[PATCH RESEND] 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);

Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Wake Liu <wakel@google.com>
---
Hi Shuah, Alexandre,

This is a resend of a patch that has been Acked by the RTC maintainer
but does not seem to have been picked up by either tree yet:
https://lore.kernel.org/all/20260430120313.4078185-1-wakel@google.com/

It applies cleanly on top of current mainline. No code changes since
the original posting; I have only folded in Alexandre's Acked-by.

Since tools/testing/selftests/rtc/ is listed under both the RTC
subsystem and the kselftest framework in MAINTAINERS, could either
Shuah (via the kselftest tree) or Alexandre (via the rtc tree) please
pick it up? I'm happy to go with whichever tree you both prefer.

We are still seeing this test fail intermittently in Android VTS
(about 15-18% of runs on some devices), so it would be great to get
this merged.

Thanks,
Wake

 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);
 
-- 
2.56.0.rc1.315.gc6ed9934b7-goog