block: Improve default elevator selection

}

/*

 * For blk-mq devices supporting IO scheduling, we default to using mq-deadline,

 * if available, for single queue devices. If deadline isn't available OR

 * deadline initialization fails OR we have multiple queues, default to "none".

 * For single queue devices, default to using mq-deadline. If we have multiple

 * queues or mq-deadline is not available, default to "none".

 */

static struct elevator_type *elevator_get_default(struct request_queue *q)

{

	if (q->nr_hw_queues != 1)

		return NULL;

	return elevator_get(q, "mq-deadline", false);

}

/*

 * Get the first elevator providing the features required by the request queue.

 * Default to "none" if no matching elevator is found.

 */

static struct elevator_type *elevator_get_by_features(struct request_queue *q)

{

	struct elevator_type *e;

	spin_lock(&elv_list_lock);

	list_for_each_entry(e, &elv_list, list) {

		if (elv_support_features(e->elevator_features,

					 q->required_elevator_features))

			break;

	}

	if (e && !try_module_get(e->elevator_owner))

		e = NULL;

	spin_unlock(&elv_list_lock);

	return e;

}

/*

 * For a device queue that has no required features, use the default elevator

 * settings. Otherwise, use the first elevator available matching the required

 * features. If no suitable elevator is find or if the chosen elevator

 * initialization fails, fall back to the "none" elevator (no elevator).

 */

void elevator_init_mq(struct request_queue *q)

{

	if (!elv_support_iosched(q))

		return;

	if (q->nr_hw_queues != 1)

		return;

	WARN_ON_ONCE(test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags));

	if (unlikely(q->elevator))

		return;

	e = elevator_get(q, "mq-deadline", false);

	if (!q->required_elevator_features)

		e = elevator_get_default(q);

	else

		e = elevator_get_by_features(q);

	if (!e)

		return;