PM / Domains: Rework default device stop governor function, v2

The existing default device stop governor function for PM domains,
default_stop_ok(), is supposed to check whether or not the device's
PM QoS latency constraint will be violated if the device is stopped
by pm_genpd_runtime_suspend().  However, the computations carried out
by it don't reflect the definition of the PM QoS latency constrait in
Documentation/ABI/testing/sysfs-devices-power.

Make default_stop_ok() follow the definition of the PM QoS latency
constrait.  In particular, make it take the device's start and stop
latencies correctly.

Add a new field, effective_constraint_ns, to struct gpd_timing_data
and use it to store the difference between the device's PM QoS
constraint and its resume latency for use by the device's parent
(the effective_constraint_ns values for the children are used for
computing the parent's one along with its PM QoS constraint).

Remove the break_even_ns field from struct gpd_timing_data, because
it's not used any more.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

drivers/base/power/domain.c

@@ -506,6 +506,7 @@ static int pm_genpd_runtime_suspend(struct device *dev)
 	if (dev_gpd_data(dev)->always_on)
 		return -EBUSY;
 
+	dev_gpd_data(dev)->td.effective_constraint_ns = -1;
 	stop_ok = genpd->gov ? genpd->gov->stop_ok : NULL;
 	if (stop_ok && !stop_ok(dev))
 		return -EBUSY;

drivers/base/power/domain_governor.c

@@ -14,21 +14,66 @@
 
 #ifdef CONFIG_PM_RUNTIME
 
+static int dev_update_qos_constraint(struct device *dev, void *data)
+{
+	s64 *constraint_ns_p = data;
+	s32 constraint_ns = -1;
+
+	if (dev->power.subsys_data && dev->power.subsys_data->domain_data)
+		constraint_ns = dev_gpd_data(dev)->td.effective_constraint_ns;
+
+	if (constraint_ns < 0) {
+		constraint_ns = dev_pm_qos_read_value(dev);
+		constraint_ns *= NSEC_PER_USEC;
+	}
+	if (constraint_ns == 0)
+		return 0;
+
+	/*
+	 * constraint_ns cannot be negative here, because the device has been
+	 * suspended.
+	 */
+	if (constraint_ns < *constraint_ns_p || *constraint_ns_p == 0)
+		*constraint_ns_p = constraint_ns;
+
+	return 0;
+}
+
 /**
  * default_stop_ok - Default PM domain governor routine for stopping devices.
  * @dev: Device to check.
  */
 bool default_stop_ok(struct device *dev)
 {
 	struct gpd_timing_data *td = &dev_gpd_data(dev)->td;
+	s64 constraint_ns;
 
 	dev_dbg(dev, "%s()\n", __func__);
 
-	if (dev->power.max_time_suspended_ns < 0 || td->break_even_ns == 0)
-		return true;
+	constraint_ns = dev_pm_qos_read_value(dev);
+	if (constraint_ns < 0)
+		return false;
+
+	constraint_ns *= NSEC_PER_USEC;
+	/*
+	 * We can walk the children without any additional locking, because
+	 * they all have been suspended at this point.
+	 */
+	if (!dev->power.ignore_children)
+		device_for_each_child(dev, &constraint_ns,
+				      dev_update_qos_constraint);
 
-	return td->stop_latency_ns + td->start_latency_ns < td->break_even_ns
-		&& td->break_even_ns < dev->power.max_time_suspended_ns;
+	if (constraint_ns > 0) {
+		constraint_ns -= td->start_latency_ns;
+		if (constraint_ns == 0)
+			return false;
+	}
+	td->effective_constraint_ns = constraint_ns;
+	/*
+	 * The children have been suspended already, so we don't need to take
+	 * their stop latencies into account here.
+	 */
+	return constraint_ns > td->stop_latency_ns || constraint_ns == 0;
 }
 
 /**

include/linux/pm_domain.h

@@ -93,7 +93,7 @@ struct gpd_timing_data {
 	s64 start_latency_ns;
 	s64 save_state_latency_ns;
 	s64 restore_state_latency_ns;
-	s64 break_even_ns;
+	s64 effective_constraint_ns;
 };
 
 struct generic_pm_domain_data {