X-Git-Url: http://akaros.cs.berkeley.edu/gitweb/?p=akaros.git;a=blobdiff_plain;f=Makefile;h=5f7681f3dde6e074dd872bef8b875834a5aa1df2;hp=9897db00a48e74851be711d16c574d6b78170274;hb=f195b3f1e068302e5ce7459fd1a5aba68503f0bf;hpb=758d2e8950e46aeb583b33b4b89a154fccf035f5

diff --git a/Makefile b/Makefile
index 9897db0..5f7681f 100644
--- a/Makefile
+++ b/Makefile
@@ -16,25 +16,18 @@
 #	of the default settings that silentoldconfig uses, but I'll leave it as-is
 #	for now, and just symlink that into kern/include.  It'll be easier for us,
 #	and also potentially easier if we ever move kern/ up a level.  Similarly,
-#	there are default Kconfigs in arch/, not in kern/arch.  I might change
-#	that, if we want to support defconfig, or just symlink it.
+#	there are default Kconfigs in arch/, not in kern/arch.  I just symlinked
+#	arch->kern/arch to keep everything simple.
 #
 # TODO:
-# 	- Connect to kconfig, have it generate our CONFIGS, instead of makelocal.
-#		- what about userspace / tests?
-#			the only one is SYSCALL_TRAP.  the others control building
-#			- keep them using the old style
-#		- might need to symlink arch -> kern/arch/ too
-#			- though def config is hardcoded to arch/$ARCH, instead of k/a/
-#
 #	- Consider merging the two target-detection bits (Linux's config, mixed, or
-#	dot target, and the symlink handling).
-#
-#	- Do we want some sort of default config?  Or the ability to change arches
-#	and keep common vars?
+#	dot target, and the symlink handling).  Also, could consider moving around
+#	the KFS and EXT2 targets.  Clean doesn't need to know about them, for
+#	instance.
 #
 #	- Review, with an eye for being better about $(srctree).  It might only be
-#	necessary in this file, if we every do the KBUILD_OUTPUT option
+#	necessary in this file, if we every do the KBUILD_OUTPUT option.  But we
+#	don't always want it (like for the implicit rule for Makefile)
 #
 #	- It's a bit crazy that we build symlinks for parlib, instead of it
 #	managing its own links based on $(ARCH)
@@ -54,6 +47,14 @@ MAKEFLAGS += -rR --no-print-directory
 PHONY := all
 all: akaros-kernel
 
+# Export the location of this top level directory
+AKAROS_ROOT = $(CURDIR)
+export AKAROS_ROOT
+
+# Setup dumping ground for object files and any temporary files we need to
+# generate for non-kbuild targets
+OBJDIR ?= obj
+
 # Symlinks
 # =========================================================================
 # We have a few symlinks so that code can include <arch/whatever.h>.  This
@@ -78,8 +79,8 @@ goals-has-build-targets := 1
 endif
 
 PHONY += symlinks clean_symlinks
-clean_symlinks:
-	@-rm -f kern/include/arch kern/boot user/parlib/include/arch
+clean_symlinks: objclean
+	@rm -f kern/include/arch kern/boot user/parlib/include/arch
 
 arch-link := $(notdir $(shell readlink kern/include/arch))
 valid-arches := $(notdir $(wildcard kern/arch/*))
@@ -119,7 +120,8 @@ symlinks:
     endif # ifneq ($(arch-link),)
 endif # ifeq ($(ARCH),)
 
-export ARCH
+SRCARCH := $(ARCH)
+export ARCH SRCARCH
 
 # Generic Kbuild Environment
 # =========================================================================
@@ -187,68 +189,6 @@ export quiet Q KBUILD_VERBOSE
 $(srctree)/scripts/Kbuild.include: ;
 include $(srctree)/scripts/Kbuild.include
 
