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

[Marek Olšák]

On Wed, Jun 7, 2017 at 12:14 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> On 02.06.2017 15:01, Marek Olšák wrote:
>>
>> 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."
>
>
> Yes, that should be sufficient. Actually, it'd be even better to only do the
> workaround when PRIM_ID is read, which is probably quite rare...
>
> Do you already have a patch for the first part?

Yes, I've already sent out a patch adding uses_primid.

Marek