[Mesa-dev] [PATCH] i965: Disable dual source blending when shader doesn't support it on gen8+

[Danylo Piliaiev]

Dual source blending behaviour is undefined when shader doesn't
have second color output, dismissing fragment in such situation
leads to a hang on gen8+ if depth test in enabled.

Since blending cannot be gracefully fixed in such case and the result
is undefined - blending is simply disabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107088

Signed-off-by: Danylo Piliaiev <danylo.piliaiev at globallogic.com>
---
 src/mesa/drivers/dri/i965/genX_state_upload.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/mesa/drivers/dri/i965/genX_state_upload.c b/src/mesa/drivers/dri/i965/genX_state_upload.c
index b279f01e..3d717bb 100644
--- a/src/mesa/drivers/dri/i965/genX_state_upload.c
+++ b/src/mesa/drivers/dri/i965/genX_state_upload.c
@@ -4791,7 +4791,24 @@ genX(upload_ps_blend)(struct brw_context *brw)
             dstA = fix_dual_blend_alpha_to_one(dstA);
          }
 
-         pb.ColorBufferBlendEnable = true;
+         const struct brw_wm_prog_data *wm_prog_data =
+            brw_wm_prog_data(brw->wm.base.prog_data);
+
+         /* The Dual Source Blending documentation says:
+          *
+          * "If SRC1 is included in a src/dst blend factor and
+          * a DualSource RT Write message is not used, results
+          * are UNDEFINED. (This reflects the same restriction in DX APIs,
+          * where undefined results are produced if “o1” is not written
+          * by a PS – there are no default values defined).
+          * If SRC1 is not included in a src/dst blend factor,
+          * dual source blending must be disabled."
+          *
+          * There is no way to gracefully fix this undefined situation
+          * so we just disable the blending to prevent possible issues.
+          */
+         pb.ColorBufferBlendEnable =
+            !color->Blend[0]._UsesDualSrc || wm_prog_data->dual_src_blend;
          pb.SourceAlphaBlendFactor = brw_translate_blend_factor(srcA);
          pb.DestinationAlphaBlendFactor = brw_translate_blend_factor(dstA);
          pb.SourceBlendFactor = brw_translate_blend_factor(srcRGB);
-- 
2.7.4