Nouveau, multimonitor setup and tearing

dummy@example.com (Altoid) — Thu, 27 Jun 2024 22:30:51 +0000

Hello:

Still trying to solve a relatively small but annoying tear/artifact in my xorg/nouveau setup in Dadedalus.

Like I posted here:

I wrote:

There is still the matter of some slight tearing which did not ocurr with the Nvidia drivers ...
... there may be some xorg.conf magic to be done as nouveau does not seem to use one by default.

I have managed to sort out an xorg.conf with just a Monitors section to exactly duplicate what xorg automagically does by itself without one for a three 19" monitor setup with two Nvidia FX580 cards and no Xinerama.

Not using Xinerama is a large bonus: now RandR can be used and other applicatons (such as Firefox) which use its output don't make a mess of the way the drop down menus work. ie: invading the next monitor's screen when deploying sideways.

~$ cat /etc/X11/xorg.conf
# xorg.test - started 20240618
# Layout for four screens - 4th. screen not connected
#
#     (0,0)---------+(1280,0)------+(2560,0)------+
# 1 > |             ||             ||             |(3840,256)---+
# 0 > |  1280x1024  ||  1280x1024  ||  1280x1024  ||  1024x768  |< 7
# 2 > |     DP-3    ||    DP-1-2   ||     DP-4    ||   DP-1-1   |< 6
# 4 > |    SM940n   ||    D1914S   ||    SM940n   ||   P150S    |< 8
#     +-------------++-------------++-------------++------------+
#     |<--- 1280 ---|
#     |<---------  2560  --------->|
#     |<-----------------  3840  ---------------->|
#     |<------------------------  4864  ----------------------->|
#

### <--- MONITORS ###                           
Section "Monitor"
    Identifier  "DP-3"
#   Option      "Primary" "true"                   # defined in Option "Position"
    Option      "DPMS" "0"
    Option      "PreferredMode" "1280x1024_75.00"
    Option      "Position" "0 0"
 EndSection

Section "Monitor"
    Identifier  "DP-1-2"
#   Option      "LeftOf" "DP-4"                    # defined in Option "Position"
    Option      "DPMS" "0"
    Option      "PreferredMode" "1280x1024_75.00"
    Option      "Position" "1280 0"
EndSection

Section "Monitor"
    Identifier  "DP-4"
#   Option      "RightOf" "DP-1-2"                 # defined in Option "Position"
    Option      "DPMS" "0"
    Option      "PreferredMode" "1280x1024_75.00"
    Option      "Position" "2560 0"
EndSection
 
#####################################################################################
# Section "Monitor"                                # not connected 
#   Identifier  "DP-4"                             # n/c check with monitor connected
#   Option      "RightOf" "DP-1-2"                 # defined in Option "Position"
#   Option      "DPMS" "0"
#   Option      "PreferredMode" "1024x768"
#   Option      "Position" "3840 256"              # n/c check with monitor connected
# EndSection
######################################################################################

### MONITORS --> ###

This makes for a 3840x1024 screen, which xrandr IDs as Screen0:

~$ xrandr
Screen 0: minimum 320 x 200, current 3840 x 1024, maximum 8192 x 8192
DVI-I-2 disconnected (normal left inverted right x axis y axis)
DP-3 connected primary 1280x1024+0+0 (normal left inverted right x axis y axis) 376mm x 301mm

Unfortunately, I have not been able to find out what settings I need for the ServerLayout, Device and Screen sections to be able to finish putting together a complete xorg.conf which I hope to be able to tweak so as to get the most of my twin card setup.

But I think I have found the origin of the diagonal tear at times present in the middle of my center monitor's screen (upper left down to lower right).

While trawling the web I came across this tidbit:

About not using Xinerama, I ran into a "BaseMosaic" option in the nvidia documentation somewhere in Appendix B:
--- snip ---
... Base Mosaic does not guarantee there will be no tearing between the display boundaries.
--- snip ---

Did not quite understand what it was about till I realised that I don't have three working 1280x1024 screens but one 3840x1024 screen driven by the combined output from the two cards, each card driving 50% of the whole screen.

And the display boundary mentioned in the cited post being the slight diagonal tear I see in the center of the 3840x1024 screen.

This is as far as I have been able to get.

Any ideas on how to continue?
I think there is room for improvement as I expect that whatever settings the xorg/nouveau combo is using are probably default/conservative.

Thanks in advance.

Best,

Dev1 Galaxy Forum / Nouveau, multimonitor setup and tearing

Nouveau, multimonitor setup and tearing