I've got a QNIX QX2710 monitor, which doesn't work out of the box with X Windows. I was able to get it to work with Fluxbox and Gnome using a custom xorg.conf I found online, but for some reason it doesn't work with KDE.

Under KDE, I'll get the splash screen where the icons light up as parts of KDE load, and the screen is just fine at first. But before it can get to the 2nd icon, my monitor just starts displaying some sort of test pattern. Which is what I got before I found a working xorg.conf, so I'm guessing KDE isn't honoring the settings in xorg.conf, but I don't know what I can do about it.

I've pasted my current xorg.conf as a file.

Note there's a lot of suggested xorg.conf's floating around for this monitor online, and it took a while for me to find one that worked. This guy's seemed promising, but it made half of the monitor dim and there were weird visual glitches all over.

FWIW, I'm using Nvidia's 331.67 on a Geforce GTX 670.

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Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    Option "NoLogo"
EndSection


Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "QNIX"
    ModelName      "QX2710"
    DisplaySize     597    336
    HorizSync       88.8 - 160.0
    VertRefresh     59.5 - 120.0
    ModeLine       "2560x1440"          241.50 2560 2608 2640 2720 1440 1443 1448 1481 +hsync -vsync
    ModeLine       "2560x1440_60"       312.25 2560 2752 3024 3488 1440 1443 1448 1493 -hsync +vsync
    Modeline       "2560x1440_75"       397.25  2560 2760 3040 3520  1440 1443 1448 1506 -hsync +vsync
    ModeLine       "2560x1440_85"       454.75 2560 2768 3048 3536 1440 1443 1448 1514 -hsync +vsync
    Modeline       "2560x1440_96"       400.00   2560 2608 2640 2744 1440 1443 1448 1512  +hsync +vsync
#    Modeline      "2560x1440_100"      400.00 2560 2608 2640 2720 1440 1443 1448 1481 +Hsync +Vsync
#    ModeLine       "2560x1440_100"     542.25 2560 2776 3056 3552 1440 1443 1448 1527 -hsync +vsync
#    ModeLine       "2560x1440_120"     661.25 2560 2784 3064 3568 1440 1443 1448 1545 -hsync +vsync
    Option         "DPMS"
EndSection
 
Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "UseEDID" "False"
    Option         "UseEDIDDPI" "False"
    Option         "UseEDIDFreqs" "False"
    Option         "ExactModeTimingsDVI" "True"
    Option         "ModeValidation" "AllowNonEdidModes, AllowNon60hzmodesDFPModes, NoEDIDDFPMaxSizeCheck, NoVertRefreshCheck, NoHorizSyncCheck, NoDFPNativeResolutionCheck, NoMaxSizeCheck, NoMaxPClkCheck, NoEDIDModes"
    Option         "NoBandWidthTest" "true"
    SubSection     "Display"
        Depth       24
        Modes      "2560x1440_96" "2560x1440_85" "2560x1440_75"
"2560x1440_60" "2560x1440"
    EndSubSection
EndSection


kscreen2 or krandr will apply a former randr mode, but they should only pick allowed ones.
since apparently "2560x1440_96" is the only supported mode, i'd try commenting all others/remove them from "Modes" in the "Display" subsection.

This is however a bit weird. The screen should inform X11 about the supported modes (and thus no other mode be selectable and you not have to write modelines in 2014 ...)

Ok, I commented-out all the modes except 2560x1440_96 in the Screen and Monitor sections, but the result is still the same. I've attached my new xorg.conf just to prove that I'm not crazy.

I thought maybe the output from xrandr would be helpful, so I ran it from a another virtual terminal when KDE was running (and not displaying properly), and again when fluxbox is running (and it DOES display properly):

xrandr with KDE:

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Screen 0: minimum 8 x 8, current 800 x 600, maximum 16384 x 16384
DVI-I-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 connected primary 800x600+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1024x768       85.0 +   87.1     75.0     70.1     60.0 
   2560x1440      96.4 
   2048x1536      85.0     75.0     60.0 
   1920x1440      85.0     75.0     60.0 
   1920x1200      60.0 
   1920x1080      59.9 
   1856x1392      75.0     60.0 
   1792x1344      75.0     60.0 
   1680x1050      84.9     74.9     69.9     60.0     59.9 
   1600x1200      85.0     75.0     70.0     65.0     60.0 
   1440x900       59.9 
   1400x1050      85.0     74.8     70.0     60.0 
   1360x768       60.0     59.8 
   1280x1024      85.0     75.0     60.0 
   1280x960       85.0     60.0 
   1152x864      100.0     85.1     85.0     75.0     75.0     70.0     60.0 
   960x600        60.0 
   960x540        60.0 
   840x525        85.0     75.0     69.9     60.0     59.9 
   832x624        74.6 
   800x600        85.1     75.0     72.2     60.3*    56.2 
   720x450        59.9 
   720x400        85.0 
   700x525        74.8     60.0 
   680x384        60.0     59.8 
   640x480        85.0     75.0     72.8     59.9 
   640x400        85.1 
   640x350        85.1 
   512x384        70.1     87.1     60.0 
   400x300        72.2 
   320x240        72.8     60.1 
   320x175        85.3 
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-0 disconnected (normal left inverted right x axis y axis)
DVI-D-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)


xrandr with fluxbox:

