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George Kiagiadakis f7c0730b7e docs: enable building html documentation with hotdoc

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Configuration

WirePlumber is a heavily modular daemon. By itself, it doesn't do anything except load the configured modules. All the rest of the logic is implemented inside those modules.

Modular design ensures that it is possible to swap the implementation of specific functionality without having to re-implement the rest of it, allowing flexibility on target-sensitive parts, such as policy management and making use of non-standard hardware.

`wireplumber.conf`

This is WirePlumber's main configuration file. It is read at startup, before connecting to the PipeWire daemon. Its purpose is to list all the modules that need to be loaded by WirePlumber.

The format of this file is custom and resembles a script with commands:

# comment
command parameter1 parameter2 ...

Lines are executed in the order they appear and each of them executes an action defined by the command. Lines starting with # are treated as comments and ignored. Possible commands are:

add-spa-lib

Associates SPA plugin names with the names of the SPA modules that they can be loaded from. This takes 2 parameters: a name pattern and a library name.

This actually does not load the SPA plugin, it only calls pw_core_add_spa_lib with the 2 paramteres given as arguments. As a consequence, it is safe to call this even if the SPA module is not actually installed on the system.

Example:
```
add-spa-lib api.alsa.* alsa/libspa-alsa
```
In this example, we let libpipewire know that any SPA plugin whose name starts with api.alsa. can be loaded from the SPA module alsa/libspa-alsa.so (relative to the standard SPA modules directory).
load-pipewire-module

Loads a libpipewire module. This is similar to the load-module commands that would appear on pipewire.conf, the configuration file of the PipeWire daemon.

This takes at least 1 parameter, the module name, and optionally any module arguments, in the format that they would be given in pipewire.conf

Format:
```
load-pipewire-module module-name some-argument some-property=value
```
Example:
```
load-pipewire-module libpipewire-module-client-device
```
This command does not affect the PipeWire daemon by any means. It exists simply to allow loading libpipewire modules in the pipewire core that runs inside WirePlumber. This is usually useful to load pipewire protocol extensions, so that you can export custom objects to PipeWire and other clients.
load-module

Loads a WirePlumber module. This takes 2 arguments and an optional parameter block.

Format:
```
load-module ABI module-name {
  "parameter": <"value">
}
```
The ABI parameter specifies the binary interface that WirePlumber shall use to load this module. Currently, the only supported ABI is C. It exists to allow future expansion, writing modules in other languages.

The module-name should be the name of the .so file without the .so extension.

Optionally, if the load-module line ends with a {, the next lines up to and including the next matching } are treated as a parameter block. This block essentially is a GVariant of type a{sv} in the GVariant Text Format. As a rule of thumb, parameter names in this block must always be strings enclosed in double quotes, the separation between names and values is done with the : character and values, regardless of their inner type, must always be enclosed in < >.

Note that starting the parameter block on the next line is an error. The starting brace ({) must always be on the load-module line.

Example:
```
load-module C libwireplumber-module-monitor {
  "factory": <"api.alsa.enum.udev">,
  "flags": <["use-adapter", "activate-devices"]>
}
```
Parameters are module-dependent. They are passed as a GVariant in the module's initialization function and it is up to the module to interpret their meaning. WirePlumber does not have any reserved parameters.

Location of configuration files

WirePlumber's default location of its configuration files is determined at compile time by the build system. Typically, it ends up being /etc/wireplumber.

In more detail, this is controlled by the --sysconfdir meson option. When this is set to an absolute path, such as /etc, the location of the configuration files is set to be $sysconfdir/wireplumber. When this is set to a relative path, such as etc, then the installation prefix (--prefix) is prepended to the path: $prefix/$sysconfdir/wireplumber

WirePlumber expects its wireplumber.conf to reside in that directory. It is possible to override that at runtime by setting the WIREPLUMBER_CONFIG_FILE environment variable:

WIREPLUMBER_CONFIG_FILE=src/config/wireplumber.conf wireplumber

It is also possible to override the whole configuration directory, so that all other configuration files are being read from a different location as well, by setting the WIREPLUMBER_CONFIG_DIR environment variable:

WIREPLUMBER_CONFIG_DIR=src/config wireplumber

Location of modules

WirePlumber modules

Like with configuration files, WirePlumber's default location of its modules is determined at compile time by the build system. Typically, it ends up being /usr/lib/wireplumber-0.1 (or /usr/lib/<arch-triplet>/wireplumber-0.1 on multiarch systems)

In more detail, this is controlled by the --libdir meson option. When this is set to an absolute path, such as /lib, the location of the modules is set to be $libdir/wireplumber-$abi_version. When this is set to a relative path, such as lib, then the installation prefix (--prefix) is prepended to the path: $prefix/$libdir/wireplumber-$abi_version.

