PM / devfreq: Rewrite devfreq_update_status() to fix multiple bugs

The current devfreq_update_status() has the following bugs:
- If previous frequency doesn't have a valid level, it does an out of bounds
  access into the trans_table and causes memory corruption.
- When the new frequency doesn't have a valid level, the time spent in the
  new frequency is counted towards the next valid frequency switch instead of
  being ignored.
- The time spent on the previous frequency is added to the new frequency's
  stats instead of the previous frequency's stats.

This patch fixes all of this.

Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

drivers/devfreq/devfreq.c

@@ -91,26 +91,35 @@ static int devfreq_get_freq_level(struct devfreq *devfreq, unsigned long freq)
  */
 static int devfreq_update_status(struct devfreq *devfreq, unsigned long freq)
 {
-	int lev, prev_lev;
+	int lev, prev_lev, ret = 0;
 	unsigned long cur_time;
 
-	lev = devfreq_get_freq_level(devfreq, freq);
-	if (lev < 0)
-		return lev;
-
 	cur_time = jiffies;
-	devfreq->time_in_state[lev] +=
+
+	prev_lev = devfreq_get_freq_level(devfreq, devfreq->previous_freq);
+	if (prev_lev < 0) {
+		ret = prev_lev;
+		goto out;
+	}
+
+	devfreq->time_in_state[prev_lev] +=
 			 cur_time - devfreq->last_stat_updated;
-	if (freq != devfreq->previous_freq) {
+
-		prev_lev = devfreq_get_freq_level(devfreq,
+	lev = devfreq_get_freq_level(devfreq, freq);
-						devfreq->previous_freq);
+	if (lev < 0) {
+		ret = lev;
+		goto out;
+	}
+
+	if (lev != prev_lev) {
 		devfreq->trans_table[(prev_lev *
 				devfreq->profile->max_state) + lev]++;
 		devfreq->total_trans++;
 	}
-	devfreq->last_stat_updated = cur_time;
 
-	return 0;
+out:
+	devfreq->last_stat_updated = cur_time;
+	return ret;
 }
 
 /**