sys/ztimer: make use of periph_timer_query_freqs

This makes use of the `periph_timer_query_freqs` feature:

1. It does choose the closest frequency supported before calling
   timer_init() in the ztimer_periph_timer backend.
2. It does make use of the actually chosen frequency when using
   `ztimer_convert_frac`.
3. It does `assert()` the frequency is within 5% of the specified when
   no frequency conversion is performed or `ztimer_convert_shift_up`
   is used.

sys/include/ztimer/periph_timer.h

@@ -52,8 +52,10 @@ typedef struct {
  * @param[in]   dev     periph timer to use
  * @param[in]   freq    frequency to configure
  * @param[in]   max_val maximum value this timer supports
+ *
+ * @return  The actual frequency the timer has been configured to
  */
-void ztimer_periph_timer_init(ztimer_periph_timer_t *clock, tim_t dev,
+uint32_t ztimer_periph_timer_init(ztimer_periph_timer_t *clock, tim_t dev,
                                   uint32_t freq, uint32_t max_val);
 
 #ifdef __cplusplus

sys/ztimer/Makefile.dep

@@ -39,6 +39,7 @@ endif
 
 ifneq (,$(filter ztimer_periph_timer,$(USEMODULE)))
   FEATURES_REQUIRED += periph_timer
+  FEATURES_OPTIONAL += periph_timer_query_freqs
 endif
 
 ifneq (,$(filter ztimer_periph_rtc,$(USEMODULE)))

sys/ztimer/init.c

@@ -39,7 +39,9 @@
 
 #include "kernel_defines.h"
 
+#include "assert.h"
 #include "board.h"
+#include "compiler_hints.h"
 #include "ztimer.h"
 #include "ztimer/convert_frac.h"
 #include "ztimer/convert_shift.h"
@@ -72,6 +74,9 @@
 #  define ZTIMER_TIMER      _ztimer_periph_timer
 #  define ZTIMER_TIMER_CLK  _ztimer_periph_timer.super
 #  define ZTIMER_TIMER_FREQ CONFIG_ZTIMER_USEC_BASE_FREQ
+#  ifndef CONFIG_ZTIMER_PERIPH_TIMER_FORCE_CONVERSION
+#    define CONFIG_ZTIMER_PERIPH_TIMER_FORCE_CONVERSION 0
+#  endif
 #endif
 
 #if MODULE_ZTIMER_PERIPH_LPTIMER
@@ -209,9 +214,9 @@ ztimer_clock_t *const ZTIMER_USEC_BASE = &ZTIMER_TIMER_CLK;
 #  else
 #    error No suitable ZTIMER_USEC config. Basic timer configuration missing?
 #  endif
-#  if ZTIMER_TIMER_FREQ == FREQ_1MHZ
+#  if (ZTIMER_TIMER_FREQ == FREQ_1MHZ) && !(CONFIG_ZTIMER_PERIPH_TIMER_FORCE_CONVERSION)
 ztimer_clock_t *const ZTIMER_USEC = &ZTIMER_TIMER_CLK;
-#  elif ZTIMER_TIMER_FREQ == 250000LU
+#  elif (ZTIMER_TIMER_FREQ == 250000LU) && !(CONFIG_ZTIMER_PERIPH_TIMER_FORCE_CONVERSION)
 static ztimer_convert_shift_t _ztimer_convert_shift_usec;
 ztimer_clock_t *const ZTIMER_USEC = &_ztimer_convert_shift_usec.super.super;
 #  else
@@ -290,8 +295,11 @@ void ztimer_init(void)
         "ztimer_init(): ZTIMER_TIMER using periph timer %u, freq %lu, width %u\n",
         CONFIG_ZTIMER_USEC_DEV, ZTIMER_TIMER_FREQ,
         CONFIG_ZTIMER_USEC_WIDTH);
+    MAYBE_UNUSED
+    uint32_t periph_timer_freq =
         ztimer_periph_timer_init(&ZTIMER_TIMER, CONFIG_ZTIMER_USEC_DEV,
-                             ZTIMER_TIMER_FREQ, WIDTH_TO_MAXVAL(CONFIG_ZTIMER_USEC_WIDTH));
+                                 ZTIMER_TIMER_FREQ,
+                                 WIDTH_TO_MAXVAL(CONFIG_ZTIMER_USEC_WIDTH));
 #  if MODULE_PM_LAYERED && !MODULE_ZTIMER_ONDEMAND
     LOG_DEBUG("ztimer_init(): ZTIMER_TIMER setting block_pm_mode to %i\n",
               CONFIG_ZTIMER_TIMER_BLOCK_PM_MODE);
@@ -335,19 +343,27 @@ void ztimer_init(void)
 
