[Mesa-dev] [PATCH] anv: Stop setting 3DSTATE_PS_EXTRA::PixelShaderHasUAV

[Jason Ekstrand]

We've had several broadwell hangs that have come down to this bit just
not working correctly.  Most recently, we've had a pile of hangs
reported with apps running under DXVK:

https://github.com/doitsujin/dxvk/issues/469

Instead, use the bit that doesn't try to imply weird D3D coherency
things and just force-enables the PS like we want.

cc: mesa-stable at lists.freedesktop.org
---
 src/intel/vulkan/genX_pipeline.c | 53 +++++++++++++-------------------
 1 file changed, 22 insertions(+), 31 deletions(-)

diff --git a/src/intel/vulkan/genX_pipeline.c b/src/intel/vulkan/genX_pipeline.c
index aa63ad0e097..4004a36f74f 100644
--- a/src/intel/vulkan/genX_pipeline.c
+++ b/src/intel/vulkan/genX_pipeline.c
@@ -1389,6 +1389,28 @@ emit_3dstate_wm(struct anv_pipeline *pipeline, struct anv_subpass *subpass,
             wm.EarlyDepthStencilControl         = EDSC_NORMAL;
          }
 
+#if GEN_GEN >= 8
+         /* Gen8 hardware tries to compute ThreadDispatchEnable for us but
+          * doesn't take into account KillPixels when no depth or stencil
+          * writes are enabled.  In order for occlusion queries to work
+          * correctly with no attachments, we need to force-enable PS thread
+          * dispatch.
+          *
+          * The BDW docs are pretty clear that that this bit isn't validated
+          * and probably shouldn't be used in production:
+          *
+          *    "This must always be set to Normal. This field should not be
+          *    tested for functional validation."
+          *
+          * Unfortunately, however, the other mechanism we have for doing this
+          * is 3DSTATE_PS_EXTRA::PixelShaderHasUAV which causes hangs on BDW.
+          * Given two bad options, we choose the one which works.
+          */
+         if ((wm_prog_data->has_side_effects || wm_prog_data->uses_kill) &&
+             !has_color_buffer_write_enabled(pipeline, blend))
+            wm.ForceThreadDispatchEnable = ForceON;
+#endif
+
          wm.BarycentricInterpolationMode =
             wm_prog_data->barycentric_interp_modes;
 
@@ -1583,37 +1605,6 @@ emit_3dstate_ps_extra(struct anv_pipeline *pipeline,
       ps.PixelShaderKillsPixel         = subpass->has_ds_self_dep ||
                                          wm_prog_data->uses_kill;
 
-      /* The stricter cross-primitive coherency guarantees that the hardware
-       * gives us with the "Accesses UAV" bit set for at least one shader stage
-       * and the "UAV coherency required" bit set on the 3DPRIMITIVE command are
-       * redundant within the current image, atomic counter and SSBO GL APIs,
-       * which all have very loose ordering and coherency requirements and
-       * generally rely on the application to insert explicit barriers when a
-       * shader invocation is expected to see the memory writes performed by the
-       * invocations of some previous primitive.  Regardless of the value of
-       * "UAV coherency required", the "Accesses UAV" bits will implicitly cause
-       * an in most cases useless DC flush when the lowermost stage with the bit
-       * set finishes execution.
-       *
-       * It would be nice to disable it, but in some cases we can't because on
-       * Gen8+ it also has an influence on rasterization via the PS UAV-only
-       * signal (which could be set independently from the coherency mechanism
-       * in the 3DSTATE_WM command on Gen7), and because in some cases it will
-       * determine whether the hardware skips execution of the fragment shader
-       * or not via the ThreadDispatchEnable signal.  However if we know that
-       * GEN8_PS_BLEND_HAS_WRITEABLE_RT is going to be set and
-       * GEN8_PSX_PIXEL_SHADER_NO_RT_WRITE is not set it shouldn't make any
-       * difference so we may just disable it here.
-       *
-       * Gen8 hardware tries to compute ThreadDispatchEnable for us but doesn't
-       * take into account KillPixels when no depth or stencil writes are
-       * enabled. In order for occlusion queries to work correctly with no
-       * attachments, we need to force-enable here.
-       */
-      if ((wm_prog_data->has_side_effects || wm_prog_data->uses_kill) &&
-          !has_color_buffer_write_enabled(pipeline, blend))
-         ps.PixelShaderHasUAV = true;
-
 #if GEN_GEN >= 9
       ps.PixelShaderComputesStencil = wm_prog_data->computed_stencil;
       ps.PixelShaderPullsBary    = wm_prog_data->pulls_bary;
-- 
2.17.1