drivers: base: Unified device connection lookup

Several frameworks - clk, gpio, phy, pmw, etc. - maintain
lookup tables for describing connections and provide custom
API for handling them. This introduces a single generic
lookup table and API for the connections.

The motivation for this commit is centralizing the
connection lookup, but the goal is to ultimately extract the
connection descriptions also from firmware by using the
fwnode_graph_* functions and other mechanisms that are
available.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 43 insertions, 0 deletions

diff --git a/Documentation/driver-api/device_connection.rst b/Documentation/driver-api/device_connection.rst
new file mode 100644
index 000000000000..affbc5566ab0
--- /dev/null
+++ b/Documentation/driver-api/device_connection.rst
@@ -0,0 +1,43 @@
+==================
+Device connections
+==================
+
+Introduction
+------------
+
+Devices often have connections to other devices that are outside of the direct
+child/parent relationship. A serial or network communication controller, which
+could be a PCI device, may need to be able to get a reference to its PHY
+component, which could be attached for example to the I2C bus. Some device
+drivers need to be able to control the clocks or the GPIOs for their devices,
+and so on.
+
+Device connections are generic descriptions of any type of connection between
+two separate devices.
+
+Device connections alone do not create a dependency between the two devices.
+They are only descriptions which are not tied to either of the devices directly.
+A dependency between the two devices exists only if one of the two endpoint
+devices requests a reference to the other. The descriptions themselves can be
+defined in firmware (not yet supported) or they can be built-in.
+
+Usage
+-----
+
+Device connections should exist before device ``->probe`` callback is called for
+either endpoint device in the description. If the connections are defined in
+firmware, this is not a problem. It should be considered if the connection
+descriptions are "built-in", and need to be added separately.
+
+The connection description consists of the names of the two devices with the
+connection, i.e. the endpoints, and unique identifier for the connection which
+is needed if there are multiple connections between the two devices.
+
+After a description exists, the devices in it can request reference to the other
+endpoint device, or they can request the description itself.
+
+API
+---
+
+.. kernel-doc:: drivers/base/devcon.c
+   : functions: device_connection_find_match device_connection_find device_connection_add device_connection_remove