drm/i915: Free batch pool when idle

At runtime, this helps ensure that the batch pools are kept trim and
fast. Then at suspend, this releases memory that we do not need to
restore. It also ties into the oom-notifier to ensure that we recover as
much kernel memory as possible during OOM.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index d1ca192..4ec5d7e 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2795,8 +2795,19 @@
 {
 	struct drm_i915_private *dev_priv =
 		container_of(work, typeof(*dev_priv), mm.idle_work.work);
+	struct drm_device *dev = dev_priv->dev;
 
-	intel_mark_idle(dev_priv->dev);
+	intel_mark_idle(dev);
+
+	if (mutex_trylock(&dev->struct_mutex)) {
+		struct intel_engine_cs *ring;
+		int i;
+
+		for_each_ring(ring, dev_priv, i)
+			i915_gem_batch_pool_fini(&ring->batch_pool);
+
+		mutex_unlock(&dev->struct_mutex);
+	}
 }
 
 /**