-# Akaros Build Environment
-# =========================================================================
-AKAROSINCLUDE   := \
-				-I$(srctree)/kern/arch/ \
-				-I$(srctree)/kern/include/
-
-CROSS_COMPILE := $(ARCH)-ros-
-
-CC	    := $(CROSS_COMPILE)gcc 
-CPP	    := $(CROSS_COMPILE)g++
-AS	    := $(CROSS_COMPILE)as
-AR	    := $(CROSS_COMPILE)ar
-LD	    := $(CROSS_COMPILE)ld
-OBJCOPY	:= $(CROSS_COMPILE)objcopy
-OBJDUMP	:= $(CROSS_COMPILE)objdump
-NM	    := $(CROSS_COMPILE)nm
-STRIP   := $(CROSS_COMPILE)strip
-
-ifeq ($(goals-has-build-targets),1)
-ifeq ($(shell which $(CROSS_COMPILE)gcc 2>/dev/null ),)
-$(error Could not find a $(CROSS_COMPILE) version of GCC/binutils. \
-        Be sure to build the cross-compiler and update your PATH)
-endif
-# Computing these without a cross compiler complains loudly
-gcc-lib := $(shell $(CC) -print-libgcc-file-name)
-NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
-# Note: calling this GCC_ROOT interferes with the host tools
-XCC_ROOT := $(dir $(shell which $(CC)))../
-endif
-
-CFLAGS_KERNEL += -O2 -pipe -MD -gstabs
-CFLAGS_KERNEL += -std=gnu99 -fgnu89-inline
-CFLAGS_KERNEL += -fno-strict-aliasing -fno-omit-frame-pointer
-CFLAGS_KERNEL += -fno-stack-protector
-CFLAGS_KERNEL += -Wall -Wno-format -Wno-unused
-CFLAGS_KERNEL += -DROS_KERNEL 
-CFLAGS_KERNEL += -include include/generated/autoconf.h
-
-# TODO: do we need this, or can we rely on the compiler's defines?
-CFLAGS_KERNEL += -D$(ARCH)
-
-# TODO: this requires our own strchr (kern/src/stdio.c), which is a potential
-# source of bugs/problems.
-# note we still pull in stdbool and stddef from the compiler
-CFLAGS_KERNEL += -fno-builtin
-
-AFLAGS_KERNEL := $(CFLAGS_KERNEL)
-
-KBUILD_BUILTIN := 1
-KBUILD_CHECKSRC := 0
-
-export AKAROSINCLUDE CROSS_COMPILE
-export CC CPP AS AR LD OBJCOPY OBJDUMP NM STRIP
-export CFLAGS_KERNEL AFLAGS_KERNEL
-export NOSTDINC_FLAGS XCC_ROOT
-export KBUILD_BUILTIN KBUILD_CHECKSRC
-
-CFLAGS_USER += -O2 -std=gnu99 -fno-stack-protector -fgnu89-inline
-CXXFLAGS_USER += -O2
-
-export CFLAGS_USER CXXFLAGS_USER
-
 # Kbuild Target/Goals Parsing
 # =========================================================================
 # Need to figure out if we're a config or not, and whether or not to include
@@ -297,6 +237,13 @@ ifeq ($(config-targets),1)
 # ===========================================================================
 # *config targets only - make sure prerequisites are updated, and descend
 # in scripts/kconfig to make the *config target
+
+# Default config file, per arch.  This path will resolve to
+# arch/$ARCH/configs/defconfig (arch -> kern/arch).  Each arch can override
+# this if they want, or just symlink to one in the main root directory.
+KBUILD_DEFCONFIG := defconfig
+export KBUILD_DEFCONFIG
+
 config: scripts_basic symlinks FORCE
 	$(Q)mkdir -p include/config
 	$(Q)$(MAKE) $(build)=scripts/kconfig $@
@@ -336,28 +283,103 @@ else
 include/config/auto.conf: ;
 endif # $(dot-config)
 
