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 .. _device_link:
 
 ============
 Device links
 ============
 
 By default, the driver core only enforces dependencies between devices
 that are borne out of a parent/child relationship within the device
 hierarchy: When suspending, resuming or shutting down the system, devices
 are ordered based on this relationship, i.e. children are always suspended
 before their parent, and the parent is always resumed before its children.
 
 Sometimes there is a need to represent device dependencies beyond the
 mere parent/child relationship, e.g. between siblings, and have the
 driver core automatically take care of them.
 
 Secondly, the driver core by default does not enforce any driver presence
 dependencies, i.e. that one device must be bound to a driver before
 another one can probe or function correctly.
 
 Often these two dependency types come together, so a device depends on
 another one both with regards to driver presence *and* with regards to
 suspend/resume and shutdown ordering.
 
 Device links allow representation of such dependencies in the driver core.
 
 In its standard or *managed* form, a device link combines *both* dependency
 types:  It guarantees correct suspend/resume and shutdown ordering between a
 "supplier" device and its "consumer" devices, and it guarantees driver
 presence on the supplier.  The consumer devices are not probed before the
 supplier is bound to a driver, and they're unbound before the supplier
 is unbound.
 
 When driver presence on the supplier is irrelevant and only correct
 suspend/resume and shutdown ordering is needed, the device link may
 simply be set up with the ``DL_FLAG_STATELESS`` flag.  In other words,
 enforcing driver presence on the supplier is optional.
 
 Another optional feature is runtime PM integration:  By setting the
 ``DL_FLAG_PM_RUNTIME`` flag on addition of the device link, the PM core
 is instructed to runtime resume the supplier and keep it active
 whenever and for as long as the consumer is runtime resumed.
 
 Usage
 =====
 
 The earliest point in time when device links can be added is after
 :c:func:`device_add()` has been called for the supplier and
 :c:func:`device_initialize()` has been called for the consumer.
 
 It is legal to add them later, but care must be taken that the system
 remains in a consistent state:  E.g. a device link cannot be added in
 the midst of a suspend/resume transition, so either commencement of
 such a transition needs to be prevented with :c:func:`lock_system_sleep()`,
 or the device link needs to be added from a function which is guaranteed
 not to run in parallel to a suspend/resume transition, such as from a
 device ``->probe`` callback or a boot-time PCI quirk.
 
 Another example for an inconsistent state would be a device link that
 represents a driver presence dependency, yet is added from the consumer's
 ``->probe`` callback while the supplier hasn't started to probe yet:  Had the
 driver core known about the device link earlier, it wouldn't have probed the
 consumer in the first place.  The onus is thus on the consumer to check
 presence of the supplier after adding the link, and defer probing on
 non-presence.  [Note that it is valid to create a link from the consumer's
 ``->probe`` callback while the supplier is still probing, but the consumer must
 know that the supplier is functional already at the link creation time (that is
 the case, for instance, if the consumer has just acquired some resources that
 would not have been available had the supplier not been functional then).]
 
 If a device link with ``DL_FLAG_STATELESS`` set (i.e. a stateless device link)
 is added in the ``->probe`` callback of the supplier or consumer driver, it is
 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 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
 driver presence dependency is needed (but only correct suspend/resume and
 shutdown ordering) and ``DL_FLAG_PM_RUNTIME`` to express that runtime PM
 integration is desired.
 
 Two other flags are specifically targeted at use cases where the device
 link is added from the consumer's ``->probe`` callback:  ``DL_FLAG_RPM_ACTIVE``
 can be specified to runtime resume the supplier and prevent it from suspending
 before the consumer is runtime suspended.  ``DL_FLAG_AUTOREMOVE_CONSUMER``
 causes the device link to be automatically purged when the consumer fails to
 probe or later unbinds.
 
 Similarly, when the device link is added from supplier's ``->probe`` callback,
 ``DL_FLAG_AUTOREMOVE_SUPPLIER`` causes the device link to be automatically
 purged when the supplier fails to probe or later unbinds.
 
 If neither ``DL_FLAG_AUTOREMOVE_CONSUMER`` nor ``DL_FLAG_AUTOREMOVE_SUPPLIER``
 is set, ``DL_FLAG_AUTOPROBE_CONSUMER`` can be used to request the driver core
 to probe for a driver for the consumer driver on the link automatically after
 a driver has been bound to the supplier device.
 
 Note, however, that any combinations of ``DL_FLAG_AUTOREMOVE_CONSUMER``,
 ``DL_FLAG_AUTOREMOVE_SUPPLIER`` or ``DL_FLAG_AUTOPROBE_CONSUMER`` with
 ``DL_FLAG_STATELESS`` are invalid and cannot be used.
 
 Limitations
 ===========
 
 Driver authors should be aware that a driver presence dependency for managed
 device links (i.e. when ``DL_FLAG_STATELESS`` is not specified on link addition)
 may cause probing of the consumer to be deferred indefinitely.  This can become
 a problem if the consumer is required to probe before a certain initcall level
 is reached.  Worse, if the supplier driver is blacklisted or missing, the
 consumer will never be probed.
 
