hwclock: allow tweaking hard-coded .5s in --systohc algorithm

The algorithm hwclock uses for setting the RTC from the system clock
uses "knowledge" about the hardware, which unfortunately only applies
to some hardware (x86-like platforms, in particular). On other
platforms where the fractional part of the RTC is cleared when the RTC
is set, this algorithm causes a consistent .5 second loss when doing
hwclock --systohc.

So allow overriding those hard-coded .5. To figure out how a specific
platform behaves, and thus whether one should use '--delay 0.0', one
can do something like

# Make sure ntpd/chronyd/ are down.
# Set system clock once.
ntpdate 0.pool.ntp.org 1.pool.ntp.org
# Write the system time to the RTC, read it back, compare to NTP time.
hwclock --utc --systohc --noadjfile
hwclock --utc --hctosys --noadjfile
ntpdate -q 0.pool.ntp.org 1.pool.ntp.org
# Repeat these three steps a few times. If the discrepancy increases
# consistently with ~0.5s, try to start over but pass --delay 0.0 to
# the --systohc command and see if the situation improves.

Origin: OE-lite
Upstream-status: Inappropriate (in this form - significant changes have happened since 2.29).

--- util-linux-2.29/sys-utils/hwclock.c.orig
+++ util-linux-2.29/sys-utils/hwclock.c
@@ -531,6 +531,8 @@ set_hardware_clock(const time_t newtime,
  * This function ought to be able to accept set times as fractional times.
  * Idea for future enhancement.
  */
+static double RTC_SET_DELAY_SECS = 0.5;	    /* 500 ms */
+
 static void
 set_hardware_clock_exact(const time_t sethwtime,
 			 const struct timeval refsystime,
@@ -584,8 +586,7 @@ set_hardware_clock_exact(const time_t sethwtime,
 	time_t newhwtime = sethwtime;
 	double target_time_tolerance_secs = 0.001;  /* initial value */
 	double tolerance_incr_secs = 0.001;	    /* initial value */
-	const double RTC_SET_DELAY_SECS = 0.5;	    /* 500 ms */
-	const struct timeval RTC_SET_DELAY_TV = { 0, RTC_SET_DELAY_SECS * 1E6 };
+	struct timeval RTC_SET_DELAY_TV = { 0, RTC_SET_DELAY_SECS * 1E6 };
 
 	struct timeval targetsystime;
 	struct timeval nowsystime;
@@ -1639,6 +1640,7 @@ int main(int argc, char **argv)
 	    predict, compare, get;
 	bool utc, testing, local_opt, update, noadjfile, directisa;
 	char *date_opt;
+	char *delay_opt;
 #ifdef __alpha__
 	bool ARCconsole, Jensen, SRM, funky_toy;
 #endif
@@ -1647,6 +1649,7 @@ int main(int argc, char **argv)
 		OPT_ADJFILE = CHAR_MAX + 1,
 		OPT_BADYEAR,
 		OPT_DATE,
+		OPT_DELAY,
 		OPT_DIRECTISA,
 		OPT_EPOCH,
 		OPT_GET,
@@ -1690,6 +1693,7 @@ int main(int argc, char **argv)
 		{"directisa",	0, 0, OPT_DIRECTISA},
 		{"test",	0, 0, OPT_TEST},
 		{"date",	1, 0, OPT_DATE},
+		{"delay",       1, 0, OPT_DELAY},
 		{"epoch",	1, 0, OPT_EPOCH},
 #ifdef __linux__
 		{"rtc",		1, 0, 'f'},
@@ -1749,6 +1753,7 @@ int main(int argc, char **argv)
 	ARCconsole = Jensen = SRM = funky_toy = badyear = FALSE;
 #endif
 	date_opt = NULL;
+	delay_opt = NULL;
 
 	while ((c = getopt_long(argc, argv,
 				"?hvVDacrsulwAJSFf:", longopts, NULL)) != -1) {
@@ -1820,6 +1825,9 @@ int main(int argc, char **argv)
 		case OPT_DATE:
 			date_opt = optarg;	/* --date */
 			break;
+		case OPT_DELAY:
+			delay_opt = optarg;	/* --delay */
+			break;
 		case OPT_EPOCH:
 			epoch_option =		/* --epoch */
 			    strtoul_or_err(optarg, _("invalid epoch argument"));
@@ -1890,6 +1898,9 @@ int main(int argc, char **argv)
 	set_cmos_access(Jensen, funky_toy);
 #endif
 
+	if (delay_opt)
+		RTC_SET_DELAY_SECS = strtod_or_err(delay_opt, "invalid --delay argument");
+
 	if (set || predict) {
 		rc = interpret_date_string(date_opt, &set_time);
 		/* (time-consuming) */