Date:	Fri, 17 Jun 2016 12:36:20 +0200
From:	Lukas Wunner <lukas@...ner.de>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org,
	linux-samsung-soc@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Joerg Roedel <joro@...tes.org>,
	Inki Dae <inki.dae@...sung.com>, Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Mark Brown <broonie@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Andreas Noever <andreas.noever@...il.com>
Subject: Re: [PATCH v2 02/10] driver core: Functional dependencies tracking
 support

Hi Marek,

On Fri, Jun 17, 2016 at 08:26:52AM +0200, Marek Szyprowski wrote:
> From: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> 
> Currently, there is a problem with handling cases where functional
> dependencies between devices are involved.
> 
> What I mean by a "functional dependency" is when the driver of device
> B needs both device A and its driver to be present and functional to
> be able to work.  This implies that the driver of A needs to be
> working for B to be probed successfully and it cannot be unbound from
> the device before the B's driver.  This also has certain consequences
> for power management of these devices (suspend/resume and runtime PM
> ordering).
> 
> Add support for representing those functional dependencies between
> devices to allow the driver core to track them and act on them in
> certain cases where they matter.

Rafael has indicated that he intends to respin this series:
https://lkml.org/lkml/2016/6/8/1061

We also have such a functional dependency for Thunderbolt on Macs:
On resume from system sleep, the PCIe hotplug ports may not resume
before the thunderbolt driver has reestablished the PCI tunnels.
Currently this is enforced by quirk_apple_wait_for_thunderbolt()
in drivers/pci/quirks.c. It would be good if we could represent
this dependency using something like Rafael's approach instead of
open coding it, however one detail in Rafael's patches is problematic:

> New links are added by calling device_link_add() which may happen
> either before the consumer device is probed or when probing it, in
> which case the caller needs to ensure that the driver of the
> supplier device is present and functional and the DEVICE_LINK_PROBE_TIME
> flag should be passed to device_link_add() to reflect that.

The thunderbolt driver cannot call device_link_add() before the
PCIe hotplug ports are bound to a driver unless we amend portdrv
to return -EPROBE_DEFER for Thunderbolt hotplug ports on Macs
if the thunderbolt driver isn't loaded.

It would therefore be beneficial if device_link_add() can be
called even *after* the consumer is bound.

Thanks,

Lukas

> 
> Link objects are deleted either explicitly, by calling
> device_link_del() on the link object in question, or automatically,
> when the consumer device is unbound from its driver or when one
> of the target devices is deleted, depending on the link type.
> 
> There are two types of link objects, persistent and non-persistent.
> The difference between them is that the persistent links stay around
> until one of the target devices is deleted, which the non-persistent
> ones are deleted when the consumer driver is unbound from its device
> (ie. they are assumed to be valid only as long as the consumer device
> has a driver bound to it).  The DEVICE_LINK_PERSISTENT flag has to
> be passed to device_link_add() so as to create a persistent link.
> 
> One of the actions carried out by device_link_add() is to reorder
> the lists used for device shutdown and system suspend/resume to
> put the consumer device along with all of its children and all of
> its consumers (and so on, recursively) to the ends of those list
> in order to ensure the right ordering between the all of the supplier
> and consumer devices.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
>  drivers/base/base.h    |  11 ++
>  drivers/base/core.c    | 386 +++++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/base/dd.c      |  42 +++++-
>  include/linux/device.h |  36 +++++
>  4 files changed, 470 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/base/base.h b/drivers/base/base.h
> index e05db388bd1c..cccb1d211541 100644
> --- a/drivers/base/base.h
> +++ b/drivers/base/base.h
> @@ -107,6 +107,9 @@ extern void bus_remove_device(struct device *dev);
>  
>  extern int bus_add_driver(struct device_driver *drv);
>  extern void bus_remove_driver(struct device_driver *drv);
> +extern void device_release_driver_internal(struct device *dev,
> +					   struct device_driver *drv,
> +					   struct device *parent);
>  
>  extern void driver_detach(struct device_driver *drv);
>  extern int driver_probe_device(struct device_driver *drv, struct device *dev);
> @@ -152,3 +155,11 @@ extern int devtmpfs_init(void);
>  #else
>  static inline int devtmpfs_init(void) { return 0; }
>  #endif
> +
> +/* Device links */
> +extern int device_links_check_suppliers(struct device *dev);
> +extern void device_links_driver_bound(struct device *dev);
> +extern void device_links_driver_gone(struct device *dev);
> +extern void device_links_no_driver(struct device *dev);
> +extern bool device_links_busy(struct device *dev);
> +extern void device_links_unbind_consumers(struct device *dev);
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 0a8bdade53f2..416341df3268 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -44,6 +44,367 @@ static int __init sysfs_deprecated_setup(char *arg)
>  early_param("sysfs.deprecated", sysfs_deprecated_setup);
>  #endif
>  
> +/* Device links support. */
> +
> +DEFINE_STATIC_SRCU(device_links_srcu);
> +static DEFINE_MUTEX(device_links_lock);
> +
> +static int device_reorder_to_tail(struct device *dev, void *not_used)
> +{
> +	struct devlink *link;
> +
> +	devices_kset_move_last(dev);
> +	device_pm_move_last(dev);
> +	device_for_each_child(dev, NULL, device_reorder_to_tail);
> +	list_for_each_entry(link, &dev->consumer_links, c_node)
> +		device_reorder_to_tail(link->consumer, NULL);
> +
> +	return 0;
> +}
> +
> +/**
> + * device_link_add - Create a link between two devices.
