hpet: fix the possibility of insane return value of hpet_calibrate() …

…against SMI

hpet_calibrate() has a possibility of miss-calibration due to SMI.  If SMI
interrupts in the while loop of calibration, then return value will be
big.  This change calibrates until stabilizing by the return value with a
small value.

[[email protected]: trivial style tweaks]
Signed-off-by: Yasunori Goto <[email protected]>
Acked-by: Clemens Ladisch <[email protected]>
Acked-by: Vojtech Pavlik <[email protected]>
Cc: Robert Picco <[email protected]>
Cc: Venkatesh Pallipadi <[email protected]>
Cc: Ingo Molnar <[email protected]>
Acked-by: Paul Gortmaker <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/char/hpet.c

@@ -713,7 +713,7 @@ static struct ctl_table_header *sysctl_header;
  */
 #define	TICK_CALIBRATE	(1000UL)
 
-static unsigned long hpet_calibrate(struct hpets *hpetp)
+static unsigned long __hpet_calibrate(struct hpets *hpetp)
 {
 	struct hpet_timer __iomem *timer = NULL;
 	unsigned long t, m, count, i, flags, start;
@@ -750,6 +750,26 @@ static unsigned long hpet_calibrate(struct hpets *hpetp)
 	return (m - start) / i;
 }
 
+static unsigned long hpet_calibrate(struct hpets *hpetp)
+{
+	unsigned long ret = -1;
+	unsigned long tmp;
+
+	/*
+	 * Try to calibrate until return value becomes stable small value.
+	 * If SMI interruption occurs in calibration loop, the return value
+	 * will be big. This avoids its impact.
+	 */
+	for ( ; ; ) {
+		tmp = __hpet_calibrate(hpetp);
+		if (ret <= tmp)
+			break;
+		ret = tmp;
+	}
+
+	return ret;
+}
+
 int hpet_alloc(struct hpet_data *hdp)
 {
 	u64 cap, mcfg;