[Mesa-dev] Question about min_index/max_index calculation

[Marek Olšák]

On Mon, Mar 26, 2018 at 6:39 PM, Erico Nunes <nunes.erico at gmail.com> wrote:

> Thanks all for the input.
>
> I don't have an in-depth knowledge of the hardware either, though as
> far as we can see the hardware does expect that we pass min_index in
> the command stream. max_index is used to calculate the sizes in other
> command stream fields (which is not the same as pipe_draw_info.count).
> The existing lima-ng implementation does the same:
> https://github.com/limadriver-ng/lima/blob/660a940c8cc151fd886559233c1a22
> 385ee5f254/limare/lib/gp.c#L584
> (draw->vertex_start).
>
> On Sat, Mar 17, 2018 at 7:23 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> > For an approximation, you can use the sizes of the bound vbo's to
> > guess a min/max index. See
> > https://cgit.freedesktop.org/mesa/mesa/tree/src/gallium/
> drivers/vc4/vc4_draw.c#n183
> > for an example.
>
> I quickly played with porting something like that to mesa-lima but I
> don't think it can solve our problem.
> Apparently our bo currently have a fairly large fixed size so I'm not
> sure we can do an equivalent calculation using bo size.
> And then, min_index is required too.
>
> On Sat, Mar 24, 2018 at 10:32 PM, Marek Olšák <maraeo at gmail.com> wrote:
> > Here is how to do it:
> >
> > if (max_index != ~0u)
> >    // index bounds are valid;
> > else
> >    // scan the index buffer manually;
> >
> > u_vbuf_get_minmax_index can be used for the scanning.
>
> mesa-lima currently does not use any vbuf function. And we currently
> can't call u_vbuf_get_minmax_index since it's static in u_vbuf.c.
>
> I suppose we can consider making u_vbuf_get_minmax_index public then.
> I tested this and it works for me. It's still a bit annoying since we
> need to create fields probably in lima_context (like
> vc4_context.max_index does) and use those instead of the ones in
> pipe_draw_info. But not a big problem I guess. We could submit this
> refactor when mesa-lima gets proposed upstream.
>
> Just for completeness, one other idea that came up was to define a new
> PIPE_CAP_xxx config used to override
> st_context.draw_needs_minmax_index, and set that in this driver. This
> way, pipe_draw_info would come correctly filled and we wouldn't need
> to explicitly assemble minmax indices during the driver draw_vbo. How
> does this sound compared to the other solution?
>

The PIPE_CAP solution sounds OK.

Marek
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