[Mesa-dev] Thoughts on fp64 for GLES?

[Elie Tournier]

On Wednesday, 13 February 2019, Ilia Mirkin <imirkin at alum.mit.edu> wrote:

> On Wed, Feb 13, 2019 at 12:47 PM Elie Tournier <tournier.elie at gmail.com>
> wrote:
> >
> > On Fri, Jan 25, 2019 at 11:52:56AM -0800, Stéphane Marchesin wrote:
> > > On Fri, Jan 25, 2019 at 2:25 AM Gert Wollny <gw.fossdev at gmail.com>
> wrote:
> > > >
> > > > Am Donnerstag, den 24.01.2019, 22:25 -0800 schrieb Stéphane
> Marchesin:
> > > > >
> > > > > Yes, it's for running virgl on top of GLES. To emulate fp64 in GL
> on
> > > > > the guest side, we need fp64 on the host...
> > > >
> > > > BTW: we could also get it emulated from the guest side. When Elie (in
> > > > CC)  initially proposed the fp64 emulation series it was for r600 and
> > > > TGSI was emitted. The created shaders are horribly long and it is
> > > > certainly not performant, but if it's just for getting OpenGL 4.0
> > > > exposed it should be good enough.
> > >
> > > Yes, Ilia suggested this on IRC yesterday. My impression is that not
> > > many applications/games need high performance fp64 (it's likely mostly
> > > compute stuff, which is not our target). I could be wrong though. If
> > > anyone knows differently, please tell us :)
> > >
> > > >
> > > > I'm not sure though how much work it would be to add this to the soft
> > > > fp64 as it has now landed for NIR, though.
> > >
> > > Yes, with virgl not using NIR, I am not sure how much work soft fp64
> > > will require.
> >
> > I spent a bit of time on the project recently.
> > My thinking so far:
> > * FP64 is bad . But everyone knows that. :)
> > * Using the current soft fp64 require to emulate int64.
> > * Soft fp64 and int64 involve function call which is, iiuc, not really
> > supported in TGSI.
> > * Soft fp64 is tied to NIR. Some pass/hack need to be port to GLSLIR.
> >
> > So the project will require a lot of work.
>
> But what's the alternative? Let's say you make a spec to expose
> "proper" fp64 in GLES. No one outside mesa will implement this (why
> bother). Certainly not the Adreno/Mali proprietary stacks of the
> world.


I'm not saying that we should get an extension.
My point was, it's a lot of work.

>
> And if you are on a stack that implements this in GLES, you might as
> well be using desktop GL anyways...
>
> So going back to the original -- what use-case are you trying to cover
> that's not already covered some other way?


iiuc, Stephane want to run GL desktop on top of GLES.
In order to expose a bigger version of GL, he need fp64 support.

>
>   -ilia
>
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