| 1 | .. _device_link:
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| 2 |
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| 3 | ============
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| 4 | Device links
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| 5 | ============
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| 6 |
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| 7 | By default, the driver core only enforces dependencies between devices
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| 8 | that are borne out of a parent/child relationship within the device
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| 9 | hierarchy: When suspending, resuming or shutting down the system, devices
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| 10 | are ordered based on this relationship, i.e. children are always suspended
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| 11 | before their parent, and the parent is always resumed before its children.
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| 12 |
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| 13 | Sometimes there is a need to represent device dependencies beyond the
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| 14 | mere parent/child relationship, e.g. between siblings, and have the
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| 15 | driver core automatically take care of them.
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| 16 |
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| 17 | Secondly, the driver core by default does not enforce any driver presence
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| 18 | dependencies, i.e. that one device must be bound to a driver before
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| 19 | another one can probe or function correctly.
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| 20 |
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| 21 | Often these two dependency types come together, so a device depends on
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| 22 | another one both with regards to driver presence *and* with regards to
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| 23 | suspend/resume and shutdown ordering.
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| 24 |
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| 25 | Device links allow representation of such dependencies in the driver core.
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| 26 |
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| 27 | In its standard or *managed* form, a device link combines *both* dependency
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| 28 | types: It guarantees correct suspend/resume and shutdown ordering between a
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| 29 | "supplier" device and its "consumer" devices, and it guarantees driver
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| 30 | presence on the supplier. The consumer devices are not probed before the
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| 31 | supplier is bound to a driver, and they're unbound before the supplier
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| 32 | is unbound.
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| 33 |
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| 34 | When driver presence on the supplier is irrelevant and only correct
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| 35 | suspend/resume and shutdown ordering is needed, the device link may
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| 36 | simply be set up with the ``DL_FLAG_STATELESS`` flag. In other words,
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| 37 | enforcing driver presence on the supplier is optional.
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| 38 |
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| 39 | Another optional feature is runtime PM integration: By setting the
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| 40 | ``DL_FLAG_PM_RUNTIME`` flag on addition of the device link, the PM core
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| 41 | is instructed to runtime resume the supplier and keep it active
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| 42 | whenever and for as long as the consumer is runtime resumed.
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| 43 |
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| 44 | Usage
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| 45 | =====
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| 46 |
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| 47 | The earliest point in time when device links can be added is after
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| 48 | :c:func:`device_add()` has been called for the supplier and
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| 49 | :c:func:`device_initialize()` has been called for the consumer.
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| 50 |
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| 51 | It is legal to add them later, but care must be taken that the system
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| 52 | remains in a consistent state: E.g. a device link cannot be added in
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| 53 | the midst of a suspend/resume transition, so either commencement of
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| 54 | such a transition needs to be prevented with :c:func:`lock_system_sleep()`,
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| 55 | or the device link needs to be added from a function which is guaranteed
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| 56 | not to run in parallel to a suspend/resume transition, such as from a
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| 57 | device ``->probe`` callback or a boot-time PCI quirk.
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| 58 |
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| 59 | Another example for an inconsistent state would be a device link that
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| 60 | represents a driver presence dependency, yet is added from the consumer's
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| 61 | ``->probe`` callback while the supplier hasn't started to probe yet: Had the
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| 62 | driver core known about the device link earlier, it wouldn't have probed the
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| 63 | consumer in the first place. The onus is thus on the consumer to check
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| 64 | presence of the supplier after adding the link, and defer probing on
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| 65 | non-presence. [Note that it is valid to create a link from the consumer's
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| 66 | ``->probe`` callback while the supplier is still probing, but the consumer must
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| 67 | know that the supplier is functional already at the link creation time (that is
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| 68 | the case, for instance, if the consumer has just acquired some resources that
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| 69 | would not have been available had the supplier not been functional then).]
