PM / sleep: Mechanism to avoid resuming runtime-suspended devices unnecessarily

Currently, some subsystems (e.g. PCI and the ACPI PM domain) have to
resume all runtime-suspended devices during system suspend, mostly
because those devices may need to be reprogrammed due to different
wakeup settings for system sleep and for runtime PM.

For some devices, though, it's OK to remain in runtime suspend
throughout a complete system suspend/resume cycle (if the device was in
runtime suspend at the start of the cycle).  We would like to do this
whenever possible, to avoid the overhead of extra power-up and power-down
events.

However, problems may arise because the device's descendants may require
it to be at full power at various points during the cycle.  Therefore the
most straightforward way to do this safely is if the device and all its
descendants can remain runtime suspended until the complete stage of
system resume.

To this end, introduce a new device PM flag, power.direct_complete
and modify the PM core to use that flag as follows.

If the ->prepare() callback of a device returns a positive number,
the PM core will regard that as an indication that it may leave the
device runtime-suspended.  It will then check if the system power
transition in progress is a suspend (and not hibernation in particular)
and if the device is, indeed, runtime-suspended.  In that case, the PM
core will set the device's power.direct_complete flag.  Otherwise it
will clear power.direct_complete for the device and it also will later
clear it for the device's parent (if there's one).

Next, the PM core will not invoke the ->suspend() ->suspend_late(),
->suspend_irq(), ->resume_irq(), ->resume_early(), or ->resume()
callbacks for all devices having power.direct_complete set.  It
will invoke their ->complete() callbacks, however, and those
callbacks are then responsible for resuming the devices as
appropriate, if necessary.  For example, in some cases they may
need to queue up runtime resume requests for the devices using
pm_request_resume().

Changelog partly based on an Alan Stern's description of the idea
(http://marc.info/?l=linux-pm&m=139940466625569&w=2).

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>

1 files changed, 51 insertions, 15 deletions

diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
index 86d5e4fb5b98..343ffad59377 100644
--- a/drivers/base/power/main.c
+++ b/drivers/base/power/main.c
@@ -479,7 +479,7 @@ static int device_resume_noirq(struct device *dev, pm_message_t state, bool asyn
 	TRACE_DEVICE(dev);
 	TRACE_RESUME(0);
 
-	if (dev->power.syscore)
+	if (dev->power.syscore || dev->power.direct_complete)
 		goto Out;
 
 	if (!dev->power.is_noirq_suspended)
@@ -605,7 +605,7 @@ static int device_resume_early(struct device *dev, pm_message_t state, bool asyn
 	TRACE_DEVICE(dev);
 	TRACE_RESUME(0);
 
-	if (dev->power.syscore)
+	if (dev->power.syscore || dev->power.direct_complete)
 		goto Out;
 
 	if (!dev->power.is_late_suspended)
@@ -735,6 +735,12 @@ static int device_resume(struct device *dev, pm_message_t state, bool async)
 	if (dev->power.syscore)
 		goto Complete;
 
+	if (dev->power.direct_complete) {
+		/* Match the pm_runtime_disable() in __device_suspend(). */
+		pm_runtime_enable(dev);
+		goto Complete;
+	}
+
 	dpm_wait(dev->parent, async);
 	dpm_watchdog_set(&wd, dev);
 	device_lock(dev);
@@ -1007,7 +1013,7 @@ static int __device_suspend_noirq(struct device *dev, pm_message_t state, bool a
 		goto Complete;
 	}
 
-	if (dev->power.syscore)
+	if (dev->power.syscore || dev->power.direct_complete)
 		goto Complete;
 
 	dpm_wait_for_children(dev, async);
@@ -1146,7 +1152,7 @@ static int __device_suspend_late(struct device *dev, pm_message_t state, bool as
 		goto Complete;
 	}
 
-	if (dev->power.syscore)
+	if (dev->power.syscore || dev->power.direct_complete)
 		goto Complete;
 
 	dpm_wait_for_children(dev, async);
@@ -1332,6 +1338,17 @@ static int __device_suspend(struct device *dev, pm_message_t state, bool async)
 	if (dev->power.syscore)
 		goto Complete;
 
+	if (dev->power.direct_complete) {
+		if (pm_runtime_status_suspended(dev)) {
+			pm_runtime_disable(dev);
+			if (pm_runtime_suspended_if_enabled(dev))
+				goto Complete;
+
+			pm_runtime_enable(dev);
+		}
+		dev->power.direct_complete = false;
+	}
+
 	dpm_watchdog_set(&wd, dev);
 	device_lock(dev);
 
@@ -1382,10 +1399,19 @@ static int __device_suspend(struct device *dev, pm_message_t state, bool async)
 
  End:
 	if (!error) {
+		struct device *parent = dev->parent;
+
 		dev->power.is_suspended = true;
-		if (dev->power.wakeup_path
-		    && dev->parent && !dev->parent->power.ignore_children)
-			dev->parent->power.wakeup_path = true;
+		if (parent) {
+			spin_lock_irq(&parent->power.lock);
+
+			dev->parent->power.direct_complete = false;
+			if (dev->power.wakeup_path
+			    && !dev->parent->power.ignore_children)
+				dev->parent->power.wakeup_path = true;
+
+			spin_unlock_irq(&parent->power.lock);
+		}
 	}
 
 	device_unlock(dev);
@@ -1487,7 +1513,7 @@ static int device_prepare(struct device *dev, pm_message_t state)
 {
 	int (*callback)(struct device *) = NULL;
 	char *info = NULL;
-	int error = 0;
+	int ret = 0;
 
 	if (dev->power.syscore)
 		return 0;
@@ -1523,17 +1549,27 @@ static int device_prepare(struct device *dev, pm_message_t state)
 		callback = dev->driver->pm->prepare;
 	}
 
-	if (callback) {
-		error = callback(dev);
-		suspend_report_result(callback, error);
-	}
+	if (callback)
+		ret = callback(dev);
 
 	device_unlock(dev);
 
-	if (error)
+	if (ret < 0) {
+		suspend_report_result(callback, ret);
 		pm_runtime_put(dev);
-
-	return error;
+		return ret;
+	}
+	/*
+	 * A positive return value from ->prepare() means "this device appears
+	 * to be runtime-suspended and its state is fine, so if it really is
+	 * runtime-suspended, you can leave it in that state provided that you
+	 * will do the same thing with all of its descendants".  This only
+	 * applies to suspend transitions, however.
+	 */
+	spin_lock_irq(&dev->power.lock);
+	dev->power.direct_complete = ret > 0 && state.event == PM_EVENT_SUSPEND;
+	spin_unlock_irq(&dev->power.lock);
+	return 0;
 }
 
 /**