How to use xrandr
=================
:toc:
Cyril Brulebois <kibi@debian.org>


Getting started
---------------

What is xrandr?
~~~~~~~~~~~~~~~

`xrandr` is a command-line tool to interact with the X `RandR`
extension [see https://www.x.org/wiki/Projects/XRandR[x.org],
https://en.wikipedia.org/wiki/RandR[wikipedia]], which allows for
live (re)configuration of the X server (_i.e._ without restarting it):
It provides automatic discovery of modes (resolutions, refresh rates,
etc.) together with the ability to configure outputs dynamically
(resize, rotate, move, etc.).

*FIXME: Status across drivers?*

What consequences for xorg.conf?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Starting with `squeeze`, removing the `xorg.conf` configuration file
entirely should work well enough, but in case that doesn’t work out,
let’s document what can be removed from it from a `RandR` point of
view.

With the driver detecting modes automatically, several configuration
options become useless most of the time in your configuration file
(xorg.conf). You might want to remove:

 * `HorizSync` and `VertRefresh` from the `Monitor` section.
 * Modes from `Display` subsection in `Screen` section.
 * `ModeLine` from the `Monitor` section.

There’s also no need to keep static dual-head configuration. Some
suggestions to get a tiny `xorg.conf`:

 * Drop dual `Device`/`Screen`/`Monitor` sections, a single one is
   needed.
 * Drop `MonitorLayout` option and `Screen` lines from the remaining
   `Device` section.
 * Drop the `ServerLayout` section(s).
 * Drop `RightOf`/`LeftOf` indication of the remaining `Screen` line
   in `ServerLayout` section.


<<<
Basic xrandr usage
------------------

Once the configuration file (`xorg.conf`) is removed or updated,
starting the server should enable some outputs by default. Their
top-left corners will be at the same part of the image, but their
modes will probably be different.

All outputs may be configured through `xrandr`. To see the available
outputs, just run `xrandr`:

----
$ xrandr
Screen 0: minimum 320 x 200, current 1280 x 800, maximum 4096 x 4096
VGA1 disconnected (normal left inverted right x axis y axis)
LVDS1 connected 1280x800+0+0 inverted X and Y axis (normal left inverted right x axis y axis) 261mm x 163mm
   1280x800       59.8*+
   1024x768       60.0
   800x600        60.3     56.2
   640x480        59.9
DVI1 disconnected (normal left inverted right x axis y axis)
TV1 disconnected (normal left inverted right x axis y axis)
----

Comments:

 * We see 4 outputs: `VGA1`, `LVDS1`, `DVI1`, `TV1`.
 * Only the internal panel (`LVDS1`) is connected and it supports 4
   modes at 60 Hz, 1 mode at 56 Hz.
 * The mode marked with a star (`*`) is the current mode.
 * The one marked with a plus (`+`) is the preferred one. Most
   monitors report a preferred mode to the driver. And the
   server/driver will generally choose it by default.

*FIXME: Mention output name conventions?*

When manipulating `VGA1` output properties, you should use:

----
$ xrandr --output VGA1 <options>
----

Adding/removing heads dynamically
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The old days where you had to restart X when plugging a new monitor
are gone. With `RandR` 1.2, you can plug/unplug monitors whenever you
want. Running the following line will query all outputs and enable
them with their default mode:

----
$ xrandr --auto
----

You may also disable one output using:

----
$ xrandr --output LVDS1 --off
----

This may be useful for some buggy application that don’t support
multiple outputs well. Also, due to CRTC limitations (see the Caveats
section below), it is often required to disable one output before
enabling another since most hardware only support 2 at the same time.

Changing the mode
~~~~~~~~~~~~~~~~~

With the above `xrandr` output, you may change the `LVDS1` mode to
`1024x768` using:

----
$ xrandr --output LVDS1 --mode 1024x768
----

The refresh rate may also be changed, either at the same time or
independently:

----
$ xrandr --output LVDS1 --mode 1024x768 --rate 75
$ xrandr --output LVDS1 --rate 75
----

To get back to the default mode:

----
$ xrandr --output LVDS1 --auto
----


<<<
Placing outputs in a virtual screen
-----------------------------------

A bit of configuration for non-KMS setups:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let’s have a look at the maximal virtual screen size, we see
`4096x4096` in this example:

----
$ xrandr|head -1
Screen 0: minimum 320 x 200, current 1280 x 800, maximum 4096 x 4096
----

With KMS (*FIXME: Link to a page which explains what KMS is*),
there's no need to specify any `Virtual` option. With DRI and without
KMS, that might be needed. Indeed, drivers will often create a default
virtual screen with small dimensions, for instance `1600x1200`, to
reduce memory consumption.

If you plan to use multiple outputs displaying different zones, you
should configure your `xorg.conf` by adding a `Virtual` line to the
`Display` subsection in the `Screen` section.

----
Section "Screen"
  …
  SubSection "Display"
    Depth 24
    Virtual 3000 2000
  EndSubSection
EndSection
----

Place outputs
~~~~~~~~~~~~~

Outputs are placed using the following options:
`--right-of`/`--left-of`/`--above`/`--below`. For instance, to place
the `VGA1` output virtually-right of the internal panel (`LVDS1`):

----
$ xrandr --output VGA1 --right-of LVDS1
----

Note that hardware and memory limitations may severely restrict the
size of your virtual screen, see the Caveats section below.


<<<
Adding new modes
----------------

Under some circumstances, some modes might be missing. For instance,
if the monitor does not report correct EDID information. Or if the
output didn't have a CRTC available at startup because another output
was using it and you disabled it in the meantime.

If a mode exist, you may add it to one output with:

----
$ xrandr --addmode VGA1 800x600
----

If the mode does not exist, you may first create it by passing a modeline:

----
$ xrandr --newmode <ModeLine>
----

You may create a modeline using the `gtf` or `cvt` tools (shipped in
the `xserver-xorg-core` package).
