Makefile question in OpenWRT build system - makefile

Question

The following code is taken from OpenWRT project. Can someone give an abstract description? Thanks in advance!
#
# Copyright (C) 2007 OpenWrt.org
#
# This is free software, licensed under the GNU General Public License v2.
# See /LICENSE for more information.
#
SUBTARGETS:=clean download prepare compile install update refresh prereq dist distcheck
subtarget-default = $(filter-out ., \
$(if $($(1)/builddirs-$(2)),$($(1)/builddirs-$(2)), \
$(if $($(1)/builddirs-default),$($(1)/builddirs-default), \
$($(1)/builddirs))))
define subtarget
$(call warn_eval,$(1),t,T,$(1)/$(2): $($(1)/) $(foreach bd,$(call subtarget-default,$(1),$(2)),$(1)/$(bd)/$(2)))
endef
lastdir=$(word $(words $(subst /, ,$(1))),$(subst /, ,$(1)))
diralias=$(if $(findstring $(1),$(call lastdir,$(1))),,$(call lastdir,$(1)))
# Parameters: <subdir>
define subdir
$(call warn,$(1),d,D $(1))
$(foreach bd,$($(1)/builddirs),
$(call warn,$(1),d,BD $(1)/$(bd))
$(foreach target,$(SUBTARGETS),
$(foreach btype,$(buildtypes-$(bd)),
$(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(btype)/$(target): $(if $(QUILT),,$($(1)/$(bd)/$(btype)/$(target)) $(call $(1)//$(btype)/$(target),$(1)/$(bd)/$(btype))))
+$$(SUBMAKE) -C $(1)/$(bd) $(btype)-$(target) $(if $(findstring $(bd),$($(1)/builddirs-ignore-$(btype)-$(target))), || $(call MESSAGE, ERROR: $(1)/$(bd) [$(btype)] failed to build.))
$$(if $(call debug,$(1)/$(bd),v),,.SILENT: $(1)/$(bd)/$(btype)/$(target))
$(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(btype)/$(target): $(1)/$(bd)/$(btype)/$(target)))
)
$(call warn_eval,$(1)/$(bd),t,T,$(1)/$(bd)/$(target): $(if $(QUILT),,$($(1)/$(bd)/$(target)) $(call $(1)//$(target),$(1)/$(bd))))
$(if $(BUILD_LOG),#mkdir -p $(BUILD_LOG_DIR)/$(1)/$(bd))
$(foreach variant,$(if $(BUILD_VARIANT),$(BUILD_VARIANT),$(if $($(1)/$(bd)/variants),$($(1)/$(bd)/variants),__default)),
+$(if $(BUILD_LOG),set -o pipefail;) $$(SUBMAKE) -C $(1)/$(bd) $(target) BUILD_VARIANT="$(filter-out __default,$(variant))" $(if $(BUILD_LOG),SILENT= 2>&1 | tee $(BUILD_LOG_DIR)/$(1)/$(bd)/$(target).txt) $(if $(findstring $(bd),$($(1)/builddirs-ignore-$(target))), || $(call MESSAGE, ERROR: $(1)/$(bd) failed to build$(if $(filter-out __default,$(variant)), (build variant: $(variant))).))
)
$$(if $(call debug,$(1)/$(bd),v),,.SILENT: $(1)/$(bd)/$(target))
# legacy targets
$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(bd)-$(target): $(1)/$(bd)/$(target))
# aliases
$(if $(call diralias,$(bd)),$(call warn_eval,$(1)/$(bd),l,T,$(1)/$(call diralias,$(bd))/$(target): $(1)/$(bd)/$(target)))
)
)
$(foreach target,$(SUBTARGETS),$(call subtarget,$(1),$(target)))
endef
# Parameters: <subdir> <name> <target> <depends> <config options> <stampfile location>
define stampfile
$(1)/stamp-$(3):=$(if $(6),$(6),$(STAGING_DIR))/stamp/.$(2)_$(3)$(if $(5),_$(call confvar,$(5)))
$$($(1)/stamp-$(3)): $(TMP_DIR)/.build $(4)
#+$(SCRIPT_DIR)/timestamp.pl -n $$($(1)/stamp-$(3)) $(1) $(4) || \
$(MAKE) $$($(1)/flags-$(3)) $(1)/$(3)
#mkdir -p $$$$(dirname $$($(1)/stamp-$(3)))
#touch $$($(1)/stamp-$(3))
$$(if $(call debug,$(1),v),,.SILENT: $$($(1)/stamp-$(3)))
.PRECIOUS: $$($(1)/stamp-$(3)) # work around a make bug
$(1)//clean:=$(1)/stamp-$(3)/clean
$(1)/stamp-$(3)/clean: FORCE
#rm -f $$($(1)/stamp-$(3))
endef

Answer 1

The subdir template generates the standard make targets for a subdirectory.
It is called with the directory name as first argument, e.g. in package/Makefile it's 'package'
it then treats the variable package/builddirs as the list of subdirectories with valid build targets, which are contained in the SUBTARGETS variable.
For running the 'compile' target of a package 'foo', it allows you to run 'make package/foo/compile' which will recursively invoke make with the right environment variables in order to build the subdirectory.
This template also has support for some optional features that can be enabled through environment variables, e.g. for logging the output to a file, ignoring dependencies, etc.
It also supports selective debugging output.
The stampfile template is simply a small makefile target wrapper for speeding up builds by keeping a recursively timestamp-checked stampfile around it