[PATCH v2 16/17] driver core: Refactor fw_devlink feature

[Saravana Kannan]

The current implementation of fw_devlink is very inefficient because it
tries to get away without creating fwnode links in the name of saving
memory usage. Past attempts to optimize runtime at the cost of memory
usage were blocked with request for data showing that the optimization
made significant improvement for real world scenarios.

We have those scenarios now. There have been several reports of boot
time increase in the order of seconds in this thread [1]. Several OEMs
and SoC manufacturers have also privately reported significant
(350-400ms) increase in boot time due to all the parsing done by
fw_devlink.

So this patch uses all the setup done by the previous patches in this
series to refactor fw_devlink to be more efficient. Most of the code has
been moved out of firmware specific (DT mostly) code into driver core.

This brings the following benefits:
- Instead of parsing the device tree multiple times during bootup,
  fw_devlink parses each fwnode node/property only once and creates
  fwnode links. The rest of the fw_devlink code then just looks at these
  fwnode links to do rest of the work.

- Makes it much easier to debug probe issue due to fw_devlink in the
  future. fw_devlink=on blocks the probing of devices if they depend on
  a device that hasn't been added yet. With this refactor, it'll be very
  easy to tell what that device is because we now have a reference to
  the fwnode of the device.

- Much easier to add fw_devlink support to ACPI and other firmware
  types. A refactor to move the common bits from DT specific code to
  driver core was in my TODO list as a prerequisite to adding ACPI
  support to fw_devlink. This series gets that done.

[1] - https://lore.kernel.org/linux-omap/ea02f57e-871d-cd16-4418-c1da4bbc4696@xxxxxx/
Signed-off-by: Saravana Kannan <saravanak@xxxxxxxxxx>
Tested-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
Tested-by: Grygorii Strashko <grygorii.strashko@xxxxxx>
---
 drivers/base/core.c    | 325 ++++++++++++++++++++++++++++++-----------
 include/linux/device.h |   5 -
 2 files changed, 238 insertions(+), 92 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 1873cecb0cc4..9edf9084fc98 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -46,8 +46,6 @@ early_param("sysfs.deprecated", sysfs_deprecated_setup);
 #endif
 
 /* Device links support. */
-static LIST_HEAD(wait_for_suppliers);
-static DEFINE_MUTEX(wfs_lock);
 static LIST_HEAD(deferred_sync);
 static unsigned int defer_sync_state_count = 1;
 static DEFINE_MUTEX(fwnode_link_lock);
@@ -803,74 +801,6 @@ struct device_link *device_link_add(struct device *consumer,
 }
 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,
-					  bool need_for_probe)
-{
-	mutex_lock(&wfs_lock);
-	list_add_tail(&consumer->links.needs_suppliers, &wait_for_suppliers);
-	consumer->links.need_for_probe = need_for_probe;
-	mutex_unlock(&wfs_lock);
-}
-
-static void device_link_wait_for_mandatory_supplier(struct device *consumer)
-{
-	device_link_wait_for_supplier(consumer, true);
-}
-
-static void device_link_wait_for_optional_supplier(struct device *consumer)
-{
-	device_link_wait_for_supplier(consumer, false);
-}
-
-/**
- * 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) {
-		int ret = fwnode_call_int_op(dev->fwnode, add_links, dev);
-		if (!ret)
-			list_del_init(&dev->links.needs_suppliers);
-		else if (ret != -ENODEV)
-			dev->links.need_for_probe = false;
-	}
-	mutex_unlock(&wfs_lock);
-}
-
 #ifdef CONFIG_SRCU
 static void __device_link_del(struct kref *kref)
 {
@@ -1195,9 +1125,8 @@ void device_links_driver_bound(struct device *dev)
 	 * the device links it needs to or make new device links as it needs
 	 * them. So, it no longer needs to wait on any suppliers.
 	 */
-	mutex_lock(&wfs_lock);
-	list_del_init(&dev->links.needs_suppliers);
-	mutex_unlock(&wfs_lock);
+	if (dev->fwnode && dev->fwnode->dev == dev)
+		fwnode_links_purge_suppliers(dev->fwnode);
 	device_remove_file(dev, &dev_attr_waiting_for_supplier);
 
 	device_links_write_lock();
@@ -1488,10 +1417,6 @@ static void device_links_purge(struct device *dev)
 	if (dev->class == &devlink_class)
 		return;
 
