kbuild: Simplify generating vmlinux

Generating vmlinux in top-level Makefile were getting a bit messy after kallsyms
support were added. Also the full link of all the .o files were duplicaed a number of times.
This patch does the following:
- Introduce built-in.o which is a prelink of most .o files
- Make the build process a bit more verbose telling when linking .tmpvmlinux*
- Use less magic when determing when to generate a new version
- Allow architectures to override the defineition of cmd_vmlinux__
- Add more comments to the MAkefile and clean up soem other comments
- Display more commends during V=1 builds

The resulting kernel boots and rn here.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -545,57 +545,83 @@ libs-y		:= $(libs-y1) $(libs-y2)
 
 # Build vmlinux
 # ---------------------------------------------------------------------------
+# vmlinux is build from the objects seleted by $(vmlinux-init) and
+# $(vmlinux-main). Most are built-in.o files from top-level directories
+# in the kernel tree, others are specified in arch/$(ARCH)Makefile.
+# Ordering when linking is important, and $(vmlinux-init) must be first.
+#
+# vmlinux
+#   ^
+#   |
+#   +-< $(vmlinux-init)
+#   |   +--< init/version.o + more
+#   |
+#   +-< built-in.o
+#   |   +--< $(vmlinux-main)
+#   |        +--< driver/built-in.o mm/built-in.o + more
+#   |
+#   +-< kallsyms.o (see description in CONFIG_KALLSYMS section)
+#
+# vmlinux version cannot be updated during normal descending-into-subdirs
+# phase since we do not yet know if we need to update vmlinux.
+# Therefore this step is delayed until just before final link of vmlinux -
+# except in kallsyms case where it is done just before adding the
+# symbols to the kernel.
+#
+# System.map is generated to document addresses of all kernel symbols
 
-#	This is a bit tricky: If we need to relink vmlinux, we want
-#	the version number incremented, which means recompile init/version.o
-#	and relink init/init.o. However, we cannot do this during the
-#       normal descending-into-subdirs phase, since at that time
-#       we cannot yet know if we will need to relink vmlinux.
-#	So we descend into init/ inside the rule for vmlinux again.
-vmlinux-objs := $(head-y) $(init-y) $(core-y) $(libs-y) $(drivers-y) $(net-y)
+vmlinux-init := $(head-y) $(init-y)
+vmlinux-main := $(core-y) $(libs-y) $(drivers-y) $(net-y)
+vmlinux-all  := $(vmlinux-init) built-in.o
+vmlinux-lds  := arch/$(ARCH)/kernel/vmlinux.lds
 
+# Rule to link vmlinux - also used during CONFIG_KALLSYMS
+# May be overridden by arch/$(ARCH)/Makefile
 quiet_cmd_vmlinux__  = LD      $@
-define cmd_vmlinux__
-	$(LD) $(LDFLAGS) $(LDFLAGS_vmlinux) $(head-y) $(init-y) \
-	--start-group \
-	$(core-y) \
-	$(libs-y) \
-	$(drivers-y) \
-	$(net-y) \
-	--end-group \
-	$(filter .tmp_kallsyms%,$^) \
-	-o $@
-endef
-
-#	set -e makes the rule exit immediately on error
+      cmd_vmlinux__  = $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux) \
+                           -T $(filter-out FORCE, $^) -o $@
 
-define rule_vmlinux__
-	+set -e;							\
-	$(if $(filter .tmp_kallsyms%,$^),,				\
-	  echo '  GEN     .version';					\
+# Generate new vmlinux version
+quiet_cmd_vmlinux_version = GEN     .version
+      cmd_vmlinux_version = set -e;                     \
 	. $(srctree)/scripts/mkversion > .tmp_version;	\
 	mv -f .tmp_version .version;			\
-	  $(MAKE) $(build)=init;					\
-	)								\
-	$(if $($(quiet)cmd_vmlinux__),					\
-	  echo '  $($(quiet)cmd_vmlinux__)' &&) 			\
-	$(cmd_vmlinux__);						\
-	echo 'cmd_$@ := $(cmd_vmlinux__)' > $(@D)/.$(@F).cmd
-endef
+	$(MAKE) $(build)=init
 	
+# Generate System.map
 quiet_cmd_sysmap = SYSMAP 
       cmd_sysmap = $(CONFIG_SHELL) $(srctree)/scripts/mksysmap
 
