Add inline functions to convert between sbintime_t and decimal time units.

Use them in some existing code that is vulnerable to roundoff errors.

The existing constant SBT_1NS is a honeypot, luring unsuspecting folks into
writing code such as long_timeout_ns*SBT_1NS to generate the argument for a
sleep call.  The actual value of 1ns in sbt units is ~4.3, leading to a
large roundoff error giving a shorter sleep than expected when multiplying
by the trucated value of 4 in SBT_1NS.  (The evil honeypot aspect becomes
clear after you waste a whole day figuring out why your sleeps return early.)

1 files changed, 2 insertions, 2 deletions

diff --git a/sys/dev/ow/owc_gpiobus.c b/sys/dev/ow/owc_gpiobus.c
index 779f2d0f6945..423dde811b9f 100644
--- a/sys/dev/ow/owc_gpiobus.c
+++ b/sys/dev/ow/owc_gpiobus.c
@@ -295,10 +295,10 @@ owc_gpiobus_read_data(device_t dev, struct ow_timing *t, int *bit)
 	do {
 		now = sbinuptime();
 		GETPIN(sc, &sample);
-	} while ((now - then) / SBT_1US < t->t_rdv + 2 && sample == 0);
+	} while (sbttous(now - then) < t->t_rdv + 2 && sample == 0);
 	critical_exit();
 
-	if ((now - then) / SBT_1NS < t->t_rdv * 1000)
+	if (sbttons(now - then) < t->t_rdv * 1000)
 		*bit = 1;
 	else
 		*bit = 0;