[Mesa-dev] Mesa (master): draw: fix flat shading and screen-space linear interpolation in clipper

[Jose Fonseca]

----- Original Message -----
> 
> 
> ----- Original Message -----
> > On Tue, Jul 10, 2012 at 3:30 PM, Jose Fonseca <jfonseca at vmware.com>
> > wrote:
> > > Yep. The interfaces are busted.
> > >
> > > Without native integers we get
> > >
> > >    DCL SV[0], INSTANCEID
> > >    ...
> > >    ARL ADDR[0].x, SV[0].xxxx
> > >
> > > and with integers we get
> > >
> > >    DCL SV[0], INSTANCEID
> > >    ...
> > >    UARL ADDR[0].x, SV[0].xxxx
> > >
> > > Olivier's fix is incorrect. It works on the above cases by
> > > guessing
> > > the type, but if we have:
> > >
> > >    DCL SV[0], INSTANCEID
> > >    ...
> > >    MOV TEMP[0].x, SV[0].xxxx
> > >    ARL ADDR[0].x, TEMP[0].xxxx
> > >
> > > and
> > >
> > >    DCL SV[0], INSTANCEID
> > >    ...
> > >    MOV TEMP[0].x, SV[0].xxxx
> > >    UARL ADDR[0].x, TEMP[0].xxxx
> > >
> > > it is impossible to guess -- the change is merely replacing a bug
> > > with another.
> > >
> > >
> > > For the record, the problem also happens without LLVM:
> > >
> > >   DRAW_USE_LLVM=0 draw-instanced -auto
> > >
> > >
> > >
> > > Anyway, AFAICT, all hardware out there that really supports
> > > INSTANCEID/VERTEXID also supports native integers, so this is a
> > > problem specific to the draw module.
> > >
> > > But given that draw module can support anything, this is actually
> > > self inflicted! In short, INSTANCEID/VERTEXID without integers is
> > > an historic artifact, that should not exist going forward.
> > >
> > > The right fix is merely making sure that PIPE_SHADER_CAP_INTEGERS
> > > is accurately advertised as 1 by the draw module (just like
> > > Stephane secretly did in 45fc069600ddbfe07a0a0cd5280161a8c7c55dd0
> > > :)
> > 
> > I just wanted to tell you Stephane's change cannot work and it even
> > has no effect at the moment. The native integer support is global
> > in
> > core Mesa. It's because integer uniforms are converted to floats
> > based
> > on the global NativeInteger flag for all shader stages and that
> > can't
> > be fixed easily, because uniforms can be shared between shaders.
> > Basically, all drivers must advertise integer support either for
> > all
> > shader stages or none.
> 
> I see..
>  
> > Given that, I only see two possible outcomes:
> > 
> > 1) Disable INSTANCEID support in DX9-level drivers using Draw
> > (that's
> > only i915g AFAIK) and only support INSTANCEID with integers.
> 
> > 2) Let drivers set the type of system values in Draw, so that Draw
> > doesn't have to guess what it should be.
> 
> My issue with 2) with this is that TGSI semantics depend on an
> external state. I think this is ugly and confusing.
> 
> If Stephane's OK with 1), I'd prefer taking it.

My current plan is to:
- make it clear that INSTANCEID/VERTEXID always means integer
- require PIPE_SHADER_CAP_INTEGERS to be advertise in the vertex shader stage in order to advertise INSTANCEID/VERTEXID in Mesa statetracker
- given that Mesa assumes integer, insert a I2F when loading INSTANCEID/VERTEXID (this meets the new semantics while avoiding a big re-architecture)

I believe this is  a good compromise:
- clear semantics
- not hard to implement
- no functionality regression for i915

It might get me some time to get it done, but I'll get to it asap.

Jose