> + * @consumer: Consumer end of the link.
> + * @supplier: Supplier end of the link.
> + * @flags: Link flags.
> + *
> + * At least one of the flags must be set.  If DEVICE_LINK_PROBE_TIME is set, the
> + * caller is expected to know that (a) the supplier device is present and active
> + * (ie. its driver is functional) and (b) the consumer device is probing at the
> + * moment and therefore the initial state of the link will be "consumer probe"
> + * in that case.  If DEVICE_LINK_PROBE_TIME is not set, DEVICE_LINK_PERSISTENT
> + * must be set (meaning that the link will not go away when the consumer driver
> + * goes away).
> + *
> + * 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
> + * on it to the ends of those lists.
> + */
> +struct devlink *device_link_add(struct device *consumer,
> +				struct device *supplier, u32 flags)
> +{
> +	struct devlink *link;
> +
> +	if (!consumer || !supplier || !flags)
> +		return NULL;
> +
> +	mutex_lock(&device_links_lock);
> +
> +	list_for_each_entry(link, &supplier->supplier_links, s_node)
> +		if (link->consumer == consumer)
> +			goto out;
> +
> +	link = kmalloc(sizeof(*link), GFP_KERNEL);
> +	if (!link)
> +		goto out;
> +
> +	get_device(supplier);
> +	link->supplier = supplier;
> +	INIT_LIST_HEAD(&link->s_node);
> +	get_device(consumer);
> +	link->consumer = consumer;
> +	INIT_LIST_HEAD(&link->c_node);
> +	link->flags = flags;
> +	link->status = (flags & DEVICE_LINK_PROBE_TIME) ?
> +			DEVICE_LINK_CONSUMER_PROBE : DEVICE_LINK_DORMANT;
> +	spin_lock_init(&link->lock);
> +
> +	/*
> +	 * Move the consumer and all of the devices depending on it to the end
> +	 * of dpm_list and the devices_kset list.
> +	 *
> +	 * We have to hold dpm_list locked throughout all that or else we may
> +	 * end up suspending with a wrong ordering of it.
> +	 */
> +	device_pm_lock();
> +	device_reorder_to_tail(consumer, NULL);
> +	device_pm_unlock();
> +
> +	list_add_tail_rcu(&link->s_node, &supplier->supplier_links);
> +	list_add_tail_rcu(&link->c_node, &consumer->consumer_links);
> +
> +	dev_info(consumer, "Linked as a consumer to %s\n", dev_name(supplier));
> +
> + out:
> +	mutex_unlock(&device_links_lock);
> +	return link;
> +}
> +EXPORT_SYMBOL_GPL(device_link_add);
> +
> +static void __devlink_free_srcu(struct rcu_head *rhead)
> +{
> +	struct devlink *link;
> +
> +	link = container_of(rhead, struct devlink, rcu_head);
> +	put_device(link->consumer);
> +	put_device(link->supplier);
> +	kfree(link);
> +}
> +
> +static void devlink_del(struct devlink *link)
> +{
> +	dev_info(link->consumer, "Dropping the link to %s\n",
> +		 dev_name(link->supplier));
> +
> +	list_del_rcu(&link->s_node);
> +	list_del_rcu(&link->c_node);
> +	call_srcu(&device_links_srcu, &link->rcu_head, __devlink_free_srcu);
> +}
> +
> +/**
> + * device_link_del - Delete a link between two devices.
