driver core: Remove device link creation limitation

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.

While at it, make device_link_add() reject flags that it does not
recognize, including DL_FLAG_MANAGED.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Saravana Kannan <saravanak@google.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Review-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/2305283.AStDPdUUnE@kreacherSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/device_link.rst

@@ -78,8 +78,8 @@ typically deleted in its ``->remove`` callback for symmetry.  That way, if the
 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

drivers/base/power/runtime.c

@@ -1624,7 +1624,7 @@ void pm_runtime_remove(struct device *dev)
  * 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 device *dev)
 	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))

include/linux/device.h

@@ -833,12 +833,13 @@ 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)
@@ -846,6 +847,7 @@ 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.