--- /dev/null
+============
+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 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 probed 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.
+
+If a 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.
+
+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 upon addition of the
+device link. ``DL_FLAG_AUTOREMOVE`` causes the device link to be automatically
+purged when the consumer fails to probe or later unbinds. This obviates
+the need to explicitly delete the link in the ``->remove`` callback or in
+the error path of the ``->probe`` callback.
+
+Limitations
+===========
+
+Driver authors should be aware that a driver presence dependency (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.
+
+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 :c:type:`struct dev_pm_domain` or :c:type:`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 :c:type:`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 :c:type:`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 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
+===
+
+.. kernel-doc:: drivers/base/core.c
+ :functions: device_link_add device_link_del
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