xrandr

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xrandr --output HDMI-0 --mode 1920x1080
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How to use xrandr

Getting started

What is xrandr?

xrandr is a command-line tool to interact with the X RandR
extension [see x.org,
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).

 

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