Display Configuration in wayland
Problem
gnome-settings-daemon is currently in charge of keeping the display configuration in GNOME, and it does so with X Resize, Rotate and Reflect extension (XRandR). With X11 out of the way, a new mechanism must be designed.
In wayland, the compositor has the ability to communicate the desired display configuration to the kernel driver directly, through the KMS interface in libdrm, so it's only necessary that gnome-settings-daemon and gnome-control-center talk with gnome-shell.
Requirements
- The API user must have a complete view of all hardware, including that not in use - this immediately rules out the wl_output interface, which only exposes active monitors
- The API user must have knowledge of the HW limitations, in terms of CRTCs (which themselves abstract the encoder HW), supported modes and supported transforms
- The API must allow for non HW properties to be configured, such as "this output is primary" or "this output is for presentation only, and no regular windows should be mapped there"
- The API must be atomic (although the effect might not be, if the driver does not support atomic modesetting)
Proposed solution
Given that it is not expected that the existing wayland protocol for display configuration (wl_output) will have the necessary extensions to replace the functionality currently exposed by XRandR, we designed a new protocol. It uses DBus so that it can be used right now, before the move to wayland, by mapping the DBus calls to XRandR calls in mutter.
The protocol exposes one object, at org.gnome.Mutter.DisplayConfig /org/gnome/Mutter/DisplayConfig and uses two calls:
GetResources() to acquire the list of CRTCs, outputs and modes present and supported by the hardware
ApplyConfiguration() to configure the display, as well as generate a configuration that can be saved persistently on disk and applied at the next possible occasion
The protocol calls are coupled by a serial argument, to ensure that the new configuration is based on an updated view of hardware.
Implementation
<interface name="org.gnome.Mutter.DisplayConfig">
<!--
GetResources:
@serial: configuration serial
@crtcs: available CRTCs
@outputs: available outputs
@modes: available modes
@max_screen_width:
@max_screen_height:
Retrieves the current layout of the hardware.
@serial is an unique identifier representing the current state
of the screen. It must be passed back to ApplyConfiguration()
and will be increased for every configuration change (so that
mutter can detect that the new configuration is based on old
state).
A CRTC (CRT controller) is a logical monitor, ie a portion
of the compositor coordinate space. It might correspond
to multiple monitors, when in clone mode, but not that
it is possible to implement clone mode also by setting different
CRTCs to the same coordinates.
The number of CRTCs represent the maximum number of monitors
that can be set to expand and it is a HW constraint; if more
monitors are connected, then necessarily some will clone. This
is complementary to the concept of the encoder (not exposed in
the API), which groups outputs that necessarily will show the
same image (again a HW constraint).
A CRTC is represented by a DBus structure with the following
layout:
* u ID: the ID in the API of this CRTC
* x winsys_id: the low-level ID of this CRTC (which might
be a XID, a KMS handle or something entirely
different)
* i x, y, width, height: the geometry of this CRTC
(might be invalid if the CRTC is not in
use)
* i current_mode: the current mode of the CRTC, or -1 if this
CRTC is not used
Note: the size of the mode will always correspond
to the width and height of the CRTC
* u current_transform: the current transform (espressed according
to the wayland protocol)
* au transforms: all possible transforms
* a{sv} properties: other high-level properties that affect this
CRTC; they are not necessarily reflected in
the hardware.
No property is specified in this version of the API.
Note: all geometry information refers to the untransformed
display.
An output represents a physical screen, connected somewhere to
the computer. Floating connectors are not exposed in the API.
An output is a DBus struct with the following fields:
* u ID: the ID in the API
* x winsys_id: the low-level ID of this output (XID or KMS handle)
* i current_crtc: the CRTC that is currently driving this output,
or -1 if the output is disabled
* au possible_crtcs: all CRTCs that can control this output
* s name: the name of the connector to which the output is attached
(like VGA1 or HDMI)
* au modes: valid modes for this output
* au clones: valid clones for this output, ie other outputs that
can be assigned the same CRTC as this one; if you
want to mirror two outputs that don't have each other
in the clone list, you must configure two different
CRTCs for the same geometry
* a{sv} properties: other high-level properties that affect this
output; they are not necessarily reflected in
the hardware.
Known properties:
- "vendor" (s): (readonly) the human readable name
of the manufacturer
- "product" (s): (readonly) the human readable name
of the display model
- "serial" (s): (readonly) the serial number of this
particular hardware part
- "display-name" (s): (readonly) a human readable name
of this output, to be shown in the UI
- "backlight" (i): (readonly, use the specific interface)
the backlight value as a percentage
(-1 if not supported)
- "primary" (b): whether this output is primary
or not
- "presentation" (b): whether this output is
for presentation only
Note: properties might be ignored if not consistenly
applied to all outputs in the same clone group. In
general, it's expected that presentation or primary
outputs will not be cloned.
A mode represents a set of parameters that are applied to
each output, such as resolution and refresh rate. It is a separate
object so that it can be referenced by CRTCs and outputs.
Multiple outputs in the same CRTCs must all have the same mode.
A mode is exposed as:
* u ID: the ID in the API
* x winsys_id: the low-level ID of this mode
* u width, height: the resolution
* d frequency: refresh rate
Output and modes are read-only objects (except for output properties),
they can change only in accordance to HW changes (such as hotplugging
a monitor), while CRTCs can be changed with ApplyConfiguration().
XXX: actually, if you insist enough, you can add new modes
through xrandr command line or the KMS API, overriding what the
kernel driver and the EDID say.
