[Mesa-dev] [PATCH] i965: Fix alpha to one with dual color blending.

[Chris Forbes]

Sigh, I should have just ignored the docs when I was poking at this years
ago.

Reviewed-by: Chris Forbes <chrisforbes at google.com>

On Mon, May 29, 2017 at 10:49 PM, Kenneth Graunke <kenneth at whitecape.org>
wrote:

> The BLEND_STATE documentation says that alpha to one must be disabled
> when dual color blending is enabled.  However, it appears that it simply
> fails to override src1 alpha to one.
>
> We can work around this by leaving alpha to one enabled, but overriding
> SRC1_ALPHA to ONE and ONE_MINUS_SRC1_ALPHA to ZERO.  This appears to be
> what the other driver does, and it looks like it works despite the
> documentation saying not to do it.
>
> Fixes spec/ext_framebuffer_multisample/alpha-to-one-dual-src-blend *
> Piglit tests.
> ---
>  src/mesa/drivers/dri/i965/genX_state_upload.c | 57
> +++++++++++++++++++++------
>  1 file changed, 44 insertions(+), 13 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/genX_state_upload.c
> b/src/mesa/drivers/dri/i965/genX_state_upload.c
> index 76d2ea887b1..145173c2fc1 100644
> --- a/src/mesa/drivers/dri/i965/genX_state_upload.c
> +++ b/src/mesa/drivers/dri/i965/genX_state_upload.c
> @@ -2435,6 +2435,20 @@ static const struct brw_tracked_state
> genX(gs_state) = {
>
>  /* ----------------------------------------------------------------------
> */
>
> +static GLenum
> +fix_dual_blend_alpha_to_one(GLenum function)
> +{
> +   switch (function) {
> +   case GL_SRC1_ALPHA:
> +      return GL_ONE;
> +
> +   case GL_ONE_MINUS_SRC1_ALPHA:
> +      return GL_ZERO;
> +   }
> +
> +   return function;
> +}
> +
>  #define blend_factor(x) brw_translate_blend_factor(x)
>  #define blend_eqn(x) brw_translate_blend_equation(x)
>
> @@ -2562,6 +2576,19 @@ genX(upload_blend_state)(struct brw_context *brw)
>                 dstA = brw_fix_xRGB_alpha(dstA);
>              }
>
> +            /* From the BLEND_STATE docs, DWord 0, Bit 29 (AlphaToOne
> Enable):
> +             * "If Dual Source Blending is enabled, this bit must be
> disabled."
> +             *
> +             * We override SRC1_ALPHA to ONE and ONE_MINUS_SRC1_ALPHA to
> ZERO,
> +             * and leave it enabled anyway.
> +             */
> +            if (ctx->Color.Blend[i]._UsesDualSrc &&
> blend.AlphaToOneEnable) {
> +               srcRGB = fix_dual_blend_alpha_to_one(srcRGB);
> +               srcA = fix_dual_blend_alpha_to_one(srcA);
> +               dstRGB = fix_dual_blend_alpha_to_one(dstRGB);
> +               dstA = fix_dual_blend_alpha_to_one(dstA);
> +            }
> +
>              entry.ColorBufferBlendEnable = true;
>              entry.DestinationBlendFactor = blend_factor(dstRGB);
>              entry.SourceBlendFactor = blend_factor(srcRGB);
> @@ -2600,16 +2627,6 @@ genX(upload_blend_state)(struct brw_context *brw)
>           entry.WriteDisableBlue  = !ctx->Color.ColorMask[i][2];
>           entry.WriteDisableAlpha = !ctx->Color.ColorMask[i][3];
>
> -         /* From the BLEND_STATE docs, DWord 0, Bit 29 (AlphaToOne
> Enable):
> -          * "If Dual Source Blending is enabled, this bit must be
> disabled."
> -          */
> -         WARN_ONCE(ctx->Color.Blend[i]._UsesDualSrc &&
> -                   _mesa_is_multisample_enabled(ctx) &&
> -                   ctx->Multisample.SampleAlphaToOne,
> -                   "HW workaround: disabling alpha to one with dual src "
> -                   "blending\n");
> -         if (ctx->Color.Blend[i]._UsesDualSrc)
> -            blend.AlphaToOneEnable = false;
>  #if GEN_GEN >= 8
>           GENX(BLEND_STATE_ENTRY_pack)(NULL, &blend_map[1 + i * 2],
> &entry);
>  #else
> @@ -4049,11 +4066,15 @@ genX(upload_ps_blend)(struct brw_context *brw)
>        /* BRW_NEW_FRAGMENT_PROGRAM | _NEW_BUFFERS | _NEW_COLOR */
>        pb.HasWriteableRT = brw_color_buffer_write_enabled(brw);
>
> +      bool alpha_to_one = false;
> +
>        if (!buffer0_is_integer) {
>           /* _NEW_MULTISAMPLE */
> -         pb.AlphaToCoverageEnable =
> -            _mesa_is_multisample_enabled(ctx) &&
> -            ctx->Multisample.SampleAlphaToCoverage;
> +
> +         if (_mesa_is_multisample_enabled(ctx)) {
> +            pb.AlphaToCoverageEnable = ctx->Multisample.
> SampleAlphaToCoverage;
> +            alpha_to_one = ctx->Multisample.SampleAlphaToOne;
> +         }
>
>           pb.AlphaTestEnable = color->AlphaEnabled;
>        }
> @@ -4098,6 +4119,16 @@ genX(upload_ps_blend)(struct brw_context *brw)
>              dstA = brw_fix_xRGB_alpha(dstA);
>           }
>
> +         /* Alpha to One doesn't work with Dual Color Blending.  Override
> +          * SRC1_ALPHA to ONE and ONE_MINUS_SRC1_ALPHA to ZERO.
> +          */
> +         if (alpha_to_one && color->Blend[0]._UsesDualSrc) {
> +            srcRGB = fix_dual_blend_alpha_to_one(srcRGB);
> +            srcA = fix_dual_blend_alpha_to_one(srcA);
> +            dstRGB = fix_dual_blend_alpha_to_one(dstRGB);
> +            dstA = fix_dual_blend_alpha_to_one(dstA);
> +         }
> +
>           pb.ColorBufferBlendEnable = true;
>           pb.SourceAlphaBlendFactor = brw_translate_blend_factor(srcA);
>           pb.DestinationAlphaBlendFactor = brw_translate_blend_factor(
> dstA);
> --
> 2.13.0
>
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