Documentation/core-api/device_link: Add initial documentation
Document device links as introduced in v4.10 with commits:
4bdb35506b ("driver core: Add a wrapper around
__device_release_driver()")
9ed9895370 ("driver core: Functional dependencies tracking
support")
8c73b42884 ("PM / sleep: Make async suspend/resume of devices use
device links")
21d5c57b37 ("PM / runtime: Use device links")
baa8809f60 ("PM / runtime: Optimize the use of device links")
Signed-off-by: Lukas Wunner <lukas@wunner.de>
[ jc: Moved from core-api to driver-api ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Lukas Wunner
Jonathan Corbet
parent
2ba90ccca7
commit
aad800403a
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@ -0,0 +1,279 @@
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============
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Device links
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============
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By default, the driver core only enforces dependencies between devices
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that are borne out of a parent/child relationship within the device
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hierarchy: When suspending, resuming or shutting down the system, devices
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are ordered based on this relationship, i.e. children are always suspended
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before their parent, and the parent is always resumed before its children.
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Sometimes there is a need to represent device dependencies beyond the
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mere parent/child relationship, e.g. between siblings, and have the
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driver core automatically take care of them.
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Secondly, the driver core by default does not enforce any driver presence
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dependencies, i.e. that one device must be bound to a driver before
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another one can probe or function correctly.
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Often these two dependency types come together, so a device depends on
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another one both with regards to driver presence *and* with regards to
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suspend/resume and shutdown ordering.
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Device links allow representation of such dependencies in the driver core.
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In its standard form, a device link combines *both* dependency types:
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It guarantees correct suspend/resume and shutdown ordering between a
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"supplier" device and its "consumer" devices, and it guarantees driver
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presence on the supplier. The consumer devices are not probed before the
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supplier is bound to a driver, and they're unbound before the supplier
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is unbound.
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When driver presence on the supplier is irrelevant and only correct
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suspend/resume and shutdown ordering is needed, the device link may
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simply be set up with the ``DL_FLAG_STATELESS`` flag. In other words,
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enforcing driver presence on the supplier is optional.
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Another optional feature is runtime PM integration: By setting the
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``DL_FLAG_PM_RUNTIME`` flag on addition of the device link, the PM core
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is instructed to runtime resume the supplier and keep it active
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whenever and for as long as the consumer is runtime resumed.
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Usage
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=====
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The earliest point in time when device links can be added is after
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:c:func:`device_add()` has been called for the supplier and
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:c:func:`device_initialize()` has been called for the consumer.
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It is legal to add them later, but care must be taken that the system
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remains in a consistent state: E.g. a device link cannot be added in
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the midst of a suspend/resume transition, so either commencement of
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such a transition needs to be prevented with :c:func:`lock_system_sleep()`,
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or the device link needs to be added from a function which is guaranteed
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not to run in parallel to a suspend/resume transition, such as from a
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device ``->probe`` callback or a boot-time PCI quirk.
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Another example for an inconsistent state would be a device link that
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represents a driver presence dependency, yet is added from the consumer's
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``->probe`` callback while the supplier hasn't probed yet: Had the driver
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core known about the device link earlier, it wouldn't have probed the
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consumer in the first place. The onus is thus on the consumer to check
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presence of the supplier after adding the link, and defer probing on
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non-presence.
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If a device link is added in the ``->probe`` callback of the supplier or
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consumer driver, it is typically deleted in its ``->remove`` callback for
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symmetry. That way, if the driver is compiled as a module, the device
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link is added on module load and orderly deleted on unload. The same
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restrictions that apply to device link addition (e.g. exclusion of a
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parallel suspend/resume transition) apply equally to deletion.
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Several flags may be specified on device link addition, two of which
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have already been mentioned above: ``DL_FLAG_STATELESS`` to express that no
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driver presence dependency is needed (but only correct suspend/resume and
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shutdown ordering) and ``DL_FLAG_PM_RUNTIME`` to express that runtime PM
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integration is desired.
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Two other flags are specifically targeted at use cases where the device
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link is added from the consumer's ``->probe`` callback: ``DL_FLAG_RPM_ACTIVE``
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can be specified to runtime resume the supplier upon addition of the
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device link. ``DL_FLAG_AUTOREMOVE`` causes the device link to be automatically
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purged when the consumer fails to probe or later unbinds. This obviates
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the need to explicitly delete the link in the ``->remove`` callback or in
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the error path of the ``->probe`` callback.
