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320 lines
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320 lines
16 KiB
.. _device_link: |
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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 or *managed* form, a device link combines *both* dependency |
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types: 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 started to probe yet: Had the |
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driver 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. [Note that it is valid to create a link from the consumer's |
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``->probe`` callback while the supplier is still probing, but the consumer must |
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know that the supplier is functional already at the link creation time (that is |
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the case, for instance, if the consumer has just acquired some resources that |
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would not have been available had the supplier not been functional then).] |
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If a device link with ``DL_FLAG_STATELESS`` set (i.e. a stateless device link) |
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is added in the ``->probe`` callback of the supplier or consumer driver, it is |
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typically deleted in its ``->remove`` callback for symmetry. That way, if the |
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driver is compiled as a module, the device link is added on module load and |
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orderly deleted on unload. The same restrictions that apply to device link |
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addition (e.g. exclusion of a parallel suspend/resume transition) apply equally |
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to deletion. Device links managed by the driver core are deleted automatically |
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by it. |
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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 and prevent it from suspending |
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before the consumer is runtime suspended. ``DL_FLAG_AUTOREMOVE_CONSUMER`` |
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causes the device link to be automatically purged when the consumer fails to |
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probe or later unbinds. |
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Similarly, when the device link is added from supplier's ``->probe`` callback, |
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``DL_FLAG_AUTOREMOVE_SUPPLIER`` causes the device link to be automatically |
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purged when the supplier fails to probe or later unbinds. |
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If neither ``DL_FLAG_AUTOREMOVE_CONSUMER`` nor ``DL_FLAG_AUTOREMOVE_SUPPLIER`` |
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is set, ``DL_FLAG_AUTOPROBE_CONSUMER`` can be used to request the driver core |
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to probe for a driver for the consumer driver on the link automatically after |
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a driver has been bound to the supplier device. |
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Note, however, that any combinations of ``DL_FLAG_AUTOREMOVE_CONSUMER``, |
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``DL_FLAG_AUTOREMOVE_SUPPLIER`` or ``DL_FLAG_AUTOPROBE_CONSUMER`` with |
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``DL_FLAG_STATELESS`` are invalid and cannot be used. |
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Limitations |
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=========== |
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Driver authors should be aware that a driver presence dependency for managed |
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device links (i.e. when ``DL_FLAG_STATELESS`` is not specified on link addition) |
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may cause probing of the consumer to be deferred indefinitely. This can become |
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a problem if the consumer is required to probe before a certain initcall level |
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is reached. Worse, if the supplier driver is blacklisted or missing, the |
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consumer will never be probed. |
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Moreover, managed device links cannot be deleted directly. They are deleted |
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by the driver core when they are not necessary any more in accordance with the |
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``DL_FLAG_AUTOREMOVE_CONSUMER`` and ``DL_FLAG_AUTOREMOVE_SUPPLIER`` flags. |
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However, stateless device links (i.e. device links with ``DL_FLAG_STATELESS`` |
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set) are expected to be removed by whoever called :c:func:`device_link_add()` |
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to add them with the help of either :c:func:`device_link_del()` or |
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:c:func:`device_link_remove()`. |
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Passing ``DL_FLAG_RPM_ACTIVE`` along with ``DL_FLAG_STATELESS`` to |
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:c:func:`device_link_add()` may cause the PM-runtime usage counter of the |
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supplier device to remain nonzero after a subsequent invocation of either |
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:c:func:`device_link_del()` or :c:func:`device_link_remove()` to remove the |
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device link returned by it. This happens if :c:func:`device_link_add()` is |
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called twice in a row for the same consumer-supplier pair without removing the |
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link between these calls, in which case allowing the PM-runtime usage counter |
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of the supplier to drop on an attempt to remove the link may cause it to be |
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suspended while the consumer is still PM-runtime-active and that has to be |
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avoided. [To work around this limitation it is sufficient to let the consumer |
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runtime suspend at least once, or call :c:func:`pm_runtime_set_suspended()` for |
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it with PM-runtime disabled, between the :c:func:`device_link_add()` and |
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:c:func:`device_link_del()` or :c:func:`device_link_remove()` calls.] |
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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 struct dev_pm_domain or struct generic_pm_domain, |
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however these are not independent devices that happen to share a power |
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switch, but rather the MMU device serves the busmaster device and is |
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useless without it. A device link creates a synthetic hierarchical |
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relationship between the devices and is thus 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 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 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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| | |
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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 the consumer device is not in the wait_for_suppliers |
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list and 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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See device_link_add(), device_link_del() and device_link_remove().
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