[PATCH] driver core: Remove device link creation limitation

[Rafael J. Wysocki]

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

If device_link_add() is called for a consumer/supplier pair with an
existing device link between them and the existing link's type is
not in agreement with the flags passed to that function by its
caller, NULL will be returned.  That is seriously inconvenient,
because it forces the callers of device_link_add() to worry about
what others may or may not do even if that is not relevant to them
for any other reasons.

It turns out, however, that this limitation can be made go away
relatively easily.

The underlying observation is that if DL_FLAG_STATELESS has been
passed to device_link_add() in flags for the given consumer/supplier
pair at least once, calling either device_link_del() or
device_link_remove() to release the link returned by it should work,
but there are no other requirements associated with that flag.  In
turn, if at least one of the callers of device_link_add() for the
given consumer/supplier pair has not passed DL_FLAG_STATELESS to it
in flags, the driver core should track the status of the link and act
on it as appropriate (ie. the link should be treated as "managed").
This means that DL_FLAG_STATELESS needs to be set for managed device
links and it should be valid to call device_link_del() or
device_link_remove() to drop references to them in certain
sutiations.

To allow that to happen, introduce a new (internal) device link flag
called DL_FLAG_MANAGED and make device_link_add() set it automatically
whenever DL_FLAG_STATELESS is not passed to it.  Also make it take
additional references to existing device links that were previously
stateless (that is, with DL_FLAG_STATELESS set and DL_FLAG_MANAGED
unset) and will need to be managed going forward and initialize
their status (which has been DL_STATE_NONE so far).

Accordingly, when a managed device link is dropped automatically
by the driver core, make it clear DL_FLAG_MANAGED, reset the link's
status back to DL_STATE_NONE and drop the reference to it associated
with DL_FLAG_MANAGED instead of just deleting it right away (to
allow it to stay around in case it still needs to be released
explicitly by someone).

With that, since setting DL_FLAG_STATELESS doesn't mean that the
device link in question is not managed any more, replace all of the
status-tracking checks against DL_FLAG_STATELESS with analogous
checks against DL_FLAG_MANAGED and update the documentation to
reflect these changes.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---
 Documentation/driver-api/device_link.rst |    4 
 drivers/base/core.c                      |  169 +++++++++++++++++--------------
 drivers/base/power/runtime.c             |    4 
 include/linux/device.h                   |    4 
 4 files changed, 102 insertions(+), 79 deletions(-)

Index: linux-pm/drivers/base/core.c
===================================================================
--- linux-pm.orig/drivers/base/core.c
+++ linux-pm/drivers/base/core.c
@@ -124,6 +124,50 @@ static int device_is_dependent(struct de
 	return ret;
 }
 