 Moreover, managed device links cannot be deleted directly.  They are deleted
 by the driver core when they are not necessary any more in accordance with the
 ``DL_FLAG_AUTOREMOVE_CONSUMER`` and ``DL_FLAG_AUTOREMOVE_SUPPLIER`` flags.
 However, stateless device links (i.e. device links with ``DL_FLAG_STATELESS``
 set) are expected to be removed by whoever called :c:func:`device_link_add()`
 to add them with the help of either :c:func:`device_link_del()` or
 :c:func:`device_link_remove()`.
 
 Passing ``DL_FLAG_RPM_ACTIVE`` along with ``DL_FLAG_STATELESS`` to
 :c:func:`device_link_add()` may cause the PM-runtime usage counter of the
 supplier device to remain nonzero after a subsequent invocation of either
 :c:func:`device_link_del()` or :c:func:`device_link_remove()` to remove the
 device link returned by it.  This happens if :c:func:`device_link_add()` is
 called twice in a row for the same consumer-supplier pair without removing the
 link between these calls, in which case allowing the PM-runtime usage counter
 of the supplier to drop on an attempt to remove the link may cause it to be
 suspended while the consumer is still PM-runtime-active and that has to be
 avoided.  [To work around this limitation it is sufficient to let the consumer
 runtime suspend at least once, or call :c:func:`pm_runtime_set_suspended()` for
 it with PM-runtime disabled, between the :c:func:`device_link_add()` and
 :c:func:`device_link_del()` or :c:func:`device_link_remove()` calls.]
 
 Sometimes drivers depend on optional resources.  They are able to operate
 in a degraded mode (reduced feature set or performance) when those resources
 are not present.  An example is an SPI controller that can use a DMA engine
 or work in PIO mode.  The controller can determine presence of the optional
 resources at probe time but on non-presence there is no way to know whether
 they will become available in the near future (due to a supplier driver
 probing) or never.  Consequently it cannot be determined whether to defer
 probing or not.  It would be possible to notify drivers when optional
 resources become available after probing, but it would come at a high cost
 for drivers as switching between modes of operation at runtime based on the
 availability of such resources would be much more complex than a mechanism
 based on probe deferral.  In any case optional resources are beyond the
 scope of device links.
 
 Examples
 ========
 
 * An MMU device exists alongside a busmaster device, both are in the same
   power domain.  The MMU implements DMA address translation for the busmaster
   device and shall be runtime resumed and kept active whenever and as long
   as the busmaster device is active.  The busmaster device's driver shall
   not bind before the MMU is bound.  To achieve this, a device link with
   runtime PM integration is added from the busmaster device (consumer)
   to the MMU device (supplier).  The effect with regards to runtime PM
   is the same as if the MMU was the parent of the master device.
 
   The fact that both devices share the same power domain would normally
   suggest usage of a struct dev_pm_domain or struct generic_pm_domain,
   however these are not independent devices that happen to share a power
   switch, but rather the MMU device serves the busmaster device and is
   useless without it.  A device link creates a synthetic hierarchical
   relationship between the devices and is thus more apt.
 
 * A Thunderbolt host controller comprises a number of PCIe hotplug ports
   and an NHI device to manage the PCIe switch.  On resume from system sleep,
   the NHI device needs to re-establish PCI tunnels to attached devices
   before the hotplug ports can resume.  If the hotplug ports were children
   of the NHI, this resume order would automatically be enforced by the
   PM core, but unfortunately they're aunts.  The solution is to add
   device links from the hotplug ports (consumers) to the NHI device
   (supplier).  A driver presence dependency is not necessary for this
   use case.
 
 * Discrete GPUs in hybrid graphics laptops often feature an HDA controller
   for HDMI/DP audio.  In the device hierarchy the HDA controller is a sibling
   of the VGA device, yet both share the same power domain and the HDA
   controller is only ever needed when an HDMI/DP display is attached to the
   VGA device.  A device link from the HDA controller (consumer) to the
   VGA device (supplier) aptly represents this relationship.
 
 * ACPI allows definition of a device start order by way of _DEP objects.
   A classical example is when ACPI power management methods on one device
   are implemented in terms of I\ :sup:`2`\ C accesses and require a specific
   I\ :sup:`2`\ C controller to be present and functional for the power
   management of the device in question to work.
 
 * In some SoCs a functional dependency exists from display, video codec and
   video processing IP cores on transparent memory access IP cores that handle
   burst access and compression/decompression.
 
 Alternatives
 ============
 
 * A struct dev_pm_domain can be used to override the bus,
   class or device type callbacks.  It is intended for devices sharing
   a single on/off switch, however it does not guarantee a specific
   suspend/resume ordering, this needs to be implemented separately.
   It also does not by itself track the runtime PM status of the involved
   devices and turn off the power switch only when all of them are runtime
   suspended.  Furthermore it cannot be used to enforce a specific shutdown
   ordering or a driver presence dependency.
 