+# Akaros Build Environment
+# =========================================================================
+AKAROSINCLUDE   := -I$(srctree)/kern/include/
+
+# CROSS_COMPILE is defined per-arch.  Each arch can set other makeflags, kbuild
+# directories, etc. 
+-include $(srctree)/kern/arch/$(ARCH)/Makefile
+
+CC	    := $(CROSS_COMPILE)gcc 
+CPP	    := $(CROSS_COMPILE)g++
+AS	    := $(CROSS_COMPILE)as
+AR	    := $(CROSS_COMPILE)ar
+LD	    := $(CROSS_COMPILE)ld
+OBJCOPY	:= $(CROSS_COMPILE)objcopy
+OBJDUMP	:= $(CROSS_COMPILE)objdump
+NM	    := $(CROSS_COMPILE)nm
+STRIP   := $(CROSS_COMPILE)strip
+KERNEL_LD ?= kernel.ld
+
+# These may have bogus values if there is no compiler.  The kernel and user
+# build targets will check cc-exists.  Hopefully no cleaning targets rely on
+# these.  Note that if you change configs, these will get computed once, before
+# silentoldconfig kicks in to regenerate auto.conf, and these values will
+# temporarily be stale.
+gcc-lib := $(shell $(CC) -print-libgcc-file-name 2>/dev/null)
+NOSTDINC_FLAGS += -nostdinc -isystem \
+                  $(shell $(CC) -print-file-name=include 2>/dev/null)
+XCC_TARGET_ROOT := $(dir $(shell which $(CC) 2> /dev/null))../$(patsubst %-,%,\
+                                                               $(CROSS_COMPILE))
+
+CFLAGS_KERNEL += -O2 -pipe -MD
+CFLAGS_KERNEL += -std=gnu99 -fgnu89-inline
+CFLAGS_KERNEL += -fno-strict-aliasing -fno-omit-frame-pointer
+CFLAGS_KERNEL += -fno-stack-protector
+CFLAGS_KERNEL += -Wall -Wno-format -Wno-unused
+CFLAGS_KERNEL += -DROS_KERNEL 
+CFLAGS_KERNEL += -include include/generated/autoconf.h -include include/common.h
+CFLAGS_KERNEL += -fplan9-extensions
+ifeq ($(CONFIG_64BIT),y)
+CFLAGS_KERNEL += -m64 -g
+else
+CFLAGS_KERNEL += -m32 -gstabs
+endif
+
+# TODO: do we need this, or can we rely on the compiler's defines?
+CFLAGS_KERNEL += -D$(ARCH)
+
+# TODO: this requires our own strchr (kern/src/stdio.c), which is a potential
+# source of bugs/problems.
+# note we still pull in stdbool and stddef from the compiler
+CFLAGS_KERNEL += -fno-builtin
+
+AFLAGS_KERNEL := $(CFLAGS_KERNEL)
+
+KBUILD_BUILTIN := 1
+KBUILD_CHECKSRC := 0
+
+export AKAROSINCLUDE CROSS_COMPILE
+export CC CPP AS AR LD OBJCOPY OBJDUMP NM STRIP
+export CFLAGS_KERNEL AFLAGS_KERNEL
+export NOSTDINC_FLAGS XCC_TARGET_ROOT
+export KBUILD_BUILTIN KBUILD_CHECKSRC
+
+CFLAGS_USER += -O2 -std=gnu99 -fno-stack-protector -fgnu89-inline
+CXXFLAGS_USER += -O2
+
+export CFLAGS_USER CXXFLAGS_USER
+
 # Akaros include stuff (includes custom make targets and user overrides)
 # =========================================================================
 
 # The user can override this, though it won't apply for any of the in-tree
 # kernel build output.  Right now, it's only passed down to tests/
-OBJDIR ?= obj
 dummy-1 := $(shell mkdir -p $(OBJDIR)/kern/)
 
 # Don't need to export these, since the Makelocal is included.
 KERNEL_OBJ := $(OBJDIR)/kern/akaros-kernel
 CMP_KERNEL_OBJ := $(KERNEL_OBJ).gz
 
-# TODO: have the KFS paths be determined in .config
-# Give it a reasonable default path for initramfs to avoid build breakage
-INITRAMFS_PATHS = kern/kfs
-FIRST_INITRAMFS_PATH = $(firstword $(INITRAMFS_PATHS))
+# Since we're doing this outside of the dot-config part, some targets, such as
+# clean, won't read in our .config/auto.conf, and won't know about the
+# KFS_PATH.  Future rules related to KFS will have issues (mkdir with no
+# argument, or a find of the entire pwd).  It's also possible someone provided
+# an empty path.  To deal with both, we'll just have a sensible default.
+kfs-paths :=  $(subst $\",,$(CONFIG_KFS_PATHS))
+ifeq ($(kfs-paths),)
+kfs-paths := kern/kfs
+endif
+
+FIRST_KFS_PATH = $(firstword $(kfs-paths))
 
-export OBJDIR INITRAMFS_PATHS FIRST_INITRAMFS_PATH
+export OBJDIR FIRST_KFS_PATH
 
 # Avoiding implicit rules
-$(srctree)/Makeconfig $(srctree)/Makelocal: ;
-include $(srctree)/Makeconfig
+$(srctree)/Makelocal: ;
+# TODO: one issue is that we import all types of targets: build, clean, etc.
+# That makes it a bit tougher to reorganize with ifeqs.
 -include $(srctree)/Makelocal
 
 # Akaros Kernel Build
@@ -372,7 +394,7 @@ include $(srctree)/Makeconfig
 # We have all-arch-dirs and all-dirs, so that we can still clean even without
 # an arch symlink.
 
