drm/i915: Move the irq_barrier for reset earlier into reset_prepare

When updating the bookkeeping following the reset, we need the seqno to
be coherent on the CPU prior to trusting its result for deciding whether
any request is completed. We need the irq_barrier before we start making
these decisions, i.e. in reset_prepare.

References: https://bugs.freedesktop.org/show_bug.cgi?id=99733
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170210185214.23463-1-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -2652,6 +2652,9 @@ int i915_gem_reset_prepare(struct drm_i915_private *dev_priv)
 		tasklet_disable(&engine->irq_tasklet);
 		tasklet_kill(&engine->irq_tasklet);
 
+		if (engine->irq_seqno_barrier)
+			engine->irq_seqno_barrier(engine);
+
 		if (engine_stalled(engine)) {
 			request = i915_gem_find_active_request(engine);
 			if (request && request->fence.error == -EIO)
@@ -2748,9 +2751,6 @@ static void i915_gem_reset_engine(struct intel_engine_cs *engine)
 {
 	struct drm_i915_gem_request *request;
 
-	if (engine->irq_seqno_barrier)
-		engine->irq_seqno_barrier(engine);
-
 	request = i915_gem_find_active_request(engine);
 	if (request && i915_gem_reset_request(request)) {
 		DRM_DEBUG_DRIVER("resetting %s to restart from tail of request 0x%x\n",