driver core: Add support for linking devices during device addition

The firmware corresponding to a device (dev.fwnode) might be able to
provide functional dependency information between a device and its
supplier and consumer devices. Tracking this functional dependency
allows optimizing device probe order and informing a supplier when all
its consumers have probed (and thereby actively managing their
resources).

The existing device links feature allows tracking and using
supplier-consumer relationships. So, this patch adds the add_links()
fwnode callback to allow firmware to create device links for each
device as the device is added.

However, when consumer devices are added, they might not have a supplier
device to link to despite needing mandatory resources/functionality from
one or more suppliers. A waiting_for_suppliers list is created to track
such consumers and retry linking them when new devices get added.

Signed-off-by: Saravana Kannan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/driver-api/device_link.rst

···
   :c:func:`driver_bound()`.)
 
 * Before a consumer device is probed, presence of supplier drivers is
-  verified by checking that links to suppliers are in ``DL_STATE_AVAILABLE``
+  verified by checking the consumer device is not in the wait_for_suppliers
+  list and by checking that links to suppliers are in ``DL_STATE_AVAILABLE``
   state.  The state of the links is updated to ``DL_STATE_CONSUMER_PROBE``.
   (Call to :c:func:`device_links_check_suppliers()` from
   :c:func:`really_probe()`.)

drivers/base/core.c

···
 #endif
 
 /* Device links support. */
+static LIST_HEAD(wait_for_suppliers);
+static DEFINE_MUTEX(wfs_lock);
 
 #ifdef CONFIG_SRCU
 static DEFINE_MUTEX(device_links_lock);
···
 }
 EXPORT_SYMBOL_GPL(device_link_add);
 
+/**
+ * device_link_wait_for_supplier - Add device to wait_for_suppliers list
+ * @consumer: Consumer device
+ *
+ * Marks the @consumer device as waiting for suppliers to become available by
+ * adding it to the wait_for_suppliers list. The consumer device will never be
+ * probed until it's removed from the wait_for_suppliers list.
+ *
+ * The caller is responsible for adding the links to the supplier devices once
+ * they are available and removing the @consumer device from the
+ * wait_for_suppliers list once links to all the suppliers have been created.
+ *
+ * This function is NOT meant to be called from the probe function of the
+ * consumer but rather from code that creates/adds the consumer device.
+ */
+static void device_link_wait_for_supplier(struct device *consumer)
+{
+	mutex_lock(&wfs_lock);
+	list_add_tail(&consumer->links.needs_suppliers, &wait_for_suppliers);
+	mutex_unlock(&wfs_lock);
+}
+
+/**
+ * device_link_add_missing_supplier_links - Add links from consumer devices to
+ *					    supplier devices, leaving any
+ *					    consumer with inactive suppliers on
+ *					    the wait_for_suppliers list
+ *
+ * Loops through all consumers waiting on suppliers and tries to add all their
+ * supplier links. If that succeeds, the consumer device is removed from
+ * wait_for_suppliers list. Otherwise, they are left in the wait_for_suppliers
+ * list.  Devices left on the wait_for_suppliers list will not be probed.
+ *
+ * The fwnode add_links callback is expected to return 0 if it has found and
+ * added all the supplier links for the consumer device. It should return an
+ * error if it isn't able to do so.
+ *
+ * The caller of device_link_wait_for_supplier() is expected to call this once
+ * it's aware of potential suppliers becoming available.
+ */
+static void device_link_add_missing_supplier_links(void)
+{
+	struct device *dev, *tmp;
+
+	mutex_lock(&wfs_lock);
+	list_for_each_entry_safe(dev, tmp, &wait_for_suppliers,
+				 links.needs_suppliers)
+		if (!fwnode_call_int_op(dev->fwnode, add_links, dev))
+			list_del_init(&dev->links.needs_suppliers);
+	mutex_unlock(&wfs_lock);
+}
+
 static void device_link_free(struct device_link *link)
 {
 	while (refcount_dec_not_one(&link->rpm_active))
···
 {
 	struct device_link *link;
 	int ret = 0;
+
+	/*
+	 * Device waiting for supplier to become available is not allowed to
+	 * probe.
+	 */
+	mutex_lock(&wfs_lock);
+	if (!list_empty(&dev->links.needs_suppliers)) {
+		mutex_unlock(&wfs_lock);
+		return -EPROBE_DEFER;
+	}
+	mutex_unlock(&wfs_lock);
 
 	device_links_write_lock();
 
