Index: sys/x86/include/x86_var.h
===================================================================
--- sys/x86/include/x86_var.h
+++ sys/x86/include/x86_var.h
@@ -116,6 +116,7 @@
 #endif
 }
 
+bool	acpi_get_fadt_bootflags(uint16_t *flagsp);
 void	*alloc_fpusave(int flags);
 void	busdma_swi(void);
 bool	cpu_mwait_usable(void);
Index: sys/x86/isa/atrtc.c
===================================================================
--- sys/x86/isa/atrtc.c
+++ sys/x86/isa/atrtc.c
@@ -32,7 +32,6 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
-#include "opt_acpi.h"
 #include "opt_isa.h"
 
 #include <sys/param.h>
@@ -55,10 +54,8 @@
 #endif
 #include <machine/intr_machdep.h>
 #include "clock_if.h"
-
-#ifdef DEV_ACPI
 #include <contrib/dev/acpica/include/acpi.h>
-#endif
+#include <machine/md_var.h>
 
 /*
  * atrtc_lock protects low-level access to individual hardware registers.
@@ -261,29 +258,12 @@
 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)
+	if (!acpi_get_fadt_bootflags(&flags))
 		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 ((flags & ACPI_FADT_NO_CMOS_RTC) != 0);
 		return (true);
-#endif
-
-	return (false);
 }
 
 static int
Index: sys/x86/x86/cpu_machdep.c
===================================================================
--- sys/x86/x86/cpu_machdep.c
+++ sys/x86/x86/cpu_machdep.c
@@ -41,6 +41,7 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include "opt_acpi.h"
 #include "opt_atpic.h"
 #include "opt_cpu.h"
 #include "opt_ddb.h"
@@ -98,6 +99,8 @@
 
 #include <isa/isareg.h>
 
+#include <contrib/dev/acpica/include/acpi.h>
+
 #define	STATE_RUNNING	0x0
 #define	STATE_MWAIT	0x1
 #define	STATE_SLEEPING	0x2
@@ -930,3 +933,23 @@
 		load_cr0(rcr0() | CR0_WP);
 }
 
+bool
+acpi_get_fadt_bootflags(uint16_t *flagsp)
+{
+#ifdef DEV_ACPI
+	ACPI_TABLE_FADT *fadt;
+	vm_paddr_t physaddr;
+
+	physaddr = acpi_find_table(ACPI_SIG_FADT);
+	if (physaddr == 0)
+		return (false);
+	fadt = acpi_map_table(physaddr, ACPI_SIG_FADT);
+	if (fadt == NULL)
+		return (false);
+	*flagsp = fadt->BootFlags;
+	acpi_unmap_table(fadt);
+	return (true);
+#else
+	return (false);
+#endif
+}
Index: sys/x86/x86/tsc.c
===================================================================
--- sys/x86/x86/tsc.c
+++ sys/x86/x86/tsc.c
@@ -50,6 +50,7 @@
 #include <machine/specialreg.h>
 #include <x86/vmware.h>
 #include <dev/acpica/acpi_hpet.h>
+#include <contrib/dev/acpica/include/acpi.h>
 
 #include "cpufreq_if.h"
 
@@ -81,8 +82,9 @@
     "Disable x86 Time Stamp Counter");
 
 static int	tsc_skip_calibration;
-SYSCTL_INT(_machdep, OID_AUTO, disable_tsc_calibration, CTLFLAG_RDTUN,
-    &tsc_skip_calibration, 0, "Disable TSC frequency calibration");
+SYSCTL_INT(_machdep, OID_AUTO, disable_tsc_calibration, CTLFLAG_RDTUN |
+    CTLFLAG_NOFETCH, &tsc_skip_calibration, 0,
+    "Disable TSC frequency calibration");
 
 static void tsc_freq_changed(void *arg, const struct cf_level *level,
     int status);
@@ -213,6 +215,7 @@
 {
 	u_int regs[4];
 	uint64_t tsc1, tsc2;
+	uint16_t bootflags;
 
 	if (cpu_high >= 6) {
 		do_cpuid(6, regs);
@@ -272,6 +275,25 @@
 		break;
 	}
 
+	if (!TUNABLE_INT_FETCH("machdep.disable_tsc_calibration",
+	    &tsc_skip_calibration)) {
+		/*
+		 * User did not give the order about calibration.
+		 * If he did, we do not try to guess.
+		 *
+		 * Otherwise, if ACPI FADT reports that the platform
+		 * is legacy-free and CPUID provides TSC frequency,
+		 * use it.  The calibration could fail anyway since
+		 * ISA timer can be absent or power gated.
+		 */
+		if (acpi_get_fadt_bootflags(&bootflags) &&
+		    (bootflags & ACPI_FADT_LEGACY_DEVICES) == 0 &&
+		    tsc_freq_cpuid()) {
+			printf("Skipping TSC calibration since no legacy "
+			    "devices reported by FADT and CPUID works\n");
+			tsc_skip_calibration = 1;
+		}
+	}
 	if (tsc_skip_calibration) {
 		if (tsc_freq_cpuid())
 			;