PM-runtime: update accounting_timestamp on enable

Initializing accounting_timestamp to something different from 0 during
pm_runtime_init() doesn't make sense and puts an artificial ordering
constraint between timekeeping_init() and pm_runtime_init().

PM-runtime should start time accounting only when it is enabled and
discard the period when disabled.

Set accounting_timestamp to now when enabling PM-runtime.
Suggested-by: N"Rafael J. Wysocki" <rjw@rjwysocki.net>
Signed-off-by: NVincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: NUlf Hansson <ulf.hansson@linaro.org>
[ rjw: Subject & changelog ]
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/runtime.c

@@ -1309,10 +1309,15 @@ void pm_runtime_enable(struct device *dev)
 
 	spin_lock_irqsave(&dev->power.lock, flags);
 
-	if (dev->power.disable_depth > 0)
+	if (dev->power.disable_depth > 0) {
 		dev->power.disable_depth--;
-	else
+
+		/* About to enable runtime pm, set accounting_timestamp to now */
+		if (!dev->power.disable_depth)
+			dev->power.accounting_timestamp = jiffies;
+	} else {
 		dev_warn(dev, "Unbalanced %s!\n", __func__);
+	}
 
 	WARN(!dev->power.disable_depth &&
 	     dev->power.runtime_status == RPM_SUSPENDED &&
@@ -1509,7 +1514,7 @@ void pm_runtime_init(struct device *dev)
 	dev->power.request_pending = false;
 	dev->power.request = RPM_REQ_NONE;
 	dev->power.deferred_resume = false;
-	dev->power.accounting_timestamp = jiffies;
+	dev->power.accounting_timestamp = 0;
 	INIT_WORK(&dev->power.work, pm_runtime_work);
 
 	dev->power.timer_expires = 0;