[Mesa-dev] [PATCH] gallium: add pipe_grid_info::partial_block

[Marek Olšák]

On Tue, Jan 8, 2019, 7:55 PM Ilia Mirkin <imirkin at alum.mit.edu wrote:

> On Tue, Jan 8, 2019 at 7:26 PM Marek Olšák <maraeo at gmail.com> wrote:
> >
> > On Tue, Jan 8, 2019 at 7:18 PM Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> >>
> >> On Tue, Jan 8, 2019 at 6:21 PM Marek Olšák <maraeo at gmail.com> wrote:
> >> >
> >> > On Tue, Jan 8, 2019 at 5:25 PM Ilia Mirkin <imirkin at alum.mit.edu>
> wrote:
> >> >>
> >> >> Why does this need to be in p_state? And who is responsible for
> >> >> setting it (and how will it be set)?
> >> >
> >> >
> >> > Oh right, there is a way to get it out of p_state.h if needed.
> >> >
> >> > It should be set to 0 by default.
> >> >
> >> > If your thread block is 8x8x1, but you need to launch 10x8x1 threads,
> set partial_block = {2, 0, 0}. It will launch the following thread blocks:
> >> > 8x8x1
> >> > 2x8x1
> >> >
> >> > It's the same as launching 16x8x1 threads and doing this at the
> beginning of the compute shader:
> >> >   if (globalThreadID.x >= 10) return;
> >>
> >> But that all sounds like something a state tracker wouldn't care
> >> about, right? In e.g. GLSL you can specify the block to be 10x8x1 and
> >> let the backend work it all out. Should st/mesa care about this (or
> >> clover or whatever)?
> >
> >
> > The block size should be a multiple of 64 on radeonsi to utilize all
> SIMD lanes. If you want to launch 8192+1 threads with the block size of 64,
> you need to launch 1 partial block with the block size of 1 at the end.
> OpenGL can't do this.
>
> Ohhhhhhhhhh. So the partial-ness applies to the last-executed block.
> If you have a local_size=(2,2,1), and you want your global grid to be,
> say, (5,4,1), with unextended GL you might run it as groups = (3,2,1)
> which would end up invoking a bunch of bits you don't want, and so
> this partial_size is a way to say that you don't want the last "line"
> to be executed at all.
>
> That makes sense, and seems like a reasonable thing to have in
> pipe_grid_info. The documentation did not make that clear the first
> time I read it, but now I'm having trouble suggesting improvements to
> it. So I think it's fine.
>

Yep. The partialness adds additional blocks to the grid with disabled
threads (lanes). I might rename it to grid_padding[3]. Or I might keep the
whole thing private in radeonsi.

It's useful for e.g. compute-based image blits when the blit box is not
aligned to the block size.

Marek

The p_state.h bits are Acked-by: Ilia Mirkin <imirkin at alum.mit.edu> .
> Can't speak as to the radeonsi bits.
>
> Cheers,
>
>   -ilia
>
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