Ubuntu – How to disable monitor auto configuration

hdmimultiple-monitorsxfcexorg

Im running Ubuntu 16.04 (dev branch), XFCE as desktop environment.

My computer is connected to two monitors. One via regular VGA cable and the other
is connected to a HDMI Switcher. That setup works fine, but every time I temporarily change the HDMI Switcher to another HDMI connection Xorg detects that the monitor has been disconnected and reconfigures Xorg to only use a single monitor, switching the HDMI Switcher back to my computer doesn't reactivate dual monitor use either, I have to manually trigger that with xrandr

How can I prevent this kind of automatic monitor detection and reconfiguration from happening? I'd like to be able to switch the HDMI Switcher without the computer doing anything.

Command used to restore the multi-monitor setup:

xrandr \
 --output HDMI-1 \
   --mode "1920x1080" \
   --panning "1920x1080+1280+0/0x0+0+0" \
 --output DVI-1-I \
   --mode "1280x1024" \
   --panning "1280x1024/0x0+0+0"

xrandr output on regular use:

Screen 0: minimum 320 x 200, current 3200 x 1080, maximum 8192 x 8192
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 connected 1920x1080+1280+0 (normal left inverted right x axis y axis) 443mm x 249mm panning 1920x1080+1280+0
   1920x1080     60.00*+  60.00    50.00    59.94    59.93  
   1920x1080i    60.00    50.00    59.94  
   1680x1050     59.95    59.88  
   1600x1024     60.17  
   1400x1050     74.76    70.00    59.98  
   1280x1024     75.02    60.02  
   1440x900      59.89    59.90  
   1280x960      60.00  
   1360x768      59.80    59.96  
   1280x800      59.91  
   1152x864      75.00    75.00    70.00    60.00  
   1280x720      60.00    50.00    59.94  
   1024x768      60.04    75.03    70.07    60.00  
   960x720       75.00    60.00  
   928x696       75.00    60.05  
   896x672       75.05    60.01  
   960x600       60.00  
   832x624       74.55  
   960x540       59.99  
   800x600       75.00    70.00    65.00    60.00    72.19    75.00    60.32    56.25  
   840x525       74.96    69.88    60.01    59.88  
   720x576       50.00  
   800x512       60.17  
   700x525       74.76    70.06    59.98  
   720x480       60.00    59.94  
   640x512       75.02    60.02  
   720x450       59.89  
   640x480       60.00    72.81    75.00    66.67    60.00    59.94  
   720x400       70.08  
   680x384       59.80    59.96  
   576x432       75.00    75.00    70.00    60.06  
   512x384       75.03    70.07    60.00  
   416x312       74.66  
   400x300       72.19    75.12    60.32    56.34  
   320x240       72.81    75.00    60.05  
DVI-I-1 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 359mm x 287mm
   1280x1024     60.02*+  75.02  
   1280x960      60.00  
   1280x800      74.93    59.81  
   1152x864      75.00  
   1280x768      74.89    59.87  
   1280x720      60.00  
   1024x768      75.08    75.03    70.07    60.00  
   1024x576      59.97  
   832x624       74.55  
   800x600       72.19    75.00    70.01    60.32    56.25  
   848x480       60.00  
   640x480       75.00    72.81    66.67    60.00  
   720x400       70.08

xrandr output when HDMI switcher is switched to a different connection:

Screen 0: minimum 320 x 200, current 1280 x 1024, maximum 8192 x 8192
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
DVI-I-1 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 359mm x 287mm
   1280x1024     60.02*+  75.02  
   1280x960      60.00  
   1280x800      74.93    59.81  
   1152x864      75.00  
   1280x768      74.89    59.87  
   1280x720      60.00  
   1024x768      75.08    75.03    70.07    60.00  
   1024x576      59.97  
   832x624       74.55  
   800x600       72.19    75.00    70.01    60.32    56.25  
   848x480       60.00  
   640x480       75.00    72.81    66.67    60.00  
   720x400       70.08

Some more things I tried:

With the monitor connected find out the modeline:

