[Mesa-dev] [PATCH 2/5] radeonsi: use guard band clipping

[Marek Olšák]

On Tue, Apr 12, 2016 at 1:53 AM, Marek Olšák <maraeo at gmail.com> wrote:
> On Mon, Apr 11, 2016 at 7:37 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
>> On 10.04.2016 17:34, Marek Olšák wrote:
>>>
>>> From: Marek Olšák <marek.olsak at amd.com>
>>>
>>> Guard band clipping speeds up rasterization for primitives that are
>>> partially off-screen.  This change in particular results in small
>>> framerate improvements in a wide range of games.
>>>
>>> Started by Grigori Goronzy <greg at chown.ath.cx>.
>>> ---
>>>   src/gallium/drivers/radeonsi/si_state.c | 73
>>> +++++++++++++++++++++++++++++++--
>>>   1 file changed, 69 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/src/gallium/drivers/radeonsi/si_state.c
>>> b/src/gallium/drivers/radeonsi/si_state.c
>>> index b5db456..bc04b46 100644
>>> --- a/src/gallium/drivers/radeonsi/si_state.c
>>> +++ b/src/gallium/drivers/radeonsi/si_state.c
>>> @@ -887,6 +887,15 @@ static void si_clip_scissor(struct pipe_scissor_state
>>> *out,
>>>         out->maxy = MIN2(out->maxy, clip->maxy);
>>>   }
>>>
>>> +static void si_scissor_make_union(struct si_signed_scissor *out,
>>> +                                 struct si_signed_scissor *in)
>>> +{
>>> +       out->minx = MIN2(out->minx, in->minx);
>>> +       out->miny = MIN2(out->miny, in->miny);
>>> +       out->maxx = MAX2(out->maxx, in->maxx);
>>> +       out->maxy = MAX2(out->maxy, in->maxy);
>>> +}
>>> +
>>>   static void si_emit_one_scissor(struct radeon_winsys_cs *cs,
>>>                                 struct si_signed_scissor *vp_scissor,
>>>                                 struct pipe_scissor_state *scissor)
>>> @@ -908,12 +917,64 @@ static void si_emit_one_scissor(struct
>>> radeon_winsys_cs *cs,
>>>                         S_028254_BR_Y(final.maxy));
>>>   }
>>>
>>> +/* the range is [-MAX, MAX] */
>>> +#define SI_MAX_VIEWPORT_RANGE 32768
>>> +
>>> +static void si_emit_guardband(struct si_context *sctx,
>>> +                             struct si_signed_scissor *vp_as_scissor)
>>> +{
>>> +       struct radeon_winsys_cs *cs = sctx->b.gfx.cs;
>>> +       struct pipe_viewport_state vp;
>>> +       float left, top, right, bottom, max_range, guardband_x,
>>> guardband_y;
>>> +
>>> +       /* Reconstruct the viewport transformation from the scissor. */
>>> +       vp.translate[0] = (vp_as_scissor->minx + vp_as_scissor->maxx) /
>>> 2.0;
>>> +       vp.translate[1] = (vp_as_scissor->miny + vp_as_scissor->maxy) /
>>> 2.0;
>>> +       vp.scale[0] = vp_as_scissor->maxx - vp.translate[0];
>>> +       vp.scale[1] = vp_as_scissor->maxy - vp.translate[1];
>>> +
>>> +       /* Treat a 0x0 viewport as 1x1 to prevent division by zero. */
>>> +       if (vp_as_scissor->minx == vp_as_scissor->maxx)
>>> +               vp.scale[0] = 0.5;
>>> +       if (vp_as_scissor->miny == vp_as_scissor->maxy)
>>> +               vp.scale[1] = 0.5;
>>> +
>>> +       /* Find the biggest guard band that is inside the supported
>>> viewport
>>> +        * range. The guard band is specified as a horizontal and vertical
>>> +        * distance from (0,0) in clip space.
>>> +        *
>>> +        * This is done by applying the inverse viewport transformation
>>> +        * on the viewport limits to get those limits in clip space.
>>> +        *
>>> +        * Use a limit one pixel smaller to allow for some precision
>>> error.
>>> +        */
>>> +       max_range = SI_MAX_VIEWPORT_RANGE - 1;
>>> +       left   = (-max_range - vp.translate[0]) / vp.scale[0];
>>> +       right  = ( max_range - vp.translate[0]) / vp.scale[0];
>>> +       top    = (-max_range - vp.translate[1]) / vp.scale[1];
>>> +       bottom = ( max_range - vp.translate[1]) / vp.scale[1];
>>> +
>>> +       assert(left < 0 && top < 0 && right > 0 && bottom > 0);
>>
>>
>> The viewport transform is supplied directly by the user, isn't it? So if
>> they provide a crazy transform, this assert might trigger. On the other
>> hand, for reasonable viewport transforms, it should even be the case that
>> left <= -1.0, right >= 1.0, and so on, right? I guess what I'm saying is
>> that the spirit of the assert is good and could probably be even
>> strengthened, but in reality it probably needs to go away.
>>
>> In a similar spirit, perhaps guardband_x/y should be bounded to be at least
>> 1.0?
>
> glViewport is clamped to ctx->Const.ViewportBounds.Min/Max. This
> assures that all guard bands are >= 1. However, if we get invalid
> coordinates from other sources, the guard band must prevent generating

That should be "if we get invalid viewport transform from other sources".

I think the assertion should stay and be strengthened to "x >= 1", because:
- we don't know how the discard guard band interacts with the clip guard band
- we should keep the assertion to prevent and detect possible
corruption caused by invalid viewport transformation

Does this sound good?

Marek