Xorg-7 Testing and Configuration

Testing Xorg



Before starting Xorg for the first time, is useful to rebuild the library cache by running ldconfig as the root user.



Before starting Xorg for the first time, is often needed to reboot the system to ensure all appropriate daemons are started and appropriate security issues are properly set. As an alternative, logging out and logging back in may work, but as of this writing has not been tested.



If Xorg hangs for some reason (for example, lacking a proper input driver), the system may stop responding to any user input. As a precaution, you can enable a magic SysRq key before testing Xorg. As the root user, issue:

echo 4 > /proc/sys/kernel/sysrq

Then if Xorg hangs, it's possible to use Alt+SysRq+R to reset the keyboard mode. Now it should be able to use Ctrl+Alt+Fx (replace x with a VT number) to switch to another VT. If it works, login and kill Xorg using command line in the new VT.

To test the Xorg installation, issue startx. This command brings up a rudimentary window manager called twm with three xterm windows and one xclock window. The xterm window in the upper left is a login terminal and running exit from this terminal will exit the X Window session. The third xterm window may be obscured on your system by the other two xterms.



When testing Xorg with the twm window manager, there will be several warnings in the Xorg log file, $HOME/.local/share/xorg/Xorg.0.log, about missing font files. In addition, there will be several warnings on the text mode terminal (usually tty1) about missing fonts. These warnings do not affect functionality, but can be removed if desired by installing the Xorg Legacy Fonts.



On systems with NVIDIA GPUs which are using the Nouveau kernel driver, you may encounter occasional GPU crashes and hangs. If this problem occurs, downgrade to the latest version of the Linux 6.1 kernel.

Generally, there is no specific configuration required for Xorg, but customization is possible. For details, see the section called “Setting up Xorg Devices” below.

Checking the Direct Rendering Infrastructure (DRI) Installation

DRI is a framework for allowing software to access graphics hardware in a safe and efficient manner. It is installed in X by default (using Mesa) if you have a supported video card.

To check if DRI drivers are installed properly, check the log file $HOME/.local/share/xorg/Xorg.0.log (or /var/log/Xorg.0.log if you have built Xorg-Server-21.1.11 with the suid bit) for statements such as:

(II) modeset(0): [DRI2] Setup complete
(II) modeset(0): [DRI2]   DRI driver: crocus
(II) modeset(0): [DRI2]   VDPAU driver: va_gl


DRI configuration may differ if you are using alternate drivers, such as traditional DDX drivers, or the proprietary drivers from NVIDIA or AMD.

Another way to determine if DRI is working properly is to use one of the two optionally installed OpenGL demo programs in Mesa-24.0.1. From an X terminal, run glxinfo -B and look for the phrase:

name of display: :0
display: :0  screen: 0
direct rendering: Yes

If direct rendering is enabled, you can add verbosity by running LIBGL_DEBUG=verbose glxinfo. This will show the drivers, device nodes and files used by the DRI system.

To confirm that DRI2 hardware acceleration is working, you can (still in the X terminal) run the command glxinfo | grep -E "(OpenGL vendor|OpenGL renderer|OpenGL version)". If that reports something other than Software Rasterizer then you have working acceleration for the user who ran the command.

If your hardware does not have any DRI2 driver available, it will use a Software Rasterizer for Direct Rendering. In such cases, you can use a new, LLVM-accelerated, Software Rasterizer called LLVMPipe. In order to build LLVMPipe just make sure that LLVM-17.0.6 is present at Mesa build time. Note that all decoding is done on the CPU instead of the GPU, so the display will run slower than with hardware acceleration. To check if you are using LLVMpipe, review the output of the glxinfo command above. An example of the output using the Software Rasterizer is shown below:

OpenGL vendor string: VMware, Inc.
OpenGL renderer string: Gallium 0.4 on llvmpipe (LLVM 3.5, 256 bits)
OpenGL version string: 3.0 Mesa 10.4.5

You can also force LLVMPipe by exporting the LIBGL_ALWAYS_SOFTWARE=1 environment variable when starting Xorg.

