driver core: Fix adding device links to probing suppliers

Currently, it is not valid to add a device link from a consumer
driver ->probe callback to a supplier that is still probing too, but
generally this is a valid use case.  For example, if the consumer has
just acquired a resource that can only be available if the supplier
is functional, adding a device link to that supplier right away
should be safe (and even desirable arguably), but device_link_add()
doesn't handle that case correctly and the initial state of the link
created by it is wrong then.

To address this problem, change the initial state of device links
added between a probing supplier and a probing consumer to
DL_STATE_CONSUMER_PROBE and update device_links_driver_bound() to
skip such links on the supplier side.

With this change, if the supplier probe completes first,
device_links_driver_bound() called for it will skip the link state
update and when it is called for the consumer, the link state will
be updated to "active".  In turn, if the consumer probe completes
first, device_links_driver_bound() called for it will change the
state of the link to "active" and when it is called for the
supplier, the link status update will be skipped.

However, in principle the supplier or consumer probe may still fail
after the link has been added, so modify device_links_no_driver() to
change device links in the "active" or "consumer probe" state to
"dormant" on the supplier side and update __device_links_no_driver()
to change the link state to "available" only if it is "consumer
probe" or "active".

Then, if the supplier probe fails first, the leftover link to the
probing consumer will become "dormant" and device_links_no_driver()
called for the consumer (when its probe fails) will clean it up.
In turn, if the consumer probe fails first, it will either drop the
link, or change its state to "available" and, in the latter case,
when device_links_no_driver() is called for the supplier, it will
update the link state to "dormant".  [If the supplier probe fails,
but the consumer probe succeeds, which should not happen as long as
the consumer driver is correct, the link still will be around, but
it will be "dormant" until the supplier is probed again.]

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/Documentation/driver-api/device_link.rst b/Documentation/driver-api/device_link.rst
index d6763272e747..5c7178189612 100644
--- a/Documentation/driver-api/device_link.rst
+++ b/Documentation/driver-api/device_link.rst
@@ -59,11 +59,15 @@ 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 probed yet:  Had the driver
-core known about the device link earlier, it wouldn't have probed the
+``->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.
+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 is added in the ``->probe`` callback of the supplier or
 consumer driver, it is typically deleted in its ``->remove`` callback for