Creating Pipewire Links with WirePlumber

Pipewire is a low-latency, graph-based processing engine for audio and video for Linux, and WirePlumber is its session manager. In this post we’ll be looking at how exactly we can create links in Pipewire using WirePlumber.

Also make sure to stay to the end, to find out how you can quickly connect two random ports using the object.path.

Actually creating the link is the easy part, we just tell WirePlumber to use the link factory to create one as shown in the code snippet below:

local link = Link("link-factory", {
 ["link.output.port"] = <source port object id>,
 ["link.input.port"] = <target port object id>,
 ["link.output.node"] = <source node object id>,
 ["link.input.node"] = <target node object id>,
})
link:activate(1)

Copy the content into a file, for example create_link.lua, replace the port and nodes ids, and then run it with wpexec create_link.lua. The challenge is to find the correct ids, and to know when to create the link.

Note that it is not necessary ti specify both, the node and the port. When only a node is specified, wireplumber will pick a random port of the appropriate direction.

The Object Manager

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If the ports already exist, and we do just want to connect them, and not also handle device connects and disconnects, we can use the object manager to search for the available ports. When creating the ObjectManager, we supply an Interest, which tells it which objects we’re interested in. The Interest needs a type, in our case port, and Constraints, which allow us to filter the objects. Constraints take an array of strings during construction, which describe the conditions. For example, to search for a port by the object.path, use the following code snippet, which will match any port that matches the pattern alsa:acp:HDSPMxa5963e:0:playback:*. The object manager collects all objects currently available asynchronously. To react when the object manager has done so, we need to activate the object manager and connect to the installed event to get notified when it has finished.

port_om = ObjectManager({
 Interest({
  type = "port",
  Constraint({
   "object.path",
   "matches",
   "alsa:acp:HDSPMxa5963e:0:playback:*",
  }),
 }),
})

port_om:activate()

source_port_om:connect("installed", function(om)
  for port in om:iterate() do
    print(port.properties["object.path"])
  end
end)

Putting both snippets together, creating a link between two ports can be accomplished as follows:

source_port_om = ObjectManager({
 Interest({
  type = "port",
  Constraint({
   "port.alias",
   "matches",
   "RME RayDAT:capture_AUX0",
  }),
 }),
})

target_port_om = ObjectManager({
 Interest({
  type = "port",
  Constraint({
   "port.alias",
   "matches",
   "RME RayDAT:playback_AUX0",
  }),
 }),
})

source_port_om:activate()
target_port_om:activate()

MyLink = nil
local function create_link()
 if MyLink == nil then
  for source_port in source_port_om:iterate() do
   for target_port in target_port_om:iterate() do
    MyLink = Link("link-factory", {
     ["link.output.port"] = source_port.properties["port.id"],
     ["link.input.port"] = target_port.properties["port.id"],
     ["link.output.node"] = source_port.properties["node.id"],
     ["link.input.node"] = target_port.properties["node.id"],
    })
    MyLink:activate(1)
   end
  end
 end
end

source_port_om:connect("installed", function(om)
 create_link()
end)

target_port_om:connect("installed", function(om)
 create_link()
end)

Reacting to Objects Added

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The previous script links the objects, if they are already in the graph. If we want to also be able to link the objects when one or both of them are not already in the graph, we have to react to the object-added event. This can be done with a code snippet like this:

source_port_om:connect("object-added", function(om)
 create_link()
end)

target_port_om:create_link("object-added", function(om)
 create_link()
end)

The event is triggered whenever an object is added that fits the constraints.

And now as promised, if you want to connect two random ports with their object.path, you can use:

local link = Link("link-factory", {
 ["link.output.node"] = "alsa:acp:HDSPMxa5963e:1:capture",
 ["link.input.node"] = "alsa:acp:HDSPMxa5963e:0:playback",
})
link:activate(1)