> + * @link: Device link to delete.
> + */
> +void device_link_del(struct devlink *link)
> +{
> +	mutex_lock(&device_links_lock);
> +	devlink_del(link);
> +	mutex_unlock(&device_links_lock);
> +}
> +EXPORT_SYMBOL_GPL(device_link_del);
> +
> +static int device_links_read_lock(void)
> +{
> +	return srcu_read_lock(&device_links_srcu);
> +}
> +
> +static void device_links_read_unlock(int idx)
> +{
> +	return srcu_read_unlock(&device_links_srcu, idx);
> +}
> +
> +static void device_links_missing_supplier(struct device *dev)
> +{
> +	struct devlink *link;
> +
> +	list_for_each_entry_rcu(link, &dev->consumer_links, c_node) {
> +		spin_lock(&link->lock);
> +
> +		if (link->status == DEVICE_LINK_CONSUMER_PROBE)
> +			link->status = DEVICE_LINK_AVAILABLE;
> +
> +		spin_unlock(&link->lock);
> +	}
> +}
> +
> +/**
> + * device_links_check_suppliers - Check supplier devices for this one.
> + * @dev: Consumer device.
> + *
> + * Check links from this device to any suppliers.  Walk the list of the device's
> + * consumer links and see if all of the suppliers are available.  If not, simply
> + * return -EPROBE_DEFER.
> + *
> + * Walk the list under SRCU and check each link's status field under its lock.
> + *
> + * We need to guarantee that the supplier will not go away after the check has
> + * been positive here.  It only can go away in __device_release_driver() and
> + * that function  checks the device's links to consumers.  This means we need to
> + * mark the link as "consumer probe in progress" to make the supplier removal
> + * wait for us to complete (or bad things may happen).
> + */
> +int device_links_check_suppliers(struct device *dev)
> +{
> +	struct devlink *link;
> +	int idx, ret = 0;
> +
> +	idx = device_links_read_lock();
> +
> +	list_for_each_entry_rcu(link, &dev->consumer_links, c_node) {
> +		spin_lock(&link->lock);
> +		if (link->status != DEVICE_LINK_AVAILABLE) {
> +			spin_unlock(&link->lock);
> +			device_links_missing_supplier(dev);
> +			ret = -EPROBE_DEFER;
> +			break;
> +		}
> +		link->status = DEVICE_LINK_CONSUMER_PROBE;
> +		spin_unlock(&link->lock);
> +	}
> +
> +	device_links_read_unlock(idx);
> +	return ret;
> +}
> +
> +/**
> + * device_links_driver_bound - Update device links after probing its driver.
> + * @dev: Device to update the links for.
> + *
> + * The probe has been successful, so update links from this device to any
> + * consumers by changing their status to "available".
> + *
> + * Also change the status of @dev's links to suppliers to "active".
> + */
> +void device_links_driver_bound(struct device *dev)
> +{
> +	struct devlink *link;
> +	int idx;
> +
> +	idx = device_links_read_lock();
> +
> +	list_for_each_entry_rcu(link, &dev->supplier_links, s_node) {
> +		spin_lock(&link->lock);
> +		WARN_ON(link->status != DEVICE_LINK_DORMANT);
> +		link->status = DEVICE_LINK_AVAILABLE;
> +		spin_unlock(&link->lock);
> +	}
> +
> +	list_for_each_entry_rcu(link, &dev->consumer_links, c_node) {
> +		spin_lock(&link->lock);
> +		WARN_ON(link->status != DEVICE_LINK_CONSUMER_PROBE);
> +		link->status = DEVICE_LINK_ACTIVE;
> +		spin_unlock(&link->lock);
> +	}
> +
> +	device_links_read_unlock(idx);
> +}
> +
> +/**
> + * device_links_driver_gone - Update links after driver removal.
> + * @dev: Device whose driver has gone away.
> + *
> + * Update links to consumers for @dev by changing their status to "dormant".
> + */
> +void device_links_driver_gone(struct device *dev)
> +{
> +	struct devlink *link;
> +	int idx;
> +
> +	idx = device_links_read_lock();
> +
> +	list_for_each_entry_rcu(link, &dev->supplier_links, s_node) {
> +		WARN_ON(!(link->flags & DEVICE_LINK_PERSISTENT));
> +		spin_lock(&link->lock);
> +		WARN_ON(link->status != DEVICE_LINK_SUPPLIER_UNBIND);
> +		link->status = DEVICE_LINK_DORMANT;
> +		spin_unlock(&link->lock);
> +	}
> +
> +	device_links_read_unlock(idx);
> +}
> +
> +/**
> + * device_links_no_driver - Update links of a device without a driver.
