From 70fb9a252317e04bd9af68ed89d4bf6caeb252e4 Mon Sep 17 00:00:00 2001 From: "Rafael J. Wysocki" Date: Thu, 7 Feb 2019 19:41:56 +0100 Subject: 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 Signed-off-by: Greg Kroah-Hartman --- Documentation/driver-api/device_link.rst | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) (limited to 'Documentation/driver-api') 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 -- cgit v1.2.3