[PATCH v3 2/5] PM: sleep: Suspend async parents after suspending children

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

In analogy with the previous change affecting the resume path,
make device_suspend() start the async suspend of the device's parent
after the device itself has been processed and make dpm_suspend() start
processing "async" leaf devices (that is, devices without children)
upfront so they don't need to wait for the "sync" devices they don't
depend on.

On the Dell XPS13 9360 in my office, this change reduces the total
duration of device suspend by approximately 100 ms (over 20%).

Suggested-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---

v2 -> v3:
   * When starting async processing early, only take parents and children
     into account.
   * Refine parent check in dpm_async_suspend_parent().
   * Use list_splice() for merging lists on errors.
   * Adjust subject and changelog.
   * Adjust comments.

v1 -> v2:
   * Adjust for the changes in patch [1/3].
   * Fix walking suppliers in dpm_async_suspend_superior().
   * Use device links read locking in dpm_async_suspend_superior() (Saravana).
   * Move all devices to the target list even if there are errors in
     dpm_suspend() so they are properly resumed during rollback (Saravana).

---
 drivers/base/power/main.c |   67 +++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 63 insertions(+), 4 deletions(-)

--- a/drivers/base/power/main.c
+++ b/drivers/base/power/main.c
@@ -1236,6 +1236,41 @@
 
 /*------------------------- Suspend routines -------------------------*/
 
+static bool dpm_leaf_device(struct device *dev)
+{
+	struct device *child;
+
+	lockdep_assert_held(&dpm_list_mtx);
+
+	child = device_find_any_child(dev);
+	if (child) {
+		put_device(child);
+
+		return false;
+	}
+
+	return true;
+}
+
+static void dpm_async_suspend_parent(struct device *dev, async_func_t func)
+{
+	guard(mutex)(&dpm_list_mtx);
+
+	/*
+	 * If the device is suspended asynchronously and the parent's callback
+	 * deletes both the device and the parent itself, the parent object may
+	 * be freed while this function is running, so avoid that by checking
+	 * if the device has been deleted already as the parent cannot be
+	 * deleted before it.
+	 */
+	if (!device_pm_initialized(dev))
+		return;
+
+	/* Start processing the device's parent if it is "async". */
+	if (dev->parent)
+		dpm_async_with_cleanup(dev->parent, func);
+}
+
 /**
  * resume_event - Return a "resume" message for given "suspend" sleep state.
  * @sleep_state: PM message representing a sleep state.
@@ -1661,6 +1696,8 @@
 	device_links_read_unlock(idx);
 }
 
+static void async_suspend(void *data, async_cookie_t cookie);
+
 /**
  * device_suspend - Execute "suspend" callbacks for given device.
  * @dev: Device to handle.
@@ -1790,7 +1827,13 @@
 
 	complete_all(&dev->power.completion);
 	TRACE_SUSPEND(error);
-	return error;
+
+	if (error || async_error)
+		return error;
+
+	dpm_async_suspend_parent(dev, async_suspend);
+
+	return 0;
 }
 
 static void async_suspend(void *data, async_cookie_t cookie)
@@ -1808,6 +1851,7 @@
 int dpm_suspend(pm_message_t state)
 {
 	ktime_t starttime = ktime_get();
+	struct device *dev;
 	int error = 0;
 
 	trace_suspend_resume(TPS("dpm_suspend"), state.event, true);
@@ -1821,12 +1865,21 @@
 
 	mutex_lock(&dpm_list_mtx);
 
+	/*
+	 * Start processing "async" leaf devices upfront so they don't need to
+	 * wait for the "sync" devices they don't depend on.
+	 */
+	list_for_each_entry_reverse(dev, &dpm_prepared_list, power.entry) {
+		dpm_clear_async_state(dev);
+		if (dpm_leaf_device(dev))
+			dpm_async_with_cleanup(dev, async_suspend);
+	}
+
 	while (!list_empty(&dpm_prepared_list)) {
-		struct device *dev = to_device(dpm_prepared_list.prev);
+		dev = to_device(dpm_prepared_list.prev);
 
 		list_move(&dev->power.entry, &dpm_suspended_list);
 
-		dpm_clear_async_state(dev);
 		if (dpm_async_fn(dev, async_suspend))
 			continue;
 
@@ -1840,8 +1893,14 @@
 
 		mutex_lock(&dpm_list_mtx);
 
-		if (error || async_error)
+		if (error || async_error) {
+			/*
+			 * Move all devices to the target list to resume them
+			 * properly.
+			 */
+			list_splice(&dpm_prepared_list, &dpm_suspended_list);
 			break;
+		}
 	}
 
 	mutex_unlock(&dpm_list_mtx);