[PATCH] selftests: rtc: fix flaky date_read_loop test

[Wake Liu <wakel@google.com>]

The test case rtc.date_read_loop in rtctest.c fails intermittently because
it checks that the RTC time does not advance by more than 1 second per loop
iteration. However, the loop sleeps for 11ms via nanosleep(), and if the
test thread is descheduled by the OS scheduler (e.g., under heavy system
load in a VM), more than 1 second can elapse between consecutive RTC reads.
This causes the next RTC time read to be 2 or more seconds ahead of the
previous read, triggering a test assertion failure.

To make the test more resilient against OS scheduling delays, measure the
real elapsed time between iterations using a monotonic clock
(clock_gettime(CLOCK_MONOTONIC)), and compute the actual number of seconds
elapsed (delta_s) between consecutive RTC reads. Then dynamically adjust
the assertion to:
ASSERT_GE(prev_rtc_read + delta_s + 1, rtc_read);

Signed-off-by: Wake Liu <wakel@google.com>
---
 tools/testing/selftests/rtc/rtctest.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/rtc/rtctest.c b/tools/testing/selftests/rtc/rtctest.c
index 8047d9879039..54eb5c255a45 100644
--- a/tools/testing/selftests/rtc/rtctest.c
+++ b/tools/testing/selftests/rtc/rtctest.c
@@ -116,6 +116,7 @@ TEST_F_TIMEOUT(rtc, date_read_loop, READ_LOOP_DURATION_SEC + 2) {
 	long iter_count = 0;
 	struct rtc_time rtc_tm;
 	time_t start_rtc_read, prev_rtc_read;
+	struct timespec prev_mono, cur_mono;
 
 	if (self->fd == -1 && errno == ENOENT)
 		SKIP(return, "Skipping test since %s does not exist", rtc_file);
@@ -126,25 +127,31 @@ TEST_F_TIMEOUT(rtc, date_read_loop, READ_LOOP_DURATION_SEC + 2) {
 
 	rc = ioctl(self->fd, RTC_RD_TIME, &rtc_tm);
 	ASSERT_NE(-1, rc);
+	clock_gettime(CLOCK_MONOTONIC, &prev_mono);
 	start_rtc_read = rtc_time_to_timestamp(&rtc_tm);
 	prev_rtc_read = start_rtc_read;
 
 	do  {
 		time_t rtc_read;
+		time_t delta_s = 0;
 
 		rc = ioctl(self->fd, RTC_RD_TIME, &rtc_tm);
 		ASSERT_NE(-1, rc);
+		clock_gettime(CLOCK_MONOTONIC, &cur_mono);
 
 		rtc_read = rtc_time_to_timestamp(&rtc_tm);
+		delta_s = cur_mono.tv_sec - prev_mono.tv_sec;
+
 		/* Time should not go backwards */
 		ASSERT_LE(prev_rtc_read, rtc_read);
-		/* Time should not increase more then 1s at a time */
-		ASSERT_GE(prev_rtc_read + 1, rtc_read);
+		/* Time should not increase more then elapsed time + 1s */
+		ASSERT_GE(prev_rtc_read + delta_s + 1, rtc_read);
 
 		/* Sleep 11ms to avoid killing / overheating the RTC */
 		nanosleep_with_retries(READ_LOOP_SLEEP_MS * 1000000);
 
 		prev_rtc_read = rtc_read;
+		prev_mono = cur_mono;
 		iter_count++;
 	} while (prev_rtc_read <= start_rtc_read + READ_LOOP_DURATION_SEC);
 
-- 
2.54.0.545.g6539524ca2-goog