PM / runtime: Optimize the use of device links

If the device has no links to suppliers that should be used for
runtime PM (links with DEVICE_LINK_PM_RUNTIME set), there is no
reason to walk the list of suppliers for that device during
runtime suspend and resume.

Add a simple mechanism to detect that case and possibly avoid the
extra unnecessary overhead.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/core.c

@@ -205,7 +205,8 @@ struct device_link *device_link_add(struct device *consumer,
 	if (!link)
 		goto out;
 
-	if ((flags & DL_FLAG_PM_RUNTIME) && (flags & DL_FLAG_RPM_ACTIVE)) {
+	if (flags & DL_FLAG_PM_RUNTIME) {
+		if (flags & DL_FLAG_RPM_ACTIVE) {
 			if (pm_runtime_get_sync(supplier) < 0) {
 				pm_runtime_put_noidle(supplier);
 				kfree(link);
@@ -214,6 +215,8 @@ struct device_link *device_link_add(struct device *consumer,
 			}
 			link->rpm_active = true;
 		}
+		pm_runtime_new_link(consumer);
+	}
 	get_device(supplier);
 	link->supplier = supplier;
 	INIT_LIST_HEAD(&link->s_node);
@@ -296,6 +299,9 @@ static void __device_link_del(struct device_link *link)
 	dev_info(link->consumer, "Dropping the link to %s\n",
 		 dev_name(link->supplier));
 
+	if (link->flags & DL_FLAG_PM_RUNTIME)
+		pm_runtime_drop_link(link->consumer);
+
 	list_del_rcu(&link->s_node);
 	list_del_rcu(&link->c_node);
 	call_srcu(&device_links_srcu, &link->rcu_head, __device_link_free_srcu);

drivers/base/power/runtime.c

@@ -305,6 +305,7 @@ static int __rpm_callback(int (*cb)(struct device *), struct device *dev)
 	__releases(&dev->power.lock) __acquires(&dev->power.lock)
 {
 	int retval, idx;
+	bool use_links = dev->power.links_count > 0;
 
 	if (dev->power.irq_safe) {
 		spin_unlock(&dev->power.lock);
@@ -318,7 +319,7 @@ static int __rpm_callback(int (*cb)(struct device *), struct device *dev)
 		 * routine returns, so it is safe to read the status outside of
 		 * the lock.
 		 */
-		if (dev->power.runtime_status == RPM_RESUMING) {
+		if (use_links && dev->power.runtime_status == RPM_RESUMING) {
 			idx = device_links_read_lock();
 
 			retval = rpm_get_suppliers(dev);
@@ -341,8 +342,9 @@ static int __rpm_callback(int (*cb)(struct device *), struct device *dev)
 		 *
 		 * Do that if resume fails too.
 		 */
-		if ((dev->power.runtime_status == RPM_SUSPENDING && !retval)
-		    || (dev->power.runtime_status == RPM_RESUMING && retval)) {
+		if (use_links
+		    && ((dev->power.runtime_status == RPM_SUSPENDING && !retval)
+		    || (dev->power.runtime_status == RPM_RESUMING && retval))) {
 			idx = device_links_read_lock();
 
  fail:
@@ -1593,6 +1595,21 @@ void pm_runtime_put_suppliers(struct device *dev)
 	device_links_read_unlock(idx);
 }
 
+void pm_runtime_new_link(struct device *dev)
+{
+	spin_lock_irq(&dev->power.lock);
+	dev->power.links_count++;
+	spin_unlock_irq(&dev->power.lock);
+}
+
+void pm_runtime_drop_link(struct device *dev)
+{
+	spin_lock_irq(&dev->power.lock);
+	WARN_ON(dev->power.links_count == 0);
+	dev->power.links_count--;
+	spin_unlock_irq(&dev->power.lock);
+}
+
 /**
  * pm_runtime_force_suspend - Force a device into suspend state if needed.
  * @dev: Device to suspend.

include/linux/pm.h

@@ -597,6 +597,7 @@ struct dev_pm_info {
 	unsigned int		use_autosuspend:1;
 	unsigned int		timer_autosuspends:1;
 	unsigned int		memalloc_noio:1;
+	unsigned int		links_count;
 	enum rpm_request	request;
 	enum rpm_status		runtime_status;
 	int			runtime_error;

include/linux/pm_runtime.h

@@ -58,6 +58,8 @@ extern void pm_runtime_set_memalloc_noio(struct device *dev, bool enable);
 extern void pm_runtime_clean_up_links(struct device *dev);
 extern void pm_runtime_get_suppliers(struct device *dev);
 extern void pm_runtime_put_suppliers(struct device *dev);
+extern void pm_runtime_new_link(struct device *dev);
+extern void pm_runtime_drop_link(struct device *dev);
 
 static inline void pm_suspend_ignore_children(struct device *dev, bool enable)
 {
@@ -192,6 +194,8 @@ static inline void pm_runtime_set_memalloc_noio(struct device *dev,
 static inline void pm_runtime_clean_up_links(struct device *dev) {}
 static inline void pm_runtime_get_suppliers(struct device *dev) {}
 static inline void pm_runtime_put_suppliers(struct device *dev) {}
+static inline void pm_runtime_new_link(struct device *dev) {}
+static inline void pm_runtime_drop_link(struct device *dev) {}
 
 #endif /* !CONFIG_PM */