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