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Screen 0: minimum 8 x 8, current 2560 x 1440, maximum 16384 x 16384
DVI-I-0 disconnected primary (normal left inverted right x axis y axis)
DVI-I-1 connected 2560x1440+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1024x768       85.0 +   87.1     75.0     70.1     60.0 
   2560x1440      96.4*
   2048x1536      85.0     75.0     60.0 
   1920x1440      85.0     75.0     60.0 
   1920x1200      60.0 
   1920x1080      59.9 
   1856x1392      75.0     60.0 
   1792x1344      75.0     60.0 
   1680x1050      84.9     74.9     69.9     60.0     59.9 
   1600x1200      85.0     75.0     70.0     65.0     60.0 
   1440x900       59.9 
   1400x1050      85.0     74.8     70.0     60.0 
   1360x768       60.0     59.8 
   1280x1024      85.0     75.0     60.0 
   1280x960       85.0     60.0 
   1152x864      100.0     85.1     85.0     75.0     75.0     70.0     60.0 
   960x600        60.0 
   960x540        60.0 
   840x525        85.0     75.0     69.9     60.0     59.9 
   832x624        74.6 
   800x600        85.1     75.0     72.2     60.3     56.2 
   720x450        59.9 
   720x400        85.0 
   700x525        74.8     60.0 
   680x384        60.0     59.8 
   640x480        85.0     75.0     72.8     59.9 
   640x400        85.1 
   640x350        85.1 
   512x384        70.1     87.1     60.0 
   400x300        72.2 
   320x240        72.8     60.1 
   320x175        85.3 
HDMI-0 disconnected (normal left inverted right x axis y axis)
DP-0 disconnected (normal left inverted right x axis y axis)
DVI-D-0 disconnected (normal left inverted right x axis y axis)
DP-1 disconnected (normal left inverted right x axis y axis)


new xorg.conf:

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Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    Option "NoLogo"
EndSection


Section "Monitor"
    Identifier     "Monitor0"
    VendorName     "QNIX"
    ModelName      "QX2710"
    DisplaySize     597    336
    HorizSync       88.8 - 160.0
    VertRefresh     59.5 - 120.0
#    ModeLine       "2560x1440"          241.50 2560 2608 2640 2720 1440 1443 1448 1481 +hsync -vsync
#    ModeLine       "2560x1440_60"       312.25 2560 2752 3024 3488 1440 1443 1448 1493 -hsync +vsync
#    Modeline       "2560x1440_75"       397.25  2560 2760 3040 3520  1440 1443 1448 1506 -hsync +vsync
#    ModeLine       "2560x1440_85"       454.75 2560 2768 3048 3536 1440 1443 1448 1514 -hsync +vsync
    Modeline       "2560x1440_96"       400.00   2560 2608 2640 2744 1440 1443 1448 1512  +hsync +vsync
#    Modeline      "2560x1440_100"      400.00 2560 2608 2640 2720 1440 1443 1448 1481 +Hsync +Vsync
#    ModeLine       "2560x1440_100"     542.25 2560 2776 3056 3552 1440 1443 1448 1527 -hsync +vsync
#    ModeLine       "2560x1440_120"     661.25 2560 2784 3064 3568 1440 1443 1448 1545 -hsync +vsync
    Option         "DPMS"
EndSection
 
Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "UseEDID" "False"
    Option         "UseEDIDDPI" "False"
    Option         "UseEDIDFreqs" "False"
    Option         "ExactModeTimingsDVI" "True"
    Option         "ModeValidation" "AllowNonEdidModes, AllowNon60hzmodesDFPModes, NoEDIDDFPMaxSizeCheck, NoVertRefreshCheck, NoHorizSyncCheck, NoDFPNativeResolutionCheck, NoMaxSizeCheck, NoMaxPClkCheck, NoEDIDModes"
    Option         "NoBandWidthTest" "true"
    SubSection     "Display"
        Depth       24
#        Modes      "2560x1440_96" "2560x1440_85" "2560x1440_75"
        Modes      "2560x1440_96"
    EndSubSection
EndSection


Well, "something"™ sets the resolution to 800x600

login to KDE, move to VT1 (ctrl+alt+f1, alternatively you can maybe login via ssh?) and run

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export DISPLAY=:0
xrandr -s 2560x1440


return to KDE (ctrl+alt+f7) and run "kcmshell4 kscreen", set the screen resolution (lower right icon in the monitor picture) to 2560x1440, apply, ok, pray, log out and back in again.

alternatively just

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sudo chmod -x `which krandrstartup`


latter will break with the next system update.

