tests: time_units: check for overflow in z_tmcvt intermediate

Prior to #41602, due to the ordering of operations (first mul,
then div), an intermediate value would overflow, resulting in
a time non-linearity.

This test ensures that time rolls-over properly.

Signed-off-by: Chris Friedt <cfriedt@meta.com>

tests/unit/time_units/CMakeLists.txt

@@ -0,0 +1,10 @@
+# Copyright 2022 Meta
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+find_package(Zephyr COMPONENTS unittest REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(time_units)
+
+FILE(GLOB app_sources *.c)
+target_sources(testbinary PRIVATE ${app_sources})

tests/unit/time_units/main.c

@@ -0,0 +1,9 @@
+/*
+ * Copyright 2022 Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/ztest.h>
+
+ZTEST_SUITE(time_units, NULL, NULL, NULL, NULL, NULL);

tests/unit/time_units/overflow.c

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2022 Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <inttypes.h>
+#include <limits.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+#include <zephyr/sys/time_units.h>
+#include <zephyr/ztest.h>
+
+/**
+ * @brief Test @ref z_tmcvt for robustness against intermediate value overflow.
+ *
+ * With input
+ * ```
+ * [t0, t1, t2] = [
+ *   UINT64_MAX / to_hz - 1,
+ *   UINT64_MAX / to_hz,
+ *   UINT64_MAX / to_hz + 1,
+ * ]
+ * ```
+ *
+ * passed through @ref z_tmcvt, we expect a linear sequence:
+ * ```
+ * [
+ *   562949953369140,
+ *   562949953399658,
+ *   562949953430175,
+ * ]
+ * ```
+ *
+ * If an overflow occurs, we see something like the following:
+ * ```
+ * [
+ *   562949953369140,
+ *   562949953399658,
+ *   8863,
+ * ]
+ * ```
+ */
+ZTEST(time_units, test_z_tmcvt_for_overflow)
+{
+	const uint32_t from_hz = 32768UL;
+	const uint32_t to_hz = 1000000000UL;
+
+	zassert_equal(562949953369140ULL,
+		      z_tmcvt(UINT64_MAX / to_hz - 1, from_hz, to_hz, true, false, false, false));
+	zassert_equal(562949953399658ULL,
+		      z_tmcvt(UINT64_MAX / to_hz, from_hz, to_hz, true, false, false, false));
+	zassert_equal(562949953430175ULL,
+		      z_tmcvt(UINT64_MAX / to_hz + 1, from_hz, to_hz, true, false, false, false));
+}

tests/unit/time_units/prj.conf

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+CONFIG_ZTEST=y
+CONFIG_ZTEST_NEW_API=y

tests/unit/time_units/testcase.yaml

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+common:
+  tags: time_units
+  type: unit
+tests:
+  utilities.time_units.z_tmcvt: {}