 * A struct generic_pm_domain is a lot more heavyweight than a
   device link and does not allow for shutdown ordering or driver presence
   dependencies.  It also cannot be used on ACPI systems.
 
 Implementation
 ==============
 
 The device hierarchy, which -- as the name implies -- is a tree,
 becomes a directed acyclic graph once device links are added.
 
 Ordering of these devices during suspend/resume is determined by the
 dpm_list.  During shutdown it is determined by the devices_kset.  With
 no device links present, the two lists are a flattened, one-dimensional
 representations of the device tree such that a device is placed behind
 all its ancestors.  That is achieved by traversing the ACPI namespace
 or OpenFirmware device tree top-down and appending devices to the lists
 as they are discovered.
 
 Once device links are added, the lists need to satisfy the additional
 constraint that a device is placed behind all its suppliers, recursively.
 To ensure this, upon addition of the device link the consumer and the
 entire sub-graph below it (all children and consumers of the consumer)
 are moved to the end of the list.  (Call to :c:func:`device_reorder_to_tail()`
 from :c:func:`device_link_add()`.)
 
 To prevent introduction of dependency loops into the graph, it is
 verified upon device link addition that the supplier is not dependent
 on the consumer or any children or consumers of the consumer.
 (Call to :c:func:`device_is_dependent()` from :c:func:`device_link_add()`.)
 If that constraint is violated, :c:func:`device_link_add()` will return
 ``NULL`` and a ``WARNING`` will be logged.
 
 Notably this also prevents the addition of a device link from a parent
 device to a child.  However the converse is allowed, i.e. a device link
 from a child to a parent.  Since the driver core already guarantees
 correct suspend/resume and shutdown ordering between parent and child,
 such a device link only makes sense if a driver presence dependency is
 needed on top of that.  In this case driver authors should weigh
 carefully if a device link is at all the right tool for the purpose.
 A more suitable approach might be to simply use deferred probing or
 add a device flag causing the parent driver to be probed before the
 child one.
 
 State machine
 =============
 
 .. kernel-doc:: include/linux/device.h
    :functions: device_link_state
 
 ::
 
                  .=============================.
                  |                             |
                  v                             |
  DORMANT <=> AVAILABLE <=> CONSUMER_PROBE => ACTIVE
     ^                                          |
     |                                          |
     '============ SUPPLIER_UNBIND <============'
 
 * The initial state of a device link is automatically determined by
   :c:func:`device_link_add()` based on the driver presence on the supplier
   and consumer.  If the link is created before any devices are probed, it
   is set to ``DL_STATE_DORMANT``.
 
 * When a supplier device is bound to a driver, links to its consumers
   progress to ``DL_STATE_AVAILABLE``.
   (Call to :c:func:`device_links_driver_bound()` from
   :c:func:`driver_bound()`.)
 
 * Before a consumer device is probed, presence of supplier drivers is
   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()`.)
   This prevents the supplier from unbinding.
   (Call to :c:func:`wait_for_device_probe()` from
   :c:func:`device_links_unbind_consumers()`.)
 
 * If the probe fails, links to suppliers revert back to ``DL_STATE_AVAILABLE``.
   (Call to :c:func:`device_links_no_driver()` from :c:func:`really_probe()`.)
 
 * If the probe succeeds, links to suppliers progress to ``DL_STATE_ACTIVE``.
   (Call to :c:func:`device_links_driver_bound()` from :c:func:`driver_bound()`.)
 
 * When the consumer's driver is later on removed, links to suppliers revert
   back to ``DL_STATE_AVAILABLE``.
   (Call to :c:func:`__device_links_no_driver()` from
   :c:func:`device_links_driver_cleanup()`, which in turn is called from
   :c:func:`__device_release_driver()`.)
 
 * Before a supplier's driver is removed, links to consumers that are not
   bound to a driver are updated to ``DL_STATE_SUPPLIER_UNBIND``.
   (Call to :c:func:`device_links_busy()` from
   :c:func:`__device_release_driver()`.)
   This prevents the consumers from binding.
   (Call to :c:func:`device_links_check_suppliers()` from
   :c:func:`really_probe()`.)
   Consumers that are bound are freed from their driver; consumers that are
   probing are waited for until they are done.
   (Call to :c:func:`device_links_unbind_consumers()` from
   :c:func:`__device_release_driver()`.)
   Once all links to consumers are in ``DL_STATE_SUPPLIER_UNBIND`` state,
   the supplier driver is released and the links revert to ``DL_STATE_DORMANT``.
   (Call to :c:func:`device_links_driver_cleanup()` from
   :c:func:`__device_release_driver()`.)
 
 API
 ===
 
 See device_link_add(), device_link_del() and device_link_remove().

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