-	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
 	 * devices (either consumers or suppliers).
@@ -1561,19 +1486,246 @@ static void fw_devlink_parse_fwtree(struct fwnode_handle *fwnode)
 		fw_devlink_parse_fwtree(child);
 }
 
-static void fw_devlink_link_device(struct device *dev)
+/**
+ * fw_devlink_create_devlink - Create a device link from a consumer to fwnode
+ * @con - Consumer device for the device link
+ * @sup_handle - fwnode handle of supplier
+ *
+ * This function will try to create a device link between the consumer device
+ * @con and the supplier device represented by @sup_handle.
+ *
+ * The supplier has to be provided as a fwnode because incorrect cycles in
+ * fwnode links can sometimes cause the supplier device to never be created.
+ * This function detects such cases and returns an error if it cannot create a
+ * device link from the consumer to a missing supplier.
+ *
+ * Returns,
+ * 0 on successfully creating a device link
+ * -EINVAL if the device link cannot be created as expected
+ * -EAGAIN if the device link cannot be created right now, but it may be
+ *  possible to do that in the future
+ */
+static int fw_devlink_create_devlink(struct device *con,
+				     struct fwnode_handle *sup_handle, u32 flags)
+{
+	struct device *sup_dev;
+	int ret = 0;
+
+	sup_dev = get_dev_from_fwnode(sup_handle);
+	if (sup_dev) {
+		/*
+		 * If this fails, it is due to cycles in device links.  Just
+		 * give up on this link and treat it as invalid.
+		 */
+		if (!device_link_add(con, sup_dev, flags))
+			ret = -EINVAL;
+
+		goto out;
+	}
+
+	/*
+	 * DL_FLAG_SYNC_STATE_ONLY doesn't block probing and supports
+	 * cycles. So cycle detection isn't necessary and shouldn't be
+	 * done.
+	 */
+	if (flags & DL_FLAG_SYNC_STATE_ONLY)
+		return -EAGAIN;
+
+	/*
+	 * If we can't find the supplier device from its fwnode, it might be
+	 * due to a cyclic dependency between fwnodes. Some of these cycles can
+	 * be broken by applying logic. Check for these types of cycles and
+	 * break them so that devices in the cycle probe properly.
+	 *
+	 * If the supplier's parent is dependent on the consumer, then
+	 * the consumer-supplier dependency is a false dependency. So,
+	 * treat it as an invalid link.
+	 */
+	sup_dev = fwnode_get_next_parent_dev(sup_handle);
+	if (sup_dev && device_is_dependent(con, sup_dev)) {
+		dev_dbg(con, "Not linking to %pfwP - False link\n",
+			sup_handle);
+		ret = -EINVAL;
+	} else {
+		/*
+		 * Can't check for cycles or no cycles. So let's try
+		 * again later.
+		 */
+		ret = -EAGAIN;
+	}
+
+out:
+	put_device(sup_dev);
+	return ret;
+}
+
+/**
+ * __fw_devlink_link_to_consumers - Create device links to consumers of a device
+ * @dev - Device that needs to be linked to its consumers
+ *
+ * This function looks at all the consumer fwnodes of @dev and creates device
+ * links between the consumer device and @dev (supplier).
+ *
+ * If the consumer device has not been added yet, then this function creates a
+ * SYNC_STATE_ONLY link between @dev (supplier) and the closest ancestor device
+ * of the consumer fwnode. This is necessary to make sure @dev doesn't get a
+ * sync_state() callback before the real consumer device gets to be added and
+ * then probed.
+ *
+ * Once device links are created from the real consumer to @dev (supplier), the
+ * fwnode links are deleted.
+ */
+static void __fw_devlink_link_to_consumers(struct device *dev)
+{
