diff options
Diffstat (limited to 'Documentation/driver-api/device_link.rst')
| -rw-r--r-- | Documentation/driver-api/device_link.rst | 42 |
1 files changed, 25 insertions, 17 deletions
diff --git a/Documentation/driver-api/device_link.rst b/Documentation/driver-api/device_link.rst index 5c7178189612..e249e074a8d2 100644 --- a/Documentation/driver-api/device_link.rst +++ b/Documentation/driver-api/device_link.rst @@ -25,8 +25,8 @@ suspend/resume and shutdown ordering. Device links allow representation of such dependencies in the driver core. -In its standard form, a device link combines *both* dependency types: -It guarantees correct suspend/resume and shutdown ordering between a +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 @@ -69,12 +69,14 @@ 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 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. +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 with ``DL_FLAG_STATELESS`` unset (i.e. managed +device links) are deleted automatically by the driver core. Several flags may be specified on device link addition, two of which have already been mentioned above: ``DL_FLAG_STATELESS`` to express that no @@ -87,8 +89,6 @@ link is added from the consumer's ``->probe`` callback: ``DL_FLAG_RPM_ACTIVE`` can be specified to runtime resume the supplier upon addition of the device link. ``DL_FLAG_AUTOREMOVE_CONSUMER`` causes the device link to be automatically purged when the consumer fails to probe or later unbinds. -This obviates the need to explicitly delete the link in the ``->remove`` -callback or in the error path of the ``->probe`` callback. Similarly, when the device link is added from supplier's ``->probe`` callback, ``DL_FLAG_AUTOREMOVE_SUPPLIER`` causes the device link to be automatically @@ -97,12 +97,20 @@ purged when the supplier fails to probe or later unbinds. Limitations =========== -Driver authors should be aware that a driver presence dependency (i.e. when -``DL_FLAG_STATELESS`` is not specified on link addition) may cause probing of -the consumer to be deferred indefinitely. This can become a problem if the -consumer is required to probe before a certain initcall level is reached. -Worse, if the supplier driver is blacklisted or missing, the consumer will -never be probed. +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()`. Sometimes drivers depend on optional resources. They are able to operate in a degraded mode (reduced feature set or performance) when those resources @@ -286,4 +294,4 @@ API === .. kernel-doc:: drivers/base/core.c - :functions: device_link_add device_link_del + :functions: device_link_add device_link_del device_link_remove |