tests/timer_api: Correct precision and fix correctness mistakes

Correct a bunch of precision/analysis errors in this test:

* Test items weren't consistent about tick alignment and resetting of
  the timestamp, so put these steps into init_timer_data() and call
  that immediately before k_timer_start().

* Many items would calculate the initial timestamp AFTER
  k_timer_start(), leading to an extra (third!) point where the timer
  computation could alias by an extra tick.  Always do this
  consistently before the timer is started (via init_timer-data()).

* Tickless systems with high tick rates can easily advance the system
  uptime while the timer ISR is running, so the system can't expect
  perfect accuracy even there (this test was originally written for
  ticked systmes where the ISR was by definition happening "at the
  same time").

  (Unfortunately our most popular high tick rate tickless system,
  nRF5, also has a clock that doesn't divide milliseconds exactly, so
  it had a special path through all these precision comparisons and
  avoided the bugs.  We finally found it on a x86 HPET system with 10
  kHz ticks.)

* The interval validation was placing a minimum bound on the interval
  time but not a maximum (this mistake was what had hidden the failure
  to reset the timestamp mentioned above).

Longer term, the millisecond precision math in these tests is at this
point an out of control complexity explosion.  We should look at
reworking the core OS tests of k_timer to use tick precision (which is
by definition exact) pervasively and leave the millisecond stuff to a
separate layer testing the alternative/legacy APIs.

Fixes #31964 (probably -- that was reported against up_squared, on
which I had trouble reproducing, but it was a common failure on
ehl_crb).

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

tests/kernel/timer/timer_api/src/main.c

@@ -17,8 +17,8 @@ struct timer_data {
 #define DURATION 100
 #define PERIOD 50
 #define EXPIRE_TIMES 4
-#define WITHIN_ERROR(var, target, epsilon)       \
-		(((var) >= (target)) && ((var) <= (target) + (epsilon)))
+#define WITHIN_ERROR(var, target, epsilon) (abs((target) - (var)) <= (epsilon))
+
 /* ms can be converted precisely to ticks only when a ms is exactly
  * represented by an integral number of ticks.  If the conversion is
  * not precise, then the reverse conversion of a difference in ms can
@@ -81,6 +81,31 @@ static void init_timer_data(void)
 {
 	tdata.expire_cnt = 0;
 	tdata.stop_cnt = 0;
+
+	k_usleep(1); /* align to tick */
+	tdata.timestamp = k_uptime_get();
+}
+
+static bool interval_check(int64_t interval, int64_t desired)
+{
+	int64_t slop = INEXACT_MS_CONVERT ? 1 : 0;
+
+	/* Tickless kernels will advance time inside of an ISR, so it
+	 * is always possible (especially with high tick rates and
+	 * slow CPUs) for us to arrive at the uptime check above too
+	 * late to see a full period elapse before the next period.
+	 * We can alias at both sides of the interval, so two
+	 * one-ticks deltas (NOT one two-tick delta!)
+	 */
+	if (IS_ENABLED(CONFIG_TICKLESS_KERNEL)) {
+		slop += 2 * k_ticks_to_ms_ceil32(1);
+	}
+
+	if (abs(interval - desired) > slop) {
+		return false;
+	}
+
+	return true;
 }
 
 /* entry routines */
@@ -91,13 +116,9 @@ static void duration_expire(struct k_timer *timer)
 
 	tdata.expire_cnt++;
 	if (tdata.expire_cnt == 1) {
-		TIMER_ASSERT((interval >= DURATION)
-			     || (INEXACT_MS_CONVERT
-				 && (interval == DURATION - 1)), timer);
+		TIMER_ASSERT(interval_check(interval, DURATION), timer);
 	} else {
-		TIMER_ASSERT((interval >= PERIOD)
-			     || (INEXACT_MS_CONVERT
-				 && (interval == PERIOD - 1)), timer);
+		TIMER_ASSERT(interval_check(interval, PERIOD), timer);
 	}
 
 	if (tdata.expire_cnt >= EXPIRE_TIMES) {
@@ -165,9 +186,7 @@ void test_timer_duration_period(void)
 {
 	init_timer_data();
 	/** TESTPOINT: init timer via k_timer_init */
-	k_usleep(1); /* align to tick */
 	k_timer_start(&duration_timer, K_MSEC(DURATION), K_MSEC(PERIOD));
-	tdata.timestamp = k_uptime_get();
 	busy_wait_ms(DURATION + PERIOD * EXPIRE_TIMES + PERIOD / 2);
 	/** TESTPOINT: check expire and stop times */
 	TIMER_ASSERT(tdata.expire_cnt == EXPIRE_TIMES, &duration_timer);
@@ -236,7 +255,6 @@ void test_timer_period_0(void)
 			      - BUSY_SLEW_THRESHOLD_TICKS(DURATION
 							  * USEC_PER_MSEC)),
 		      K_NO_WAIT);
-	tdata.timestamp = k_uptime_get();
 	busy_wait_ms(DURATION + 1);
 
 	/** TESTPOINT: ensure it is one-short timer */
@@ -454,6 +472,7 @@ void test_timer_status_sync(void)
 		TIMER_ASSERT(tdata.expire_cnt == (i + 1), &status_sync_timer);
 	}
 
+	init_timer_data();
 	k_timer_start(&status_sync_timer, K_MSEC(DURATION), K_MSEC(PERIOD));
 	busy_wait_ms(PERIOD*2);
 	zassert_true(k_timer_status_sync(&status_sync_timer), NULL);
@@ -482,9 +501,7 @@ void test_timer_k_define(void)
 {
 	init_timer_data();
 	/** TESTPOINT: init timer via k_timer_init */
-	k_usleep(1); /* align to tick */
 	k_timer_start(&ktimer, K_MSEC(DURATION), K_MSEC(PERIOD));
-	tdata.timestamp = k_uptime_get();
 	busy_wait_ms(DURATION + PERIOD * EXPIRE_TIMES + PERIOD / 2);
 
 	/** TESTPOINT: check expire and stop times */
@@ -620,7 +637,6 @@ void test_timer_remaining(void)
 
 
 	init_timer_data();
-	k_usleep(1); /* align to tick */
 	k_timer_start(&remain_timer, K_MSEC(DURATION), K_NO_WAIT);
 	busy_wait_ms(DURATION / 2);
 	rem_ticks = k_timer_remaining_ticks(&remain_timer);
@@ -700,7 +716,7 @@ void test_timeout_abs(void)
 	 * ticks, so we have to check that at least one case is
 	 * satisfied.
 	 */
-	k_usleep(1); /* align to tick */
+	init_timer_data();
 	k_timer_start(&remain_timer, t, K_FOREVER);
 	cap_ticks = k_uptime_ticks();
 	rem_ticks = k_timer_remaining_ticks(&remain_timer);