···
 static void device_links_purge(struct device *dev)
 {
 	struct device_link *link, *ln;
+
+	mutex_lock(&wfs_lock);
+	list_del(&dev->links.needs_suppliers);
+	mutex_unlock(&wfs_lock);
 
 	/*
 	 * Delete all of the remaining links from this device to any other
···
 #endif
 	INIT_LIST_HEAD(&dev->links.consumers);
 	INIT_LIST_HEAD(&dev->links.suppliers);
+	INIT_LIST_HEAD(&dev->links.needs_suppliers);
 	dev->links.status = DL_DEV_NO_DRIVER;
 }
 EXPORT_SYMBOL_GPL(device_initialize);
···
 
 	if (dev->fwnode && !dev->fwnode->dev)
 		dev->fwnode->dev = dev;
+
+	/*
+	 * Check if any of the other devices (consumers) have been waiting for
+	 * this device (supplier) to be added so that they can create a device
+	 * link to it.
+	 *
+	 * This needs to happen after device_pm_add() because device_link_add()
+	 * requires the supplier be registered before it's called.
+	 *
+	 * But this also needs to happe before bus_probe_device() to make sure
+	 * waiting consumers can link to it before the driver is bound to the
+	 * device and the driver sync_state callback is called for this device.
+	 */
+	device_link_add_missing_supplier_links();
+
+	if (fwnode_has_op(dev->fwnode, add_links)
+	    && fwnode_call_int_op(dev->fwnode, add_links, dev))
+		device_link_wait_for_supplier(dev);
 
 	bus_probe_device(dev);
 	if (parent)

include/linux/device.h

···
  * struct dev_links_info - Device data related to device links.
  * @suppliers: List of links to supplier devices.
  * @consumers: List of links to consumer devices.
+ * @needs_suppliers: Hook to global list of devices waiting for suppliers.
  * @status: Driver status information.
  */
 struct dev_links_info {
 	struct list_head suppliers;
 	struct list_head consumers;
+	struct list_head needs_suppliers;
 	enum dl_dev_state status;
 };
 

include/linux/fwnode.h

···
  *			       endpoint node.
  * @graph_get_port_parent: Return the parent node of a port node.
  * @graph_parse_endpoint: Parse endpoint for port and endpoint id.
+ * @add_links:	Called after the device corresponding to the fwnode is added
+ *		using device_add(). The function is expected to create device
+ *		links to all the suppliers of the device that are available at
+ *		the time this function is called.  The function must NOT stop
+ *		at the first failed device link if other unlinked supplier
+ *		devices are present in the system.  If some suppliers are not
+ *		yet available, this function will be called again when other
+ *		devices are added to allow creating device links to any newly
+ *		available suppliers.
+ *
+ *		Return 0 if device links have been successfully created to all
+ *		the suppliers of this device or if the supplier information is
+ *		not known. Return an error if and only if the supplier
+ *		information is known but some of the suppliers are not yet
+ *		available to create device links to.
  */
 struct fwnode_operations {
 	struct fwnode_handle *(*get)(struct fwnode_handle *fwnode);
···
 	(*graph_get_port_parent)(struct fwnode_handle *fwnode);
 	int (*graph_parse_endpoint)(const struct fwnode_handle *fwnode,
 				    struct fwnode_endpoint *endpoint);
+	int (*add_links)(const struct fwnode_handle *fwnode,
+			 struct device *dev);
 };
 
 #define fwnode_has_op(fwnode, op)				\