Again, if you have built the Mesa OpenGL demos, you can also run the test program glxgears. This program brings up a window with three gears turning. The X terminal will display how many frames were drawn every five seconds, so this will give a rough benchmark. The window is scalable, and the frames drawn per second is highly dependent on the size of the window. On some hardware, glxgears will run synchronized with the vertical refresh signal and the frame rate will be approximately the same as the monitor refresh rate.

Debugging Xorg

When starting xorg, there are a couple of ways to check what any issues you may have. If the system comes up, you can see what driver is being used by running xdriinfo. If there are issues or you just want to check, look at Xorg.0.log.

The location of Xorg.0.log depends on how Xorg is installed. If the instructions in the book are followed closely and Xorg is started from the command line, it will be located in the $HOME/.local/share/xorg/ directory. If Xorg is started by a display manager (e.g. lightdm-1.32.0, sddm-0.20.0, or GDM-45.0.1) or if $XORG_PREFIX/bin/Xorg has the suid bit set, it will be located in the /var/log/ directory.

Xorg.0.log Issues

When you look at Xorg.0.log, check for entries like (EE) or (WW). Below are some common entries:

(WW) Open ACPI failed (/var/run/acpid.socket)

This warning is because acpid-2.0.34 is not installed. If you are not on a laptop, it can be safely ignored. On a laptop, install acpid-2.0.34 to enable actions like recognizing when the lid is closed.

(WW) VGA arbiter: cannot open kernel arbiter, no multi-card support

This warning is displayed when a regular user starts Xorg. The library libpciaccess.so issues this warning when it tries to open /dev/vga_arbiter. If there is no more than one legacy PCI (not PCIe) graphic cards on the system, it can safely be ignored. If really necessary, the permissions of this device can be changed by adding a udev rule and adding the local user to the video group. As the root user:

cat > /etc/udev/rules.d/99-vga-arbiter.rules << EOF
# /etc/udev/rules.d/99-vga-arbiter.rules: Set vga_arbiter group/mode

ACTION=="add", KERNEL=="vga_arbiter", GROUP="video" MODE="0660"

usermod -a -G video <user running xorg>

Hybrid Graphics

Hybrid Graphics is still in experimental state for Linux. Xorg Developers have developed a technology called PRIME that can be used for switching between integrated and muxless discrete GPU at will. Automatic switching is not possible at the moment.

In order to use PRIME for GPU switching, make sure that you are using Linux Kernel 3.4 or later (recommended). You will need latest DRI and DDX drivers for your hardware and Xorg Server 1.13 or later.

Xorg Server should load both GPU drivers automatically. You can check that by running:

xrandr --listproviders

There should be two (or more) providers listed, for example:

Providers: number : 2
Provider 0: id: 0x7d cap: 0xb, Source Output, Sink Output, Sink Offload crtcs: 3 outputs: 4 associated providers: 1 name:modesetting
Provider 1: id: 0x56 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 6 outputs: 1 associated providers: 1 name:modesetting

In order to be able to run a GLX application on a discrete GPU, you will need to run the following command, where <provider> is the id of the more powerful discrete card, and <sink> is the id of card which has a display connected:

xrandr --setprovideroffloadsink <provider> <sink>


With the Xorg modesetting driver, which is DRI3 capable, the above command is no longer necessary. It does no harm however.

Then, you will need to export the DRI_PRIME=1 environment variable each time you want the powerful GPU to be used. For example,

DRI_PRIME=1 glxinfo | grep -E "(OpenGL vendor|OpenGL renderer|OpenGL version)"

will show OpenGL vendor, renderer and version for the discrete GPU.

If the last command reports same OpenGL renderer with and without DRI_PRIME=1, you will need to check your installation.

Setting up Xorg Devices

For most hardware configurations, modern Xorg will automatically get the server configuration correct without any user intervention. There are, however, some cases where auto-configuration will be incorrect. Following are some example manual configuration items that may be of use in these instances.

Setting up X Input Devices

For most input devices, no additional configuration will be necessary. This section is provided for informational purposes only.

A sample default XKB setup could look like the following (executed as the root user):

cat > /etc/X11/xorg.conf.d/xkb-defaults.conf << "EOF"
Section "InputClass"
    Identifier "XKB Defaults"
    MatchIsKeyboard "yes"
    Option "XkbLayout" "fr"
    Option "XkbOptions" "terminate:ctrl_alt_bksp"

The XkbLayout line is an example for a French (AZERTY) keyboard. Change it to your keyboard model. That line is not needed for a QWERTY (US) keyboard.