-core-y += kern/src/
+core-y += kern/src/ kern/drivers/ $(AKAROS_EXTERNAL_DIRS)
 arch-y += kern/arch/$(ARCH)/
 
 akaros-dirs     := $(patsubst %/,%,$(filter %/, $(core-y) $(arch-y)))
@@ -384,48 +406,98 @@ core-y          := $(patsubst %/, %/built-in.o, $(core-y))
 arch-y          := $(patsubst %/, %/built-in.o, $(arch-y))
 
 kbuild_akaros_main := $(core-y) $(arch-y)
-akaros-deps := $(kbuild_akaros_main)  kern/arch/$(ARCH)/kernel.ld
+akaros-deps := $(kbuild_akaros_main)  kern/arch/$(ARCH)/$(KERNEL_LD)
 
 kern_cpio := $(OBJDIR)/kern/initramfs.cpio
 kern_cpio_obj := $(kern_cpio).o
-ifneq ($(EXT2_BDEV),)
-ext2_bdev_obj := $(OBJDIR)/kern/$(shell basename $(EXT2_BDEV)).o
+
+# ext2 will crash at runtime if we don't have a block device.  try to catch the
+# errors now.  if it is a bad one, you're just out of luck.
+ifneq ($(CONFIG_EXT2FS),)
+ext2-bdev := $(patsubst "%",%,$(CONFIG_EXT2_BDEV))
+ifeq ($(ext2-bdev),)
+$(error EXT2 selected with no block device [$(ext2-bdev)], fix your .config)
+endif
+ext2_bdev_obj = $(OBJDIR)/kern/$(shell basename $(ext2-bdev)).o
+endif
+
+# a bit hacky: we want to make sure the directories exist, and error out
+# otherwise.  we also want to error out before the initramfs target, otherwise
+# we might not get the error (if initramfs files are all up to date).  the
+# trickiest thing here is that kfs-paths-check could be stale and require an
+# oldconfig.  running make twice should suffice.
+kfs-paths-check := $(shell for i in $(kfs-paths); do \
+                               if [ ! -d "$$i" ]; then \
+                                   echo "Can't find KFS directory $$i"; \
+	                               $(MAKE) -f $(srctree)/Makefile \
+								           silentoldconfig > /dev/null; \
+                                   exit -1; \
+                               fi; \
+                           done; echo "ok")
+
+ifneq (ok,$(kfs-paths-check))
+$(error $(kfs-paths-check), try make one more time in case of stale configs)
 endif
 
-kern_initramfs_files := $(shell mkdir -p $(INITRAMFS_PATHS); \
-                          find $(INITRAMFS_PATHS))
+kern_initramfs_files := $(shell find $(kfs-paths))
 
+# Need to make an empty cpio, then append each kfs-path's contents
 $(kern_cpio) initramfs: $(kern_initramfs_files)
 	@echo "  Building initramfs:"
-	@if [ "$(INITRAMFS_BIN)" != "" ]; then \
-        sh $(INITRAMFS_BIN); \
+	@if [ "$(CONFIG_KFS_CPIO_BIN)" != "" ]; then \
+        sh $(CONFIG_KFS_CPIO_BIN); \
     fi
-	$(Q)for i in $(INITRAMFS_PATHS); do cd $$i; \
+	@cat /dev/null | cpio --quiet -oH newc -O $(kern_cpio)
+	$(Q)for i in $(kfs-paths); do cd $$i; \
         echo "    Adding $$i to initramfs..."; \
-        find -L . | cpio --quiet -oH newc > \
-			$(CURDIR)/$(kern_cpio); \
+        find -L . | cpio --quiet -oAH newc -O $(CURDIR)/$(kern_cpio); \
         cd $$OLDPWD; \
     done;
 
 ld_emulation = $(shell $(OBJDUMP) -i | grep -v BFD | grep ^[a-z] |head -n1)
 ld_arch = $(shell $(OBJDUMP) -i | grep -v BFD | grep "^  [a-z]" | head -n1)
 