+	struct fwnode_handle *fwnode = dev->fwnode;
+	struct fwnode_link *link, *tmp;
+
+	list_for_each_entry_safe(link, tmp, &fwnode->consumers, s_hook) {
+		u32 dl_flags = fw_devlink_get_flags();
+		struct device *con_dev;
+		bool own_link = true;
+		int ret;
+
+		con_dev = get_dev_from_fwnode(link->consumer);
+		/*
+		 * If consumer device is not available yet, make a "proxy"
+		 * SYNC_STATE_ONLY link from the consumer's parent device to
+		 * the supplier device. This is necessary to make sure the
+		 * supplier doesn't get a sync_state() callback before the real
+		 * consumer can create a device link to the supplier.
+		 *
+		 * This proxy link step is needed to handle the case where the
+		 * consumer's parent device is added before the supplier.
+		 */
+		if (!con_dev) {
+			con_dev = fwnode_get_next_parent_dev(link->consumer);
+			/*
+			 * However, if the consumer's parent device is also the
+			 * parent of the supplier, don't create a
+			 * consumer-supplier link from the parent to its child
+			 * device. Such a dependency is impossible.
+			 */
+			if (con_dev &&
+			    fwnode_is_ancestor_of(con_dev->fwnode, fwnode)) {
+				put_device(con_dev);
+				con_dev = NULL;
+			} else {
+				own_link = false;
+				dl_flags = DL_FLAG_SYNC_STATE_ONLY;
+			}
+		}
+
+		if (!con_dev)
+			continue;
+
+		ret = fw_devlink_create_devlink(con_dev, fwnode, dl_flags);
+		put_device(con_dev);
+		if (!own_link || ret == -EAGAIN)
+			continue;
+
+		list_del(&link->s_hook);
+		list_del(&link->c_hook);
+		kfree(link);
+	}
+}
+
+/**
+ * __fw_devlink_link_to_suppliers - Create device links to suppliers of a device
+ * @dev - The consumer device that needs to be linked to its suppliers
+ * @fwnode - Root of the fwnode tree that is used to create device links
+ *
+ * This function looks at all the supplier fwnodes of fwnode tree rooted at
+ * @fwnode and creates device links between @dev (consumer) and all the
+ * supplier devices of the entire fwnode tree at @fwnode.
+ *
+ * The function creates normal (non-SYNC_STATE_ONLY) device links between @dev
+ * and the real suppliers of @dev. Once these device links are created, the
+ * fwnode links are deleted. When such device links are successfully created,
+ * this function is called recursively on those supplier devices. This is
+ * needed to detect and break some invalid cycles in fwnode links.  See
+ * fw_devlink_create_devlink() for more details.
+ *
+ * In addition, it also looks at all the suppliers of the entire fwnode tree
+ * because some of the child devices of @dev that have not been added yet
+ * (because @dev hasn't probed) might already have their suppliers added to
+ * driver core. So, this function creates SYNC_STATE_ONLY device links between
+ * @dev (consumer) and these suppliers to make sure they don't execute their
+ * sync_state() callbacks before these child devices have a chance to create
+ * their device links. The fwnode links that correspond to the child devices
+ * aren't delete because they are needed later to create the device links
+ * between the real consumer and supplier devices.
+ */
+static void __fw_devlink_link_to_suppliers(struct device *dev,
+					   struct fwnode_handle *fwnode)
 {
-	int fw_ret;
+	bool own_link = (dev->fwnode == fwnode);
+	struct fwnode_link *link, *tmp;
+	struct fwnode_handle *child = NULL;
+	u32 dl_flags;
+
+	if (own_link)
+		dl_flags = fw_devlink_get_flags();
+	else
+		dl_flags = DL_FLAG_SYNC_STATE_ONLY;
 