+static void device_link_init_status(struct device_link *link,
+				    struct device *consumer,
+				    struct device *supplier)
+{
+	switch (supplier->links.status) {
+	case DL_DEV_PROBING:
+		switch (consumer->links.status) {
+		case DL_DEV_PROBING:
+			/*
+			 * A consumer driver can create a link to a supplier
+			 * that has not completed its probing yet as long as it
+			 * knows that the supplier is already functional (for
+			 * example, it has just acquired some resources from the
+			 * supplier).
+			 */
+			link->status = DL_STATE_CONSUMER_PROBE;
+			break;
+		default:
+			link->status = DL_STATE_DORMANT;
+			break;
+		}
+		break;
+	case DL_DEV_DRIVER_BOUND:
+		switch (consumer->links.status) {
+		case DL_DEV_PROBING:
+			link->status = DL_STATE_CONSUMER_PROBE;
+			break;
+		case DL_DEV_DRIVER_BOUND:
+			link->status = DL_STATE_ACTIVE;
+			break;
+		default:
+			link->status = DL_STATE_AVAILABLE;
+			break;
+		}
+		break;
+	case DL_DEV_UNBINDING:
+		link->status = DL_STATE_SUPPLIER_UNBIND;
+		break;
+	default:
+		link->status = DL_STATE_DORMANT;
+		break;
+	}
+}
+
 static int device_reorder_to_tail(struct device *dev, void *not_used)
 {
 	struct device_link *link;
@@ -179,9 +223,9 @@ void device_pm_move_to_tail(struct devic
  * of the link.  If DL_FLAG_PM_RUNTIME is not set, DL_FLAG_RPM_ACTIVE will be
  * ignored.
  *
- * If DL_FLAG_STATELESS is set in @flags, the link is not going to be managed by
- * the driver core and, in particular, the caller of this function is expected
- * to drop the reference to the link acquired by it directly.
+ * If DL_FLAG_STATELESS is set in @flags, the caller of this function is
+ * expected to release the link returned by it directly with the help of either
+ * device_link_del() or device_link_remove().
  *
  * If that flag is not set, however, the caller of this function is handing the
  * management of the link over to the driver core entirely and its return value
@@ -201,9 +245,16 @@ void device_pm_move_to_tail(struct devic
  * be used to request the driver core to automaticall probe for a consmer
  * driver after successfully binding a driver to the supplier device.
  *
- * The combination of DL_FLAG_STATELESS and either DL_FLAG_AUTOREMOVE_CONSUMER
- * or DL_FLAG_AUTOREMOVE_SUPPLIER set in @flags at the same time is invalid and
- * will cause NULL to be returned upfront.
+ * The combination of DL_FLAG_STATELESS and one of DL_FLAG_AUTOREMOVE_CONSUMER,
+ * DL_FLAG_AUTOREMOVE_SUPPLIER, or DL_FLAG_AUTOPROBE_CONSUMER set in @flags at
+ * the same time is invalid and will cause NULL to be returned upfront.
+ * However, if a device link between the given @consumer and @supplier pair
+ * exists already when this function is called for them, the existing link will
+ * be returned regardless of its current type and status (the link's flags may
+ * be modified then).  The caller of this function is then expected to treat
+ * the link as though it has just been created, so (in particular) if
+ * DL_FLAG_STATELESS was passed in @flags, the link needs to be released
+ * explicitly when not needed any more (as stated above).
  *
  * A side effect of the link creation is re-ordering of dpm_list and the
  * devices_kset list by moving the consumer device and all devices depending
@@ -223,7 +274,8 @@ struct device_link *device_link_add(stru
 	    (flags & DL_FLAG_STATELESS &&
 	     flags & (DL_FLAG_AUTOREMOVE_CONSUMER |
 		      DL_FLAG_AUTOREMOVE_SUPPLIER |
-		      DL_FLAG_AUTOPROBE_CONSUMER)) ||
+		      DL_FLAG_AUTOPROBE_CONSUMER |
+		      DL_FLAG_MANAGED)) ||
 	    (flags & DL_FLAG_AUTOPROBE_CONSUMER &&
 	     flags & (DL_FLAG_AUTOREMOVE_CONSUMER |
 		      DL_FLAG_AUTOREMOVE_SUPPLIER)))
@@ -236,6 +288,9 @@ struct device_link *device_link_add(stru
 		}
 	}
 
+	if (!(flags & DL_FLAG_STATELESS))
+		flags |= DL_FLAG_MANAGED;
+
 	device_links_write_lock();
 	device_pm_lock();
 
@@ -262,15 +317,6 @@ struct device_link *device_link_add(stru
 		if (link->consumer != consumer)
 			continue;
 
