Nouveau - May 2017 - multiple cards and monitors with xrandr and opengl

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P.S. This seems to be a bit outdated..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?

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

On 05/07/2017 11:12 PM, Ilia Mirkin wrote:
> 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.
Another possibility is running into memory bandwidth limitations.
>
> Cheers,
>
>   -ilia
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