Fine Tuning Display Settings

If you want to set the monitor resolution for Xorg, first run xrandr in a X terminal to list the supported resolutions and the corresponding refresh rates. For example, it outputs the following for one monitor:

Screen 0: minimum 16 x 16, current 5760 x 2160, maximum 32767 x 32767
DP-1 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
   3840x2160     59.98*+
   2048x1536     59.95
   1920x1440     59.90
   1600x1200     59.87
   1440x1080     59.99
   1400x1050     59.98
   1280x1024     59.89
   1280x960      59.94
   1152x864      59.96
   1024x768      59.92
   800x600       59.86
   640x480       59.38

From the output we can see the monitor is identified DP-1. Select a suitable resolution from the output list, for example 1920x1440. Then as the root user, create a configuration file for the Xorg server:

cat > /etc/X11/xorg.conf.d/monitor-DP-1.conf << "EOF"
Section "Monitor"
    Identifier  "DP-1"
    Option      "PerferredMode" "1920x1440"

Sometimes xrandr may fail to detect some resolution settings supported by the monitor. It usually happens with virtual monitors of virtual machine managers like qemu-8.2.1 or VMWare: a virtual monitor actually supports all pairs of integers in a range as the resolution, but xrandr will only list a few. To use a resolution not listed by xrandr, first run cvt to get the mode line for the resolution. For example:

cvt 1600 900
# 1600x900 59.95 Hz (CVT 1.44M9) hsync: 55.99 kHz; pclk: 118.25 MHz
Modeline "1600x900_60.00"  118.25  1600 1696 1856 2112  900 903 908 934 -hsync +vsync

As the root user, create a Xorg server configuration file containing this mode line, and specify the mode as preferred mode:

cat > /etc/X11/xorg.conf.d/monitor-DP-1.conf << "EOF"
Section "Monitor"
    Identifier  "DP-1"
    Modeline    "1600x900_60.00"  118.25  1600 1696 1856 2112  900 903 908 934 -hsync +vsync
    Option      "PerferredMode"   "1600x900_60.00"

Some high-end LCD monitors support a refresh rate higher than 100 Hz but xrandr may fail to recognize the supported refresh rate and use 60 Hz instead. This issue would prevent you from utilizing the full capability of the monitor, and may cause the screen to flicker or show artifacts like meshes or grids. To resolve the issue, again use cvt to get the mode line with a custom refresh rate:

cvt 3840 2160 144
# 3840x2160 143.94 Hz (CVT) hsync: 338.25 kHz; pclk: 1829.25 MHz
Modeline "3840x2160_144.00"  1829.25  3840 4200 4624 5408  2160 2163 2168 2350 -hsync +vsync

Then paste it into the Xorg server configuration file and set it as the preferred mode.

Another common setup is having multiple server layouts for use in different environments. Though the server will automatically detect the presence of another monitor, it may get the order incorrect:

cat > /etc/X11/xorg.conf.d/server-layout.conf << "EOF"
Section "ServerLayout"
    Identifier     "DefaultLayout"
    Screen      0  "Screen0" 0 0
    Screen      1  "Screen1" LeftOf "Screen0"
    Option         "Xinerama"

When you drag a window in twm (or any non-compositing window manager) horizontally, you may observe that the vertical borders of the window are broken into multiple segments. This is an example of the visual artifacts called screen tearing. To resolve the screen tearing problems, create a configuration file that enables the TearFree option. Note that you must have the Tearfree patch applied from Xorg-Server-21.1.11 for this to function properly, and it may increase memory allocation and reduce performance.

cat > /etc/X11/xorg.conf.d/20-tearfree.conf << "EOF"
Section "Device"
   Identifier "Graphics Adapter"
   Driver     "modesetting"
   Option     "TearFree" "true"

With modern Xorg, little or no additional graphic card configuration is necessary. If you should need extra options passed to your video driver, add them into the Device section as well. The options supported by the modesetting driver are documented in the man page modesetting(4).