docs: add linking documentation

docs/rst/policies/linking.rst

@@ -1,5 +1,140 @@
 .. _policies_linking:
 
 Linking Policy
---------------
+==============
 
+Introduction
+------------
+
+The linking policy in WirePlumber is the logic charged to link a PipeWire stream
+node with a PipeWire device node (most cases), or with another PipeWire stream
+node (monitoring applications).
+
+PipeWire stream nodes always have one of the following media classes:
+
+- Stream/Output/Audio: For audio playback applications (Eg pw-play).
+- Stream/Input/Audio: For audio capture applications (Eg pw-record).
+- Stream/Input/Video: For video capture applications (Eg cheese).
+
+And Pipewire device nodes always have one of the following media classes:
+
+- Audio/Sink: For audio playback devices (Eg Speakers).
+- Audio/Source: For audio capture devices (Eg Microphones).
+- Video/Source: For video capture devices (Eg Cameras).
+
+By default, since in most cases we want to link a stream node with a device
+node, the linking policy logic when linking 2 nodes always follows the following
+assignments:
+
+.. graphviz::
+
+   digraph nodes {
+      rankdir=LR;
+      APS [shape=box label=<audio playback stream<BR/>(Stream/Output/Audio)>];
+      APD [shape=box label=<audio playback device<BR/>(Audio/Sink)>];
+      ACS [shape=box label=<audio capture stream<BR/>(Stream/Input/Audio)>];
+      ACD [shape=box label=<audio capture device<BR/>(Audio/Source)>];
+      VCS [shape=box label=<video capture stream<BR/>(Stream/Input/Video)>];
+      VCD [shape=box label=<video capture device<BR/>(Video/Source)>];
+      APS -> APD;
+      ACD -> ACS;
+      VCD -> VCS;
+   }
+
+
+After that, once the media class of a device node has been select for a
+particular stream node, and there are more than 1 device node matching such
+media class, WirePlumber will select one based on a set of priorities:
+
+First, it will check if there is a default configured device node for the
+selected device media class. If there is one, and the node exists, it will link
+the stream node with such configured default node. Users can easily configure
+default device nodes for all the 3 different device media classes using tools
+such as ``pavucontrol`` or ``wpctl``. The logic is implemented in the
+``linking/find-default-target.lua`` Lua script.
+
+If there isn't any default node configured, or there is a default node
+configured but the node does not exist, WirePlumber will instead select the
+best device node available. The best device node is the node with highest
+session priority and available routes to the physical device. The logic is
+implemented in the ``linking/find-best-target.lua`` Lua script.
+
+If the best node could not be found because the system does not have any,
+WirePlumber won't link the stream and will send a "no target node available"
+error to the client.
+
+
+Stream node linking properties
+------------------------------
+
+The above default linking logic behavior can be changed by setting specific
+properties on the nodes.
+
+.. note::
+
+   These properties must be set in the **stream** nodes (not the device nodes),
+   otherwise they won't have any effect.
+
+- **target.object**:
+
+  The name of the desired node for this stream to be linked with.
+  If this property is present, WirePlumber will try to find such node, see if it
+  can be linked with the stream, and if so, will use it instead of the default
+  node or best node. The logic is implemented in the ``linking/find-defined-target.lua``
+  Lua script. Since this property is not set by default, WirePlumber will always
+  link stream nodes to the default or best device node found. This property can be
+  easily set using tools such as ``pw-play`` with the ``--target`` flag.
+
+  Note that any node name can be specified there, even if the name is not a device
+  node name, but another stream node name. If this is the case, WirePlumber will
+  link 2 stream nodes together. An example of this case is the monitoring nodes
+  created by ``pavucontrol`` to monitor audio of all audio devices and streams.
+
+- **node.dont-reconnect**:
+
+  Boolean indicating whether the stream node should not be reconnected to a new
+  node if its current linked node (target) was destroyed or not. By default it
+  is set to ``false``, so if the property is not present in the stream node, WirePlumber
+  will always try to reconnect the stream node to a new target instead of sending
+  an error to the client. The logic is implemented in the ``linking/prepare-link.lua``
+  Lua script.
+
+- **target.dont-move**:
+
+  Boolean indicating whether the stream node should not be movable or not at runtime
+  using the metadata. If a stream node is not movable, it means that users cannot
+  relink the stream node to a new target at runtime (using tools such as ``pavucontrol``
+  or ``pw-metadata``) when the stream node is already linked to a different node. By
+  default it is set to ``false``, so if the property is not present, WirePlumber will
+  always move, and therefore link the stream node to a new target if it is defined and
+  updated in the ``target.object`` metadata key.
+
+- **target.dont-fallback**:
+
+  Boolean indicating whether the stream node should not fallback to a different
+  target if its defined target does not exist (the one defined with the ``target.object``
+  property) or not. Therefore, if this property is set to ``true``, WirePlumber sends
+  a "defined target not found" error to the client and will also destroy the stream
+  node. By default it is set to ``false``, so if the property is not present in the
+  stream node, WirePlumber will always fallback to the default or best target if
+  the defined target was not found.
+
+- **target.linger**:
+
+  Boolean indicating whether the stream node should linger or not if its defined
+  target was not found and the ``target.dont-fallback`` is set to true. Therefore, if
+  this property is set to ``true``, the defined target was not found, and the
+  ``target.dont-fallback`` is set to true, WirePlumber won't send a "defined target not found"
+  error to the client, and won't destroy the stream node. This is useful if we want
+  the stream to wait (without processing any data) until its defined target becomes
+  available. By default it is set to ``false``, so if the property is not present in the
+  stream node, WirePlumber will always destroy the node and send an error to the client
+  if its target was not found and ``target.dont-fallback`` was set to true.
+
+
+Linking settings
+----------------
+
+Apart from the above properties, there are also global settings for the linking
+policy. See :ref:`config_settings` for more information, the linking settings
+are prefixed with ``linking.``.