[PATCH] x86: Do not try to disable hpet if it hasn't been initialized before

[Jeremy Fitzhardinge <jeremy@goop.org>]

[ x86 folk: While this patch is just a generic x86 bugfix, it really only affects
  the Xen pv-on-hvm patch series that I'm going to post shortly.  Do you want to
  commit it separately via tip.git, or are you OK with me sending it to Linus
  directly? - J ]

hpet_disable is called unconditionally on machine reboot if hpet support
is compiled in the kernel.
hpet_disable only checks if the machine is hpet capable but doesn't make
sure that hpet has been initialized.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Venkatesh Pallipadi <venki@google.com>

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index a198b7c..9e90dac 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -964,16 +964,18 @@ fs_initcall(hpet_late_init);
 
 void hpet_disable(void)
 {
-	if (is_hpet_capable()) {
-		unsigned int cfg = hpet_readl(HPET_CFG);
+	unsigned int cfg;
 
-		if (hpet_legacy_int_enabled) {
-			cfg &= ~HPET_CFG_LEGACY;
-			hpet_legacy_int_enabled = 0;
-		}
-		cfg &= ~HPET_CFG_ENABLE;
-		hpet_writel(cfg, HPET_CFG);
+	if (!is_hpet_capable() || !hpet_address || !hpet_virt_address)
+		return;
+
+	cfg = hpet_readl(HPET_CFG);
+	if (hpet_legacy_int_enabled) {
+		cfg &= ~HPET_CFG_LEGACY;
+		hpet_legacy_int_enabled = 0;
 	}
+	cfg &= ~HPET_CFG_ENABLE;
+	hpet_writel(cfg, HPET_CFG);
 }
 
 #ifdef CONFIG_HPET_EMULATE_RTC