-		/*
-		 * Don't return a stateless link if the caller wants a stateful
-		 * one and vice versa.
-		 */
-		if (WARN_ON((flags & DL_FLAG_STATELESS) != (link->flags & DL_FLAG_STATELESS))) {
-			link = NULL;
-			goto out;
-		}
-
 		if (flags & DL_FLAG_PM_RUNTIME) {
 			if (!(link->flags & DL_FLAG_PM_RUNTIME)) {
 				pm_runtime_new_link(consumer);
@@ -281,6 +327,7 @@ struct device_link *device_link_add(stru
 		}
 
 		if (flags & DL_FLAG_STATELESS) {
+			link->flags |= DL_FLAG_STATELESS;
 			kref_get(&link->kref);
 			goto out;
 		}
@@ -299,6 +346,11 @@ struct device_link *device_link_add(stru
 			link->flags &= ~(DL_FLAG_AUTOREMOVE_CONSUMER |
 					 DL_FLAG_AUTOREMOVE_SUPPLIER);
 		}
+		if (!(link->flags & DL_FLAG_MANAGED)) {
+			kref_get(&link->kref);
+			link->flags |= DL_FLAG_MANAGED;
+			device_link_init_status(link, consumer, supplier);
+		}
 		goto out;
 	}
 
@@ -325,48 +377,10 @@ struct device_link *device_link_add(stru
 	kref_init(&link->kref);
 
 	/* Determine the initial link state. */
-	if (flags & DL_FLAG_STATELESS) {
+	if (flags & DL_FLAG_STATELESS)
 		link->status = DL_STATE_NONE;
-	} else {
-		switch (supplier->links.status) {
-		case DL_DEV_PROBING:
-			switch (consumer->links.status) {
-			case DL_DEV_PROBING:
-				/*
-				 * A consumer driver can create a link to a
-				 * supplier that has not completed its probing
-				 * yet as long as it knows that the supplier is
-				 * already functional (for example, it has just
-				 * acquired some resources from the supplier).
-				 */
-				link->status = DL_STATE_CONSUMER_PROBE;
-				break;
-			default:
-				link->status = DL_STATE_DORMANT;
-				break;
-			}
-			break;
-		case DL_DEV_DRIVER_BOUND:
-			switch (consumer->links.status) {
-			case DL_DEV_PROBING:
-				link->status = DL_STATE_CONSUMER_PROBE;
-				break;
-			case DL_DEV_DRIVER_BOUND:
-				link->status = DL_STATE_ACTIVE;
-				break;
-			default:
-				link->status = DL_STATE_AVAILABLE;
-				break;
-			}
-			break;
-		case DL_DEV_UNBINDING:
-			link->status = DL_STATE_SUPPLIER_UNBIND;
-			break;
-		default:
-			link->status = DL_STATE_DORMANT;
-			break;
-		}
-	}
+	else
+		device_link_init_status(link, consumer, supplier);
 
 	/*
 	 * Some callers expect the link creation during consumer driver probe to
@@ -528,7 +542,7 @@ static void device_links_missing_supplie
  * mark the link as "consumer probe in progress" to make the supplier removal
  * wait for us to complete (or bad things may happen).
  *
- * Links with the DL_FLAG_STATELESS flag set are ignored.
+ * Links with the DL_FLAG_MANAGED flag unset are ignored.
  */
 int device_links_check_suppliers(struct device *dev)
 {
@@ -538,7 +552,7 @@ int device_links_check_suppliers(struct
 	device_links_write_lock();
 
 	list_for_each_entry(link, &dev->links.suppliers, c_node) {
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		if (link->status != DL_STATE_AVAILABLE) {
@@ -563,7 +577,7 @@ int device_links_check_suppliers(struct
  *
  * Also change the status of @dev's links to suppliers to "active".
  *
- * Links with the DL_FLAG_STATELESS flag set are ignored.
+ * Links with the DL_FLAG_MANAGED flag unset are ignored.
  */
 void device_links_driver_bound(struct device *dev)
 {
@@ -572,7 +586,7 @@ void device_links_driver_bound(struct de
 	device_links_write_lock();
 
 	list_for_each_entry(link, &dev->links.consumers, s_node) {
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		/*
@@ -593,7 +607,7 @@ void device_links_driver_bound(struct de
 	}
 
 	list_for_each_entry(link, &dev->links.suppliers, c_node) {
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		WARN_ON(link->status != DL_STATE_CONSUMER_PROBE);
@@ -605,6 +619,13 @@ void device_links_driver_bound(struct de
 	device_links_write_unlock();
 }
 
