[Mesa-dev] [PATCH 3/4] radeonsi: fix gl_PrimitiveID in tessellation with instanced draws on SI

[Marek Olšák]

On Wed, May 3, 2017 at 3:58 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> From: Nicolai Hähnle <nicolai.haehnle at amd.com>
>
> Cc: mesa-stable at lists.freedesktop.org
> ---
>  src/gallium/drivers/radeonsi/si_state_draw.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/src/gallium/drivers/radeonsi/si_state_draw.c b/src/gallium/drivers/radeonsi/si_state_draw.c
> index e6a9ee0..3d1d1f8 100644
> --- a/src/gallium/drivers/radeonsi/si_state_draw.c
> +++ b/src/gallium/drivers/radeonsi/si_state_draw.c
> @@ -181,20 +181,34 @@ static void si_emit_derived_tess_state(struct si_context *sctx,
>
>         /* Not necessary for correctness, but improves performance. The
>          * specific value is taken from the proprietary driver.
>          */
>         *num_patches = MIN2(*num_patches, 40);
>
>         /* SI bug workaround - limit LS-HS threadgroups to only one wave. */
>         if (sctx->b.chip_class == SI) {
>                 unsigned one_wave = 64 / MAX2(num_tcs_input_cp, num_tcs_output_cp);
>                 *num_patches = MIN2(*num_patches, one_wave);
> +
> +               if (sctx->screen->b.info.max_se == 1) {
> +                       /* The VGT HS block increments the patch ID unconditionally
> +                        * within a single threadgroup. This results in incorrect
> +                        * patch IDs when instanced draws are used.
> +                        *
> +                        * The intended solution is to restrict threadgroups to
> +                        * a single instance by setting SWITCH_ON_EOI, which
> +                        * should cause IA to split instances up. However, this
> +                        * doesn't work correctly on SI when there is no other
> +                        * SE to switch to.
> +                        */
> +                       *num_patches = 1;
> +               }

Hi Nicolai,

This commit massively decreases tessellation performance on SI 1-SE
parts. We need a different solution. Would this work: "Set num_patches
to the greatest divisor of the the number of patches per instance."

Thanks,
Marek