+# Our makefile doesn't detect a change in subarch, and old binary objects that
+# don't need to be updated won't get rebuilt, but they also can't link with the
+# new subarch (32 bit vs 64 bit).  If we detect the wrong type, we'll force a
+# rebuild.
+existing-cpio-emul := $(shell objdump -f $(kern_cpio_obj) 2> /dev/null | \
+                        grep format | sed 's/.*format //g')
+ifneq ($(existing-cpio-emul),)
+ifneq ($(existing-cpio-emul),$(ld_emulation))
+$(kern_cpio_obj): cpio-rebuild
+cpio-rebuild:
+	$(Q)rm $(kern_cpio_obj)
+endif
+endif
+
 $(kern_cpio_obj): $(kern_cpio)
-	$(Q)$(OBJCOPY) -I binary -B $(ld_arch) -O $(ld_emulation) $^ $@
+	$(Q)$(OBJCOPY) -I binary -B $(ld_arch) -O $(ld_emulation) $< $@
+
+existing-ext2b-emul := $(shell objdump -f $(kern_cpio_obj) 2> /dev/null | \
+                         grep format | sed 's/.*format //g')
+ifneq ($(existing-ext2b-emul),)
+ifneq ($(existing-ext2b-emul),$(ld_emulation))
+$(ext2_bdev_obj): ext2b-rebuild
+ext2b-rebuild:
+	$(Q)rm $(ext2_bdev_obj)
+endif
+endif
 
-$(ext2_bdev_obj): $(EXT2_BDEV)
-	$(Q)$(OBJCOPY) -I binary -B $(ld_arch) -O $(ld_emulation) $^ $@
+$(ext2_bdev_obj): $(ext2-bdev)
+	$(Q)$(OBJCOPY) -I binary -B $(ld_arch) -O $(ld_emulation) $< $@
 
-# TODO super-bugged objdump!  Passing -S (intermix source) and having auto.conf
-# be regenerated while the Makefile is running causes a segfault.  This would
-# happen after touching .config and remaking.  For now, just pass -d...
+# Not the worlds most elegant link command.  link-kernel takes the obj output
+# name, then the linker script, then everything else you'd dump on the ld
+# command line, including linker options and objects to link together.
+# 
+# After the script is done, we run the arch-specific command directly.
 quiet_cmd_link-akaros = LINK    $@
-      cmd_link-akaros = $(LD) -T kern/arch/$(ARCH)/kernel.ld -o $@ \
-                              $(akaros-deps) \
-                              $(gcc-lib) \
-                              $(kern_cpio_obj) \
-                              $(ext2_bdev_obj) ; \
-                              $(OBJDUMP) -d $@ > $@.asm
+      cmd_link-akaros = $(CONFIG_SHELL) scripts/link-kernel.sh $@ \
+                        kern/arch/$(ARCH)/$(KERNEL_LD) $(LDFLAGS_KERNEL) \
+                        $(akaros-deps) $(gcc-lib) $(kern_cpio_obj) \
+                        $(ext2_bdev_obj); \
+                        $(ARCH_POST_LINK_CMD)
 
 # For some reason, the if_changed doesn't work with FORCE (like it does in
 # Linux).  It looks like it can't find the .cmd file or something (also
@@ -439,10 +511,15 @@ $(sort $(akaros-deps)): $(akaros-dirs) ;
 
 # Recursively Kbuild all of our directories.  If we're changing arches
 # mid-make, we might have issues ( := on akaros-dirs, etc).
-PHONY += $(akaros-dirs)
-$(akaros-dirs): scripts symlinks
+PHONY += $(akaros-dirs) cc_exists
+$(akaros-dirs): scripts symlinks cc-exists
 	$(Q)$(MAKE) $(build)=$@
 
+cc-exists:
+	@if [ "`which $(CROSS_COMPILE)gcc`" = "" ]; then echo \
+	    Could not find a $(CROSS_COMPILE) version of GCC/binutils. \
+	    Be sure to build the cross-compiler and update your PATH; exit 1; fi
+
 $(CMP_KERNEL_OBJ): $(KERNEL_OBJ)
 	@echo "Compressing kernel image"
 	$(Q)gzip -c $^ > $@
@@ -463,16 +540,15 @@ endif #ifeq ($(mixed-targets),1)
 # List all userspace directories here, and state any dependencies between them,
 # such as how pthread depends on parlib.
 
