posix: clock: fix seconds calculation

Chris Friedt · cfriedt · commit 0388a90e7b41 · 2023-04-07T06:29:11.000-04:00
The previous method used to calculate seconds in `clock_gettime()`
seemed to have an inaccuracy that grew with time causing the
seconds to be off by an order of magnitude when ticks would roll
over.

This change fixes the method used to calculate seconds.

Signed-off-by: Chris Friedt &lt;cfriedt@meta.com&gt;
diff --git a/lib/posix/clock.c b/lib/posix/clock.c
@@ -48,8 +48,8 @@ int z_impl_clock_gettime(clockid_t clock_id, struct timespec *ts)
 	}
 
 	uint64_t ticks = k_uptime_ticks();
-	uint64_t elapsed_secs = k_ticks_to_ms_floor64(ticks) / MSEC_PER_SEC;
-	uint64_t nremainder = ticks - k_ms_to_ticks_floor64(MSEC_PER_SEC * elapsed_secs);
+	uint64_t elapsed_secs = ticks / CONFIG_SYS_CLOCK_TICKS_PER_SEC;
+	uint64_t nremainder = ticks - elapsed_secs * CONFIG_SYS_CLOCK_TICKS_PER_SEC;
 
 	ts->tv_sec = (time_t) elapsed_secs;
 	/* For ns 32 bit conversion can be used since its smaller than 1sec. */

Original file line number	Diff line number	Diff line change
`@@ -48,8 +48,8 @@ int z_impl_clock_gettime(clockid_t clock_id, struct timespec *ts)`
`48`	`48`	`}`
`49`	`49`
`50`	`50`	`uint64_t ticks = k_uptime_ticks();`
`51`		`- uint64_t elapsed_secs = k_ticks_to_ms_floor64(ticks) / MSEC_PER_SEC;`
`52`		`- uint64_t nremainder = ticks - k_ms_to_ticks_floor64(MSEC_PER_SEC * elapsed_secs);`
	`51`	`+ uint64_t elapsed_secs = ticks / CONFIG_SYS_CLOCK_TICKS_PER_SEC;`
	`52`	`+ uint64_t nremainder = ticks - elapsed_secs * CONFIG_SYS_CLOCK_TICKS_PER_SEC;`
`53`	`53`
`54`	`54`	`ts->tv_sec = (time_t) elapsed_secs;`
`55`	`55`	`/* For ns 32 bit conversion can be used since its smaller than 1sec. */`