[RFT/RFC][PATCH] driver core: Fix PM-runtime for links added during consumer probe

[Rafael J. Wysocki]

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

Commit 4c06c4e6cf63 ("driver core: Fix possible supplier PM-usage
counter imbalance") introduced a regression that causes suppliers
to be suspended prematurely for device links added during consumer
driver probe if the initial PM-runtime status of the consumer is
"suspended" and the consumer is resumed after adding the link and
before pm_runtime_put_suppliers() is called.  In that case,
pm_runtime_put_suppliers() will drop the rpm_active refcount for
the link by one and (since rpm_active is equal to two after the
preceding consumer resume) the supplier's PM-runtime usage counter
will be decremented, which may cause the supplier to suspend even
though the consumer's PM-runtime status is "active".

For this reason, partially revert commit 4c06c4e6cf63 as the problem
it tried to fix needs to be addressed somewhat differently, and
change pm_runtime_get_suppliers() and pm_runtime_put_suppliers() so
that the latter only drops rpm_active references acquired by the
former.  [This requires adding a new field to struct device_link,
but I coulnd't find a cleaner way to address the issue that would
work in all cases.]

Fixes: 4c06c4e6cf63 ("driver core: Fix possible supplier PM-usage counter imbalance")
Reported-by: Jon Hunter <jonathanh@xxxxxxxxxx>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---

On top of driver-core/driver-core-next.

Jon, Ulf, please test!

---
 drivers/base/core.c          |    4 ++--
 drivers/base/power/runtime.c |   29 ++++++-----------------------
 include/linux/device.h       |    1 +
 include/linux/pm_runtime.h   |    4 ----
 4 files changed, 9 insertions(+), 29 deletions(-)

Index: linux-pm/drivers/base/core.c
===================================================================
--- linux-pm.orig/drivers/base/core.c
+++ linux-pm/drivers/base/core.c
@@ -277,7 +277,7 @@ struct device_link *device_link_add(stru
 				link->flags |= DL_FLAG_PM_RUNTIME;
 			}
 			if (flags & DL_FLAG_RPM_ACTIVE)
-				pm_runtime_active_link(link, supplier);
+				refcount_inc(&link->rpm_active);
 		}
 
 		if (flags & DL_FLAG_STATELESS) {
@@ -310,7 +310,7 @@ struct device_link *device_link_add(stru
 
 	if (flags & DL_FLAG_PM_RUNTIME) {
 		if (flags & DL_FLAG_RPM_ACTIVE)
-			pm_runtime_active_link(link, supplier);
+			refcount_inc(&link->rpm_active);
 
 		pm_runtime_new_link(consumer);
 	}
Index: linux-pm/drivers/base/power/runtime.c
===================================================================
--- linux-pm.orig/drivers/base/power/runtime.c
+++ linux-pm/drivers/base/power/runtime.c
@@ -1656,6 +1656,7 @@ void pm_runtime_get_suppliers(struct dev
 
 	list_for_each_entry_rcu(link, &dev->links.suppliers, c_node)
 		if (link->flags & DL_FLAG_PM_RUNTIME) {
+			link->supplier_preactivated = true;
 			refcount_inc(&link->rpm_active);
 			pm_runtime_get_sync(link->supplier);
 		}
@@ -1675,9 +1676,11 @@ void pm_runtime_put_suppliers(struct dev
 	idx = device_links_read_lock();
 
 	list_for_each_entry_rcu(link, &dev->links.suppliers, c_node)
-		if (link->flags & DL_FLAG_PM_RUNTIME &&
-		    refcount_dec_not_one(&link->rpm_active))
-			pm_runtime_put(link->supplier);
+		if (link->supplier_preactivated) {
+			link->supplier_preactivated = false;
+			if (refcount_dec_not_one(&link->rpm_active))
+				pm_runtime_put(link->supplier);
+		}
 
 	device_links_read_unlock(idx);
 }
@@ -1689,26 +1692,6 @@ void pm_runtime_new_link(struct device *
 	spin_unlock_irq(&dev->power.lock);
 }
 
-/**
- * pm_runtime_active_link - Set up new device link as active for PM-runtime.
- * @link: Device link to be set up as active.
- * @supplier: Supplier end of the link.
- *
- * Add 2 to the rpm_active refcount of @link and increment the PM-runtime
- * usage counter of @supplier once more in case the link is being added while
- * the consumer driver is probing and pm_runtime_put_suppliers() will be called
- * subsequently.
- *
- * Note that this doesn't prevent rpm_put_suppliers() from decreasing the link's
- * rpm_active refcount down to one, so runtime suspend of the consumer end of
- * @link is not affected.
- */
-void pm_runtime_active_link(struct device_link *link, struct device *supplier)
-{
-	refcount_add(2, &link->rpm_active);
-	pm_runtime_get_noresume(supplier);
-}
-
 void pm_runtime_drop_link(struct device *dev)
 {
 	spin_lock_irq(&dev->power.lock);
Index: linux-pm/include/linux/device.h
===================================================================
--- linux-pm.orig/include/linux/device.h
+++ linux-pm/include/linux/device.h
@@ -861,6 +861,7 @@ struct device_link {
 #ifdef CONFIG_SRCU
 	struct rcu_head rcu_head;
 #endif
+	bool supplier_preactivated; /* Owned by consumer probe. */
 };
 
 /**
Index: linux-pm/include/linux/pm_runtime.h
===================================================================
--- linux-pm.orig/include/linux/pm_runtime.h
+++ linux-pm/include/linux/pm_runtime.h
@@ -59,8 +59,6 @@ extern void pm_runtime_clean_up_links(st
 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_active_link(struct device_link *link,
-				   struct device *supplier);
 extern void pm_runtime_drop_link(struct device *dev);
 
 static inline void pm_suspend_ignore_children(struct device *dev, bool enable)
@@ -180,8 +178,6 @@ static inline void pm_runtime_clean_up_l
 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_active_link(struct device_link *link,
-					  struct device *supplier) {}
 static inline void pm_runtime_drop_link(struct device *dev) {}
 
 #endif /* !CONFIG_PM */