sys/xtimer-ztimer: rework dependecies to ease backend switch

This adds a xtimer_no_ztimer_default that is currently always
selected in Makefile, but that can be switched off in Kconfig.
Removing its inclusion will allow switching the default xtimer
backend to ztimer, while allowing for an easy way back.

makefiles/default_modules.deps.mk

@@ -13,3 +13,9 @@ endif
 ifneq (,$(filter auto_init_saul,$(USEMODULE)))
   USEMODULE += saul_init_devs
 endif
+
+ifneq (,$(filter xtimer,$(USEMODULE)))
+  ifeq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
+    USEMODULE += div
+  endif
+endif

makefiles/pseudomodules.inc.mk

@@ -257,6 +257,7 @@ PSEUDOMODULES += vfs_default
 PSEUDOMODULES += wakaama_objects_%
 PSEUDOMODULES += wifi_enterprise
 PSEUDOMODULES += xtimer_on_ztimer
+PSEUDOMODULES += xtimer_no_ztimer_default
 PSEUDOMODULES += zptr
 PSEUDOMODULES += ztimer
 PSEUDOMODULES += ztimer_%

sys/xtimer/Kconfig

@@ -8,17 +8,22 @@
 menuconfig MODULE_XTIMER
     bool "xtimer"
     depends on HAS_PERIPH_TIMER
-    depends on TEST_KCONFIG
-    # use timer peripheral unless ztimer compatibility module is used
-    select MODULE_PERIPH_TIMER if HAS_PERIPH_TIMER && !MODULE_XTIMER_ON_ZTIMER && !MODULE_ZTIMER_XTIMER_COMPAT && !MODULE_ZTIMER64_XTIMER_COMPAT
-
+    select MODULE_PERIPH_TIMER
     select MODULE_DIV if !MODULE_ZTIMER_XTIMER_COMPAT
+    select ZTIMER_USEC if !MODULE_XTIMER_NO_ZTIMER_DEFAULT
     help
         Include xtimer module. xtimer requires a low-level timer implementation
         that can be provided either by a peripheral timer or the ztimer module
         when MODULE_XTIMER_ON_ZTIMER is enabled.
 
+if MODULE_XTIMER
+
+config MODULE_XTIMER_NO_ZTIMER_DEFAULT
+    bool "xtimer does not select ztimer"
+    default y
+
 config MODULE_AUTO_INIT_XTIMER
     bool "Auto-init xtimer"
     default y if MODULE_AUTO_INIT && !MODULE_ZTIMER_XTIMER_COMPAT
-    depends on MODULE_XTIMER
+
+endif # MODULE_XTIMER

sys/xtimer/Makefile.dep

@@ -1,14 +1,31 @@
-ifeq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
-  # xtimer is used, ztimer xtimer wrapper is not
-  DEFAULT_MODULE += auto_init_xtimer
-  USEMODULE += div
-  ifeq (,$(filter xtimer_on_ztimer,$(USEMODULE)))
-    # ztimer is not used, so use *periph_timer as low-level timer*.
-    FEATURES_REQUIRED += periph_timer
-  else
-    # will use *ztimer_usec as low-level timer*
+DEFAULT_MODULE += auto_init_xtimer
+# In whatever combination of backends periph_timer will be required
+# - xtimer (only) -> periph_timer
+# - xtimer_on_ztimer -> ztimer_usec -> ztimer_periph_timer -> periph_timer
+# - ztimer_xtimer_compat -> ztimer_usec -> ztimer_periph_timer -> periph_timer
+# But requesting it here avoid an ifeq dependency that would be subject to
+# dependency inclusion order issues
+FEATURES_REQUIRED += periph_timer
+
+USEMODULE += xtimer_no_ztimer_default
+ifeq (,$(filter xtimer_no_ztimer_default,$(USEMODULE)))
+  ifeq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
+    USEMODULE += xtimer_on_ztimer
   endif
 else
-  # ztimer_xtimer_compat is used, all of *xtimer's API will be mapped on ztimer
-  # or ztimer64 *
+  ifneq (,$(filter ztimer,$(USEMODULE)))
+    ifeq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
+      USEMODULE += xtimer_on_ztimer
+    endif
+  endif
+endif
+
+ifneq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
+  DISABLE_MODULE += auto_init_xtimer
+  USEMODULE += div
+endif
+
+# make xtimer use ztimer_usec as low level timer
+ifneq (,$(filter xtimer_on_ztimer,$(USEMODULE)))
+  USEMODULE += ztimer_usec
 endif

sys/ztimer/Kconfig

@@ -152,6 +152,7 @@ config MODULE_ZTIMER_XTIMER_COMPAT
     bool "map xtimer calls to ztimer"
     select MODULE_DIV
     select MODULE_ZTIMER_USEC
+    select MODULE_ZTIMER_PERIPH_TIMER
     help
         This is a wrapper of xtimer API on ztimer_usec and is currently
         incomplete. Unless doing testing, use xtimer on ztimer.

sys/ztimer/Makefile.dep

@@ -13,13 +13,6 @@ endif
 
 # unless ztimer_xtimer_compat is used, make xtimer use ztimer_usec as backend.
 ifneq (,$(filter ztimer,$(USEMODULE)))
-  ifneq (,$(filter xtimer,$(USEMODULE)))
-    ifeq (,$(filter ztimer_xtimer_compat ztimer64_xtimer_compat,$(USEMODULE)))
-      USEMODULE += xtimer_on_ztimer
-    else
-      USEMODULE += div
-    endif
-  endif
   ifneq (,$(filter evtimer,$(USEMODULE)))
     USEMODULE += evtimer_on_ztimer
   endif
@@ -34,11 +27,6 @@ ifneq (,$(filter ztimer64_xtimer_compat,$(USEMODULE)))
   USEMODULE += ztimer_xtimer_compat
 endif
 
-# make xtimer use ztimer_usec as low level timer
-ifneq (,$(filter xtimer_on_ztimer,$(USEMODULE)))
-  USEMODULE += ztimer_usec
-endif
-
 # make evtimer use ztimer_msec as low level timer
 ifneq (,$(filter evtimer_on_ztimer,$(USEMODULE)))
   USEMODULE += ztimer_msec