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| 70 |
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| 71 | If a device link with ``DL_FLAG_STATELESS`` set (i.e. a stateless device link)
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| 72 | is added in the ``->probe`` callback of the supplier or consumer driver, it is
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| 73 | typically deleted in its ``->remove`` callback for symmetry. That way, if the
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| 74 | driver is compiled as a module, the device link is added on module load and
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| 75 | orderly deleted on unload. The same restrictions that apply to device link
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| 76 | addition (e.g. exclusion of a parallel suspend/resume transition) apply equally
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| 77 | to deletion. Device links managed by the driver core are deleted automatically
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| 78 | by it.
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| 79 |
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| 80 | Several flags may be specified on device link addition, two of which
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| 81 | have already been mentioned above: ``DL_FLAG_STATELESS`` to express that no
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| 82 | driver presence dependency is needed (but only correct suspend/resume and
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| 83 | shutdown ordering) and ``DL_FLAG_PM_RUNTIME`` to express that runtime PM
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| 84 | integration is desired.
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| 85 |
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| 86 | Two other flags are specifically targeted at use cases where the device
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| 87 | link is added from the consumer's ``->probe`` callback: ``DL_FLAG_RPM_ACTIVE``
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| 88 | can be specified to runtime resume the supplier and prevent it from suspending
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| 89 | before the consumer is runtime suspended. ``DL_FLAG_AUTOREMOVE_CONSUMER``
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| 90 | causes the device link to be automatically purged when the consumer fails to
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| 91 | probe or later unbinds.
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| 92 |
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| 93 | Similarly, when the device link is added from supplier's ``->probe`` callback,
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| 94 | ``DL_FLAG_AUTOREMOVE_SUPPLIER`` causes the device link to be automatically
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| 95 | purged when the supplier fails to probe or later unbinds.
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| 96 |
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| 97 | If neither ``DL_FLAG_AUTOREMOVE_CONSUMER`` nor ``DL_FLAG_AUTOREMOVE_SUPPLIER``
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| 98 | is set, ``DL_FLAG_AUTOPROBE_CONSUMER`` can be used to request the driver core
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| 99 | to probe for a driver for the consumer driver on the link automatically after
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| 100 | a driver has been bound to the supplier device.
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| 101 |
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| 102 | Note, however, that any combinations of ``DL_FLAG_AUTOREMOVE_CONSUMER``,
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| 103 | ``DL_FLAG_AUTOREMOVE_SUPPLIER`` or ``DL_FLAG_AUTOPROBE_CONSUMER`` with
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| 104 | ``DL_FLAG_STATELESS`` are invalid and cannot be used.
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| 105 |
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| 106 | Limitations
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| 107 | ===========
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| 108 |
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| 109 | Driver authors should be aware that a driver presence dependency for managed
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| 110 | device links (i.e. when ``DL_FLAG_STATELESS`` is not specified on link addition)
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| 111 | may cause probing of the consumer to be deferred indefinitely. This can become
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| 112 | a problem if the consumer is required to probe before a certain initcall level
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| 113 | is reached. Worse, if the supplier driver is blacklisted or missing, the
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| 114 | consumer will never be probed.
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| 115 |
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| 116 | Moreover, managed device links cannot be deleted directly. They are deleted
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| 117 | by the driver core when they are not necessary any more in accordance with the
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| 118 | ``DL_FLAG_AUTOREMOVE_CONSUMER`` and ``DL_FLAG_AUTOREMOVE_SUPPLIER`` flags.
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| 119 | However, stateless device links (i.e. device links with ``DL_FLAG_STATELESS``
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| 120 | set) are expected to be removed by whoever called :c:func:`device_link_add()`
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| 121 | to add them with the help of either :c:func:`device_link_del()` or
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| 122 | :c:func:`device_link_remove()`.