-LDFLAGS_vmlinux += -T arch/$(ARCH)/kernel/vmlinux.lds
+# Link of vmlinux
+# If CONFIG_KALLSYMS is set .version is already updated
+# Generate System.map and verify that the content is consistent
+
+define rule_vmlinux__
+	$(if $(CONFIG_KALLSYMS),,$(call cmd,vmlinux_version))
+	
+	$(call cmd,vmlinux__)
+	$(Q)echo 'cmd_$@ := $(cmd_vmlinux__)' > $(@D)/.$(@F).cmd
+	
+	$(Q)$(if $($(quiet)cmd_sysmap),                 \
+	  echo '  $($(quiet)cmd_sysmap) System.map' &&) \
+	$(cmd_sysmap) $@ System.map;                    \
+	if [ $$? -ne 0 ]; then                          \
+		rm -f $@;                               \
+		/bin/false;                             \
+	fi;
+	$(verify_kallsyms)
+endef
+
 
-#	Generate section listing all symbols and add it into vmlinux
+ifdef CONFIG_KALLSYMS
+# Generate section listing all symbols and add it into vmlinux $(kallsyms.o)
 # It's a three stage process:
 # o .tmp_vmlinux1 has all symbols and sections, but __kallsyms is
 #   empty
 #   Running kallsyms on that gives us .tmp_kallsyms1.o with
-#	  the right size
+#   the right size - vmlinux version updated during this step
 # o .tmp_vmlinux2 now has a __kallsyms section of the right size,
-#	  but due to the added section, some addresses have shifted
+#   but due to the added section, some addresses have shifted.
 #   From here, we generate a correct .tmp_kallsyms2.o
 # o The correct .tmp_kallsyms2.o is linked into the final vmlinux.
 # o Verify that the System.map from vmlinux matches the map from
@@ -605,8 +631,6 @@ LDFLAGS_vmlinux += -T arch/$(ARCH)/kernel/vmlinux.lds
 #   temporary bypass to allow the kernel to be built while the
 #   maintainers work out what went wrong with kallsyms.
 
-ifdef CONFIG_KALLSYMS
-
 ifdef CONFIG_KALLSYMS_EXTRA_PASS
 last_kallsyms := 3
 else
@@ -615,16 +639,28 @@ endif
 
 kallsyms.o := .tmp_kallsyms$(last_kallsyms).o
 
-define rule_verify_kallsyms
+define verify_kallsyms
 	$(Q)$(if $($(quiet)cmd_sysmap),                       \
 	  echo '  $($(quiet)cmd_sysmap) .tmp_System.map' &&)  \
 	  $(cmd_sysmap) .tmp_vmlinux$(last_kallsyms) .tmp_System.map
 	$(Q)cmp -s System.map .tmp_System.map ||              \
-		(echo Inconsistent kallsyms data, try setting CONFIG_KALLSYMS_EXTRA_PASS ; rm .tmp_kallsyms* ; false)
+		(echo Inconsistent kallsyms data;             \
+		 echo Try setting CONFIG_KALLSYMS_EXTRA_PASS; \
+		 rm .tmp_kallsyms* ; /bin/false )
+endef
+
+# Update vmlinux version before link
+cmd_ksym_ld = $(cmd_vmlinux__)
+define rule_ksym_ld
+	$(call cmd,vmlinux_version)
+	$(call cmd,vmlinux__)
+	$(Q)echo 'cmd_$@ := $(cmd_vmlinux__)' > $(@D)/.$(@F).cmd
 endef
 
+# Generate .S file with all kernel symbols
 quiet_cmd_kallsyms = KSYM    $@
-cmd_kallsyms = $(NM) -n $< | $(KALLSYMS) $(foreach x,$(CONFIG_KALLSYMS_ALL),--all-symbols) > $@
+      cmd_kallsyms = $(NM) -n $< | $(KALLSYMS) \
+                     $(if $(CONFIG_KALLSYMS_ALL),--all-symbols) > $@
 
 .tmp_kallsyms1.o .tmp_kallsyms2.o .tmp_kallsyms3.o: %.o: %.S scripts FORCE
 	$(call if_changed_dep,as_o_S)
@@ -632,43 +668,39 @@ cmd_kallsyms = $(NM) -n $< | $(KALLSYMS) $(foreach x,$(CONFIG_KALLSYMS_ALL),--al
 .tmp_kallsyms%.S: .tmp_vmlinux% $(KALLSYMS)
 	$(call cmd,kallsyms)
 
-.tmp_vmlinux1: $(vmlinux-objs) arch/$(ARCH)/kernel/vmlinux.lds FORCE
-	$(call if_changed_rule,vmlinux__)
+# .tmp_vmlinux1 must be complete except kallsyms, so update vmlinux version
+.tmp_vmlinux1: $(vmlinux-lds) $(vmlinux-all) FORCE
+	$(call if_changed_rule,ksym_ld)
 
-.tmp_vmlinux2: $(vmlinux-objs) .tmp_kallsyms1.o arch/$(ARCH)/kernel/vmlinux.lds FORCE
-	$(call if_changed_rule,vmlinux__)
+.tmp_vmlinux2: $(vmlinux-lds) $(vmlinux-all) .tmp_kallsyms1.o FORCE
+	$(call if_changed,vmlinux__)
 
