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Old 10-13-10, 10:15 AM   #1
yann_ast
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Join Date: Oct 2010
Posts: 2
Default How to bind X display to graphics hardware?

I have a Red Hat 5 64 bits PC with 2 QUADRO FX 3800.
I use a software A that renders images in an off-screen buffer. When A starts, it opens a useless black xwindow, which is attached to a display, which is attached to a graphics hardware.

In order to avoid having this xwindow beeing open and visible on the main desktop, I thought we could set the DISPLAY environment variable (X display) to something different from :0.0 before running A from a terminal.

So I have connected 2 monitors to one of the graphics cards, and a third monitor to the second graphics card, generated the xorg.conf with "nvidia-xconfig --enable-all-gpus --separate-x-screens",restarted the X server, and could check that I could run A on the second monitor by opening a terminal from the main monitor, setting DISPLAY to :0.1 and run A. Idem for the third monitor and DISPLAY set to :0.2. I could still do it after unplugging the 2nd and 3rd monitors. --> success

However, when I restarted the X server with the 2 second monitors unplugged, the DISPLAY :0.1 and 0.2 could not be opened anymore. --> failure

I suspect that the displays are created by the X server at startup time. So I tried different configurations of the xorg.conf such as MonitorConnected, UseEDID. But none of them allowed me to have :
- Linux PC starts with 2 graphics cards + 1 Monitor connected to one of the 2 graphics cards
- The following DISPLAYS can be open :0.0, :0.1, :0.3 such as
:0.0 displays to the monitor and is attached to graphics card 1
:0.1 displays to NO monitor and is attached to graphics card 1
:0.2 displays to NO monitor and is attached to graphics card 2

Please help !
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Old 10-25-10, 10:24 AM   #2
yann_ast
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Join Date: Oct 2010
Posts: 2
Default Re: How to bind X display to graphics hardware?

I finally got a solution with the help of Nvidia support team. Here is a sum up:

Let's assume we have 2 identical Nvidia graphics cards with 3 physical ports for monitors on each card.
Let's assume that we want to use 3 Xscreens or DISPLAYs (as linux environment variable), but physically connect only 1 monitor to one of the graphics cards.
Physically connecting 3 monitors to the Nvidia cards (2 monitors on 1 card and 1 monitor on the other card) and then running the command line `nvidia-xconfig --enable-all-gpus --seperate-x-screens` creates /etx/X11/xorg.conf which gives us 3 monitors working on 3 seperate Xscreens, and therefore 3 seperate DISPLAYs. To check it, on can use the command line `nvidia-settings -q screens -q gpus` which gives :
nvidia-settings -q screens -q gpus

3 X Screens on obelix:0

[0] obelix:0.0 (Quadro FX 3800)
[1] obelix:0.1 (Quadro FX 3800)
[2] obelix:0.2 (Quadro FX 3800)
2 GPUs on obelix:0

[0] obelix:0[gpu:0] (Quadro FX 3800)
[1] obelix:0[gpu:1] (Quadro FX 3800)
The DISPLAYs are 0.0, 0.1 and 0.2

Now the problem is that one does NOT want to connect 3 monitors but only 1.
In our use case which is using a software in off-screen rendering mode, a useless black Xwindow is created when the application is launched. Until graphics hardware becomes completely separate from the concept of having an attached display, this situation will remain. But one still can open that useless Xwindow in a dedicated DISPLAY (attached to a specific graphics hardware), without needing to visualise this DISPLAY with a monitor (because the only thing to see is a useless black window). This has also the advantage to isolate the graphical application process in a specific DISPLAY.

In order to do this, reusing the previously generated xorg.conf is not enough. When the X server starts, the Nvidia driver auto-detects if monitors are connected. If they are not, 2 DISPLAYs instead of 3 are created.
The trick consists in modifying the xorg.conf file. More specifically using
Option "ConnectedMonitor" "DFP,DFP"
in "Device0" which in our case corresponds to the one with a physically connected monitor.
This makes the driver act like 2 DFPs (flat LCD screens) are connected to that board.
Note that the "ConnectedMonitor" option overrides any monitors that are actually detected by the board, so if there is a CRT (cathodic tube) connected, setting "DFP,DFP" will cause the CRT to not be driven correctly. Instead it would probably best to use "CRT,CRT".
Please have a look at the final xorg.conf file attached to this post.
Attached Files
File Type: txt xorg.conf.txt (3.5 KB, 89 views)
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