Greetings

(tried this over on Linux software and no responses - - - maybe here?)

Running debian buster (testing) using lxqt. Have 3 nvidia 570 gpus and 4 1920x1080 monitors.

Found that there were some issues with proprietary drivers nvidia 375.66 - - - was having one gpu halt and upon reboot things seemed to work again. The time between reboots was getting down to less than 12 hrs when I landed up with a system with corrupted networking on a reboot.
So in the process of re-install I decided to try the Nouveau drivers instead. So now I have a one gpu 2 monitor system available. Got to very much like the four monitors for my work layout so I want the other two monitors working.

Using xrandr I can see Provider 0,1 and 2.
DVI-I-1 and DVI-I-2 are connected.
DVI-I-2-5 and DVI-I-2-6 are listed as disconnected.

I have been able to create a virtual screen of 3840 x 3000 which is how I had things before this reinstall
using $xrandr --fb 3840x3000 .
Tried $xrandr --setprovideroutputsource 1 0
no changes
looked for pci addresses
found 01:00.0. 02:00.0 and 03:00.0

Looked in the kernel log and all I can see is a error code of -19 (for both Provider 1 and 2) which, after a lot of digging, seems to be saying "no such device"
Tried $ xrandr --addmode DVI-I-2-5 1920x1080
then DVI-I-2-5 still shows as disconnected but with a resolution of 1920x1080 60.00
Next I tried $ xrandr --output VGA-2 --mode 1920x1080 --pos 1280x1280 --rotate inverted
result - - warning: output VGA-2 not found; ignoring

It used to be that this setup happened in xorg.conf (and was a monolithic file), then things were changed to files placed in /etc/xorg.conf.d (as individual descriptions - - - ie. devices, monitors and AIUI screens) and in my system these kind of files are now in /etc/X11/Xsession.d but there aren't any to do with graphics issues.

Any information I haven't provided that would help others help me?

I can't find any guides on how to setup xrandr to use multiple gpus AND more than 2 or 3 monitors (usually from only 1 gpu).
Most of the randr information I can find is for at newest randr 1.3 and I have 1.5. Randr 1.4 is where the multiple gpu option was introduced so it should be possible but . . . .

I've found
https://cgit.freedesktop.org/xorg/pr...oto-1.5.0#n160
likely everything I need is hidden in this file but for the life of me I can't figure out what commands to use. If I were an expert I wouldn't be asking but I'm not and there are no examples to follow (or try).

Suggestions please?

(Maybe do I have to go for newer proprietary drivers? (rather not I sort of like the no issues right now with Nouveau!)

TIA

xrandr does not enable multiple gpus. The system hardware must be able to use multiple gpus. My laptop has 2 gpus, but I can't use both at once. I hope this helps.

This, below, is in the document I mentioned above - - - - perhaps you might explain section 4 to me?

Version 1.4 adds an optional Border property.

• An optional Border property. This property allows a client to
specify that the viewport of the CRTC is smaller than the active
display region described its mode. This is useful, for example,
for compensating for the overscan behavior of certain
televisions.

Version 1.4 adds a new object called a provider object. A provider object
represents a GPU or virtual device providing services to the X server.
Providers have a set of abilities and a set of possible roles.

Provider objects are used to control multi-GPU systems. Provider roles can
be dynamically configured to provide support for:

1) Output slaving: plug in a USB device, but have its output rendered
using the main GPU. On some dual-GPU laptops, the second GPU isn't
connected to the LVDS panel, so we need to use the first GPU as an output
slave for the second GPU.

2) offload - For dual-GPU laptops, allow direct rendered applications to be run
on the second GPU and display on the first GPU.

3) GPU switching - Allow switching between two GPUs as the main screen
renderer.

4) multiple GPU rendering - This replaces Xinerama.

I think that means multiple monitors on the same gpu. That is what Xinerama was for.

Xinerama is an extension to the X Window System that enables X applications and window managers to use two or more physical displays as one large virtual display.