$ xvidtune -show 
"1920x1080"   148.50   1920 2008 2052 2200   1080 1084 1089 1125 -hsync +vsync

Then with the monitor disconnected:

$ xrandr --newmode "1920x1080-new"   148.50   1920 2008 2052 2200   1080 1084 1089 1125 -hsync +vsync
$ xrandr --addmode HDMI-1 1920x1080-new
$ xrandr --output HDMI-1 --mode 1920x1080-new

This allows me to activate the display when disconnected, which would otherwise fail due to non-existent mode 1920×1080. The problem however remains, if I switch the HDMI Switcher the display gets disabled, even if I have it set to a custom mode.

This are the udev events when disconnecting the display with the HDMI Switcher:

$ udevadm monitor --property
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

KERNEL[41678.571099] change   /devices/pci0000:00/0000:00:02.0/0000:01:00.0/drm/card0 (drm)
ACTION=change
DEVNAME=/dev/dri/card0
DEVPATH=/devices/pci0000:00/0000:00:02.0/0000:01:00.0/drm/card0
DEVTYPE=drm_minor
HOTPLUG=1
MAJOR=226
MINOR=0
SEQNUM=3248
SUBSYSTEM=drm

UDEV  [41678.573432] change   /devices/pci0000:00/0000:00:02.0/0000:01:00.0/drm/card0 (drm)
ACTION=change
DEVNAME=/dev/dri/card0
DEVPATH=/devices/pci0000:00/0000:00:02.0/0000:01:00.0/drm/card0
DEVTYPE=drm_minor
HOTPLUG=1
ID_FOR_SEAT=drm-pci-0000_01_00_0
ID_PATH=pci-0000:01:00.0
ID_PATH_TAG=pci-0000_01_00_0
MAJOR=226
MINOR=0
SEQNUM=3248
SUBSYSTEM=drm
TAGS=:uaccess:master-of-seat:seat:
USEC_INITIALIZED=31279519

This are the udev events when reenabling the display with the HDMI Switcher:

$ udevadm monitor --property
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

KERNEL[41696.104481] change   /devices/pci0000:00/0000:00:02.0/0000:01:00.0/drm/card0 (drm)
ACTION=change
DEVNAME=/dev/dri/card0
DEVPATH=/devices/pci0000:00/0000:00:02.0/0000:01:00.0/drm/card0
DEVTYPE=drm_minor
HOTPLUG=1
MAJOR=226
MINOR=0
SEQNUM=3249
SUBSYSTEM=drm

UDEV  [41696.105685] change   /devices/pci0000:00/0000:00:02.0/0000:01:00.0/drm/card0 (drm)
ACTION=change
DEVNAME=/dev/dri/card0
DEVPATH=/devices/pci0000:00/0000:00:02.0/0000:01:00.0/drm/card0
DEVTYPE=drm_minor
HOTPLUG=1
ID_FOR_SEAT=drm-pci-0000_01_00_0
ID_PATH=pci-0000:01:00.0
ID_PATH_TAG=pci-0000_01_00_0
MAJOR=226
MINOR=0
SEQNUM=3249
SUBSYSTEM=drm
TAGS=:seat:master-of-seat:uaccess:
USEC_INITIALIZED=31279519

.

$ sudo lshw -c display
  *-display                 
       description: VGA compatible controller
       product: Redwood XT [Radeon HD 5670/5690/5730]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:01:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:55 memory:c0000000-cfffffff memory:fea20000-fea3ffff ioport:e000(size=256) memory:c0000-dffff

Best Answer

This is not a final answer, just a pointer into the right direction. On XFCE the xfsettingsd programm is responsible for handling of the RRScreenChangeNotify event and triggering the mode switch, the exact function can be found in xfce4-settings-4.12.0/xfsettingsd/displays.c line 77, the function is called xfce_displays_helper_screen_on_event. Disabling the function will make the mode no longer change when a new monitor connects.

Debug information can be obtained with:

DISPLAY=:0 XFSETTINGSD_DEBUG=512 xfsettingsd --replace --no-daemon
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