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authorRafael J. Wysocki <rafael.j.wysocki@intel.com>2019-02-07 19:41:56 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2019-02-08 11:26:14 +0100
commit70fb9a252317e04bd9af68ed89d4bf6caeb252e4 (patch)
tree8e9024b062cd12f25a6785921e328933994f52bf /Documentation/driver-api
parent4080ab083000a1e9656b0d1607e238e7001e0c84 (diff)
downloadlinux-70fb9a252317e04bd9af68ed89d4bf6caeb252e4.tar.bz2
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>
Diffstat (limited to 'Documentation/driver-api')
-rw-r--r--Documentation/driver-api/device_link.rst21
1 files changed, 18 insertions, 3 deletions
diff --git a/Documentation/driver-api/device_link.rst b/Documentation/driver-api/device_link.rst
index c764755121c7..0effb792a3af 100644
--- a/Documentation/driver-api/device_link.rst
+++ b/Documentation/driver-api/device_link.rst
@@ -86,9 +86,10 @@ 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 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.
+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
@@ -121,6 +122,20 @@ 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