drm/i915/gen9: add WaClearFlowControlGpgpuContextSave

This allows writes to EU flow control registers. Together
with SIP code from the user-mode driver this resolves a
hang seen in some pre-emption scenarios. Note that this
patch is just the kernel mode part of this workaround.

v2. Oops, add FLOW_CONTROL_ENABLE macro to i915_reg.h.
Signed-off-by: Tim Gore <tim.gore@intel.com>
Reviewed-by: Arun Siluvery <arun.siluvery@linux.intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458144826-17269-1-git-send-email-tim.gore@intel.com

drivers/gpu/drm/i915/i915_reg.h

@@ -7105,6 +7105,7 @@ enum skl_disp_power_wells {
 #define   GEN9_CCS_TLB_PREFETCH_ENABLE	(1<<3)
 
 #define GEN8_ROW_CHICKEN		_MMIO(0xe4f0)
+#define   FLOW_CONTROL_ENABLE		(1<<15)
 #define   PARTIAL_INSTRUCTION_SHOOTDOWN_DISABLE	(1<<8)
 #define   STALL_DOP_GATING_DISABLE		(1<<5)
 

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -925,8 +925,10 @@ static int gen9_init_workarounds(struct intel_engine_cs *engine)
 	I915_WRITE(GAM_ECOCHK, I915_READ(GAM_ECOCHK) |
 		   ECOCHK_DIS_TLB);
 
+	/* WaClearFlowControlGpgpuContextSave:skl,bxt */
 	/* WaDisablePartialInstShootdown:skl,bxt */
 	WA_SET_BIT_MASKED(GEN8_ROW_CHICKEN,
+			  FLOW_CONTROL_ENABLE |
 			  PARTIAL_INSTRUCTION_SHOOTDOWN_DISABLE);
 
 	/* Syncing dependencies between camera and graphics:skl,bxt */