Nouveau - Mar 2015 - SLI development

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

Good day!

- From Nouveau feature matrix page:
>SLI or even multicard setups are very rare among developers.
>You should start hacking with us, if you have such a setup.
I have two GTX 770 cards running in SLI[1] and would like to help bringing
SLI capability into Nouveau.

I would gladly write code for it and I am a competent C programmer per se
(C is my main language). But, I don't have much experience with graphics
programming and have very little experience when it comes to "talking"
with
hardware directly. Thus, I am not confident I would manage to do it
properly. I would like to at least try, nonetheless.

Any pointers on where to start and/or what things to research first?

[1] running in SLI mode when under Windows, but that hardly ever
happens anymore.

- -- 
Gediminas Jakutis
LDK Varčiai www.varciai.lt

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2
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=ZO/6
-----END PGP SIGNATURE-----

On Fri, Mar 20, 2015 at 1:40 PM, Gediminas Jakutis <gediminas at
varciai.lt> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA256
>
> Good day!
>
> - From Nouveau feature matrix page:
>>SLI or even multicard setups are very rare among developers.
>>You should start hacking with us, if you have such a setup.
>
> I have two GTX 770 cards running in SLI[1] and would like to help bringing
> SLI capability into Nouveau.
>
> I would gladly write code for it and I am a competent C programmer per se
> (C is my main language). But, I don't have much experience with
graphics
> programming and have very little experience when it comes to
"talking" with
> hardware directly. Thus, I am not confident I would manage to do it
> properly. I would like to at least try, nonetheless.
>
> Any pointers on where to start and/or what things to research first?
One of the main issues with SLI is that it's unclear what to do. We
have all the tools in place to be able to do it, just don't know what
to do with that capability :)

Let's say you wanted to, oh, I don't know... render a triangle. A
really big triangle, to a 8Kx8K texture (or 16Kx16K). How would you
leverage multiple graphics cards to help with that?

One way to look at it is to take a line from tiled renderers and just
consider that you have 2 tiles (or however many GPU's you have), do a
binning pass, and then re-render the fragment shader across all GPU's.
[Or screw the binning pass, just do the render on both GPU's but to
different viewports. That could present complications for shaders that
want to operate on global state with things like ARB_ssbo/etc though.
But you could just kick them off the SLI path if they do anything you
don't like.]

Another approach is to keep track of which draw depends on what, and
if you're executing a draw, allow another draw to execute on another
GPU if it has no dependencies on that first draw's output. Perhaps
there are other things still you might do.

Not that I want to discourage you from working on this, but there will
be a LOT or roadblocks, from trivial things to actually getting mesa
to feed 2 GPUs to a single driver, to the fact that the driver isn't
really structured to take advantage of this.

If you're looking to contribute to nouveau, I'd recommend you join
#nouveau on freenode and discuss there. Perhaps there are other
projects you'd be equally interested in but are more immediately
actionable. And then you could build up the knowledge to eventually
tackle SLI.

Cheers,

  -ilia

On Friday, March 20, 2015 07:40:27 PM Gediminas Jakutis
wrote:
> Good day!
> 
> From Nouveau feature matrix page:
> >SLI or even multicard setups are very rare among developers.
> >You should start hacking with us, if you have such a setup.
> 
> I have two GTX 770 cards running in SLI[1] and would like to help
> bringing SLI capability into Nouveau.
> 
> I would gladly write code for it and I am a competent C programmer per
> se (C is my main language). But, I don't have much experience with
> graphics programming and have very little experience when it comes to
> "talking" with hardware directly. Thus, I am not confident I
would
> manage to do it properly. I would like to at least try, nonetheless.
> 
> Any pointers on where to start and/or what things to research first?
> 
> [1] running in SLI mode when under Windows, but that hardly ever
> happens anymore.
> 
> --
> Gediminas Jakutis
> LDK Varčiai www.varciai.lt
> 
> _______________________________________________
> Nouveau mailing list
> Nouveau at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/nouveau
I would be happy to learn SLI too and help in those projects.