drm/i915/cnl: WaThrottleEUPerfToAvoidTDBackPressure:cnl(pre-prod)

Wa for B-stepping only.

A for a hang issue that requires throttling EU performace
to 12.5% to avoid back pressure to thread dispatch

v2: Rebased. No change from v1.
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Oscar Mateo <oscar.mateo@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170906220325.24524-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/i915_reg.h

@@ -8061,6 +8061,7 @@ enum {
 #define   FLOW_CONTROL_ENABLE		(1<<15)
 #define   PARTIAL_INSTRUCTION_SHOOTDOWN_DISABLE	(1<<8)
 #define   STALL_DOP_GATING_DISABLE		(1<<5)
+#define   THROTTLE_12_5				(7<<2)
 
 #define GEN7_ROW_CHICKEN2		_MMIO(0xe4f4)
 #define GEN7_ROW_CHICKEN2_GT2		_MMIO(0xf4f4)

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1079,6 +1079,10 @@ static int cnl_init_workarounds(struct intel_engine_cs *engine)
 	WA_SET_BIT_MASKED(CNL_HDC_CHICKEN0,
 			  HDC_FORCE_CONTEXT_SAVE_RESTORE_NON_COHERENT);
 
+	/* WaThrottleEUPerfToAvoidTDBackPressure:cnl(pre-prod) */
+	if (IS_CNL_REVID(dev_priv, CNL_REVID_B0, CNL_REVID_B0))
+		WA_SET_BIT_MASKED(GEN8_ROW_CHICKEN, THROTTLE_12_5);
+
 	/* WaDisableReplayBufferBankArbitrationOptimization:cnl */
 	WA_SET_BIT_MASKED(COMMON_SLICE_CHICKEN2,
 			  GEN8_SBE_DISABLE_REPLAY_BUF_OPTIMIZATION);