-.tmp_vmlinux3: $(vmlinux-objs) .tmp_kallsyms2.o arch/$(ARCH)/kernel/vmlinux.lds FORCE
-	$(call if_changed_rule,vmlinux__)
+.tmp_vmlinux3: $(vmlinux-lds) $(vmlinux-all) .tmp_kallsyms2.o FORCE
+	$(call if_changed,vmlinux__)
 
 # Needs to visit scripts/ before $(KALLSYMS) can be used.
 $(KALLSYMS): scripts ;
 
-endif
+endif # ifdef CONFIG_KALLSYMS
 
-# Finally the vmlinux rule
-# This rule is also used to generate System.map
-# and to verify that the content of kallsyms are consistent
+# vmlinux image - including updated kernel symbols
+vmlinux: $(vmlinux-lds) $(vmlinux-init) built-in.o $(kallsyms.o) FORCE
+	$(call if_changed_rule,vmlinux__)
 
-define rule_vmlinux
-	$(rule_vmlinux__);
-	$(Q)$(if $($(quiet)cmd_sysmap),                  \
-	  echo '  $($(quiet)cmd_sysmap) System.map' &&)  \
-	$(cmd_sysmap) $@ System.map;                     \
-	if [ $$? -ne 0 ]; then                           \
-		rm -f $@;                                \
-		/bin/false;                              \
-	fi;
-	$(rule_verify_kallsyms)
-endef
+# Link $(vmlinux-main) to speed up rest of build phase. No need to
+# relink this part too many times.
+# Use start/end-group to make sure to resolve all possible symbols
+quiet_rule_vmlinux_partial = LD      $@
+       cmd_vmlinux_partial = $(LD) $(LDFLAGS) $(LDFLAGS_$(*F)) -r \
+                                   --start-group $(filter-out FORCE, $^) \
+				   --end-group -o $@
 				   
-vmlinux: $(vmlinux-objs) $(kallsyms.o) arch/$(ARCH)/kernel/vmlinux.lds FORCE
-	$(call if_changed_rule,vmlinux)
+built-in.o: $(vmlinux-main) FORCE
+	$(call if_changed,vmlinux_partial)
 
 # The actual objects are generated when descending, 
 # make sure no implicit rule kicks in
-
-$(sort $(vmlinux-objs)) arch/$(ARCH)/kernel/vmlinux.lds: $(vmlinux-dirs) ;
+$(sort $(vmlinux-init) $(vmlinux-main)) $(vmlinux-lds): $(vmlinux-dirs) ;
 
 # Handle descending into subdirectories listed in $(vmlinux-dirs)
 # Preset locale variables to speed up the build process. Limit locale
@@ -1188,13 +1220,22 @@ ifneq ($(cmd_files),)
   include $(cmd_files)
 endif
 
+# Execute command and generate cmd file
+if_changed = $(if $(strip $? \
+		          $(filter-out $(cmd_$(1)),$(cmd_$@))\
+			  $(filter-out $(cmd_$@),$(cmd_$(1)))),\
+	@set -e; \
+	$(if $($(quiet)cmd_$(1)),echo '  $(subst ','\'',$($(quiet)cmd_$(1)))';) \
+	$(cmd_$(1)); \
+	echo 'cmd_$@ := $(subst $$,$$$$,$(subst ','\'',$(cmd_$(1))))' > $(@D)/.$(@F).cmd)
+
+
 # execute the command and also postprocess generated .d dependencies
 # file
-
 if_changed_dep = $(if $(strip $? $(filter-out FORCE $(wildcard $^),$^)\
 		          $(filter-out $(cmd_$(1)),$(cmd_$@))\
 			  $(filter-out $(cmd_$@),$(cmd_$(1)))),\
-	@set -e; \
+	$(Q)set -e; \
 	$(if $($(quiet)cmd_$(1)),echo '  $(subst ','\'',$($(quiet)cmd_$(1)))';) \
 	$(cmd_$(1)); \
 	scripts/basic/fixdep $(depfile) $@ '$(subst $$,$$$$,$(subst ','\'',$(cmd_$(1))))' > $(@D)/.$(@F).tmp; \
@@ -1208,7 +1249,7 @@ if_changed_dep = $(if $(strip $? $(filter-out FORCE $(wildcard $^),$^)\
 if_changed_rule = $(if $(strip $? \
 		               $(filter-out $(cmd_$(1)),$(cmd_$(@F)))\
 			       $(filter-out $(cmd_$(@F)),$(cmd_$(1)))),\
-	               @$(rule_$(1)))
+	               $(Q)$(rule_$(1)))
 
 # If quiet is set, only print short version of command