPS = .*.d)
files in the same folder as Makefile.

But head.S is very special, it was used by the Makefile in the parent
folder: "ALL_OBJS := $(TARGET_SUBARCH)/head.o".

In this case, the build system could not find the dependency in DEPS.
When we update the header files, the build system is unware of this
update. If we re-build the Xen without doing make clean or touching
the head.S, the build system will not recompile the head.S.

Signed-off-by: Wei Chen <Wei.Chen@linaro.org>
---
In my mind, the better way to fix this bug is converting the DEPS from
ALL_OBJS. But I am afraid of the impact. I am not sure whether there 
are some dependencies are not generated from obj files.
---
xen/Rules.mk          | 3 ++-
 xen/arch/arm/Makefile | 2 ++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/xen/Rules.mk b/xen/Rules.mk
index 961d533..1247b61 100644
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -92,9 +92,10 @@ LDFLAGS += $(LDFLAGS-y)
 
 include $(BASEDIR)/arch/$(TARGET_ARCH)/Rules.mk
 
+DEPS = .*.d
+
 include Makefile
 
-DEPS = .*.d
 define gendep
     ifneq ($(1),$(subst /,:,$(1)))
         DEPS += $(dir $(1)).$(notdir $(1)).d
diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
index af4d0e1..9e38da3 100644
--- a/xen/arch/arm/Makefile
+++ b/xen/arch/arm/Makefile
@@ -51,6 +51,8 @@ endif
 
 ALL_OBJS := $(TARGET_SUBARCH)/head.o $(ALL_OBJS)
 
+DEPS += $(TARGET_SUBARCH)/.head.o.d
+
 $(TARGET): $(TARGET)-syms $(TARGET).axf
 	$(OBJCOPY) -O binary -S $< $@
 ifeq ($(CONFIG_ARM_64),y)
-- 
2.7.4


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