Add documentation for D-Bus control

docs/src/SUMMARY.md

@@ -15,4 +15,6 @@
   - [Running as a Service](./config/services/README.md)
     - [Running as systemd service](./config/services/Systemd.md)
     - [Running as launchd service (MacOS)](./config/services/MacOS.md)
+- Other
+  - [D-Bus control](./other/D-Bus-control.md)
 

docs/src/other/D-Bus-control.md

@@ -0,0 +1,57 @@
+# D-Bus control
+
+`Spotifyd` can be configured to bind to D-Bus, exposing controls including standard MPRIS interfaces.
+
+To configure D-Bus, see `use_mpris` and `dbus_type` in the [configuration file](../config/File.md). `Spotifyd` must also be built with the `dbus_mpris` feature, which is available in the `full` flavour of the [provided binaries](https://github.com/Spotifyd/spotifyd/releases).
+
+The D-Bus service name will be in the format `org.mpris.MediaPlayer2.spotifyd.instance<number>`, where `<number>` is the process id.
+
+## Interfaces
+
+### MPRIS
+
+The `org.mpris.MediaPlayer2` and `org.mpris.MediaPlayer2.Player` interfaces from the [MPRIS specification](https://specifications.freedesktop.org/mpris-spec/latest/) are implemented.
+
+Note the `Volume` property of the `org.mpris.MediaPlayer2.Player` interface is read-only, despite supporting writes in the specification.
+
+### Spotifyd Controls
+
+The `rs.spotifyd.Controls` interface includes additional non-standard controls.
+
+- Method `TransferPlayback`: transfers Spotify playback to `spotifyd`
+- Method `VolumeUp`: increases player volume
+- Method `VolumeDown`: decreases player volume
+
+## Usage
+
+`Spotifyd` can be controlled by applications which support MPRIS such as the [playerctl](https://github.com/altdesktop/playerctl) command-line utility.
+
+The `dbus-send` command can also be used to control `spotifyd`. For example:
+
+- Find the service registered by `spotifyd`: `dbus-send --print-reply --dest=org.freedesktop.DBus /org/freedesktop/DBus org.freedesktop.DBus.ListNames | grep spotifyd`
+- Transfer playback to `spotifyd`: `dbus-send --print-reply --dest=org.mpris.MediaPlayer2.spotifyd.instancexxx /rs/spotifyd/Controls rs.spotifyd.Controls.TransferPlayback`
+- Get metadata for the current track: `dbus-send --print-reply --dest=org.mpris.MediaPlayer2.spotifyd.instancexxx /org/mpris/MediaPlayer2 org.freedesktop.DBus.Properties.Get string:org.mpris.MediaPlayer2.Player string:Metadata`
+
+## Troubleshooting
+
+### "Failed to initialize DBus connection" on a headless system
+
+Where no graphical session is available, the system bus can be used by setting the `dbus_type` configuration option to `system`.
+
+### "Failed to register dbus player name" using the system bus
+
+`Spotifyd` may not have permission to register the D-Bus service due to D-Bus security policies. It should be granted permission to own any service with the prefix "org.mpris.MediaPlayer2.spotifyd".
+
+For example, this statement can be added to the default policy in `/usr/share/dbus-1/system.conf`.
+
+```xml
+<allow own_prefix="org.mpris.MediaPlayer2.spotifyd"/>
+```
+
+It may also be necessary to add a statement to allow clients to send messages.
+
+```xml
+<allow send_destination_prefix="org.mpris.MediaPlayer2.spotifyd"/>
+```
+
+Make sure to reload the D-Bus configuration after making changes. For example `sudo systemctl reload dbus`.