sys: time_units: Add Kconfig option for algorithm selection

Add maximum timeout used for conversion to Kconfig. Option is used
to determine which conversion algorithm to use: faster but overflowing
earlier or slower without early overflow.

Signed-off-by: Krzysztof Chruscinski <[email protected]>
(cherry picked from commit 50c7c7b)

Author: nordic-krch

include/sys/time_units.h

@@ -59,6 +59,21 @@ static TIME_CONSTEXPR inline int sys_clock_hw_cycles_per_sec(void)
 #endif
 }
 
+/** @internal
+ * Macro determines if fast conversion algorithm can be used. It checks if
+ * maximum timeout represented in source frequency domain and multiplied by
+ * target frequency fits in 64 bits.
+ *
+ * @param from_hz Source frequency.
+ * @param to_hz Target frequency.
+ *
+ * @retval true Use faster algorithm.
+ * @retval false Use algorithm preventing overflow of intermediate value.
+ */
+#define Z_TMCVT_USE_FAST_ALGO(from_hz, to_hz) \
+	((ceiling_fraction(CONFIG_SYS_CLOCK_MAX_TIMEOUT_DAYS * 24ULL * 3600ULL * from_hz, \
+			   UINT32_MAX) * to_hz) <= UINT32_MAX)
+
 /* Time converter generator gadget.  Selects from one of three
  * conversion algorithms: ones that take advantage when the
  * frequencies are an integer ratio (in either direction), or a full
@@ -126,8 +141,18 @@ static TIME_CONSTEXPR ALWAYS_INLINE uint64_t z_tmcvt(uint64_t t, uint32_t from_h
 	} else {
 		if (result32) {
 			return (uint32_t)((t * to_hz + off) / from_hz);
+		} else if (const_hz && Z_TMCVT_USE_FAST_ALGO(from_hz, to_hz)) {
+			/* Faster algorithm but source is first multiplied by target frequency
+			 * and it can overflow even though final result would not overflow.
+			 * Kconfig option shall prevent use of this algorithm when there is a
+			 * risk of overflow.
+			 */
+			return ((t * to_hz + off) / from_hz);
 		} else {
-			return (t * to_hz + off) / from_hz;
+			/* Slower algorithm but input is first divided before being multiplied
+			 * which prevents overflow of intermediate value.
+			 */
+			return (t / from_hz) * to_hz + ((t % from_hz) * to_hz + off) / from_hz;
 		}
 	}
 }

kernel/Kconfig

@@ -613,6 +613,17 @@ config TIMEOUT_64BIT
 	  availability of absolute timeout values (which require the
 	  extra precision).
 
+config SYS_CLOCK_MAX_TIMEOUT_DAYS
+	int "Max timeout (in days) used in conversions"
+	default 365
+	help
+	  Value is used in the time conversion static inline function to determine
+	  at compile time which algorithm to use. One algorithm is faster, takes
+	  less code but may overflow if multiplication of source and target
+	  frequency exceeds 64 bits. Second algorithm prevents that. Faster
+	  algorithm is selected for conversion if maximum timeout represented in
+	  source frequency domain multiplied by target frequency fits in 64 bits.
+
 config XIP
 	bool "Execute in place"
 	help

subsys/testsuite/ztest/include/ztest.h

@@ -38,6 +38,7 @@
 #define CONFIG_MP_NUM_CPUS 1
 #define CONFIG_SYS_CLOCK_TICKS_PER_SEC 100
 #define CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC 10000000
+#define CONFIG_SYS_CLOCK_MAX_TIMEOUT_DAYS 365
 #define ARCH_STACK_PTR_ALIGN 8
 /* FIXME: Properly integrate with Zephyr's arch specific code */
 #define CONFIG_X86 1