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| 123 |
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| 124 | Passing ``DL_FLAG_RPM_ACTIVE`` along with ``DL_FLAG_STATELESS`` to
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| 125 | :c:func:`device_link_add()` may cause the PM-runtime usage counter of the
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| 126 | supplier device to remain nonzero after a subsequent invocation of either
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| 127 | :c:func:`device_link_del()` or :c:func:`device_link_remove()` to remove the
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| 128 | device link returned by it. This happens if :c:func:`device_link_add()` is
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| 129 | called twice in a row for the same consumer-supplier pair without removing the
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| 130 | link between these calls, in which case allowing the PM-runtime usage counter
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| 131 | of the supplier to drop on an attempt to remove the link may cause it to be
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| 132 | suspended while the consumer is still PM-runtime-active and that has to be
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| 133 | avoided. [To work around this limitation it is sufficient to let the consumer
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| 134 | runtime suspend at least once, or call :c:func:`pm_runtime_set_suspended()` for
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| 135 | it with PM-runtime disabled, between the :c:func:`device_link_add()` and
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| 136 | :c:func:`device_link_del()` or :c:func:`device_link_remove()` calls.]
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| 137 |
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| 138 | Sometimes drivers depend on optional resources. They are able to operate
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| 139 | in a degraded mode (reduced feature set or performance) when those resources
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| 140 | are not present. An example is an SPI controller that can use a DMA engine
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| 141 | or work in PIO mode. The controller can determine presence of the optional
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| 142 | resources at probe time but on non-presence there is no way to know whether
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| 143 | they will become available in the near future (due to a supplier driver
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| 144 | probing) or never. Consequently it cannot be determined whether to defer
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| 145 | probing or not. It would be possible to notify drivers when optional
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| 146 | resources become available after probing, but it would come at a high cost
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| 147 | for drivers as switching between modes of operation at runtime based on the
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| 148 | availability of such resources would be much more complex than a mechanism
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| 149 | based on probe deferral. In any case optional resources are beyond the
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| 150 | scope of device links.
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| 151 |
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| 152 | Examples
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| 153 | ========
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| 154 |
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| 155 | * An MMU device exists alongside a busmaster device, both are in the same
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| 156 | power domain. The MMU implements DMA address translation for the busmaster
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| 157 | device and shall be runtime resumed and kept active whenever and as long
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| 158 | as the busmaster device is active. The busmaster device's driver shall
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| 159 | not bind before the MMU is bound. To achieve this, a device link with
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| 160 | runtime PM integration is added from the busmaster device (consumer)
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| 161 | to the MMU device (supplier). The effect with regards to runtime PM
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| 162 | is the same as if the MMU was the parent of the master device.
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| 163 |
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| 164 | The fact that both devices share the same power domain would normally
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| 165 | suggest usage of a struct dev_pm_domain or struct generic_pm_domain,
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| 166 | however these are not independent devices that happen to share a power
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| 167 | switch, but rather the MMU device serves the busmaster device and is
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| 168 | useless without it. A device link creates a synthetic hierarchical
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| 169 | relationship between the devices and is thus more apt.
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| 170 |
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| 171 | * A Thunderbolt host controller comprises a number of PCIe hotplug ports
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| 172 | and an NHI device to manage the PCIe switch. On resume from system sleep,
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| 173 | the NHI device needs to re-establish PCI tunnels to attached devices
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| 174 | before the hotplug ports can resume. If the hotplug ports were children
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| 175 | of the NHI, this resume order would automatically be enforced by the
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| 176 | PM core, but unfortunately they're aunts. The solution is to add
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| 177 | device links from the hotplug ports (consumers) to the NHI device
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| 178 | (supplier). A driver presence dependency is not necessary for this
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| 179 | use case.
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| 180 |
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| 181 | * Discrete GPUs in hybrid graphics laptops often feature an HDA controller
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| 182 | for HDMI/DP audio. In the device hierarchy the HDA controller is a sibling
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| 183 | of the VGA device, yet both share the same power domain and the HDA
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| 184 | controller is only ever needed when an HDMI/DP display is attached to the
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| 185 | VGA device. A device link from the HDA controller (consumer) to the
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| 186 | VGA device (supplier) aptly represents this relationship.
