ztimer: pull ztimer_periph_rtt for ZTIMER_MSEC if available

sys/Makefile.include

@@ -133,10 +133,8 @@ ifneq (,$(filter test_utils_result_output,$(USEMODULE)))
   include $(RIOTBASE)/sys/test_utils/result_output/Makefile.include
 endif
 
-# Convert xtimer into a pseudo module if its API is already implemented by
-# ztimer's compatibility wrapper
-ifneq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
-  PSEUDOMODULES += xtimer
+ifneq (,$(filter ztimer,$(USEMODULE)))
+  include $(RIOTBASE)/sys/ztimer/Makefile.include
 endif
 
 ifneq (,$(filter prng,$(USEMODULE)))

sys/ztimer/Kconfig

@@ -15,7 +15,11 @@ menuconfig MODULE_ZTIMER
 
 if MODULE_ZTIMER
 
+config ZTIMER_CUSTOM_BACKEND_CONFIGURATION
+    bool "Override default backend selection"
+
 menu "Backends"
+    visible if ZTIMER_CUSTOM_BACKEND_CONFIGURATION
 
 config MODULE_ZTIMER_PERIPH_RTC
     bool "RTC peripheral"
@@ -26,7 +30,6 @@ config MODULE_ZTIMER_PERIPH_RTT
     bool "RTT peripheral"
     depends on HAS_PERIPH_RTT
     select MODULE_PERIPH_RTT
-    default y if !MODULE_ZTIMER_PERIPH_TIMER
 
 config MODULE_ZTIMER_PERIPH_PTP
     bool "PTP peripheral"
@@ -37,7 +40,6 @@ config MODULE_ZTIMER_PERIPH_TIMER
     bool "Timer peripheral"
     depends on HAS_PERIPH_TIMER
     select MODULE_PERIPH_TIMER
-    default y
 
 endmenu # Backends
 
@@ -45,19 +47,57 @@ menu "Clocks"
 
 config MODULE_ZTIMER_USEC
     bool "Microseconds"
-    depends on MODULE_ZTIMER_PERIPH_TIMER
+    select MODULE_ZTIMER_PERIPH_TIMER
 
 config MODULE_ZTIMER_MSEC
     bool "Milliseconds"
-    depends on MODULE_ZTIMER_PERIPH_TIMER || MODULE_ZTIMER_PERIPH_RTT
+
+choice
+    bool "Backend"
+    depends on MODULE_ZTIMER_MSEC
+    default ZTIMER_MSEC_BACKEND_RTT
+
+config ZTIMER_MSEC_BACKEND_TIMER
+    bool "Timer"
+    select MODULE_ZTIMER_PERIPH_TIMER
+
+config ZTIMER_MSEC_BACKEND_RTT
+    bool "RTT"
+    depends on HAS_PERIPH_RTT
+    select MODULE_ZTIMER_PERIPH_RTT
+
+endchoice
 
 config MODULE_ZTIMER_SEC
     bool "Seconds"
-    depends on MODULE_ZTIMER_PERIPH_TIMER || MODULE_ZTIMER_PERIPH_RTT || MODULE_ZTIMER_PERIPH_RTC
+
+choice
+    bool "Backend"
+    depends on MODULE_ZTIMER_SEC
+    default ZTIMER_SEC_BACKEND_RTC if !BOARD_NATIVE && \
+                                      !CPU_COMMON_SAM0 && \
+                                      !CPU_COMMON_EFM32 && \
+                                      !CPU_COMMON_STM32F1
+    default ZTIMER_SEC_BACKEND_RTT
+
+config ZTIMER_SEC_BACKEND_TIMER
+    bool "Timer"
+    select MODULE_ZTIMER_PERIPH_TIMER
+
+config ZTIMER_SEC_BACKEND_RTT
+    bool "RTT"
+    depends on HAS_PERIPH_RTT
+    select MODULE_ZTIMER_PERIPH_RTT
+
+config ZTIMER_SEC_BACKEND_RTC
+    bool "RTC"
+    depends on HAS_PERIPH_RTC
+    select MODULE_ZTIMER_PERIPH_RTC
+
+endchoice
 
 endmenu # Clocks
 
-
 menu "Frequency conversion"
 
 config MODULE_ZTIMER_CONVERT_MULDIV64

sys/ztimer/Makefile.dep

@@ -89,4 +89,22 @@ endif
 
 ifneq (,$(filter ztimer_msec,$(USEMODULE)))
   USEMODULE += ztimer
+  FEATURES_OPTIONAL += periph_rtt
+  # HACK: periph_rtt will get used only in the next iteration but an updated
+  # state for FEATURES_USED is needed here so include `features_check.inc.mk`
+  # here instead.
+  # An other option would be to check FEATURES_PROVIDED this would avoid the
+  # order of inclusion problem but it would no take into account possible conflicts
+  # and is also currently not allowed in the build system.
+  # An other alternative would be to delay to the next loop, but this produce a
+  # case where another loop is not executed and the conditional not evaluated
+  # If these kind of usecases pop up before Kconfig migration is completed
+  # then another alternative would be introduce a variable to require an extra
+  # loop independent of USEMODULE, FEATURES_REQUIRED and USEPKG
+  include $(RIOTMAKE)/features_check.inc.mk
+  ifneq (,$(filter periph_rtt,$(FEATURES_USED)))
+    USEMODULE += ztimer_periph_rtt
+  else
+    USEMODULE += ztimer_periph_timer
+  endif
 endif

sys/ztimer/Makefile.include

@@ -0,0 +1,15 @@
+# Convert xtimer into a pseudo module if its API is already implemented by
+# ztimer's compatibility wrapper
+ifneq (,$(filter ztimer_xtimer_compat,$(USEMODULE)))
+  PSEUDOMODULES += xtimer
+endif
+
+# By defaul use highest possible RTT_FREQUENCY for platforms that allow it. This
+# might not be the most optimized for conversion guarantees that ztimer_periph_rtt
+# will have a capable backend.
+ifneq (,$(filter ztimer_periph_rtt,$(USEMODULE)))
+  ifneq (,$(filter stm32 nrf52 sam% kinetis efm32,$(CPU)))
+    RTT_FREQUENCY ?= RTT_MAX_FREQUENCY
+    CFLAGS += -DRTT_FREQUENCY=$(RTT_FREQUENCY)
+  endif
+endif

tests/pkg_umorse/Makefile.ci

@@ -0,0 +1,2 @@
+BOARD_INSUFFICIENT_MEMORY := \
+    nucleo-l011k4 \

tests/ztimer_periodic/Makefile.ci

@@ -0,0 +1,2 @@
+BOARD_INSUFFICIENT_MEMORY := \
+    nucleo-l011k4 \