> + * @dev: Device without a drvier.
> + *
> + * Delete all non-persistent links from this device to any suppliers.
> + * Persistent links stay around, but their status is changed to "available",
> + * unless they already are in the "supplier unbind in progress" state in which
> + * case they need not be updated.
> + */
> +void device_links_no_driver(struct device *dev)
> +{
> +	struct devlink *link, *ln;
> +
> +	mutex_lock(&device_links_lock);
> +
> +	list_for_each_entry_safe_reverse(link, ln, &dev->consumer_links, c_node)
> +		if (link->flags & DEVICE_LINK_PERSISTENT) {
> +			spin_lock(&link->lock);
> +
> +			if (link->status != DEVICE_LINK_SUPPLIER_UNBIND)
> +				link->status = DEVICE_LINK_AVAILABLE;
> +
> +			spin_unlock(&link->lock);
> +		} else {
> +			devlink_del(link);
> +		}
> +
> +	mutex_unlock(&device_links_lock);
> +}
> +
> +/**
> + * device_links_busy - Check if there are any busy links to consumers.
> + * @dev: Device to check.
> + *
> + * Check each consumer of the device and return 'true' it if its link's status
> + * is one of "consumer probe" or "active" (meaning that the given consumer is
> + * probing right now or its driver is present).  Otherwise, change the link
> + * state to "supplier unbind" to prevent the consumer from being probed
> + * successfully going forward.
> + *
> + * Return 'false' if there are no probing or active consumers.
> + */
> +bool device_links_busy(struct device *dev)
> +{
> +	struct devlink *link;
> +	int idx;
> +	bool ret = false;
> +
> +	idx = device_links_read_lock();
> +
> +	list_for_each_entry_rcu(link, &dev->supplier_links, s_node) {
> +		spin_lock(&link->lock);
> +		if (link->status == DEVICE_LINK_CONSUMER_PROBE
> +		    || link->status == DEVICE_LINK_ACTIVE) {
> +			spin_unlock(&link->lock);
> +			ret = true;
> +			break;
> +		}
> +		link->status = DEVICE_LINK_SUPPLIER_UNBIND;
> +		spin_unlock(&link->lock);
> +	}
> +
> +	device_links_read_unlock(idx);
> +	return ret;
> +}
> +
> +/**
> + * device_links_unbind_consumers - Force unbind consumers of the given device.
> + * @dev: Device to unbind the consumers of.
> + *
> + * Walk the list of links to consumers for @dev and if any of them is in the
> + * "consumer probe" state, wait for all device probes in progress to complete
> + * and start over.
> + *
> + * If that's not the case, change the status of the link to "supplier unbind"
> + * and check if the link was in the "active" state.  If so, force the consumer
> + * driver to unbind and start over (the consumer will not re-probe as we have
> + * changed the state of the link already).
> + */
> +void device_links_unbind_consumers(struct device *dev)
> +{
> +	struct devlink *link;
> +	int idx;
> +
> + start:
> +	idx = device_links_read_lock();
> +
> +	list_for_each_entry_rcu(link, &dev->supplier_links, s_node) {
> +		enum devlink_status status;
> +
> +		spin_lock(&link->lock);
> +		status = link->status;
> +		if (status == DEVICE_LINK_CONSUMER_PROBE) {
> +			spin_unlock(&link->lock);
> +
> +			device_links_read_unlock(idx);
> +
> +			wait_for_device_probe();
> +			goto start;
> +		}
> +		link->status = DEVICE_LINK_SUPPLIER_UNBIND;
> +		if (status == DEVICE_LINK_ACTIVE) {
> +			struct device *consumer = link->consumer;
> +
> +			get_device(consumer);
> +			spin_unlock(&link->lock);
> +
> +			device_links_read_unlock(idx);
> +
> +			device_release_driver_internal(consumer, NULL,
> +						       consumer->parent);
> +			put_device(consumer);
> +			goto start;
> +		}
> +		spin_unlock(&link->lock);
> +	}
> +
> +	device_links_read_unlock(idx);
> +}
> +
> +/* Device links support end. */
> +
>  int (*platform_notify)(struct device *dev) = NULL;
>  int (*platform_notify_remove)(struct device *dev) = NULL;
>  static struct kobject *dev_kobj;
> @@ -711,6 +1072,8 @@ void device_initialize(struct device *dev)
>  #ifdef CONFIG_GENERIC_MSI_IRQ
>  	INIT_LIST_HEAD(&dev->msi_list);
>  #endif
> +	INIT_LIST_HEAD(&dev->supplier_links);
> +	INIT_LIST_HEAD(&dev->consumer_links);
>  }
>  EXPORT_SYMBOL_GPL(device_initialize);
>  
> @@ -1233,6 +1596,7 @@ void device_del(struct device *dev)
>  {
>  	struct device *parent = dev->parent;
>  	struct class_interface *class_intf;
> +	struct devlink *link, *ln;
>  
>  	/* Notify clients of device removal.  This call must come
>  	 * before dpm_sysfs_remove().
