PM / domains: Rework governor code to be more consistent

The genpd governor currently uses negative PM QoS values to indicate
the "no suspend" condition and 0 as "no restriction", but it doesn't
use them consistently.  Moreover, it tries to refresh QoS values for
already suspended devices in a quite questionable way.

For the above reasons, rework it to be a bit more consistent.

First off, note that dev_pm_qos_read_value() in
dev_update_qos_constraint() and __default_power_down_ok() is
evaluated for devices in suspend.  Moreover, that only happens if the
effective_constraint_ns value for them is negative (meaning "no
suspend").  It is not evaluated in any other cases, so effectively
the QoS values are only updated for devices in suspend that should
not have been suspended in the first place.  In all of the other
cases, the QoS values taken into account are the effective ones from
the time before the device has been suspended, so generally devices
need to be resumed and suspended again for new QoS values to take
effect anyway.  Thus evaluating dev_update_qos_constraint() in
those two places doesn't make sense at all, so drop it.

Second, initialize effective_constraint_ns to 0 ("no constraint")
rather than to (-1) ("no suspend"), which makes more sense in
general and in case effective_constraint_ns is never updated
(the device is in suspend all the time or it is never suspended)
it doesn't affect the device's parent and so on.

Finally, rework default_suspend_ok() to explicitly handle the
"no restriction" and "no suspend" special cases.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Tero Kristo <t-kristo@ti.com>
Reviewed-by: Ramesh Thomas <ramesh.thomas@intel.com>

drivers/base/power/domain.c

@@ -1328,7 +1328,7 @@ static struct generic_pm_domain_data *genpd_alloc_dev_data(struct device *dev,
 
 	gpd_data->base.dev = dev;
 	gpd_data->td.constraint_changed = true;
-	gpd_data->td.effective_constraint_ns = -1;
+	gpd_data->td.effective_constraint_ns = 0;
 	gpd_data->nb.notifier_call = genpd_dev_pm_qos_notifier;
 
 	spin_lock_irq(&dev->power.lock);

drivers/base/power/domain_governor.c

@@ -14,22 +14,33 @@
 static int dev_update_qos_constraint(struct device *dev, void *data)
 {
 	s64 *constraint_ns_p = data;
-	s32 constraint_ns = -1;
+	s64 constraint_ns;
 
-	if (dev->power.subsys_data && dev->power.subsys_data->domain_data)
+	if (dev->power.subsys_data && dev->power.subsys_data->domain_data) {
+		/*
+		 * Only take suspend-time QoS constraints of devices into
+		 * account, because constraints updated after the device has
+		 * been suspended are not guaranteed to be taken into account
+		 * anyway.  In order for them to take effect, the device has to
+		 * be resumed and suspended again.
+		 */
 		constraint_ns = dev_gpd_data(dev)->td.effective_constraint_ns;
-
+	} else {
-	if (constraint_ns < 0) {
+		/*
+		 * The child is not in a domain and there's no info on its
+		 * suspend/resume latencies, so assume them to be negligible and
+		 * take its current PM QoS constraint (that's the only thing
+		 * known at this point anyway).
+		 */
 		constraint_ns = dev_pm_qos_read_value(dev);
-		constraint_ns *= NSEC_PER_USEC;
+		if (constraint_ns > 0)
+			constraint_ns *= NSEC_PER_USEC;
 	}
+
+	/* 0 means "no constraint" */
 	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;
 
@@ -76,14 +87,32 @@ static bool default_suspend_ok(struct device *dev)
 		device_for_each_child(dev, &constraint_ns,
 				      dev_update_qos_constraint);
 
-	if (constraint_ns > 0) {
+	if (constraint_ns == 0) {
+		/* "No restriction", so the device is allowed to suspend. */
+		td->effective_constraint_ns = 0;
+		td->cached_suspend_ok = true;
+	} else if (constraint_ns < 0) {
+		/*
+		 * This triggers if one of the children that don't belong to a
+		 * domain has a negative PM QoS constraint and it's better not
+		 * to suspend then.  effective_constraint_ns is negative already
+		 * and cached_suspend_ok is false, so bail out.
+		 */
+		return false;
+	} else {
 		constraint_ns -= td->suspend_latency_ns +
 				td->resume_latency_ns;
-		if (constraint_ns == 0)
+		/*
+		 * effective_constraint_ns is negative already and
+		 * cached_suspend_ok is false, so if the computed value is not
+		 * positive, return right away.
+		 */
+		if (constraint_ns <= 0)
 			return false;
+
+		td->effective_constraint_ns = constraint_ns;
+		td->cached_suspend_ok = true;
 	}
-	td->effective_constraint_ns = constraint_ns;
-	td->cached_suspend_ok = constraint_ns >= 0;
 
 	/*
 	 * The children have been suspended already, so we don't need to take
@@ -144,18 +173,16 @@ static bool __default_power_down_ok(struct dev_pm_domain *pd,
 		 */
 		td = &to_gpd_data(pdd)->td;
 		constraint_ns = td->effective_constraint_ns;
-		/* default_suspend_ok() need not be called before us. */
+		/*
-		if (constraint_ns < 0) {
+		 * Negative values mean "no suspend at all" and this runs only
-			constraint_ns = dev_pm_qos_read_value(pdd->dev);
+		 * when all devices in the domain are suspended, so it must be
-			constraint_ns *= NSEC_PER_USEC;
+		 * 0 at least.
-		}
+		 *
+		 * 0 means "no constraint"
+		 */
 		if (constraint_ns == 0)
 			continue;
 
-		/*
-		 * constraint_ns cannot be negative here, because the device has
-		 * been suspended.
-		 */
 		if (constraint_ns <= off_on_time_ns)
 			return false;