drm/i915/gem: Wait until the context is finally retired before releasing engines

If we want to percolate information back from the HW, up through the GEM
context, we need to wait until the intel_context is scheduled out for
the last time. This is handled by the retirement of the intel_context's
barrier, i.e. by listening to the pulse after the notional unpin. So
wait until the intel_context is finally retired before releasing the
engine, so that we can inspect the final context state and pass it on.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406155840.1728-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -570,23 +570,19 @@ static void engines_idle_release(struct i915_gem_context *ctx,
 	engines->ctx = i915_gem_context_get(ctx);
 
 	for_each_gem_engine(ce, engines, it) {
-		struct dma_fence *fence;
-		int err = 0;
+		int err;
 
 		/* serialises with execbuf */
 		set_bit(CONTEXT_CLOSED_BIT, &ce->flags);
 		if (!intel_context_pin_if_active(ce))
 			continue;
 
-		fence = i915_active_fence_get(&ce->timeline->last_request);
-		if (fence) {
-			err = i915_sw_fence_await_dma_fence(&engines->fence,
-							    fence, 0,
-							    GFP_KERNEL);
-			dma_fence_put(fence);
-		}
+		/* Wait until context is finally scheduled out and retired */
+		err = i915_sw_fence_await_active(&engines->fence,
+						 &ce->active,
+						 I915_ACTIVE_AWAIT_BARRIER);
 		intel_context_unpin(ce);
-		if (err < 0)
+		if (err)
 			goto kill;
 	}