From fce21e7e25d8e11fb3b59c217fc8ec63709a7877 Mon Sep 17 00:00:00 2001
From: Marius Strobl <marius@FreeBSD.org>
Date: Sat, 4 Jun 2005 23:24:50 +0000
Subject: [PATCH] After some input from bde@ and rereading the datasheet use a
 MTX_SPIN mutex instead of a MTX_DEF one in order to defer preemption while
 reading the date and time registers. If we don't manage to read them within
 the time slot where we are guaranteed that no updates occur we might actually
 read them during an update in which case the output is undefined.

---
 sys/dev/mc146818/mc146818.c | 22 +++++++++++-----------
 sys/kern/subr_witness.c     |  1 +
 sys/sparc64/sparc64/rtc.c   |  2 +-
 3 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/sys/dev/mc146818/mc146818.c b/sys/dev/mc146818/mc146818.c
index 6e6d478e59c..3bd4ce32138 100644
--- a/sys/dev/mc146818/mc146818.c
+++ b/sys/dev/mc146818/mc146818.c
@@ -80,9 +80,9 @@ mc146818_attach(device_t dev)
 			sc->sc_setcent = mc146818_def_setcent;
 	}
 
-	mtx_lock(&sc->sc_mtx);
+	mtx_lock_spin(&sc->sc_mtx);
 	if (!(*sc->sc_mcread)(dev, MC_REGD) & MC_REGD_VRT) {
-		mtx_unlock(&sc->sc_mtx);
+		mtx_unlock_spin(&sc->sc_mtx);
 		device_printf(dev, "%s: battery low\n", __func__);
 		return (ENXIO);
 	}
@@ -94,7 +94,7 @@ mc146818_attach(device_t dev)
 	sc->sc_regb |= (sc->sc_flag & MC146818_BCD) ? 0 : MC_REGB_BINARY;
 	sc->sc_regb |= (sc->sc_flag & MC146818_12HR) ? 0 : MC_REGB_24HR;
 	(*sc->sc_mcwrite)(dev, MC_REGB, sc->sc_regb);
-	mtx_unlock(&sc->sc_mtx);
+	mtx_unlock_spin(&sc->sc_mtx);
 
 	clock_register(dev, 1000000);	/* 1 second resolution. */
 
@@ -116,16 +116,16 @@ mc146818_gettime(device_t dev, struct timespec *ts)
 
 	timeout = 1000000;	/* XXX how long should we wait? */
 
-	mtx_lock(&sc->sc_mtx);
 	/*
 	 * If MC_REGA_UIP is 0 we have at least 244us before the next
 	 * update. If it's 1 an update is imminent.
 	 */
 	for (;;) {
+		mtx_lock_spin(&sc->sc_mtx);
 		if (!((*sc->sc_mcread)(dev, MC_REGA) & MC_REGA_UIP))
 			break;
+		mtx_unlock_spin(&sc->sc_mtx);
 		if (--timeout < 0) {
-			mtx_unlock(&sc->sc_mtx);
 			device_printf(dev, "%s: timeout\n", __func__);
 			return (EBUSY);
 		}
@@ -148,7 +148,7 @@ mc146818_gettime(device_t dev, struct timespec *ts)
 		year += cent * 100;
 	} else if (year < POSIX_BASE_YEAR)
 		year += 100;
-	mtx_unlock(&sc->sc_mtx);
+	mtx_unlock_spin(&sc->sc_mtx);
 
 	ct.year = year;
 
@@ -166,19 +166,19 @@ mc146818_getsecs(device_t dev, int *secp)
 
 	timeout = 1000000;	/* XXX how long should we wait? */
 
-	mtx_lock(&sc->sc_mtx);
 	for (;;) {
+		mtx_lock_spin(&sc->sc_mtx);
 		if (!((*sc->sc_mcread)(dev, MC_REGA) & MC_REGA_UIP)) {
 			sec = FROMREG((*sc->sc_mcread)(dev, MC_SEC));
+			mtx_unlock_spin(&sc->sc_mtx);
 			break;
 		}
+		mtx_unlock_spin(&sc->sc_mtx);
 		if (--timeout == 0) {
-			mtx_unlock(&sc->sc_mtx);
 			device_printf(dev, "%s: timeout\n", __func__);
 			return (EBUSY);
 		}
 	}
-	mtx_unlock(&sc->sc_mtx);
 
 #undef FROMREG
 
@@ -206,7 +206,7 @@ mc146818_settime(device_t dev, struct timespec *ts)
 	ts->tv_nsec = 0;
 	clock_ts_to_ct(ts, &ct);
 
-	mtx_lock(&sc->sc_mtx);
+	mtx_lock_spin(&sc->sc_mtx);
 	/* Disable RTC updates and interrupts (if enabled). */
 	(*sc->sc_mcwrite)(dev, MC_REGB,
 	    ((sc->sc_regb & (MC_REGB_BINARY | MC_REGB_24HR)) | MC_REGB_SET));
@@ -232,7 +232,7 @@ mc146818_settime(device_t dev, struct timespec *ts)
 
 	/* Reenable RTC updates and interrupts. */
 	(*sc->sc_mcwrite)(dev, MC_REGB, sc->sc_regb);
-	mtx_unlock(&sc->sc_mtx);
+	mtx_unlock_spin(&sc->sc_mtx);
 
 #undef TOREG
 
diff --git a/sys/kern/subr_witness.c b/sys/kern/subr_witness.c
index 7c71f539cc8..082c8b80697 100644
--- a/sys/kern/subr_witness.c
+++ b/sys/kern/subr_witness.c
@@ -375,6 +375,7 @@ static struct witness_order_list_entry order_lists[] = {
 #endif
 #ifdef __sparc64__
 	{ "ipi", &lock_class_mtx_spin },
+	{ "rtc_mtx", &lock_class_mtx_spin },
 #endif
 #endif
 	{ "clk", &lock_class_mtx_spin },
diff --git a/sys/sparc64/sparc64/rtc.c b/sys/sparc64/sparc64/rtc.c
index 234d1b7f6f5..be8e009df0b 100644
--- a/sys/sparc64/sparc64/rtc.c
+++ b/sys/sparc64/sparc64/rtc.c
@@ -149,7 +149,7 @@ rtc_attach(device_t dev)
 	sc = device_get_softc(dev);
 	bzero(sc, sizeof(struct mc146818_softc));
 
-	mtx_init(&sc->sc_mtx, "rtc_mtx", NULL, MTX_DEF);
+	mtx_init(&sc->sc_mtx, "rtc_mtx", NULL, MTX_SPIN);
 
 	if (strcmp(device_get_name(device_get_parent(dev)), "isa") == 0)
 		rtype = SYS_RES_IOPORT;