driver core: Add support for linking devices during device addition

The firmware corresponding to a device (dev.fwnode) might be able to
provide functional dependency information between a device and its
supplier and consumer devices.  Tracking this functional dependency
allows optimizing device probe order and informing a supplier when all
its consumers have probed (and thereby actively managing their
resources).

The existing device links feature allows tracking and using
supplier-consumer relationships. So, this patch adds the add_links()
fwnode callback to allow firmware to create device links for each
device as the device is added.

However, when consumer devices are added, they might not have a supplier
device to link to despite needing mandatory resources/functionality from
one or more suppliers. A waiting_for_suppliers list is created to track
such consumers and retry linking them when new devices get added.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20190904211126.47518-3-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/Documentation/driver-api/device_link.rst b/Documentation/driver-api/device_link.rst
index 1b5020ec6517..bc2d89af88ce 100644
--- a/Documentation/driver-api/device_link.rst
+++ b/Documentation/driver-api/device_link.rst
@@ -281,7 +281,8 @@ State machine
   :c:func:`driver_bound()`.)
 
 * Before a consumer device is probed, presence of supplier drivers is
-  verified by checking that links to suppliers are in ``DL_STATE_AVAILABLE``
+  verified by checking the consumer device is not in the wait_for_suppliers
+  list and by checking that links to suppliers are in ``DL_STATE_AVAILABLE``
   state.  The state of the links is updated to ``DL_STATE_CONSUMER_PROBE``.
   (Call to :c:func:`device_links_check_suppliers()` from
   :c:func:`really_probe()`.)