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| 187 |
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| 188 | * ACPI allows definition of a device start order by way of _DEP objects.
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| 189 | A classical example is when ACPI power management methods on one device
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| 190 | are implemented in terms of I\ :sup:`2`\ C accesses and require a specific
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| 191 | I\ :sup:`2`\ C controller to be present and functional for the power
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| 192 | management of the device in question to work.
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| 193 |
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| 194 | * In some SoCs a functional dependency exists from display, video codec and
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| 195 | video processing IP cores on transparent memory access IP cores that handle
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| 196 | burst access and compression/decompression.
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| 197 |
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| 198 | Alternatives
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| 199 | ============
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| 200 |
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| 201 | * A struct dev_pm_domain can be used to override the bus,
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| 202 | class or device type callbacks. It is intended for devices sharing
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| 203 | a single on/off switch, however it does not guarantee a specific
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| 204 | suspend/resume ordering, this needs to be implemented separately.
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| 205 | It also does not by itself track the runtime PM status of the involved
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| 206 | devices and turn off the power switch only when all of them are runtime
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| 207 | suspended. Furthermore it cannot be used to enforce a specific shutdown
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| 208 | ordering or a driver presence dependency.
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| 209 |
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| 210 | * A struct generic_pm_domain is a lot more heavyweight than a
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| 211 | device link and does not allow for shutdown ordering or driver presence
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| 212 | dependencies. It also cannot be used on ACPI systems.
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| 213 |
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| 214 | Implementation
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| 215 | ==============
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| 216 |
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| 217 | The device hierarchy, which -- as the name implies -- is a tree,
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| 218 | becomes a directed acyclic graph once device links are added.
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| 219 |
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| 220 | Ordering of these devices during suspend/resume is determined by the
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| 221 | dpm_list. During shutdown it is determined by the devices_kset. With
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| 222 | no device links present, the two lists are a flattened, one-dimensional
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| 223 | representations of the device tree such that a device is placed behind
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| 224 | all its ancestors. That is achieved by traversing the ACPI namespace
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| 225 | or OpenFirmware device tree top-down and appending devices to the lists
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| 226 | as they are discovered.
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| 227 |
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| 228 | Once device links are added, the lists need to satisfy the additional
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| 229 | constraint that a device is placed behind all its suppliers, recursively.
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| 230 | To ensure this, upon addition of the device link the consumer and the
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| 231 | entire sub-graph below it (all children and consumers of the consumer)
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| 232 | are moved to the end of the list. (Call to :c:func:`device_reorder_to_tail()`
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| 233 | from :c:func:`device_link_add()`.)
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| 234 |
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| 235 | To prevent introduction of dependency loops into the graph, it is
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| 236 | verified upon device link addition that the supplier is not dependent
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| 237 | on the consumer or any children or consumers of the consumer.
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| 238 | (Call to :c:func:`device_is_dependent()` from :c:func:`device_link_add()`.)
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| 239 | If that constraint is violated, :c:func:`device_link_add()` will return
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| 240 | ``NULL`` and a ``WARNING`` will be logged.
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| 241 |
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| 242 | Notably this also prevents the addition of a device link from a parent
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| 243 | device to a child. However the converse is allowed, i.e. a device link
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| 244 | from a child to a parent. Since the driver core already guarantees
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| 245 | correct suspend/resume and shutdown ordering between parent and child,
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| 246 | such a device link only makes sense if a driver presence dependency is
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| 247 | needed on top of that. In this case driver authors should weigh
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| 248 | carefully if a device link is at all the right tool for the purpose.
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| 249 | A more suitable approach might be to simply use deferred probing or
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| 250 | add a device flag causing the parent driver to be probed before the
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| 251 | child one.