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Limitations
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===========
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Driver authors should be aware that a driver presence dependency (i.e. when
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``DL_FLAG_STATELESS`` is not specified on link addition) may cause probing of
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the consumer to be deferred indefinitely. This can become a problem if the
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consumer is required to probe before a certain initcall level is reached.
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Worse, if the supplier driver is blacklisted or missing, the consumer will
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never be probed.
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Sometimes drivers depend on optional resources. They are able to operate
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in a degraded mode (reduced feature set or performance) when those resources
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are not present. An example is an SPI controller that can use a DMA engine
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or work in PIO mode. The controller can determine presence of the optional
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resources at probe time but on non-presence there is no way to know whether
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they will become available in the near future (due to a supplier driver
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probing) or never. Consequently it cannot be determined whether to defer
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probing or not. It would be possible to notify drivers when optional
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resources become available after probing, but it would come at a high cost
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for drivers as switching between modes of operation at runtime based on the
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availability of such resources would be much more complex than a mechanism
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based on probe deferral. In any case optional resources are beyond the
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scope of device links.
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Examples
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========
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* An MMU device exists alongside a busmaster device, both are in the same
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power domain. The MMU implements DMA address translation for the busmaster
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device and shall be runtime resumed and kept active whenever and as long
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as the busmaster device is active. The busmaster device's driver shall
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not bind before the MMU is bound. To achieve this, a device link with
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runtime PM integration is added from the busmaster device (consumer)
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to the MMU device (supplier). The effect with regards to runtime PM
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is the same as if the MMU was the parent of the master device.
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The fact that both devices share the same power domain would normally
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suggest usage of a :c:type:`struct dev_pm_domain` or :c:type:`struct
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generic_pm_domain`, however these are not independent devices that
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happen to share a power switch, but rather the MMU device serves the
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busmaster device and is useless without it. A device link creates a
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synthetic hierarchical relationship between the devices and is thus
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more apt.
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* A Thunderbolt host controller comprises a number of PCIe hotplug ports
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and an NHI device to manage the PCIe switch. On resume from system sleep,
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the NHI device needs to re-establish PCI tunnels to attached devices
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before the hotplug ports can resume. If the hotplug ports were children
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of the NHI, this resume order would automatically be enforced by the
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PM core, but unfortunately they're aunts. The solution is to add
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device links from the hotplug ports (consumers) to the NHI device
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(supplier). A driver presence dependency is not necessary for this
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use case.
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* Discrete GPUs in hybrid graphics laptops often feature an HDA controller
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for HDMI/DP audio. In the device hierarchy the HDA controller is a sibling
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of the VGA device, yet both share the same power domain and the HDA
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controller is only ever needed when an HDMI/DP display is attached to the
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VGA device. A device link from the HDA controller (consumer) to the
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VGA device (supplier) aptly represents this relationship.
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* ACPI allows definition of a device start order by way of _DEP objects.
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A classical example is when ACPI power management methods on one device
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are implemented in terms of I\ :sup:`2`\ C accesses and require a specific
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I\ :sup:`2`\ C controller to be present and functional for the power
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management of the device in question to work.
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* In some SoCs a functional dependency exists from display, video codec and
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video processing IP cores on transparent memory access IP cores that handle
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burst access and compression/decompression.
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Alternatives
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============
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* A :c:type:`struct dev_pm_domain` can be used to override the bus,
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class or device type callbacks. It is intended for devices sharing
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a single on/off switch, however it does not guarantee a specific
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suspend/resume ordering, this needs to be implemented separately.
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It also does not by itself track the runtime PM status of the involved
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devices and turn off the power switch only when all of them are runtime
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suspended. Furthermore it cannot be used to enforce a specific shutdown
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ordering or a driver presence dependency.
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* A :c:type:`struct generic_pm_domain` is a lot more heavyweight than a
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device link and does not allow for shutdown ordering or driver presence
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dependencies. It also cannot be used on ACPI systems.
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Implementation
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==============
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The device hierarchy, which -- as the name implies -- is a tree,
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becomes a directed acyclic graph once device links are added.
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Ordering of these devices during suspend/resume is determined by the
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dpm_list. During shutdown it is determined by the devices_kset. With
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no device links present, the two lists are a flattened, one-dimensional
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representations of the device tree such that a device is placed behind
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all its ancestors. That is achieved by traversing the ACPI namespace
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or OpenFirmware device tree top-down and appending devices to the lists
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as they are discovered.
