[Nouveau] multiple cards and monitors with xrandr and opengl

[Karol Herbst]

On 8 May 2017 12:10:13 a.m. GMT+02:00, Ben Skeggs <skeggsb at gmail.com> wrote:
>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 P.S. This seems to be a bit outdated..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.
>

And PCIe bandwidth, especially if only on 2.5 GT/s speed. So if this is done through Reverse Prime, reclocking is a must.

>>
>> Cheers,
>>
>>   -ilia
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