[Mesa-dev] [PATCH 30/30] mesa/st: require linear interpolation for ARB_texture_float

[Ilia Mirkin]

On Mon, Nov 19, 2018 at 10:40 AM Erik Faye-Lund
<erik.faye-lund at collabora.com> wrote:
>
> On Mon, 2018-11-19 at 10:02 -0500, Ilia Mirkin wrote:
> > Unfortunately this will drop GL 3.0 from Adreno A3xx. I think we'd
> > rather fake linear interpolation with F32 textures which are never
> > used than lose GL 3.0 there...
>
> Right...
>
> I guess this means that this GPU never really did support OpenGL 3.0,
> and will make some applications misbehave. There's definately
> applications out there that will lead to surprisingly bad problems when
> features like these are not supported.
>
> For instance if an application tries to take a local gradient by
> sampling a texture twice with a tiny epsilon (a common trick in
> tangent-free normal mapping, for instance), it will essentially get
> garbage, which can cause close to useless rendering.
>
> I've worked on applications that would have had problems like these if
> drivers report the wrong version, but could work correctly if they
> report the right version.
>
> Either way, I don't believe faking like that belongs in core Mesa. So
> if the Freedreno developers really want this kind of behavior, perhaps
> something like this could be a better move?
>
> ---8<---
> diff --git a/src/gallium/drivers/freedreno/freedreno_screen.c
> b/src/gallium/drivers/freedreno/freedreno_screen.c
> index 88d91a91234..de811371f05 100644
> --- a/src/gallium/drivers/freedreno/freedreno_screen.c
> +++ b/src/gallium/drivers/freedreno/freedreno_screen.c
> @@ -260,6 +260,11 @@ fd_screen_get_param(struct pipe_screen *pscreen,
> enum pipe_cap param)
>                 return 0;
>
>         case PIPE_CAP_TEXTURE_FLOAT_LINEAR:
> +               /* HACK: A330 doesn't support linear interpolation of
> FP32 textures, but
> +                * to keep OpenGL 3.0 support, we lie about it here.
> +                */
> +               return is_a3xx(screen) || is_a4xx(screen) ||
> is_a5xx(screen) || is_a6xx(screen);
> +
>         case PIPE_CAP_CUBE_MAP_ARRAY:
>         case PIPE_CAP_SAMPLER_VIEW_TARGET:
>         case PIPE_CAP_TEXTURE_QUERY_LOD:
> ---8<---
>
> Alternatively, they could ask users to override the GL-version for
> applications that need GL 3.0, but doesn't have problems with the lack
> of FP32-interpolation...

GL 3.0 brings SO much stuff in though, and GL 3.1 brings core profiles.

Your proposed solution will also expose the OES_bla ext, which we
definitely don't want to do. I'd instead keep it loose. The hardware
that doesn't support this stuff is generally targeted at ES. However
it's convenient to have desktop GL both for test coverage (piglit) as
well as regular use.

Tons of desktop stuff doesn't work in Adreno. Starting with different
cull modes for front and back. Setting polygon mode for quads to lines
shows you the internal line. Edge mode isn't supported. Probably 10000
other things.

But it's still very useful to have GL 3.x advertised.