PM / QoS: Introcuce latency tolerance device PM QoS type

Add a new latency tolerance device PM QoS type to be use for
specifying active state (RPM_ACTIVE) memory access (DMA) latency
tolerance requirements for devices.  It may be used to prevent
hardware from choosing overly aggressive energy-saving operation
modes (causing too much latency to appear) for the whole platform.

This feature reqiures hardware support, so it only will be
available for devices having a new .set_latency_tolerance()
callback in struct dev_pm_info populated, in which case the
routine pointed to by it should implement whatever is necessary
to transfer the effective requirement value to the hardware.

Whenever the effective latency tolerance changes for the device,
its .set_latency_tolerance() callback will be executed and the
effective value will be passed to it.  If that value is negative,
which means that the list of latency tolerance requirements for
the device is empty, the callback is expected to switch the
underlying hardware latency tolerance control mechanism to an
autonomous mode if available.  If that value is PM_QOS_LATENCY_ANY,
in turn, and the hardware supports a special "no requirement"
setting, the callback is expected to use it.  That allows software
to prevent the hardware from automatically updating the device's
latency tolerance in response to its power state changes (e.g. during
transitions from D3cold to D0), which generally may be done in the
autonomous latency tolerance control mode.

If .set_latency_tolerance() is present for the device, a new
pm_qos_latency_tolerance_us attribute will be present in the
devivce's power directory in sysfs.  Then, user space can use
that attribute to specify its latency tolerance requirement for
the device, if any.  Writing "any" to it means "no requirement, but
do not let the hardware control latency tolerance" and writing
"auto" to it allows the hardware to be switched to the autonomous
mode if there are no other requirements from the kernel side in the
device's list.

This changeset includes a fix from Mika Westerberg.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/kernel/power/qos.c b/kernel/power/qos.c
index e23ae38..884b770 100644
--- a/kernel/power/qos.c
+++ b/kernel/power/qos.c
@@ -173,6 +173,7 @@ int pm_qos_update_target(struct pm_qos_constraints *c, struct plist_node *node,
 {
 	unsigned long flags;
 	int prev_value, curr_value, new_value;
+	int ret;
 
 	spin_lock_irqsave(&pm_qos_lock, flags);
 	prev_value = pm_qos_get_value(c);
@@ -208,13 +209,15 @@ int pm_qos_update_target(struct pm_qos_constraints *c, struct plist_node *node,
 
 	trace_pm_qos_update_target(action, prev_value, curr_value);
 	if (prev_value != curr_value) {
-		blocking_notifier_call_chain(c->notifiers,
-					     (unsigned long)curr_value,
-					     NULL);
-		return 1;
+		ret = 1;
+		if (c->notifiers)
+			blocking_notifier_call_chain(c->notifiers,
+						     (unsigned long)curr_value,
+						     NULL);
 	} else {
-		return 0;
+		ret = 0;
 	}
+	return ret;
 }
 
 /**