kbuild: better abstract vmlinux sequential prerequisites

When CONFIG_TRIM_UNUSED_KSYMS=y and CONFIG_BUILD_DOCSRC=y it is possible
to get the following error:

ERROR: "cn_del_callback" [Documentation/connector/cn_test.ko] undefined!
ERROR: "cn_add_callback" [Documentation/connector/cn_test.ko] undefined!
ERROR: "cn_netlink_send" [Documentation/connector/cn_test.ko] undefined!
../scripts/Makefile.modpost:91: recipe for target '__modpost' failed

It is not sufficient to do "vmlinux-dirs += Documentation" as this also
depends on the headers_check target, and all of this needs to be done
before adjust_autoksyms.sh is executed.

Let's sort this out by gathering those sequential prerequisites in a make
target of their own, separate from the vmlinux target. And by doing so,
the special autoksyms_recursive target is no longer needed.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>

Makefile

@@ -926,17 +926,11 @@ export KBUILD_ALLDIRS := $(sort $(filter-out arch/%,$(vmlinux-alldirs)) arch Doc
 
 vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN)
 
-# Final link of vmlinux
-      cmd_link-vmlinux = $(CONFIG_SHELL) $< $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux)
-quiet_cmd_link-vmlinux = LINK    $@
-
-# Include targets which we want to
-# execute if the rest of the kernel build went well.
-vmlinux: scripts/link-vmlinux.sh $(vmlinux-deps) FORCE
-ifdef CONFIG_TRIM_UNUSED_KSYMS
-	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \
-	  "$(MAKE) KBUILD_MODULES=1 -f $(srctree)/Makefile autoksyms_recursive"
-endif
+# Include targets which we want to execute sequentially if the rest of the
+# kernel build went well. If CONFIG_TRIM_UNUSED_KSYMS is set, this might be
+# evaluated more than once.
+PHONY += vmlinux_prereq
+vmlinux_prereq: $(vmlinux-deps) FORCE
 ifdef CONFIG_HEADERS_CHECK
 	$(Q)$(MAKE) -f $(srctree)/Makefile headers_check
 endif
@@ -946,17 +940,22 @@ endif
 ifdef CONFIG_GDB_SCRIPTS
 	$(Q)ln -fsn `cd $(srctree) && /bin/pwd`/scripts/gdb/vmlinux-gdb.py
 endif
-	+$(call if_changed,link-vmlinux)
-
-autoksyms_recursive: $(vmlinux-deps)
+ifdef CONFIG_TRIM_UNUSED_KSYMS
 	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \
-	  "$(MAKE) KBUILD_MODULES=1 -f $(srctree)/Makefile autoksyms_recursive"
-PHONY += autoksyms_recursive
+	  "$(MAKE) KBUILD_MODULES=1 -f $(srctree)/Makefile vmlinux_prereq"
+endif
 
 # standalone target for easier testing
 include/generated/autoksyms.h: FORCE
 	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh true
 
+# Final link of vmlinux
+      cmd_link-vmlinux = $(CONFIG_SHELL) $< $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux)
+quiet_cmd_link-vmlinux = LINK    $@
+
+vmlinux: scripts/link-vmlinux.sh vmlinux_prereq FORCE
+	+$(call if_changed,link-vmlinux)
+
 # Build samples along the rest of the kernel
 ifdef CONFIG_SAMPLES
 vmlinux-dirs += samples