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

[Kenneth Graunke]

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