driver core: Document limitation related to DL_FLAG_RPM_ACTIVE
If device_link_add() is called twice in a row to create a stateless device link for the same consumer-supplier pair without an attempt to delete the link between these calls, and the second caller passes DL_FLAG_RPM_ACTIVE to it in flags, calling either device_link_del() or device_link_remove() immediately after that will leave the link's supplier device with nonzero PM-runtime usage counter, which may prevent the supplier from being runtime-suspended going forward until the link is deleted by another invocation of device_link_del() or device_link_remove() for it. Even though this is confusing and may lead to subtle issues, trying to avoid it in the framework also may cause problems to appear, so document it as a known limitation. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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@ -86,9 +86,10 @@ integration is desired.
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Two other flags are specifically targeted at use cases where the device
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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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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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can be specified to runtime resume the supplier and prevent it from suspending
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device link. ``DL_FLAG_AUTOREMOVE_CONSUMER`` causes the device link to be
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before the consumer is runtime suspended. ``DL_FLAG_AUTOREMOVE_CONSUMER``
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automatically purged when the consumer fails to probe or later unbinds.
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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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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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``DL_FLAG_AUTOREMOVE_SUPPLIER`` causes the device link to be automatically
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@ -121,6 +122,20 @@ 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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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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: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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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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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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are not present. An example is an SPI controller that can use a DMA engine
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