[Nouveau] TLD instruction usage in non-linked sampler mode

[Andy Ritger]

On Wed, Mar 07, 2018 at 04:20:08PM -0500, Ilia Mirkin wrote:
> Hi Andy,
> 
> Thanks for checking! I do see an issue on Tesla as well (at least G92,
> and I believe someone else reported on a GT215 or GT218). However I
> haven't confirmed that it's the identical issue to what I see on Fermi
> with quite as much certainty as what I've checked on a GF108. (For the
> G92, the texture buffer object test fails in the same way it does on
> Fermi, but there could be other reasons for that.)
> 
> To be clear, when I say "unbound" for Fermi / Tesla, I mean
> BIND_TSC(ACTIVE=0) (method 0x2400, bit 0 == 0). Kepler doesn't have
> that, so presumably you mean TLD will look at TSC ID == 0,
> irrespective of the TSC ID encoded into the texture handle.

Actually, I may have been wrong about Kepler.  More below.

> The sRGB thing is ... rather annoying. Especially since texelFetch has
> some kind of extra-specially-odd rule around sRGB conversion. I don't
> remember what it is, but it feels like this would potentially make it
> impossible to follow. But I guess going back in time and fixing the hw
> is also difficult.

It probably doesn't help this case, but FYI sRGB conversion should
only happen if sRGBConversion is set in both texture header (bit 10)
and texture sampler (bit 13).

> Can you confirm whether it will always look at slot 0, even in
> LINKED_TSC mode (i.e. method 0x1234 == 1)? We don't currently make use
> of that mode in nouveau, but I'm thinking about doing it on Tesla /
> Fermi so that we can get more than 16 textures per stage on there.

I wasn't able to find anything definitive.  A few of us attempted to
infer the behavior from what different NVIDIA proprietary drivers do.
The best we can guess is:

    Fermi method 0x1234 == 0    : sampler #0
    Fermi method 0x1234 == 1    : not sure if sampler #0 or texture ID?
    Kepler method 0x1234 == 0   : Use the MSBs of the handle
    Kepler method 0x1234 == 1   : Use the LSBs (texture ID) of the handle

The Fermi LINKED_TSC case was the one we were least sure of from code
inspection.  Hopefully you can determine that case empirically?

Sorry I'm not more help here.

- Andy

> Thanks again for the info!
> 
>   -ilia
> 
> On Wed, Mar 7, 2018 at 3:21 PM, Andy Ritger <aritger at nvidia.com> wrote:
> > Hi Ilia.
> >
> > It looks like there is a hardware bug on Fermi and Kepler where TLD
> > unconditionally uses the sampler from slot 0.  This is supposedly fixed
> > in Maxwell.  What I could find internally suggests the bug wasn't present
> > on Tesla, but let me know if your observations contradict that.
> >
> > The only work around is to have a sampler defined and bound.  Further,
> > be careful to have reasonable state in the entry in slot #0: as I
> > understand it, the one piece of sampler state that will influence TLD
> > is sRGBConversion (bit 13).
> >
> > I hope that helps,
> > - Andy
> >
> >
> > On Thu, Mar 01, 2018 at 11:47:18PM -0500, Ilia Mirkin wrote:
> >> Hello,
> >>
> >> This question is in the context of Tesla / Fermi generations, which
> >> have explicit bindings for textures / samplers. It might also apply to
> >> Kepler+, not quite as sure due to the bindless nature.
> >>
> >> I've been trying to understand how the TLD operation works (which is
> >> used to implement texelFetch in GLSL). It does not appear to the op
> >> takes an explicit sampler id at all (unlike all the other texturing
> >> operations). In unlinked TSC mode (i.e. method 0x1234 == 0), my
> >> observation is that it desperately wants for a valid sampler to be
> >> bound to sampler slot 0. Of course I don't think TLD actually needs
> >> anything from the sampler, which makes this all the more odd.
> >>
> >> Is that a correct assessment of the operation of the TLD instruction?
> >> Is there any way to make it just not care about the sampler binding?
> >> Does the DirectX driver just always keep something bound to sampler
> >> slot 0? (And what happens on Kepler+? Does it always look at TSC ID ==
> >> 0?)
> >>
> >> (I kind of assume that all these problems go away in linked TSC mode,
> >> since it'd naturally just look up the TSC entry associated with the
> >> bound TIC.)
> >>
> >> Thanks,
> >>
> >>   -ilia