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| 252 |
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| 253 | State machine
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| 254 | =============
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| 255 |
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| 256 | .. kernel-doc:: include/linux/device.h
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| 257 | :functions: device_link_state
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| 258 |
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| 259 | ::
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| 260 |
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| 261 | .=============================.
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| 262 | | |
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| 263 | v |
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| 264 | DORMANT <=> AVAILABLE <=> CONSUMER_PROBE => ACTIVE
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| 265 | ^ |
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| 266 | | |
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| 267 | '============ SUPPLIER_UNBIND <============'
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| 268 |
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| 269 | * The initial state of a device link is automatically determined by
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| 270 | :c:func:`device_link_add()` based on the driver presence on the supplier
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| 271 | and consumer. If the link is created before any devices are probed, it
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| 272 | is set to ``DL_STATE_DORMANT``.
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| 273 |
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| 274 | * When a supplier device is bound to a driver, links to its consumers
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| 275 | progress to ``DL_STATE_AVAILABLE``.
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| 276 | (Call to :c:func:`device_links_driver_bound()` from
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| 277 | :c:func:`driver_bound()`.)
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| 278 |
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| 279 | * Before a consumer device is probed, presence of supplier drivers is
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| 280 | verified by checking the consumer device is not in the wait_for_suppliers
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| 281 | list and by checking that links to suppliers are in ``DL_STATE_AVAILABLE``
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| 282 | state. The state of the links is updated to ``DL_STATE_CONSUMER_PROBE``.
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| 283 | (Call to :c:func:`device_links_check_suppliers()` from
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| 284 | :c:func:`really_probe()`.)
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| 285 | This prevents the supplier from unbinding.
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| 286 | (Call to :c:func:`wait_for_device_probe()` from
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| 287 | :c:func:`device_links_unbind_consumers()`.)
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| 288 |
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| 289 | * If the probe fails, links to suppliers revert back to ``DL_STATE_AVAILABLE``.
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| 290 | (Call to :c:func:`device_links_no_driver()` from :c:func:`really_probe()`.)
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| 291 |
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| 292 | * If the probe succeeds, links to suppliers progress to ``DL_STATE_ACTIVE``.
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| 293 | (Call to :c:func:`device_links_driver_bound()` from :c:func:`driver_bound()`.)
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| 294 |
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| 295 | * When the consumer's driver is later on removed, links to suppliers revert
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| 296 | back to ``DL_STATE_AVAILABLE``.
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| 297 | (Call to :c:func:`__device_links_no_driver()` from
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| 298 | :c:func:`device_links_driver_cleanup()`, which in turn is called from
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| 299 | :c:func:`__device_release_driver()`.)
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| 300 |
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| 301 | * Before a supplier's driver is removed, links to consumers that are not
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| 302 | bound to a driver are updated to ``DL_STATE_SUPPLIER_UNBIND``.
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| 303 | (Call to :c:func:`device_links_busy()` from
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| 304 | :c:func:`__device_release_driver()`.)
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| 305 | This prevents the consumers from binding.
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| 306 | (Call to :c:func:`device_links_check_suppliers()` from
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| 307 | :c:func:`really_probe()`.)
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| 308 | Consumers that are bound are freed from their driver; consumers that are
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| 309 | probing are waited for until they are done.
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| 310 | (Call to :c:func:`device_links_unbind_consumers()` from
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| 311 | :c:func:`__device_release_driver()`.)
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| 312 | Once all links to consumers are in ``DL_STATE_SUPPLIER_UNBIND`` state,
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| 313 | the supplier driver is released and the links revert to ``DL_STATE_DORMANT``.
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| 314 | (Call to :c:func:`device_links_driver_cleanup()` from
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| 315 | :c:func:`__device_release_driver()`.)
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| 316 |
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| 317 | API
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| 318 | ===
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| 319 |
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| 320 | See device_link_add(), device_link_del() and device_link_remove().
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