diff --git a/core/envsetup.mk b/core/envsetup.mk
index 199fe9b..867ba1d 100644
--- a/core/envsetup.mk
+++ b/core/envsetup.mk
@@ -11,9 +11,65 @@
 #         This can be useful if you set OUT_DIR to be a different directory
 #         than other outputs of your build system.
 
+# Returns all words in $1 up to and including $2
+define find_and_earlier
+  $(strip $(if $(1),
+    $(firstword $(1))
+    $(if $(filter $(firstword $(1)),$(2)),,
+      $(call find_and_earlier,$(wordlist 2,$(words $(1)),$(1)),$(2)))))
+endef
+
+#$(warning $(call find_and_earlier,A B C,A))
+#$(warning $(call find_and_earlier,A B C,B))
+#$(warning $(call find_and_earlier,A B C,C))
+#$(warning $(call find_and_earlier,A B C,D))
+
+define version-list
+$(1)PR1 $(1)PD1 $(1)PD2 $(1)PM1 $(1)PM2
+endef
+
+ALL_VERSIONS := O P Q R S T U V W X Y Z
+ALL_VERSIONS := $(foreach v,$(ALL_VERSIONS),$(call version-list,$(v)))
+
+# Filters ALL_VERSIONS down to the range [$1, $2], and errors if $1 > $2 or $3 is
+# not in [$1, $2]
+# $(1): min platform version
+# $(2): max platform version
+# $(3): default platform version
+define allowed-platform-versions
+$(strip \
+  $(if $(filter $(ALL_VERSIONS),$(1)),,
+    $(error Invalid MIN_PLATFORM_VERSION '$(1)'))
+  $(if $(filter $(ALL_VERSIONS),$(2)),,
+    $(error Invalid MAX_PLATFORM_VERSION '$(2)'))
+  $(if $(filter $(ALL_VERSIONS),$(3)),,
+    $(error Invalid DEFAULT_PLATFORM_VERSION '$(3)'))
+
+  $(eval allowed_versions_ := $(call find_and_earlier,$(ALL_VERSIONS),$(2)))
+
+  $(if $(filter $(allowed_versions_),$(1)),,
+    $(error MIN_PLATFORM_VERSION '$(1)' must be before MAX_PLATFORM_VERSION '$(2)'))
+
+  $(eval allowed_versions_ := $(1) \
+    $(filter-out $(call find_and_earlier,$(allowed_versions_),$(1)),$(allowed_versions_)))
+
+  $(if $(filter $(allowed_versions_),$(3)),,
+    $(error DEFAULT_PLATFORM_VERSION '$(3)' must be between MIN_PLATFORM_VERSION '$(1)' and MAX_PLATFORM_VERSION '$(2)'))
+
+  $(allowed_versions_))
+endef
+
+#$(warning $(call allowed-platform-versions,OPR1,PPR1,OPR1))
+#$(warning $(call allowed-platform-versions,OPM1,PPR1,OPR1))
+
 # Set up version information.
 include $(BUILD_SYSTEM)/version_defaults.mk
 
+ENABLED_VERSIONS := $(call find_and_earlier,$(ALL_VERSIONS),$(TARGET_PLATFORM_VERSION))
+
+$(foreach v,$(ENABLED_VERSIONS), \
+  $(eval IS_AT_LEAST_$(v) := true))
+
 # ---------------------------------------------------------------
 # If you update the build system such that the environment setup
 # or buildspec.mk need to be updated, increment this number, and
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 62e5499..e11e8b3 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -38,52 +38,27 @@
   include $(INTERNAL_BUILD_ID_MAKEFILE)
 endif
 
-# Returns all words in $1 up to and including $2
-define find_and_earlier
-  $(strip $(if $(1),
-    $(firstword $(1))
-    $(if $(filter $(firstword $(1)),$(2)),,
-      $(call find_and_earlier,$(wordlist 2,$(words $(1)),$(1)),$(2)))))
-endef
-
-#$(warning $(call find_and_earlier,A B C,A))
-#$(warning $(call find_and_earlier,A B C,B))
-#$(warning $(call find_and_earlier,A B C,C))
-#$(warning $(call find_and_earlier,A B C,D))
-
-define version-list
-$(1)PR1 $(1)PD1 $(1)PD2 $(1)PM1 $(1)PM2
-endef
-
-ALL_VERSIONS := O P
-ALL_VERSIONS := $(foreach v,$(ALL_VERSIONS),$(call version-list,$(v)))
-
 DEFAULT_PLATFORM_VERSION := OPR1
+MIN_PLATFORM_VERSION := OPR1
+MAX_PLATFORM_VERSION := PPR1
 
-# HACK: forward P to PPR1 until the build server config is updated
-ifeq (P,$(TARGET_PLATFORM_VERSION))
-  TARGET_PLATFORM_VERSION := PPR1
-endif
+ALLOWED_VERSIONS := $(call allowed-platform-versions,\
+  $(MIN_PLATFORM_VERSION),\
+  $(MAX_PLATFORM_VERSION),\
+  $(DEFAULT_PLATFORM_VERSION))
 
-ifeq (,$(TARGET_PLATFORM_VERSION))
-  # Default targeted platform version
-  # TODO: PLATFORM_VERSION, PLATFORM_SDK_VERSION, etc. should be conditional
-  # on this
+ifndef TARGET_PLATFORM_VERSION
   TARGET_PLATFORM_VERSION := $(DEFAULT_PLATFORM_VERSION)
 endif
 
-ifeq (,$(filter $(ALL_VERSIONS), $(TARGET_PLATFORM_VERSION)))
-$(warning Invalid TARGET_PLATFORM_VERSION '$(TARGET_PLATFORM_VERSION)', must be one of)
-$(warning $(ALL_VERSIONS))
-$(error Stopping...)
+ifeq (,$(filter $(ALLOWED_VERSIONS), $(TARGET_PLATFORM_VERSION)))
+  $(warning Invalid TARGET_PLATFORM_VERSION '$(TARGET_PLATFORM_VERSION)', must be one of)
+  $(error $(ALLOWED_VERSIONS))
 endif
 
-ENABLED_VERSIONS := $(call find_and_earlier,$(ALL_VERSIONS),$(TARGET_PLATFORM_VERSION))
-
-$(foreach v,$(ENABLED_VERSIONS), \
-  $(eval IS_AT_LEAST_$(v) := true))
-
 # Default versions for each TARGET_PLATFORM_VERSION
+# TODO: PLATFORM_VERSION, PLATFORM_SDK_VERSION, etc. should be conditional
+# on this
 
 # This is the canonical definition of the platform version,
 # which is the version that we reveal to the end user.