drm/i915/cnl: WaForceContextSaveRestoreNonCoherent

To avoid a potential hang condition with TLB invalidation
we need to enable masked bit 5 of MMIO 0xE5F0 at boot.

Same workaround was in place for previous platforms,
but the register offset has changed for CNL.
But also BSpec doesn't mention the bit 15 as set on gen9
platforms and mark bit as reserved on CNL.

v2: Improve commit message accepting Oscar's suggestion.

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Oscar Mateo <oscar.mateo@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170823203504.10012-1-rodrigo.vivi@intel.com

drivers/gpu/drm/i915/i915_reg.h

@@ -7024,6 +7024,7 @@ enum {
 
 /* GEN8 chicken */
 #define HDC_CHICKEN0				_MMIO(0x7300)
+#define CNL_HDC_CHICKEN0			_MMIO(0xE5F0)
 #define  HDC_FORCE_CSR_NON_COHERENT_OVR_DISABLE	(1<<15)
 #define  HDC_FENCE_DEST_SLM_DISABLE		(1<<14)
 #define  HDC_DONOT_FETCH_MEM_WHEN_MASKED	(1<<11)

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1070,6 +1070,10 @@ static int cnl_init_workarounds(struct intel_engine_cs *engine)
 	struct drm_i915_private *dev_priv = engine->i915;
 	int ret;
 
+	/* WaForceContextSaveRestoreNonCoherent:cnl */
+	WA_SET_BIT_MASKED(CNL_HDC_CHICKEN0,
+			  HDC_FORCE_CONTEXT_SAVE_RESTORE_NON_COHERENT);
+
 	/* WaDisableReplayBufferBankArbitrationOptimization:cnl */
 	WA_SET_BIT_MASKED(COMMON_SLICE_CHICKEN2,
 			  GEN8_SBE_DISABLE_REPLAY_BUF_OPTIMIZATION);