Index: share/man/man4/atrtc.4
===================================================================
--- share/man/man4/atrtc.4
+++ share/man/man4/atrtc.4
@@ -33,13 +33,18 @@
 .Sh SYNOPSIS
 This driver is a mandatory part of i386/amd64 kernels.
 .Pp
-The following tunable is settable from the
+The following tunables are settable from the
 .Xr loader 8 :
 .Bl -ohang
 .It Va hint.atrtc. Ns Ar X Ns Va .clock
 controls event timers functionality support.
 Setting to 0, disables it.
 Default value is 1.
+.It Va hw.atrtc.enable
+Forces enabling or disabling of the device(s).
+Setting to 0 disables it, setting to 1 enables it, bypassing any platform
+configuration hints.
+Default value is -1 (autodetect).
 .El
 .Sh DESCRIPTION
 This driver uses RTC hardware to supply kernel with time-of-day clock
Index: sys/x86/isa/atrtc.c
===================================================================
--- sys/x86/isa/atrtc.c
+++ sys/x86/isa/atrtc.c
@@ -32,6 +32,7 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include "opt_acpi.h"
 #include "opt_isa.h"
 
 #include <sys/param.h>
@@ -55,6 +56,10 @@
 #include <machine/intr_machdep.h>
 #include "clock_if.h"
 
+#ifdef DEV_ACPI
+#include <contrib/dev/acpica/include/acpi.h>
+#endif
+
 /*
  * clock_lock protects low-level access to individual hardware registers.
  * atrtc_time_lock protects the entire sequence of accessing multiple registers
@@ -63,6 +68,10 @@
 #define	RTC_LOCK	do { if (!kdb_active) mtx_lock_spin(&clock_lock); } while (0)
 #define	RTC_UNLOCK	do { if (!kdb_active) mtx_unlock_spin(&clock_lock); } while (0)
 
+/* Force RTC enabled/disabled. */
+static int atrtc_enabled = -1;
+TUNABLE_INT("hw.atrtc.enabled", &atrtc_enabled);
+
 struct mtx atrtc_time_lock;
 MTX_SYSINIT(atrtc_lock_init, &atrtc_time_lock, "atrtc", MTX_DEF);
 
@@ -241,11 +250,43 @@
 	{ 0 }
 };
 
+static bool
+atrtc_acpi_disabled(void)
+{
+#ifdef DEV_ACPI
+	ACPI_TABLE_FADT *fadt;
+	vm_paddr_t physaddr;
+	uint16_t flags;
+
+	physaddr = acpi_find_table(ACPI_SIG_FADT);
+	if (physaddr == 0)
+		return (false);
+
+	fadt = acpi_map_table(physaddr, ACPI_SIG_FADT);
+	if (fadt == NULL) {
+		printf("at_rtc: unable to map FADT ACPI table\n");
+		return (false);
+	}
+
+	flags = fadt->BootFlags;
+	acpi_unmap_table(fadt);
+
+	if (flags & ACPI_FADT_NO_CMOS_RTC)
+		return (true);
+#endif
+
+	return (false);
+}
+
 static int
 atrtc_probe(device_t dev)
 {
 	int result;
-	
+
+	if ((atrtc_enabled == -1 && atrtc_acpi_disabled()) ||
+	    (atrtc_enabled == 0))
+		return (ENXIO);
+
 	result = ISA_PNP_PROBE(device_get_parent(dev), dev, atrtc_ids);
 	/* ENOENT means no PnP-ID, device is hinted. */
 	if (result == ENOENT) {