It is possible to override this directory at runtime by setting the WIREPLUMBER_MODULE_DIR environment variable:

WIREPLUMBER_MODULE_DIR=build/modules wireplumber

PipeWire and SPA modules

PipeWire and SPA modules are not loaded from the same location as WirePlumber's modules. They are loaded from the location that PipeWire loads them.

It is also possible to override these locations by using environment variables: SPA_PLUGIN_DIR and PIPEWIRE_MODULE_DIR. For more details, refer to PipeWire's documentation.

module-monitor

This module internally loads a SPA "device" object which enumerates all the devices of a certain subsystem. Then it listens for "node" objects that are being created by this device and exports them to PipeWire, after adjusting their properties to provide enough context.

module-monitor does not read any configuration files, however, it supports configuration through parameters defined in the main wireplumber.conf. Possible parameters are:

factory

A string that specifies the name of the SPA factory that loads the intial "device" object.

Well-known factories are:
- "api.alsa.enum.udev" - Discovers ALSA devices via udev
- "api.v4l2.enum.udev" - Discovers V4L2 devices via udev
- "api.bluez5.enum.dbus" - Discovers bluetooth devices by calling bluez5 API via D-Bus
flags

An array of strings that enable specific functionality in the monitor. Possible flags include:
- "use-adapter"
  
  Instructs the monitor to wrap all the created nodes in an "adapter" SPA node, which provides automatic port splitting/merging and format/rate conversion. This should be always enabled for audio device nodes.
- "local-nodes"
  
  Instructs the monitor to run all the created nodes locally in in the WirePlumber process, instead of the default behavior which is to create the nodes in the PipeWire process. This is useful for bluetooth nodes, which should run outside of the main PipeWire process for performance reasons.
- "activate-devices"
  
  Instructs the monitor to automatically set the device profile to "On", so that the nodes are created. If not specified, the profile must be set externally by the user before any nodes appear.

module-config-endpoint

This module creates endpoints when WirePlumber detects new nodes in the pipewire graph. Nodes themselves can be created in two ways: Device modes are being created by "monitors" that watch a specific subsystem (udev, bluez, etc...) for devices. Client nodes are being created by client applications that try to stream to/from pipewire. As soon as a node is created, the module-config-endpoint iterates through all the .endpoint configuration files, in the order that is determined by the match-node.priority field, and tries to match the node to the node description in the [match-node] table. Upon a successful match, a new endpoint that follows the description in the [endpoint] table is created.

`*.endpoint` configuration files

These files are TOML v0.5 files. At the top-level, they must contain exactly 2 tables: [match-node] and [endpoint]

The [match-node] table contains properties that match a pipewire node that exists on the graph. Possible fields of this table are:

priority

Specifies the order in which the .endpoint files are being searched for a match with a node. If a node matches the description of more than one .endpoint file, the one with the highest priority wins.

The type of this field is unsigned integer. Bigger numbers mean higher priority.
properties

This is a TOML array of tables, where each table must contain two fields: name and value, both being strings. Each table describes a match against one of the pipewire properties of the node. For a successful node match, all the described properties must match with the node.

The value of the name field must match exactly the name of the pipewire property, while the value of the value field can contain '*' (wildcard) and '?' (joker), adhering to the rules of the GLib g_pattern_match() function.

When writing .endpoint files, a useful utility that you can use to list device node properties is:
```
$ wireplumber-cli device-node-props
```
Another way to figure out some of these properties for ALSA nodes is by parsing the aplay/arecord output. For example, this line from aplay -l is interpreted as follows:
```
card 0: PCH [HDA Intel PCH], device 2: ALC3246 [ALC3246 Analog]
```
```
{ name = "api.alsa.path", value = "hw:0,2" },
{ name = "api.alsa.card", value = "0" },
{ name = "api.alsa.card.id", value = "PCH" },
{ name = "api.alsa.card.name", value = "HDA Intel PCH" },
{ name = "api.alsa.pcm.device", value = "2" },
{ name = "api.alsa.pcm.id", value = "ALC3246" },
{ name = "api.alsa.pcm.name", value = "ALC3246 Analog" },
```