-user-dirs = parlib pthread benchutil
-pthread: parlib
-
-ifeq ($(ARCH),i686)
-user-dirs += c3po
-c3po: parlib
-endif
+user-dirs = parlib pthread benchutil iplib ndblib bsd
+benchutil: parlib
+pthread: parlib benchutil
+iplib: parlib
+ndblib: iplib
+bsd: parlib iplib
 
 PHONY += install-libs $(user-dirs)
-install-libs: $(user-dirs) symlinks
+install-libs: $(user-dirs) symlinks cc-exists
 
 $(user-dirs):
 	@cd user/$@ && $(MAKE) && $(MAKE) install
@@ -480,7 +556,7 @@ $(user-dirs):
 
 PHONY += userclean $(clean-user-dirs)
 clean-user-dirs := $(addprefix _clean_user_,$(user-dirs))
-userclean: $(clean-user-dirs) testclean
+userclean: $(clean-user-dirs) testclean utestclean
 
 $(clean-user-dirs):
 	@cd user/$(patsubst _clean_user_%,%,$@) && $(MAKE) clean
@@ -489,19 +565,37 @@ tests/: tests
 tests: install-libs
 	@$(MAKE) -f tests/Makefile
 
+PHONY += utest
+utest: $(user-dirs)
+	@cd user/$@ && $(MAKE) 
+
 testclean:
 	@$(MAKE) -f tests/Makefile clean
 
-install-tests:
+utestclean:
+	@cd user/utest && $(MAKE) clean
+
+# KFS related stuff
+PHONY += fill-kfs unfill-kfs
+XCC_SO_FILES = $(addprefix $(XCC_TARGET_ROOT)/lib/, *.so*)
+
+$(OBJDIR)/.dont-force-fill-kfs:
+	$(Q)rm -rf $(addprefix $(FIRST_KFS_PATH)/lib/, $(notdir $(XCC_SO_FILES)))
+	@echo "Cross Compiler 'so' files removed from KFS"
+	@$(MAKE) -f tests/Makefile uninstall
+	@echo "Apps from /test removed from KFS"
+	@cd user/utest && $(MAKE) uninstall
+	@echo "User space tests removed from KFS"
+	@touch $(OBJDIR)/.dont-force-fill-kfs
+
+fill-kfs: $(OBJDIR)/.dont-force-fill-kfs install-libs
+	@mkdir -p $(FIRST_KFS_PATH)/lib
+	$(Q)cp -uP $(XCC_SO_FILES) $(FIRST_KFS_PATH)/lib
+	@echo "Cross Compiler 'so' files installed to KFS"
 	@$(MAKE) -f tests/Makefile install
-
-# TODO: cp -u all of the .sos, but flush it on an arch change (same with tests)
-fill-kfs: install-libs install-tests
-	@mkdir -p $(FIRST_INITRAMFS_PATH)/lib
-	@cp $(addprefix $(XCC_ROOT)/$(ARCH)-ros/lib/, \
-	  libc.so.6 ld.so.1 libm.so libgcc_s.so.1) $(FIRST_INITRAMFS_PATH)/lib
-	$(Q)$(STRIP) --strip-debug $(addprefix $(FIRST_INITRAMFS_PATH)/lib/, \
-	                                       libc.so.6 ld.so.1)
+	@echo "Apps from /test installed to KFS"
+	@cd user/utest && $(MAKE) install
+	@echo "User space tests installed to KFS"
 
 # Use doxygen to make documentation for ROS (Untested since 2010 or so)
 doxygen-dir := $(CUR_DIR)/Documentation/doxygen
@@ -519,7 +613,7 @@ doxyclean:
 
 objclean:
 	@echo + clean [OBJDIR]
-	@rm -rf $(OBJDIR)/*
+	@rm -rf $(OBJDIR)
 
 realclean: userclean mrproper doxyclean objclean
 
@@ -582,6 +676,10 @@ mrproper: $(mrproper-dirs) clean clean_symlinks
 PHONY += FORCE
 FORCE:
 
+# Don't put the srctree on this, make is looking for Makefile, not
+# /full/path/to/Makefile.
+Makefile: ; # avoid implicit rule on Makefile
+
 # Declare the contents of the .PHONY variable as phony.  We keep that
 # information in a variable so we can use it in if_changed and friends.
 .PHONY: $(PHONY)