[Nouveau] multiple cards and monitors with xrandr and opengl

[Ilia Mirkin]

On Sun, May 7, 2017 at 7:17 AM, Sampsa Riikonen <sampsa.riikonen at iki.fi> wrote:
> Dear Devs,
>
> We have achieved a desktop of up to six monitors, with openGL running
> succesfully on the desktop, with the following setup/features:
>
> * Ubuntu 16+
> * Xrandr
> * Noveau driver
> * Two gtx750 graphic cards
>
> Each (identical) graphic card has 2xHDMI + 2xDVI connectors, which we
> connect to the monitor array.
>
> So far it works with six monitors, but we'd like to achieve eight.
> However, throwing in more monitors, results in monitors starting to drop
> out (black screen).  However, xrandr sees them correctly.
>
> The nv$dia proprietary driver is not an option - they have artificially
> restricted the amount of monitors in the "base mosaic" feature (ref.
> https://www.phoronix.com/forums/forum/linux-graphics-x-org-drivers/x-org-drm/46705-linux-multi-head-success-with-8-monitors-in-1-x-screen-without-xinerama
> ).
>
> What might cause the monitors to drop out?  I can provide some logs
> later next week (which file should I look at?)
>
> Could using nv$dia's proprietary firmware in noveau help?
>
> Regards,
>
> Sampsa
>
> P.S. This seems to be a bit outdated.. ?
>
> https://nouveau.freedesktop.org/wiki/MultiMonitorDesktop/
>
> .. it implies that xinerama must be used to combine screens from two
> different graphics cards into a single screen which implies that opengl
> should not work, right?

Using reverse prime with a ton of screens has had a questionable
success rate. Using Xinerama Just Works (tm). But you do lose direct
rendering (but you still get OpenGL via indirect GLX, assuming the X
server has that enabled). However in the past, this hasn't been an
issue for the people trying to create such arrangements.

With reverse prime, I believe it's all just one big framebuffer, so
you run into fb size limits. I think we restrict it to 16k x 16k in
size.

Cheers,

  -ilia