sys/ztimer: add 'ztimer_no_periph_rtt'

This is a temporary fix for Issue #17060. It allows to disable
auto inclusion of `ztimer_periph_rtt` in cases where another
module or application requires direct access.

Limitations:
- as ifeq are involved order of inclusion matters, therefore
  these modules should be included early in the build at application
  level and not in modules `Makefile.dep`
- this does not disallow direct inclusions of `ztimer_periph_rtt`,
  since this only disables auto inclusion of these modules

This is a temporary solution since this is already possible with
Kconfig, but not in make.

drivers/include/periph/rtt.h

@@ -22,6 +22,10 @@
  * On many CPUs, certain power states might need to be blocked in rtt_init(), so
  * that it is ensured that the RTT will function properly while it is enabled.
  *
+ * @warning     This module will be automatically be used as a backend for
+ *              ZTIMER_SEC and ZTIMER_MSEC. If direct access to RTT is needed
+ *              then include `ztimer_no_periph_rtt` to avoid auto-selection,
+ *              i.e.: `USEMODULE += ztimer_no_periph_rtt`.
  * @{
  * @file
  * @brief       Low-level RTT (Real Time Timer) peripheral driver interface

sys/include/ztimer.h

@@ -197,6 +197,11 @@
  *              if if these are missing it will use same basic timer
  *              as ZTIMER_USEC does.
  *
+ * If `periph_rtt` is required with direct access by another MODULE or
+ * application, `ztimer_no_periph_rtt` can be included to avoid automatic
+ * selection of `ztimer_periph_rtt` as a backend for ZTIMER_SEC and ZTIMER_MSEC.
+ * i.e.: `USEMODULE += ztimer_no_periph_rtt`.
+ *
  * These pointers are defined in `ztimer.h` and can be used like this:
  *
  *     ztimer_now(ZTIMER_USEC);

sys/ztimer/Kconfig

@@ -10,6 +10,12 @@ menu "ztimer - High level timer abstraction layer"
 config ZTIMER_CUSTOM_BACKEND_CONFIGURATION
     bool "Override default backend selection"
 
+config MODULE_ZTIMER_NO_PERIPH_RTT
+    bool "Disable ztimer_periph_rtt auto-inclusion"
+    help
+        This module disables the auto-inclusion of ztimer_periph_rtt as a backend
+        for ztimer_msec and ztimer_sec.
+
 menu "Backends"
     visible if ZTIMER_CUSTOM_BACKEND_CONFIGURATION
 
@@ -56,7 +62,7 @@ config MODULE_ZTIMER_MSEC
 choice
     bool "Backend"
     depends on MODULE_ZTIMER_MSEC
-    default ZTIMER_MSEC_BACKEND_RTT
+    default ZTIMER_MSEC_BACKEND_RTT if !MODULE_ZTIMER_NO_PERIPH_RTT
 
 config ZTIMER_MSEC_BACKEND_TIMER
     bool "Timer"
@@ -85,7 +91,7 @@ config ZTIMER_SEC_BACKEND_TIMER
 config ZTIMER_SEC_BACKEND_RTT
     bool "RTT"
     depends on HAS_PERIPH_RTT
-    select MODULE_ZTIMER_PERIPH_RTT
+    select MODULE_ZTIMER_PERIPH_RTT if !MODULE_ZTIMER_NO_PERIPH_RTT
 
 config ZTIMER_SEC_BACKEND_RTC
     bool "RTC"

sys/ztimer/Makefile.dep

@@ -87,35 +87,26 @@ ifneq (,$(filter ztimer_usec,$(USEMODULE)))
   USEMODULE += ztimer_periph_timer
 endif
 
-ifneq (,$(filter ztimer_msec,$(USEMODULE)))
+ifneq (,$(filter ztimer_msec ztimer_sec,$(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,$(FEATURES_USED)))
-    USEMODULE += ztimer_periph_rtt
-  else
-    USEMODULE += ztimer_periph_timer
-  endif
-endif
-
-ifneq (,$(filter ztimer_sec,$(USEMODULE)))
-  USEMODULE += ztimer
-  FEATURES_OPTIONAL += periph_rtt
-  # HACK: see above
-  ifneq (,$(filter periph_rtt,$(FEATURES_USED)))
-    USEMODULE += ztimer_periph_rtt
-  else
-    USEMODULE += ztimer_periph_timer
+  ifeq (,$(filter ztimer_no_periph_rtt,$(USEMODULE)))
+    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
 endif