[Mesa-dev] [PATCH] i915: Always emit W on gen3

[Ian Romanick]

Reviewed-by: Ian Romanick <ian.d.romanick at intel.com>

On 06/20/2017 10:22 AM, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Unlike the older gen2 hardware, gen3 performs perspective
> correct interpolation even for the primary/secondary colors.
> To do that it naturally needs us to emit W for the vertices.
> 
> Currently we emit W only when at least one texture coordinate
> set gets emitted. This means the interpolation of color will
> change depending on whether texcoords/varyings are used or not.
> That's probably not what anyone would expect, so let's just
> always emit W to get consistent behaviour. Trying to avoid
> emitting W seems like more hassle than it's worth, especially
> as bspec seems to suggest that the hardware will perform the
> perspective division anyway.
> 
> This used to be broken until it was accidentally fixed it in
> commit c349031c27b7 ("i915: Fix texcoord vs. varying collision
> in fragment programs") by introducing a bug that made the driver
> always emit W. After fixing that bug in commit c1eedb43f32f
> ("i915: Fix wpos_tex vs. -1 comparison") we went back to the
> old behaviour and caused an apparent regression.
> 
> Fixes: c1eedb43f32f ("i915: Fix wpos_tex vs. -1 comparison")
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101451
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
>  src/mesa/drivers/dri/i915/i915_fragprog.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i915/i915_fragprog.c b/src/mesa/drivers/dri/i915/i915_fragprog.c
> index e19ca60bd334..3657b2d82565 100644
> --- a/src/mesa/drivers/dri/i915/i915_fragprog.c
> +++ b/src/mesa/drivers/dri/i915/i915_fragprog.c
> @@ -1252,12 +1252,10 @@ i915ValidateFragmentProgram(struct i915_context *i915)
>     intel->coloroffset = 0;
>     intel->specoffset = 0;
>  
> -   if (inputsRead & VARYING_BITS_TEX_ANY || p->wpos_tex != I915_WPOS_TEX_INVALID) {
> -      EMIT_ATTR(_TNL_ATTRIB_POS, EMIT_4F_VIEWPORT, S4_VFMT_XYZW, 16);
> -   }
> -   else {
> -      EMIT_ATTR(_TNL_ATTRIB_POS, EMIT_3F_VIEWPORT, S4_VFMT_XYZ, 12);
> -   }
> +   /* Always emit W to get consistent perspective
> +    * correct interpolation of primary/secondary colors.
> +    */
> +   EMIT_ATTR(_TNL_ATTRIB_POS, EMIT_4F_VIEWPORT, S4_VFMT_XYZW, 16);
>  
>     /* Handle gl_PointSize builtin var here */
>     if (ctx->Point._Attenuated || ctx->VertexProgram.PointSizeEnabled)
>