[Intel-gfx] [PATCH] drm/i915: Enable provoking vertex fix on Gen9+ systems.

[Chris Wilson]

Quoting Kenneth Graunke (2018-06-14 22:53:28)
> The SF and clipper units mishandle the provoking vertex in some cases,
> which can cause misrendering with shaders that use flat shaded inputs.
> 
> There are chicken bits in 3D_CHICKEN3 (for SF) and FF_SLICE_CHICKEN
> (for the clipper) that work around the issue.  These registers are
> unfortunately not part of the logical context (even the power context).

Indeed they are not. And that explains why we lose them in CI, my kbl
never seems to powerdown and so never loses them. But an LRI from
userspace to them is enough for their contents to be lost, and we never
recover.

So that rules out using the context image to store these registers, and
we need to add a payload to every batch. We may get away with just using
the per-context wa, to load the registers every time we switch into a
context (and so also on load from idle).

Instead we need:

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index e935f8c45db8..46c93ccd36f3 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1565,11 +1565,21 @@ static u32 *gen9_init_indirectctx_bb(struct intel_engine_cs *engine, u32 *batch)
        /* WaFlushCoherentL3CacheLinesAtContextSwitch:skl,bxt,glk */
        batch = gen8_emit_flush_coherentl3_wa(engine, batch);
 
+       *batch++ = MI_LOAD_REGISTER_IMM(3);
+
        /* WaDisableGatherAtSetShaderCommonSlice:skl,bxt,kbl,glk */
-       *batch++ = MI_LOAD_REGISTER_IMM(1);
        *batch++ = i915_mmio_reg_offset(COMMON_SLICE_CHICKEN2);
        *batch++ = _MASKED_BIT_DISABLE(
                        GEN9_DISABLE_GATHER_AT_SET_SHADER_COMMON_SLICE);
+
+       /* BSpec: 11391 */
+       *batch++ = i915_mmio_reg_offset(FF_SLICE_CHICKEN);
+       *batch++ = _MASKED_BIT_ENABLE(FF_SLICE_CHICKEN_CL_PROVOKING_VERTEX_FIX);
+
+       /* BSpec: 11299 */
+       *batch++ = i915_mmio_reg_offset(_3D_CHICKEN3);
+       *batch++ = _MASKED_BIT_ENABLE(_3D_CHICKEN_SF_PROVOKING_VERTEX_FIX);
+
        *batch++ = MI_NOOP;
 
        /* WaClearSlmSpaceAtContextSwitch:kbl */

How crummy.
-Chris