I tried your suggestion of launching KDE, switching to another virtual console, and running:

export DISPLAY=:0
xrandr -s 2560x1440

and got this error:

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Size 2560x1440 not found in available modes

I ran just plain "xrandr" again to check the listed modes, and sure enough the 2560x1440 resolution was still in there.

Don't know if this helps, but on a lark I tried running:

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xrandr -s 1024x768 ; xrandr

and:

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xrandr -s 2048x1536 ; xrandr

and both of these times, there were no error messages, but also both times, xrandr was displaying the 800x600 as the active resolution -the '*' in the output for xrandr indicates the current one, right?

Of course, I kept checking on what KDE was up to after each command, and still just test pattern...

"Interesting" - pot. the KDE startup script stumbles on this behavior.

-> Tried "sudo chmod -x `which krandrstartup`" seeking to prevent it running?

On the xrandr weirdness: Maybe the new API is more convincing?

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xrandr --output DVI-I-1 --mode 2560x1440

or alternatively

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xrandr --output DVI-I-1 --auto

It would appear that krandrstartup wasn't installed on my system at all. KDE had been working with a pair of Asus monitors before I replaced them with the Qnix, but I tracked krandrstartup to the kde-workspace-randr package via packages.debian.org, and installed it. I'm using Debian testing, by the way. Didn't seem to make a difference, before or after I did the "chmod -x" on it.

Note that I've currently got Gnome running in the first X display and then launched KDE from another virtual console so I could compose this message and run your suggestions at the same time. Thus KDE is on DISPLAY=:1 --verified by running "DISPLAY=:1 xrandr" and checking that the low resolution was the current mode.

"DISPLAY=:1 xrandr --output DVI-I-1 --mode 2560x1440" yields:

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X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  7 (RRSetScreenSize)
  Serial number of failed request:  40
  Current serial number in output stream:  41

"DISPLAY=:1 xrandr --output DVI-I-1 --auto" yields:

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X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  7 (RRSetScreenSize)
  Serial number of failed request:  40
  Current serial number in output stream:  41

I don't know if this is a clue, but:

I discovered that, when running fluxbox or gnome, I can bring up the KDE control panel (actually titled System Settings, apparently) by running "systemsettings" from an xterm. To be clear, the KDE control panel appears in fluxbox or gnome; it's not miraculously appearing in a malfunctioning KDE X session running from another virtual console.

When I open the "Display and Monitor" part of the KDE control panel, I noticed the following output in the xterm I launched it from:

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QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
Connecting to deprecated signal QDBusConnectionInterface::serviceOwnerChanged(QString,QString,QString)
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
QDBusConnection: session D-Bus connection created before QCoreApplication. Application may misbehave.
kbuildsycoca4 running...
BackendLoader::init: "/usr/lib/x86_64-linux-gnu/kde4/plugins"
This backend is only for XRandR 1.1, your version is:  1 . 4

Is that last message, about XRandR versions, noteworthy?

Seems harmless, https://bugs.kde.org/show_bug.cgi?id=326776

The X11 Error seems far more like a problem to me (--auto fails!)

I assume anything in XRandr does not care about one or more Options in your xorg.conf.

Can you in fluxbox do eg.

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xrandr --output DVI-I-1 --mode 800x600; sleep 10; xrandr --output DVI-I-1 --auto


Be prepared to loose the screen with this.

Hey! Sorry it took me so long to get back. Been a busy week. Please don't give up on me.

I wasn't quite sure if you were looking to try to hit the KDE session or the fluxbox session with the commands:

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xrandr --output DVI-I-1 --mode 800x600; sleep 10; xrandr --output DVI-I-1 --auto

...so I tried both:

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export DISPLAY=:0
xrandr --output DVI-I-1 --mode 800x600; sleep 10; xrandr --output DVI-I-1 --auto
export DISPLAY=:1
xrandr --output DVI-I-1 --mode 800x600; sleep 10; xrandr --output DVI-I-1 --auto

Each time, nothing happened except for generating this error message:
xrandr: cannot find mode 800x600

I ran them both from inside an xterm inside fluxbox. The fluxbox screen didn't show any sign of change, and when I checked on the KDE session it was still doing the same thing as before.