-# will check if $(cmd_foo) changed, or any of the prequisites changed,
-# and if so will execute $(rule_foo)
-if_changed_rule = $(if $(strip $(filter-out $(PHONY),$?) \
- $(call arg-check, $(cmd_$(1)), $(cmd_$@)) ),\
- @set -e; \
- $(rule_$(1)))
+# Will check if $(cmd_foo) or any of the prerequisites changed,
+# and if so will execute $(rule_foo).
+if_changed_rule = $(if $(strip $(any-prereq) $(arg-check) ), \
+ @set -e; \
+ $(rule_$(1)))
+
+###
+# why - tell why a a target got build
+# enabled by make V=2
+# Output (listed in the order they are checked):
+# (1) - due to target is PHONY
+# (2) - due to target missing
+# (3) - due to: file1.h file2.h
+# (4) - due to command line change
+# (5) - due to missing .cmd file
+# (6) - due to target not in $(targets)
+# (1) PHONY targets are always build
+# (2) No target, so we better build it
+# (3) Prerequisite is newer than target
+# (4) The command line stored in the file named dir/.target.cmd
+# differed from actual command line. This happens when compiler
+# options changes
+# (5) No dir/.target.cmd file (used to store command line)
+# (6) No dir/.target.cmd file and target not listed in $(targets)
+# This is a good hint that there is a bug in the kbuild file
+ifeq ($(KBUILD_VERBOSE),2)
+why = \
+ $(if $(filter $@, $(PHONY)),- due to target is PHONY, \
+ $(if $(wildcard $@), \
+ $(if $(strip $(any-prereq)),- due to: $(any-prereq), \
+ $(if $(arg-check), \
+ $(if $(cmd_$@),- due to command line change, \
+ $(if $(filter $@, $(targets)), \
+ - due to missing .cmd file, \
+ - due to $(notdir $@) not in $$(targets) \
+ ) \
+ ) \
+ ) \
+ ), \
+ - due to target missing \
+ ) \
+ )
+
+echo-why = $(call escsq, $(strip $(why)))
+endif