The [endpoint] table contains a description of the endpoint to be created. Possible fields of this table are:

type

Required. Specifies the factory to be used for construction. The only well-known factory at the moment of writing is: pw-audio-softdsp-endpoint
direction

Required. Can be set to either "sink" or "source". Specifies the direction of the media flow of this endpoint. A source is an endpoint that produces data (i.e. an audio capture device or a playback application) and a sink is an endpoint that consumes data (audio playback device or capture application).
name

Optional. The name of the newly created endpoint. If not specified, the endpoint is named after the node (from the node.name property of the node).
media_class

Optional. A string that specifies an override for the media.class property of the node. It can be used in special circumstances to declare that an endpoint is dealing with a different type of data. This is only useful in combination with a policy implementation that is aware of this media class.
priority

Optional. An unsigned integer that specifies the order in which endpoints are chosen to be the default of a specific device group. Possible device groups are (determined by the endpoint's media.class):
- Audio/Sink
- Audio/Source
- Video/Source
Every time a new device endpoint is created, wireplumber picks the "default" of the group that it belongs to, based on this priority number: the endpoint with the biggest priority number wins.

If not specified, the default priority of an endpoint is equal to zero (i.e. the lowest priority).
streams

Optional. Specifies the name of a .streams file that contains the descriptions of the streams to create for this endpoint. This currently specific to the implementation of the pw-audio-softdsp-endpoint and might change in the future.

`*.streams` configuration files

These files contain lists of streams with their names and priorities. They are TOML v0.5 files.

Each .streams file must contain exactly one top-level array of tables, called streams. Every table must contain exactly two fields: name and priority.

The name of each stream is used to create the streams on new endpoints.

The priority of each stream is being interpreted by the policy module to apply restrictions on which app can use the stream at a given time.

module-config-policy

This module implements demo-quality policy management that is partly driven by configuration files. The configuration files that this module reads are described below:

`*.endpoint-link`

These files contain rules to link endpoints with each other. They are TOML v0.5 files.

Endpoints are normally created by another module, such as module-config-endpoint which is described above. As soon as an endpoint is created, the module-config-policy uses the information gathered from the .endpoint-link files in order to create a link to another endpoint.

.endpoint-link files can contain 3 top-level tables:

[match-endpoint], required
[target-endpoint], optional
[endpoint-link], required

The [match-endpoint] table contains properties that match an endpoint that exists on the graph. Possible fields of this table are:

priority

Specifies the order in which the .endpoint-link files are being searched for a match with an endpoint. If an endpoint matches the description of more than one .endpoint-link file, the one with the highest priority wins.

The type of this field is unsigned integer. Bigger numbers mean higher priority.
direction

Required. Can be set to either "sink" or "source". Specifies the direction of the media flow of this endpoint. A source is an endpoint that produces data (i.e. an audio capture device or a playback application) and a sink is an endpoint that consumes data (audio playback device or capture application).
name

Optional. The name of the endpoint. It is possible to use wildcards here to match only parts of the name.
media_class

Optional. A string that specifies the media.class that the endpoint must have in order to match.
properties

This is a TOML array of tables, where each table must contain two fields: name and value, both being strings. Each table describes a match against one of the pipewire properties of the endpoint. For a successful endpoint match, all the described properties must match with the endpoint.

The [target-endpoint] table contains properties that match an endpoint that exists on the graph. The purpose of this table is to match a second endpoint that the original matching endpoint from [match-endpoint] will be linked to. If not specified, module-config-policy will look for the session "default" endpoint for the type of media that the matching endpoint produces or consumes and will use that as a target. Possible fields of this table are:

direction, name, media_class, properties

All these fields are permitted and behave exactly as described above for the [match-endpoint] table.
stream

This field specifies a stream name that the link will use on the target endpoint. If it is not specified, the stream name is acquired from the media.role property of the matching endpoint. If specified, the value of this field overrides the media.role.

The [endpoint-link] table specifies properties of the link. Possible fields of this table are:

keep

A boolean field. If set to true, the link is always kept active and ignores policy rules regarding corking or stream priority. This link will also not affect the rules for other links. For example, if a keep=true link is activating a high priority stream, lower priority streams can still work on the same target endpoint for links with keep=false.

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