Not getting 3d acceleration on one of two GeForce graphic cards

Question

Running Debian 8. I have a GeForce 6600 GT and a GeForce GTX 460 SE. I was using the 340.65-2 driver, whatever the standard nvidia driver is in the repos, but that doesn't support the 6600. So I downgraded to version 304, (the legacy driver) and now I can run my three monitors without any problems, except for one. Any GL accelerated windows on the 6600 are just black. For example, glxgears gives me a black screen. OpenGL works great on the two screens being driven by the 460, but no luck with the 6600. How can I fix this?

Not interested in buying a new video card, or likely, two new video cards. I'm saving up for two very high end cards that can drive 4k displays, but that's 5 years out at least. In the mean time, I would like to get my current setup working. This has worked in the past, probably several years ago on Debian 7. I managed to play openarena across all three monitors without any issues. So it's possible :) It stopped working after I did a fresh install of Debian 8, and I haven't had a chance to work on it until recently.

.xsession-errors shows only:

Xlib:  extension "RANDR" missing on display ":0.0".

but not seeing anything pop up when I try to run glxgears on the "bad" display.

Xorg.0.log shows:

The GPU driving screen 1 is incompatible with the rest of the
    GPUs composing the desktop.  OpenGL rendering will be
    disabled on screen 1.

But I know I have had this working in the past.

Answer 1

This is a known problem with xinerama (which is used for multi monitor support). The NVIDIA support page says:

Using identical GPUs is recommended. Some combinations of non-identical, but similar, GPUs are supported. If a GPU is incompatible with the rest of a Xinerama desktop then no OpenGL rendering will appear on the screens driven by that GPU. Rendering will still appear normally on screens connected to other supported GPUs. In this situation the X log file will include a message of the form:

(WW) NVIDIA(2): The GPU driving screen 2 is incompatible with the rest of

(WW) NVIDIA(2): the GPUs composing the desktop. OpenGL rendering will

(WW) NVIDIA(2): be disabled on screen 2.

You could try to disable xinerama in the NVIDIA driver and see if you get it to work using xrandr instead of xinerama (for example using the arandr GUI for xrandr).

Check this page and this answer for information on how to set up xrandr.

If xrandr does not work for your setup under Debian 8, you may have to go back to Debian 7 which (as far as I know) uses xrandr by default.

Answer 2

The error message in your Xorg.0.log explains quite clearly why OpenGL rendering is not working on your 6600. I suspect nvidia developers made an engineering decision to limit backwards compatibility to reduce development complexity or debugging/QA cost.

To solve your problem using the nvidia binary-only drivers, I'd recommend reverting to the version you described using with your Debian 7 install that met your needs. If necessary, you could then creep forward through revisions to find newest version before the support was dropped.

A second option is to use the nouveau open-source driver. According to FeatureMatrix on the nouveau driver webpage, both your NV40(Curie) (GeForce 6600) and NVC0(Fermi) (GeForce GTX 460 SE) cards are supported by the driver. The front page does caution that 3D Acceleration on Fermi generation cards may be slow but that may be out of date. Personally, in your situation, I would try this driver to see if it fit your needs. The nouveau drivers are most likely to be supported for your estimated next five years until you can purchase new video cards. Fortunately, the driver is already packaged in Debian as xserver-xorg-video-nouveau. Installing it will require removing the installed nvidia drivers and, most likely, a reboot.

Good Luck!