drm/i915: Eliminate the trylock for awaiting an earlier request

static int

i915_request_await_start(struct i915_request *rq, struct i915_request *signal)

{

	struct intel_timeline *tl;

	struct dma_fence *fence;

	int err;

	GEM_BUG_ON(i915_request_timeline(rq) ==

		   rcu_access_pointer(signal->timeline));

	rcu_read_lock();

	tl = rcu_dereference(signal->timeline);

	if (i915_request_started(signal) || !kref_get_unless_zero(&tl->kref))

		tl = NULL;

	rcu_read_unlock();

	if (!tl) /* already started or maybe even completed */

		return 0;

	fence = ERR_PTR(-EAGAIN);

	if (mutex_trylock(&tl->mutex)) {

	fence = NULL;

	rcu_read_lock();

	spin_lock_irq(&signal->lock);

	if (!i915_request_started(signal) &&

		    !list_is_first(&signal->link, &tl->requests)) {

			signal = list_prev_entry(signal, link);

			fence = dma_fence_get(&signal->fence);

	    !list_is_first(&signal->link,

			   &rcu_dereference(signal->timeline)->requests)) {

		struct i915_request *prev = list_prev_entry(signal, link);

		/*

		 * Peek at the request before us in the timeline. That

		 * request will only be valid before it is retired, so

		 * after acquiring a reference to it, confirm that it is

		 * still part of the signaler's timeline.

		 */

		if (i915_request_get_rcu(prev)) {

			if (list_next_entry(prev, link) == signal)

				fence = &prev->fence;

			else

				i915_request_put(prev);

		}

		mutex_unlock(&tl->mutex);

	}

	intel_timeline_put(tl);

	if (IS_ERR_OR_NULL(fence))

		return PTR_ERR_OR_ZERO(fence);

	spin_unlock_irq(&signal->lock);

	rcu_read_unlock();

	if (!fence)

		return 0;

	err = 0;

	if (intel_timeline_sync_is_later(i915_request_timeline(rq), fence))