+static void device_link_drop_managed(struct device_link *link)
+{
+	link->flags &= ~DL_FLAG_MANAGED;
+	WRITE_ONCE(link->status, DL_STATE_NONE);
+	kref_put(&link->kref, __device_link_del);
+}
+
 /**
  * __device_links_no_driver - Update links of a device without a driver.
  * @dev: Device without a drvier.
@@ -615,18 +636,18 @@ void device_links_driver_bound(struct de
  * unless they already are in the "supplier unbind in progress" state in which
  * case they need not be updated.
  *
- * Links with the DL_FLAG_STATELESS flag set are ignored.
+ * Links with the DL_FLAG_MANAGED flag unset are ignored.
  */
 static void __device_links_no_driver(struct device *dev)
 {
 	struct device_link *link, *ln;
 
 	list_for_each_entry_safe_reverse(link, ln, &dev->links.suppliers, c_node) {
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		if (link->flags & DL_FLAG_AUTOREMOVE_CONSUMER)
-			__device_link_del(&link->kref);
+			device_link_drop_managed(link);
 		else if (link->status == DL_STATE_CONSUMER_PROBE ||
 			 link->status == DL_STATE_ACTIVE)
 			WRITE_ONCE(link->status, DL_STATE_AVAILABLE);
@@ -643,7 +664,7 @@ static void __device_links_no_driver(str
  * %__device_links_no_driver() to update links to suppliers for it as
  * appropriate.
  *
- * Links with the DL_FLAG_STATELESS flag set are ignored.
+ * Links with the DL_FLAG_MANAGED flag unset are ignored.
  */
 void device_links_no_driver(struct device *dev)
 {
@@ -652,7 +673,7 @@ void device_links_no_driver(struct devic
 	device_links_write_lock();
 
 	list_for_each_entry(link, &dev->links.consumers, s_node) {
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		/*
@@ -680,7 +701,7 @@ void device_links_no_driver(struct devic
  * invoke %__device_links_no_driver() to update links to suppliers for it as
  * appropriate.
  *
- * Links with the DL_FLAG_STATELESS flag set are ignored.
+ * Links with the DL_FLAG_MANAGED flag unset are ignored.
  */
 void device_links_driver_cleanup(struct device *dev)
 {
@@ -689,7 +710,7 @@ void device_links_driver_cleanup(struct
 	device_links_write_lock();
 
 	list_for_each_entry_safe(link, ln, &dev->links.consumers, s_node) {
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		WARN_ON(link->flags & DL_FLAG_AUTOREMOVE_CONSUMER);
@@ -702,7 +723,7 @@ void device_links_driver_cleanup(struct
 		 */
 		if (link->status == DL_STATE_SUPPLIER_UNBIND &&
 		    link->flags & DL_FLAG_AUTOREMOVE_SUPPLIER)
-			__device_link_del(&link->kref);
+			device_link_drop_managed(link);
 
 		WRITE_ONCE(link->status, DL_STATE_DORMANT);
 	}
@@ -724,7 +745,7 @@ void device_links_driver_cleanup(struct
  *
  * Return 'false' if there are no probing or active consumers.
  *
- * Links with the DL_FLAG_STATELESS flag set are ignored.
+ * Links with the DL_FLAG_MANAGED flag unset are ignored.
  */
 bool device_links_busy(struct device *dev)
 {
@@ -734,7 +755,7 @@ bool device_links_busy(struct device *de
 	device_links_write_lock();
 
 	list_for_each_entry(link, &dev->links.consumers, s_node) {
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		if (link->status == DL_STATE_CONSUMER_PROBE
@@ -764,7 +785,7 @@ bool device_links_busy(struct device *de
  * driver to unbind and start over (the consumer will not re-probe as we have
  * changed the state of the link already).
  *
- * Links with the DL_FLAG_STATELESS flag set are ignored.
+ * Links with the DL_FLAG_MANAGED flag unset are ignored.
  */
 void device_links_unbind_consumers(struct device *dev)
 {
@@ -776,7 +797,7 @@ void device_links_unbind_consumers(struc
 	list_for_each_entry(link, &dev->links.consumers, s_node) {
 		enum device_link_state status;
 