-	device_link_add_missing_supplier_links();
+	list_for_each_entry_safe(link, tmp, &fwnode->suppliers, c_hook) {
+		int ret;
+		struct device *sup_dev;
+		struct fwnode_handle *sup = link->supplier;
+
+		ret = fw_devlink_create_devlink(dev, sup, dl_flags);
+		if (!own_link || ret == -EAGAIN)
+			continue;
 
-	if (fw_devlink_flags && fwnode_has_op(dev->fwnode, add_links)) {
-		fw_ret = fwnode_call_int_op(dev->fwnode, add_links, dev);
-		if (fw_ret == -ENODEV && !fw_devlink_is_permissive())
-			device_link_wait_for_mandatory_supplier(dev);
-		else if (fw_ret)
-			device_link_wait_for_optional_supplier(dev);
+		list_del(&link->s_hook);
+		list_del(&link->c_hook);
+		kfree(link);
+
+		/* If no device link was created, nothing more to do. */
+		if (ret)
+			continue;
+
+		/*
+		 * If a device link was successfully created to a supplier, we
+		 * now need to try and link the supplier to all its suppliers.
+		 *
+		 * This is needed to detect and delete false dependencies in
+		 * fwnode links that haven't been converted to a device link
+		 * yet. See comments in fw_devlink_create_devlink() for more
+		 * details on the false dependency.
+		 *
+		 * Without deleting these false dependencies, some devices will
+		 * never probe because they'll keep waiting for their false
+		 * dependency fwnode links to be converted to device links.
+		 */
+		sup_dev = get_dev_from_fwnode(sup);
+		__fw_devlink_link_to_suppliers(sup_dev, sup_dev->fwnode);
+		put_device(sup_dev);
 	}
+
+	/*
+	 * Make "proxy" SYNC_STATE_ONLY device links to represent the needs of
+	 * all the descendants. This proxy link step is needed to handle the
+	 * case where the supplier is added before the consumer's parent device
+	 * (@dev).
+	 */
+	while ((child = fwnode_get_next_available_child_node(fwnode, child)))
+		__fw_devlink_link_to_suppliers(dev, child);
+}
+
+static void fw_devlink_link_device(struct device *dev)
+{
+	struct fwnode_handle *fwnode = dev->fwnode;
+
+	if (!fw_devlink_flags)
+		return;
+
+	fw_devlink_parse_fwtree(fwnode);
+
+	mutex_lock(&fwnode_link_lock);
+	__fw_devlink_link_to_consumers(dev);
+	__fw_devlink_link_to_suppliers(dev, fwnode);
+	mutex_unlock(&fwnode_link_lock);
 }
 
 /* Device links support end. */
@@ -2431,7 +2583,6 @@ void device_initialize(struct device *dev)
 #endif
 	INIT_LIST_HEAD(&dev->links.consumers);
 	INIT_LIST_HEAD(&dev->links.suppliers);
-	INIT_LIST_HEAD(&dev->links.needs_suppliers);
 	INIT_LIST_HEAD(&dev->links.defer_sync);
 	dev->links.status = DL_DEV_NO_DRIVER;
 }
diff --git a/include/linux/device.h b/include/linux/device.h
index 1e771ea4dca6..89bb8b84173e 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -351,18 +351,13 @@ enum dl_dev_state {
  * 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.
  * @defer_sync: Hook to global list of devices that have deferred sync_state.
- * @need_for_probe: If needs_suppliers is on a list, this indicates if the
- *		    suppliers are needed for probe or not.
  * @status: Driver status information.
  */
 struct dev_links_info {
 	struct list_head suppliers;
 	struct list_head consumers;
-	struct list_head needs_suppliers;
 	struct list_head defer_sync;
-	bool need_for_probe;
 	enum dl_dev_state status;
 };
 
-- 
2.29.2.454.gaff20da3a2-goog