PipeWire / WirePlumber
AGL uses the PipeWire daemon service to provide audio playback and capture capabilities. PipeWire is accompanied by a secondary service, WirePlumber (also referred to as the session manager), which provides policy management, device discovery, configuration and more.
Applications can connect to the PipeWire service through its UNIX socket, by using either the libpipewire or libwireplumber libraries as a front-end to that socket.
Upstream documentation for these components can be found at the links below:
The main entry point for applications to access the audio system is the API offered by libpipewire. The functionality offered by libpipewire is vast and it is beyond the scope of this document to describe it all.
For playback and capture, applications should use struct pw_stream and its associated methods. See PipeWire: Tutorial - Part 4: Playing a tone for a starting point.
For convenience, applications that use GStreamer can use the PipeWire GStreamer elements to plug the functionality offered by struct pw_stream directly in the GStreamer pipeline. These elements are called pipewiresrc and pipewiresink
> gst-launch-1.0 audiotestsrc ! pipewiresink
Through these elements, it is possible to specify the application role by setting it in the stream-properties property of the element, as shown below:
> gst-launch-1.0 audiotestsrc ! pipewiresink stream-properties="p,media.role=Multimedia""
or, in the C API:
gst_util_set_object_arg (sink, "stream-properties", "p,media.role=Multimedia");
Of course, it is also possible to use alsasink and alsasrc and route audio through the virtual ALSA device that is mentioned below. This is also the default behavior of playbin and similar auto-plugging elements, because the PipeWire GStreamer elements are not autoplugged (this may change in a future version).
PipeWire offers a virtual ALSA device (called pipewire) that redirects audio to PipeWire through an ALSA PCM plugin. This device is the default one, so unless you explicitly specify a device in your ALSA client application, audio will go through PipeWire instead.
> aplay sound.wav # the default device is 'pipewire' > aplay -D pipewire sound.wav
In order to specify the application role while using the ALSA compatibility device, pass the role as a device parameter like this:
> aplay -D pipewire:ROLE=Navigation turnleft.wav
See the separate agl-service-audiomixer documentation.
The WirePlumber library provides API that wraps libpipewire and makes it easier to work with when you are writing control applications, such as a volume mixer. The audiomixer service is in fact implemented using libwireplumber.
WirePlumber also provides support for lua-based scripting. Standalone scripts, executed with the wpexec tool, may be used as a means to rapidly make use of the API provided by libwireplumber
wpctl: allows inspecting the devices, choosing which source & sink are the default ones and allows volume/mute adjustments to be made on the command line. Try
wpctl helpto get started with it
wpexec: allows running wireplumber lua scripts standalone, which is useful to implement custom scripts to interact with PipeWire
pw-cli: this is the main tool for interacting with pipewire directly
pw-dump: dumps all the objects in the pipewire graph to a big JSON. The output of this tool is very useful to include in bug reports. It is also suitable for implementing scripts that parse information with jq
pw-dot is a useful debug tool that dumps the objects in a dot graph for easy visualization
pw-cat / pw-play / pw-record: This is a set of tools similar to aplay/arecord, for simple audio operations
pw-top: This is a performance measurement tool that shows realtime timing information about the audio pipeline. Before running this tool, you will need to uncomment the loading of "libpipewire-module-profiler" in /etc/pipewire/pipewire.conf and restart pipewire
The PipeWire service,
pipewire.service, is activated on demand, via systemd socket activation,
pipewire.socket. The WirePlumber service,
wireplumber.service, is bound to the pipewire
service and therefore started and stopped together with the PipeWire service.
If you wish to manually stop or restart both services, you can do so by using systemctl, operating on the .socket unit:
> systemctl restart pipewire.socket > systemctl stop pipewire.socket
The PipeWire daemon can be configured to be more verbose by editing
/etc/pipewire/pipewire.conf and setting
log.level=n (n=0-5) in section
Similarly, the WirePlumber daemon can be configured to be more verbose by editing
/etc/wireplumber/wireplumber.conf and setting
log.level=n (n=0-5) in section
All messages will be available in the systemd journal, inspectable with journalctl.
For applications, at runtime,
PIPEWIRE_DEBUG can be set to a value between 0 and 5,
with 5 being the most verbose and 0 the least verbose.
For applications that use libwireplumber the equivalent environment variable is
WIREPLUMBER_DEBUG, which also takes values between 0 and 5.
The default debug level for the daemons is 2, while for applications it's 0 (with few exceptions).
More information is also available on WirePlumber's documentation on debug logging