> @@ -1240,6 +1604,28 @@ void device_del(struct device *dev)
>  	if (dev->bus)
>  		blocking_notifier_call_chain(&dev->bus->p->bus_notifier,
>  					     BUS_NOTIFY_DEL_DEVICE, dev);
> +
> +	/*
> +	 * Delete all of the remaining links from this device to any other
> +	 * devices (either consumers or suppliers).
> +	 *
> +	 * This requires that all links be dormant, so warn if that's no the
> +	 * case.
> +	 */
> +	mutex_lock(&device_links_lock);
> +
> +	list_for_each_entry_safe_reverse(link, ln, &dev->consumer_links, c_node) {
> +		WARN_ON(link->status != DEVICE_LINK_DORMANT);
> +		devlink_del(link);
> +	}
> +
> +	list_for_each_entry_safe_reverse(link, ln, &dev->supplier_links, s_node) {
> +		WARN_ON(link->status != DEVICE_LINK_DORMANT);
> +		devlink_del(link);
> +	}
> +
> +	mutex_unlock(&device_links_lock);
> +
>  	dpm_sysfs_remove(dev);
>  	if (parent)
>  		klist_del(&dev->p->knode_parent);
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index d9e76e9205c7..7c0abeba89e9 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -249,6 +249,7 @@ static void driver_bound(struct device *dev)
>  		 __func__, dev_name(dev));
>  
>  	klist_add_tail(&dev->p->knode_driver, &dev->driver->p->klist_devices);
> +	device_links_driver_bound(dev);
>  
>  	device_pm_check_callbacks(dev);
>  
> @@ -399,6 +400,7 @@ probe_failed:
>  		blocking_notifier_call_chain(&dev->bus->p->bus_notifier,
>  					     BUS_NOTIFY_DRIVER_NOT_BOUND, dev);
>  pinctrl_bind_failed:
> +	device_links_no_driver(dev);
>  	devres_release_all(dev);
>  	driver_sysfs_remove(dev);
>  	dev->driver = NULL;
> @@ -489,6 +491,10 @@ int driver_probe_device(struct device_driver *drv, struct device *dev)
>  	if (!device_is_registered(dev))
>  		return -ENODEV;
>  
> +	ret = device_links_check_suppliers(dev);
> +	if (ret)
> +		return ret;
> +
>  	pr_debug("bus: '%s': %s: matched device %s with driver %s\n",
>  		 drv->bus->name, __func__, dev_name(dev), drv->name);
>  
> @@ -756,7 +762,7 @@ EXPORT_SYMBOL_GPL(driver_attach);
>   * __device_release_driver() must be called with @dev lock held.
>   * When called for a USB interface, @dev->parent lock must be held as well.
>   */
> -static void __device_release_driver(struct device *dev)
> +static void __device_release_driver(struct device *dev, struct device *parent)
>  {
>  	struct device_driver *drv;
>  
> @@ -765,6 +771,25 @@ static void __device_release_driver(struct device *dev)
>  		if (driver_allows_async_probing(drv))
>  			async_synchronize_full();
>  
> +		while (device_links_busy(dev)) {
> +			device_unlock(dev);
> +			if (parent)
> +				device_unlock(parent);
> +
> +			device_links_unbind_consumers(dev);
> +			if (parent)
> +				device_lock(parent);
> +
> +			device_lock(dev);
> +			/*
> +			 * A concurrent invocation of the same function might
> +			 * have released the driver successfully while this one
> +			 * was waiting, so check for that.
