Add Wireplumber policiy samples to Audio Management concept

This demonstrates simple definition of policy roles and a sample policy
script written in Lua.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>

content/concepts/audio-management.md

@@ -454,8 +454,9 @@ the user will not hear.
 
 An internal priority module can be written. This module would associate a
 priority to all differents streams' metadata. It is loaded statically from the
-config file. See [Routing data structure example] for an example of data
-structure.
+config file. See
+[Routing data structure example]( {{< ref "#routing-data-structure-example" >}} )
+for an example of data structure.
 
 #### Hybrid routing module maps stream metadata to external audio router calls
 
@@ -580,6 +581,120 @@ Since the CE domain do not know what is happening in the automotive domain.
 | traffic_info          | INFO1         | alert_sink    | mix   |
 | gps                   | INFO2         | main_sink     | mix   |
 
+### WirePlumber policy samples
+
+All the policies in WirePlumber are completely scriptable and written in Lua.
+The Lua API Documentation can be found
+[here](https://pipewire.pages.freedesktop.org/wireplumber/lua_api.html).
+
+The default roles, priorities and related actions are defined in
+`/etc/wireconfig/policy.lua.d/50-endpoints-config.lua` and
+can be re-written to support the standalone setup defined in
+[Routing data structure example]( {{< ref "#routing-data-structure-example" >}} ):
+```
+default_policy.policy.roles = {
+  -- main sink
+  ["Multimedia"]    = { ["priority"] = 0, ["action.default"] = "cork", ["alias"] = { "Movie", "Music", "Game" }, },
+  ["GPS"]           = { ["priority"] = 5, ["action.default"] = "duck", },
+  ["Phone"]         = { ["priority"] = 7, ["action.default"] = "cork", ["alias"] = { "CustomRingtone" }, },
+
+  -- alert sink
+  ["New_email"]     = { ["priority"] = 1, ["action.default"] = "mix", },
+  ["Traffic_info"]  = { ["priority"] = 6, ["action.default"] = "mix", },
+  ["Ringtone"]      = { ["priority"] = 7, ["action.default"] = "mix", },
+}
+
+default_policy.endpoints = {
+  ["endpoint.multimedia"]    = { ["media.class"] = "Audio/Sink", ["role"] = "Multimedia", },
+  ["endpoint.gps"]           = { ["media.class"] = "Audio/Sink", ["role"] = "GPS", },
+  ["endpoint.phone"]         = { ["media.class"] = "Audio/Sink", ["role"] = "Phone", },
+  ["endpoint.ringtone"]      = { ["media.class"] = "Audio/Sink", ["role"] = "Ringtone", },
+  ["endpoint.new_email"]     = { ["media.class"] = "Audio/Sink", ["role"] = "New_email", },
+  ["endpoint.traffic_info"]  = { ["media.class"] = "Audio/Sink", ["role"] = "Traffic_info", },
+}
+```
+And, for example, a policy to automatically switch Bluetooth from A2DP to
+HSP/HFP profile when a specific application starts, e.g. Zoom, could be defined
+like:
+```
+#!/usr/bin/wpexec
+--
+-- WirePlumber
+--
+-- Copyright © 2021 Collabora Ltd.
+--    @author George Kiagiadakis <george.kiagiadakis@collabora.com>
+--
+-- SPDX-License-Identifier: MIT
+--
+-- This is an example of a standalone policy making script. It can be executed
+-- either on top of another instance of wireplumber or pipewire-media-session,
+-- as a standalone executable, or it can be placed in WirePlumber's scripts
+-- directory and loaded together with other scripts.
+--
+-- The script basically watches for a client application called
+-- "ZOOM VoiceEngine", and when it appears (i.e. Zoom starts), it switches
+-- the profile of all connected bluetooth devices to the "headset-head-unit"
+-- (a.k.a HSP Headset Audio) profile. When Zoom exits, it switches again the
+-- profile of all bluetooth devices to A2DP Sink.
+--
+-- The script can be customized further to look for other clients and/or
+-- change the profile of a specific device, by customizing the constraints.
+-----------------------------------------------------------------------------
+
+devices_om = ObjectManager {
+  Interest { type = "device",
+    Constraint { "device.api", "=", "bluez5" },
+  }
+}
+
+clients_om = ObjectManager {
+  Interest { type = "client",
+    Constraint { "application.name", "=", "ZOOM VoiceEngine" },
+  }
+}
+
+function set_profile(profile_name)
+  for device in devices_om:iterate() do
+    local index = nil
+    local desc = nil
+
+    for profile in device:iterate_params("EnumProfile") do
+      local p = profile:parse()
+      if p.properties.name == profile_name then
+        index = p.properties.index
+        desc = p.properties.description
+        break
+      end
+    end
+
+    if index then
+      local pod = Pod.Object {
+        "Spa:Pod:Object:Param:Profile", "Profile",
+        index = index
+      }
+
+      print("Setting profile of '"
+            .. device.properties["device.description"]
+            .. "' to: " .. desc)
+      device:set_params("Profile", pod)
+    end
+  end
+end
+
+clients_om:connect("object-added", function (om, client)
+  print("Client '" .. client.properties["application.name"] .. "' connected")
+  set_profile("headset-head-unit")
+end)
+
+clients_om:connect("object-removed", function (om, client)
+  print("Client '" .. client.properties["application.name"] .. "' disconnected")
+  set_profile("a2dp-sink")
+end)
+
+devices_om:activate()
+clients_om:activate()
+```
+
 ### Testability
 
 The key point to keep in mind for testing is that several applications can