[Intel-gfx] [PATCH] drm/i915/cnl: WaDisableGatherAtSetShaderCommonSlice

[Rodrigo Vivi]

Purely empirical. I don't have a better description of the need
of this workaround. It is not on BSpec and it is not on wa_database.

However it brings a huge stability to CNL and fix many issues that
Mesa was facing.

Empirical history: when Rafael and I were trying to understand
the misterious WaSendPushConstantsFromMMIO we just had a description
of it that was "If not using RS, we must send two MMIO registers at
context create to trigger push constants at 3D primitive"

And in another place we just saw that WaSendPushConstantsFromMMIO
was only adding COMMON_SLICE_CHICKEN2 in a white list.

So we looked to the programmin notes of COMMON_SLICE_CHICKEN2
and we notice that this bit 12 is marked in association with
2 other MMIO registers for SKL+. Apparently for SKL+ we should
check few MMIOs to decide for set or reset of this bit 12.

Also "gather" is related to gather and packing of consntant elements
into "push constants".

So we give a shot with this workaround in place and achieved
a good stability.

Cc: Rafael Antognolli <rafael.antognolli at intel.com>
Cc: Mika Kuoppala <mika.kuoppala at intel.com>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 7d6da130b184..aa1705319d71 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1231,6 +1231,23 @@ static u32 *gen9_init_indirectctx_bb(struct intel_engine_cs *engine, u32 *batch)
 	return batch;
 }
 
+static u32 *gen10_init_indirectctx_bb(struct intel_engine_cs *engine,
+				      u32 *batch)
+{
+	/* WaDisableGatherAtSetShaderCommonSlice:cnl */
+	*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);
+	*batch++ = MI_NOOP;
+
+	/* Pad to end of cacheline */
+	while ((unsigned long)batch % CACHELINE_BYTES)
+		*batch++ = MI_NOOP;
+
+	return batch;
+}
+
 #define CTX_WA_BB_OBJ_SIZE (PAGE_SIZE)
 
 static int lrc_setup_wa_ctx(struct intel_engine_cs *engine)
@@ -1284,7 +1301,9 @@ static int intel_init_workaround_bb(struct intel_engine_cs *engine)
 
 	switch (INTEL_GEN(engine->i915)) {
 	case 10:
-		return 0;
+		wa_bb_fn[0] = gen10_init_indirectctx_bb;
+		wa_bb_fn[1] = NULL;
+		break;
 	case 9:
 		wa_bb_fn[0] = gen9_init_indirectctx_bb;
 		wa_bb_fn[1] = NULL;
-- 
2.13.5