Xorg/Multiple monitors

Important
Anything passing for a modern Xorg has autoconfiguration and RandR extension which means that it should just work and any configuration can be done either via xrandr command line tool or a graphical tool provided by your desktop environment assuming you're using one. Nevertheless this article can prove useful for legacy or advanced use cases.

This page describes the setup of multiple monitors under Xorg. Multihead support in X has luckily progressed in recent versions, so it is pretty easy to set up. Usually it is as simple as defining monitors in Xorg configuration files.

Installation

USE flags

Be aware, most desktop environments, window managers, and graphical toolkit require the xinerama USE flag to be enabled in order to support multiple monitors. When preparing to add second (or third!) monitor to the system, be sure to add this flag to /etc/portage/make.conf:

FILE /etc/portage/make.conf

USE="xinerama"

Emerge

Next rebuild the @world set.

root #emerge --ask --update --deep --newuse @world

Configuration

Simple

The basic procedure is defining one "Monitor" section per Monitor and then tying everything together in a "Device" section that specifies the video chip driving the monitor(s). For the following examples, Xorg's configuration is presumed to be stored in /etc/X11/xorg.conf.d (no /etc/X11/xorg.conf file will be used). Alternatively, if the user desires, all files defined in the xorg.conf.d folder can be combined together to form an xorg.conf. The following example is the basic version for two monitors of the same resolution.

FILE /etc/X11/xorg.conf.d/41-dvi-monitor.confRight monitor

Section "Monitor"
    Identifier  "DVI"
EndSection

FILE /etc/X11/xorg.conf.d/42-hdmi-monitor.confLeft monitor

Section "Monitor"
    Identifier  "DP"
    Option      "LeftOf"        "DVI"
EndSection

Each "Monitor" section must contain an Identifier, everything else is optional. The identifier may be freely chosen. The placement of the monitors is done relatively with one of the keywords RightOf, LeftOf, Above, or Below. Additionally there is a Relative keyword for fancy cases, however this article avoids using it. Each monitor can receive custom options (e.g. resolution) in its configuration.

Next the monitors need to be mapped to the outputs. This is done in the Device section describing the video chip and driver. The Device section of the two monitor example (using an Intel video card) would look like the following.

FILE /etc/X11/xorg.conf.d/50-video.confDevice section

Section "Device"
    Identifier  "Device0"
    Option      "Monitor-DVI2" "DVI"
    Option      "Monitor-DP1" "DP"
    Driver      "intel"
EndSection

Important
The Driver keyword has to be updated using the system's video card. Common options are intel, radeon or nouveau.

The names of the monitors are in the form "Monitor-<port>". The port each monitor was on was determined by running the xrandr -q command. This should be everything needed for a basic setup.

More complex cases

Sometimes more complex layouts will be needed. A third example of a monitor file, with resolution options could look like the following.

FILE /etc/X11/xorg.conf.d/43-lvds-monitor.confAdvanced configuration with resolution defined

Section "Monitor"
    Modeline    "1280x800x60" 71.00 1280 1328 1360 1440 800 803 809 823
    Option      "PreferredMode" "1280x800"
    Identifier  "LVDS"
    Option      "RightOf"       "DVI"
    VendorName  "CMO"
EndSection

It is possible to see in the example above that manufacturer names can be used to get a single config file attached to a single device.

Next a more complete example of the video configuration for x11-drivers/xf86-video-intel package will be provided:

FILE /etc/X11/xorg.conf.d/50-video.confAdvanced intel example

Section "Device"
    Option      "Monitor-DVI2" "DVI"
    Option      "Monitor-DP1" "DP"
    Option      "Monitor-LVDS2" "LVDS"
    Option      "AccelMethod" "sna"
    Option      "TearFree" "true"
    Option      "DRI" "True"
    Option      "XvMCSurfaces" "6"
    Identifier  "GMA-X4500"
    Driver      "intel"
EndSection

The following was made for the x11-drivers/xf86-video-ati radeon driver. Notice the change of the video output name on the same laptop.

FILE /etc/X11/xorg.conf.d/50-video.confRadeon driver example

Section "Device"
    Option      "Monitor-DVI-0" "DVI"
    Option      "Monitor-HDMI-0" "DP"
    Option      "Monitor-LVDS" "LVDS"
    Option      "AccelMethod" "EXA"
    Option      "ClockGating" "On"
    Option      "ForceLowPowerMode" "On"
    Option      "VGAAccess" "On"
    Identifier  "HD3450"
    Driver      "radeon"
    Option      "MonitorLayout" "LVDS, AUTO"
EndSection

Note
If the x11-drivers/ati-drivers package is being used, to increase the Virtual in xorg.conf it must be an exact multiple of the highest resolution monitor. For instance, if the largest monitor is 1920x1080, enter 3840 2160 for Virtual. This will enable xrandr to work properly. This is a problem and appears to be a bug.

Different resolutions

Xrandr-1.3 is a little deficient in this regard. X can precisely probe the screen devices, but xrandr will get/display the minimum resolution fitted to the smallest screen attached to the machine. This is annoying because it creates a debug situation. Xorg.0.log has to be read in order to get possible monitor resolutions, define a few modes with xrandr, and then choose a video mode with xrandr. There is nothing user friendly about the process. The process usually takes matter of minutes, but is annoying when a user desires to attach a bigger screen to a computer for a simple event like watching a movie. The latter can be achieved, somewhat, with the following example.

FILE /etc/X11/xorg.conf.d/44-vga-monitor.confPreferred video modes example

Section "Monitor"
    Option      "SyncOnGreen" "True"
    Modeline    "1280x1024x85" 157.50 1280 1344 1504 1728 1024 1025 1028 1072
    Modeline    "1600x1200x85" 229.50 1600 1664 1856 2160 1200 1201 1204 1250
    Modeline    "1600x1200x75" 202.50 1600 1664 1856 2160 1200 1201 1204 1250
    Option      "PreferredMode" "1600x1200"
    Identifier  "VGA"
    VendorName  "GSM"
EndSection

Troubleshooting

Bug: Anyone can notice the level of fine probing X server does (or rather x11-drivers/xf86-input-evdev). Numerous handy details are provided (numerous screen resolutions, gamma values, and other obscure values). One can pick any defined video mode on the list and X server display it. Now, whenever anyone desires to change the video mode of a particular display xrandr will complain about non-existant modes. What? The video modes vanished somewhere? The only that remain are the ones that can fit to the biggest resolution on the smallest display. To get more video modes copy/paste some preferred video modes like in the example above.