 /* Step 5: initialize ztimers requested */
 #if MODULE_ZTIMER_USEC
-#  if ZTIMER_TIMER_FREQ != FREQ_1MHZ
+#  if (ZTIMER_TIMER_FREQ != FREQ_1MHZ) || CONFIG_ZTIMER_PERIPH_TIMER_FORCE_CONVERSION
-#    if ZTIMER_TIMER_FREQ == FREQ_250KHZ
+#    if (ZTIMER_TIMER_FREQ == FREQ_250KHZ) && !(CONFIG_ZTIMER_PERIPH_TIMER_FORCE_CONVERSION)
+    if (IS_ACTIVE(DEVELHELP) && ((periph_timer_freq < 237500) || (periph_timer_freq > 262500))) {
+        LOG_WARNING("ZTIMER_USEC from %" PRIu32 " Hz clock with \"left-shift by 2\" frequency conversion\n",
+                    periph_timer_freq);
+    }
     LOG_DEBUG("ztimer_init(): ZTIMER_USEC convert_shift %lu to 1000000\n",
-              ZTIMER_TIMER_FREQ);
+              periph_timer_freq);
     ztimer_convert_shift_up_init(&_ztimer_convert_shift_usec,
                                  ZTIMER_USEC_BASE, 2);
 #    else
     LOG_DEBUG("ztimer_init(): ZTIMER_USEC convert_frac %lu to 1000000\n",
               ZTIMER_TIMER_FREQ);
     ztimer_convert_frac_init(&_ztimer_convert_frac_usec, ZTIMER_USEC_BASE,
-                             FREQ_1MHZ, ZTIMER_TIMER_FREQ);
+                             FREQ_1MHZ, periph_timer_freq);
 #    endif
 #  else
+    if (IS_ACTIVE(DEVELHELP) && ((periph_timer_freq < 950000) || (periph_timer_freq > 1050000))) {
+        LOG_WARNING("ZTIMER_USEC from %" PRIu32 " Hz clock without frequency conversion\n",
+                    periph_timer_freq);
+    }
     LOG_DEBUG("ztimer_init(): ZTIMER_USEC without conversion\n");
 #  endif
 

sys/ztimer/periph_timer.c

@@ -21,7 +21,9 @@
  */
 
 #include "assert.h"
+#include "compiler_hints.h"
 #include "irq.h"
+#include "modules.h"
 #include "ztimer/periph_timer.h"
 
 #ifndef ZTIMER_PERIPH_TIMER_OFFSET
@@ -104,12 +106,16 @@ static const ztimer_ops_t _ztimer_periph_timer_ops = {
 #endif
 };
 
-void ztimer_periph_timer_init(ztimer_periph_timer_t *clock, tim_t dev,
+uint32_t ztimer_periph_timer_init(ztimer_periph_timer_t *clock, tim_t dev,
                                   uint32_t freq, uint32_t max_val)
 {
     clock->dev = dev;
     clock->super.ops = &_ztimer_periph_timer_ops;
     clock->super.max_value = max_val;
+
+    if (IS_USED(MODULE_PERIPH_TIMER_QUERY_FREQS)) {
+        freq = timer_get_closest_freq(dev, freq);
+    }
     int ret = timer_init(dev, freq, _ztimer_periph_timer_callback, clock);
 
     (void)ret;
@@ -127,4 +133,6 @@ void ztimer_periph_timer_init(ztimer_periph_timer_t *clock, tim_t dev,
      * the first ztimer_acquire() call starts the peripheral */
     timer_stop(dev);
 #endif
+
+    return freq;
 }