ACPI/hpet: prevent boot hang when hpet=force used on ICH-4M

Linux tells ICH4 users that they can (manually) invoke
"hpet=force" to enable the undocumented ICH-4M HPET.
The HPET becomes available for both clocksource and clockevents.

But as of ff69f2bba6
(acpi: fix of pmtimer overflow that make Cx states time incorrect)
the HPET may be used via clocksource for idle accounting, and
hpet=force on an ICH4 box hangs boot.

It turns out that touching the MMIO HPET withing
the ARB_DIS part of C3 will hang the hardware.

The fix is to simply move the timer access outside
the ARB_DIS region.  This is a no-op on modern hardware
because ARB_DIS is no longer used.

http://bugzilla.kernel.org/show_bug.cgi?id=13087

Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/processor_idle.c

@@ -955,6 +955,7 @@ static int acpi_idle_enter_bm(struct cpuidle_device *dev,
 	 */
 	acpi_state_timer_broadcast(pr, cx, 1);
 
+	kt1 = ktime_get_real();
 	/*
 	 * disable bus master
 	 * bm_check implies we need ARB_DIS
@@ -976,10 +977,7 @@ static int acpi_idle_enter_bm(struct cpuidle_device *dev,
 		ACPI_FLUSH_CPU_CACHE();
 	}
 
-	kt1 = ktime_get_real();
 	acpi_idle_do_entry(cx);
-	kt2 = ktime_get_real();
-	idle_time =  ktime_to_us(ktime_sub(kt2, kt1));
 
 	/* Re-enable bus master arbitration */
 	if (pr->flags.bm_check && pr->flags.bm_control) {
@@ -988,6 +986,8 @@ static int acpi_idle_enter_bm(struct cpuidle_device *dev,
 		c3_cpu_count--;
 		spin_unlock(&c3_lock);
 	}
+	kt2 = ktime_get_real();
+	idle_time =  ktime_to_us(ktime_sub(kt2, kt1));
 
 #if defined (CONFIG_GENERIC_TIME) && defined (CONFIG_X86)
 	/* TSC could halt in idle, so notify users */