[Nouveau] multiple cards and monitors with xrandr and opengl

[Ben Skeggs]

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 P.S. This seems to be a bit outdated..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