xrandr three screens

Question

I have 2 cards and three screens. I got them working before after a lot of difficulty. The solution was to add a setting to the boot config to increase memory allowed for display. Then, I had to do a complete reinstall. After searching for a long time, I found the setting and got three screens working again. I just had to do a reinstall again. Now, I've been searching and searching. I cannot find the setting to add to the boot config to increase the amount of memory allowed for display. Does anyone happen to know what it is? Perhaps this time I will write it down and try not to lose it.

Answer 1

I got this working. It is very difficult if you don't know a lot about xorg.conf files. Here is my setup: 2 nvidia cards. The main one has 4 outputs, but only 2 may be used in Linux at any time (kernel limitation apparently). The second one has 3 outputs, but I only use one. To make the three screens that I am using work, I must define all 7 outputs in xorg.conf (Why? I don't know. If I remove just one definition of one output, it fails to work.) So, in the ServerLayout section, I have:

Identifier "default display"
Screen 0 "Screen0" 0 0
Screen 1 "Screen1" RightOf "Screen0"
Screen 2 "Screen2" RightOf "Screen1"
...repeated through Screen 6, which is RightOf Screen5
InputDevice "Mouse0" "CorePointer"
InputDevice "Keyboard0" "CoreKeyboard"

The Files section, Module section, and InputDevice sections don't make a difference here. I have 7 Monitor sections, all with bare descriptions, just changing the monitor number.

Identifier "Monitor0"
VendorName "Vendor"
ModelName "model"

I have 7 Device sections, all with bare descriptions. It is important to get the BusID correct. I have four of them for Card0 on PCI:1:0:0 and three for Card1 on PCI:2:0:0. They look like:

Identifier "Card0"
Driver "modesetting"
BusID "PCI:1:0:0"

Notice, the driver is not nouveau or nvidia. Neither one of those will work (I even tried nv).

Of course, I have 7 Screen sections. I made these the minimal settings, but I did hard-code a request for depth 24. They all look the same, just changing the number for each one:

Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
SubSection "Display"
    Viewport 0 0
    Depth 24
EndSubSection

Now, when I run, I get all three screens up. The two on the primary card are joined with xrandr. The third monitor active, but not part of the main display. So, I cannot drag windows to it from the main two screens. But, I can open a window in that display if I want to. Now, I am going to experiment with using xinerama to make all three screens one desktop instead of two on a desktop and one off on its own.