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doc: Add monitor configuration documentation

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This describes how the `monitors.xml` file work, and how to override the
newly introduced configurable policy.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2030>
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Jonas Ådahl 2021-10-01 09:52:15 +02:00 committed by Marge Bot
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Monitor configuration
=====================
File locations
--------------
Monitor configurations are stored as XML files called `monitors.xml` on the file
system. There are two types of locations for the XML file: the system level and
the user level.
The directories for system level configuration is defined in accordance to the
$XDG_CONFIG_DIRS environment variable defined in the XDG Base Directory
Specification. The default is `/etc/xdg/monitors.xml`.
The directory for the user level configuration is defined in accordance to the
$XDG_CONFIG_HOME environment variable defined in the XDG Base Directory
Specification. The default is `~/.config/monitors.xml`
File contents
-------------
A configuration file consists of an XML document with the root element
`<monitors version="2">`. In this document multiple configurations are stored as
individual `<configuration/>` elements containing all the details of the monitor
setup. The `version` attribute must be set to `"2"`.
Each configuration corresponds to a specific hardware setup, where a given set
of monitors are connected to the computer. There can only be one configuration
per hardware setup.
Writing configuration
---------------------
Monitor configurations are managed by Mutter via the Display panel in Settings,
which uses a D-Bus API to communicate with Mutter. Each time a new configuration
is applied and accepted, the user level configuration file is replaced with
updated content.
Previously defined monitor configurations for hardware state other than the
current are left intact.
Configuration policy
--------------------
The monitor configuration policy determines how Mutter configures monitors. This
can mean for example in what order configuration files should be preferred, or
whether configuration via Settings (i.e. D-Bus) should be allowed.
The default policy is to prioritize configurations defined in the user level
configuration file, and to allow configuring via D-Bus.
Changing the policy is possible by manually adding a `<policy/>` element inside
the `<monitors version="2"/>` element in the `monitors.xml` file. Note that
there may only be one `<policy/>` element in each configuration file.
### Changing configuration file priority policy
To change the order of configuration file priority, or to disable configuration
files completely, add a `<stores/>` element inside the `<policy/>` element
described above.
In this element, the file policy is defined by a `<stores/>` element, which
lists stores with the order according to prioritization. Each store is specified
using a `<store/>` element with either `system` or `user` as the content.
#### Example of only reading monitor configuration from the system level file:
```xml
<monitors version="2">
<policy>
<stores>
<store>system</store>
</stores>
</policy>
</monitors>
```
#### Example of reversing the priority of monitor configuration:
```xml
<monitors version="2">
<policy>
<stores>
<store>user</store>
<store>system</store>
</stores>
</policy>
</monitors>
```
### Changing D-Bus configuration policy
D-Bus configureability can be configured using a `<dbus/>` element in the
`<policy/>` element. It's content should either be `yes` or `no` depending on
whether monitor configuration via D-Bus should be enabled or disable.
#### Example of how to disable monitor configuration via D-Bus:
```xml
<monitors version="2">
<policy>
<dbus>no</dbus>
</policy>
</monitors>
```