drm/i915: Reduce nested prepare_remote_context() to a trylock

	GEM_BUG_ON(rq->hw_context == ce);

	GEM_BUG_ON(rq->hw_context == ce);

	if (rcu_access_pointer(rq->timeline) != tl) { /* timeline sharing! */

	if (rcu_access_pointer(rq->timeline) != tl) { /* timeline sharing! */

		err = mutex_lock_interruptible_nested(&tl->mutex,

		/*

						      SINGLE_DEPTH_NESTING);

		 * Ideally, we just want to insert our foreign fence as

		if (err)

		 * a barrier into the remove context, such that this operation

			return err;

		 * occurs after all current operations in that context, and

		 * all future operations must occur after this.

		 *

		 * Currently, the timeline->last_request tracking is guarded

		 * by its mutex and so we must obtain that to atomically

		 * insert our barrier. However, since we already hold our

		 * timeline->mutex, we must be careful against potential

		 * inversion if we are the kernel_context as the remote context

		 * will itself poke at the kernel_context when it needs to

		 * unpin. Ergo, if already locked, we drop both locks and

		 * try again (through the magic of userspace repeating EAGAIN).

		 */

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

			return -EAGAIN;

		/* Queue this switch after current activity by this context. */

		/* Queue this switch after current activity by this context. */

		err = i915_active_fence_set(&tl->last_request, rq);

		err = i915_active_fence_set(&tl->last_request, rq);