Usually, it only matters with old cards with broken drivers, or
old monitors with broken EDIDs, but it happens more often with
projectors (if for example the kernel driver doesn't add the
640x480 - 800x600 - 1024x768 default modes). Probably something
that we need to handle in mutter anyway.
-->
<method name="GetResources">
<arg name="serial" direction="out" type="u" />
<arg name="crtcs" direction="out" type="a(uxiiiiiuaua{sv})" />
<arg name="outputs" direction="out" type="a(uxiausauaua{sv})" />
<arg name="modes" direction="out" type="a(uxuud)" />
<arg name="max_screen_width" direction="out" type="i" />
<arg name="max_screen_height" direction="out" type="i" />
</method>
<!--
ApplyConfiguration:
@serial: configuration serial
@persistent: whether this configuration should be saved on disk
@crtcs: new data for CRTCs
@outputs: new data for outputs
Applies the requested configuration changes.
@serial must match the serial from the last GetResources() call,
or org.freedesktop.DBus.AccessDenied will be generated.
If @persistent is true, mutter will attempt to replicate this
configuration the next time this HW layout appears.
@crtcs represents the new logical configuration, as a list
of structures containing:
- u ID: the API ID from the corresponding GetResources() call
- i new_mode: the API ID of the new mode to configure the CRTC
with, or -1 if the CRTC should be disabled
- i x, y: the new coordinates of the top left corner
the geometry will be completed with the size information
from @new_mode
- u transform: the desired transform
- au outputs: the API ID of outputs that should be assigned to
this CRTC
- a{sv} properties: properties whose value should be changed
Note: CRTCs not referenced in the array will be disabled.
@outputs represent the output property changes as:
- u ID: the API ID of the output to change
- a{sv} properties: properties whose value should be changed
Note: both for CRTCs and outputs, properties not included in
the dictionary will not be changed.
Note: unrecognized properties will have no effect, but if the
configuration change succeeds the property will be reported
by the next GetResources() call, and if @persistent is true,
it will also be saved to disk.
If the configuration is invalid according to the previous
GetResources() call, for example because a CRTC references
an output it cannot drive, or not all outputs support the
chosen mode, the error org.freedesktop.DBus.InvalidArgs will
be generated.
If the configuration cannot be applied for any other reason
(eg. the screen size would exceed texture limits), the error
org.freedesktop.DBus.Error.LimitsExceeded will be generated.
-->
<method name="ApplyConfiguration">
<arg name="serial" direction="in" type="u" />
<arg name="persistent" direction="in" type="b" />
<arg name="crtcs" direction="in" type="a(uiiiuaua{sv})" />
<arg name="outputs" direction="in" type="a(ua{sv})" />
</method>
<!--
ChangeBacklight:
@serial: configuration serial
@output: the API id of the output
@value: the new backlight value
Changes the backlight of @output to @value, which is
expressed as a percentage and rounded to the HW limits.
-->
<method name="ChangeBacklight">
<arg name="serial" direction="in" type="u" />
<arg name="output" direction="in" type="u" />
<arg name="value" direction="in" type="i" />
</method>
<!--
GetCrtcGamma:
@serial: configuration serial
@crtc: API id of the crtc
@red: red gamma ramp
@green: green gamma ramp
@blue: blue gamma ramp
Requests the current gamma ramps of @crtc.
-->
<method name="GetCrtcGamma">
<arg name="serial" direction="in" type="u" />
<arg name="crtc" direction="in" type="u" />
<arg name="red" direction="out" type="aq" />
<arg name="green" direction="out" type="aq" />
<arg name="blue" direction="out" type="aq" />
</method>
<!--
SetCrtcGamma:
@serial: configuration serial
@crtc: API id of the crtc
@red: red gamma ramp
@green: green gamma ramp
@blue: blue gamma ramp
Changes the gamma ramps of @crtc.
-->
<method name="SetCrtcGamma">
<arg name="serial" direction="in" type="u" />
<arg name="crtc" direction="in" type="u" />
<arg name="red" direction="in" type="aq" />
<arg name="green" direction="in" type="aq" />
<arg name="blue" direction="in" type="aq" />
</method>
<!--
PowerSaveMode:
Contains the current power saving mode for the screen, and
allows changing it.
Possible values:
- 0: on
- 1: standby
- 2: suspend
- 3: off
- -1: unknown (unsupported)
A client should not attempt to change the powersave mode
from -1 (unknown) to any other value, and viceversa.
Note that the actual effects of the different values
depend on the hardware and the kernel driver in use, and
it's perfectly possible that all values different than on
have the same effect.
Also, setting the PowerSaveMode to 3 (off) may or may
not have the same effect as disabling all outputs by
setting no CRTC on them with ApplyConfiguration(), and
may or may not cause a configuration change.
Also note that this property might become out of date
if changed through different means (for example using the
XRandR interface directly).
-->
<property name="PowerSaveMode" type="i" access="readwrite" />
</interface>The interface is implemented by mutter 3.9.90.
Note that this interface is considered semi-private and may change in the future. If you want to modify display configuration, you should use the GnomeRR API in gnome-desktop.
Out of scope
Although it is desirable and probably necessary to use HW transforms to implement screen rotation, the proposed API is by far too complex for that.
Rather, a new Rotate() interface might be added, which would also take care of the necessary animation and only at the end apply the HW transform (possibly triggering the apps to provide transformed buffers).