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Once device links are added, the lists need to satisfy the additional
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constraint that a device is placed behind all its suppliers, recursively.
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To ensure this, upon addition of the device link the consumer and the
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entire sub-graph below it (all children and consumers of the consumer)
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are moved to the end of the list. (Call to :c:func:`device_reorder_to_tail()`
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from :c:func:`device_link_add()`.)
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To prevent introduction of dependency loops into the graph, it is
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verified upon device link addition that the supplier is not dependent
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on the consumer or any children or consumers of the consumer.
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(Call to :c:func:`device_is_dependent()` from :c:func:`device_link_add()`.)
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If that constraint is violated, :c:func:`device_link_add()` will return
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``NULL`` and a ``WARNING`` will be logged.
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Notably this also prevents the addition of a device link from a parent
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device to a child. However the converse is allowed, i.e. a device link
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from a child to a parent. Since the driver core already guarantees
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correct suspend/resume and shutdown ordering between parent and child,
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such a device link only makes sense if a driver presence dependency is
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needed on top of that. In this case driver authors should weigh
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carefully if a device link is at all the right tool for the purpose.
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A more suitable approach might be to simply use deferred probing or
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add a device flag causing the parent driver to be probed before the
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child one.
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State machine
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=============
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.. kernel-doc:: include/linux/device.h
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:functions: device_link_state
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::
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.=============================.
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v |
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DORMANT <=> AVAILABLE <=> CONSUMER_PROBE => ACTIVE
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^ |
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'============ SUPPLIER_UNBIND <============'
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* The initial state of a device link is automatically determined by
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:c:func:`device_link_add()` based on the driver presence on the supplier
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and consumer. If the link is created before any devices are probed, it
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is set to ``DL_STATE_DORMANT``.
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* When a supplier device is bound to a driver, links to its consumers
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progress to ``DL_STATE_AVAILABLE``.
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(Call to :c:func:`device_links_driver_bound()` from
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:c:func:`driver_bound()`.)
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* Before a consumer device is probed, presence of supplier drivers is
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verified by checking that links to suppliers are in ``DL_STATE_AVAILABLE``
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state. The state of the links is updated to ``DL_STATE_CONSUMER_PROBE``.
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(Call to :c:func:`device_links_check_suppliers()` from
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:c:func:`really_probe()`.)
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This prevents the supplier from unbinding.
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(Call to :c:func:`wait_for_device_probe()` from
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:c:func:`device_links_unbind_consumers()`.)
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* If the probe fails, links to suppliers revert back to ``DL_STATE_AVAILABLE``.
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(Call to :c:func:`device_links_no_driver()` from :c:func:`really_probe()`.)
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* If the probe succeeds, links to suppliers progress to ``DL_STATE_ACTIVE``.
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(Call to :c:func:`device_links_driver_bound()` from :c:func:`driver_bound()`.)
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* When the consumer's driver is later on removed, links to suppliers revert
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back to ``DL_STATE_AVAILABLE``.
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(Call to :c:func:`__device_links_no_driver()` from
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:c:func:`device_links_driver_cleanup()`, which in turn is called from
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:c:func:`__device_release_driver()`.)
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* Before a supplier's driver is removed, links to consumers that are not
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bound to a driver are updated to ``DL_STATE_SUPPLIER_UNBIND``.
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(Call to :c:func:`device_links_busy()` from
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:c:func:`__device_release_driver()`.)
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This prevents the consumers from binding.
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(Call to :c:func:`device_links_check_suppliers()` from
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:c:func:`really_probe()`.)
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Consumers that are bound are freed from their driver; consumers that are
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probing are waited for until they are done.
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(Call to :c:func:`device_links_unbind_consumers()` from
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:c:func:`__device_release_driver()`.)
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Once all links to consumers are in ``DL_STATE_SUPPLIER_UNBIND`` state,
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the supplier driver is released and the links revert to ``DL_STATE_DORMANT``.
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(Call to :c:func:`device_links_driver_cleanup()` from
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:c:func:`__device_release_driver()`.)
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API
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===
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.. kernel-doc:: drivers/base/core.c
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:functions: device_link_add device_link_del
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@ -16,6 +16,7 @@ available subsections can be seen below.
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basics
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infrastructure
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device_link
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message-based
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sound
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frame-buffer
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