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		status = link->status;
Index: linux-pm/drivers/base/power/runtime.c
===================================================================
--- linux-pm.orig/drivers/base/power/runtime.c
+++ linux-pm/drivers/base/power/runtime.c
@@ -1624,7 +1624,7 @@ void pm_runtime_remove(struct device *de
  * runtime PM references to the device, drop the usage counter of the device
  * (as many times as needed).
  *
- * Links with the DL_FLAG_STATELESS flag set are ignored.
+ * Links with the DL_FLAG_MANAGED flag unset are ignored.
  *
  * Since the device is guaranteed to be runtime-active at the point this is
  * called, nothing else needs to be done here.
@@ -1641,7 +1641,7 @@ void pm_runtime_clean_up_links(struct de
 	idx = device_links_read_lock();
 
 	list_for_each_entry_rcu(link, &dev->links.consumers, s_node) {
-		if (link->flags & DL_FLAG_STATELESS)
+		if (!(link->flags & DL_FLAG_MANAGED))
 			continue;
 
 		while (refcount_dec_not_one(&link->rpm_active))
Index: linux-pm/include/linux/device.h
===================================================================
--- linux-pm.orig/include/linux/device.h
+++ linux-pm/include/linux/device.h
@@ -829,12 +829,13 @@ enum device_link_state {
 /*
  * Device link flags.
  *
- * STATELESS: The core won't track the presence of supplier/consumer drivers.
+ * STATELESS: The core will not remove this link automatically.
  * AUTOREMOVE_CONSUMER: Remove the link automatically on consumer driver unbind.
  * PM_RUNTIME: If set, the runtime PM framework will use this link.
  * RPM_ACTIVE: Run pm_runtime_get_sync() on the supplier during link creation.
  * AUTOREMOVE_SUPPLIER: Remove the link automatically on supplier driver unbind.
  * AUTOPROBE_CONSUMER: Probe consumer driver automatically after supplier binds.
+ * MANAGED: The core tracks presence of supplier/consumer drivers (internal).
  */
 #define DL_FLAG_STATELESS		BIT(0)
 #define DL_FLAG_AUTOREMOVE_CONSUMER	BIT(1)
@@ -842,6 +843,7 @@ enum device_link_state {
 #define DL_FLAG_RPM_ACTIVE		BIT(3)
 #define DL_FLAG_AUTOREMOVE_SUPPLIER	BIT(4)
 #define DL_FLAG_AUTOPROBE_CONSUMER	BIT(5)
+#define DL_FLAG_MANAGED			BIT(6)
 
 /**
  * struct device_link - Device link representation.
Index: linux-pm/Documentation/driver-api/device_link.rst
===================================================================
--- linux-pm.orig/Documentation/driver-api/device_link.rst
+++ linux-pm/Documentation/driver-api/device_link.rst
@@ -78,8 +78,8 @@ typically deleted in its ``->remove`` ca
 driver is compiled as a module, the device link is added on module load and
 orderly deleted on unload.  The same restrictions that apply to device link
 addition (e.g. exclusion of a parallel suspend/resume transition) apply equally
-to deletion.  Device links with ``DL_FLAG_STATELESS`` unset (i.e. managed
-device links) are deleted automatically by the driver core.
+to deletion.  Device links managed by the driver core are deleted automatically
+by it.
 
 Several flags may be specified on device link addition, two of which
 have already been mentioned above:  ``DL_FLAG_STATELESS`` to express that no