> +			 */
> +			if (dev->driver != drv)
> +				return;
> +		}
> +
>  		pm_runtime_get_sync(dev);
>  
>  		driver_sysfs_remove(dev);
> @@ -780,6 +805,9 @@ static void __device_release_driver(struct device *dev)
>  			dev->bus->remove(dev);
>  		else if (drv->remove)
>  			drv->remove(dev);
> +
> +		device_links_driver_gone(dev);
> +		device_links_no_driver(dev);
>  		devres_release_all(dev);
>  		dev->driver = NULL;
>  		dev_set_drvdata(dev, NULL);
> @@ -796,16 +824,16 @@ static void __device_release_driver(struct device *dev)
>  	}
>  }
>  
> -static void device_release_driver_internal(struct device *dev,
> -					   struct device_driver *drv,
> -					   struct device *parent)
> +void device_release_driver_internal(struct device *dev,
> +				    struct device_driver *drv,
> +				    struct device *parent)
>  {
>  	if (parent)
>  		device_lock(parent);
>  
>  	device_lock(dev);
>  	if (!drv || drv == dev->driver)
> -		__device_release_driver(dev);
> +		__device_release_driver(dev, parent);
>  
>  	device_unlock(dev);
>  	if (parent)
> @@ -818,6 +846,10 @@ static void device_release_driver_internal(struct device *dev,
>   *
>   * Manually detach device from driver.
>   * When called for a USB interface, @dev->parent lock must be held.
> + *
> + * If this function is to be called with @dev->parent lock held, ensure that
> + * the device's consumers are unbound in advance or that their locks can be
> + * acquired under the @dev->parent lock.
>   */
>  void device_release_driver(struct device *dev)
>  {
> diff --git a/include/linux/device.h b/include/linux/device.h
> index 38f02814d53a..647204bd74a0 100644
> --- a/include/linux/device.h
> +++ b/include/linux/device.h
> @@ -706,6 +706,34 @@ struct device_dma_parameters {
>  	unsigned long segment_boundary_mask;
>  };
>  
> +enum devlink_status {
> +	DEVICE_LINK_DORMANT = 0,	/* Link not in use. */
> +	DEVICE_LINK_AVAILABLE,		/* Supplier driver is present. */
> +	DEVICE_LINK_ACTIVE,		/* Consumer driver is present too. */
> +	DEVICE_LINK_CONSUMER_PROBE,	/* Consumer is probing. */
> +	DEVICE_LINK_SUPPLIER_UNBIND,	/* Supplier is unbinding. */
> +};
> +
> +/*
> + * Device link flags.
> + *
> + * PERSISTENT: Do not delete the link on consumer device driver unbind.
> + * PROBE_TIME: Assume supplier device functional when creating the link.
> + */
> +#define DEVICE_LINK_PERSISTENT	(1 << 0)
> +#define DEVICE_LINK_PROBE_TIME	(1 << 1)
> +
> +struct devlink {
> +	struct device *supplier;
> +	struct list_head s_node;
> +	struct device *consumer;
> +	struct list_head c_node;
> +	enum devlink_status status;
> +	u32 flags;
> +	spinlock_t lock;
> +	struct rcu_head rcu_head;
> +};
> +
>  /**
>   * struct device - The basic device structure
>   * @parent:	The device's "parent" device, the device to which it is attached.
> @@ -731,6 +759,8 @@ struct device_dma_parameters {
>   * 		on.  This shrinks the "Board Support Packages" (BSPs) and
>   * 		minimizes board-specific #ifdefs in drivers.
>   * @driver_data: Private pointer for driver specific info.
> + * @supplier_links: Links to consumer devices.
> + * @consumer_links: Links to supplier devices.
>   * @power:	For device power management.
>   * 		See Documentation/power/devices.txt for details.
>   * @pm_domain:	Provide callbacks that are executed during system suspend,
> @@ -797,6 +827,8 @@ struct device {
>  					   core doesn't touch it */
>  	void		*driver_data;	/* Driver data, set and get with
>  					   dev_set/get_drvdata */
> +	struct list_head	supplier_links;
> +	struct list_head	consumer_links;
>  	struct dev_pm_info	power;
>  	struct dev_pm_domain	*pm_domain;
>  
> @@ -1113,6 +1145,10 @@ extern void device_shutdown(void);
>  /* debugging and troubleshooting/diagnostic helpers. */
>  extern const char *dev_driver_string(const struct device *dev);
>  
> +/* Device links interface. */
> +struct devlink *device_link_add(struct device *consumer,
> +				struct device *supplier, u32 flags);
> +void device_link_del(struct devlink *link);
>  
>  #ifdef CONFIG_PRINTK
>  
> -- 
> 1.9.1
> 
> --
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