block: Remove elevator required features

The only elevator feature ever implemented is ELEVATOR_F_ZBD_SEQ_WRITE
for signaling that a scheduler implements zone write locking to tightly
control the dispatching order of write operations to zoned block
devices. With the removal of zone write locking support in mq-deadline
and the reliance of all block device drivers on the block layer zone
write plugging to control ordering of write operations to zones, the
elevator feature ELEVATOR_F_ZBD_SEQ_WRITE is completely unused.
Remove it, and also remove the now unused code for filtering the
possible schedulers for a block device based on required features.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Hans Holmberg <hans.holmberg@wdc.com>
Tested-by: Dennis Maisenbacher <dennis.maisenbacher@wdc.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20240408014128.205141-23-dlemoal@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-settings.c

@@ -1052,22 +1052,6 @@ void blk_queue_write_cache(struct request_queue *q, bool wc, bool fua)
 }
 EXPORT_SYMBOL_GPL(blk_queue_write_cache);
 
-/**
- * blk_queue_required_elevator_features - Set a queue required elevator features
- * @q:		the request queue for the target device
- * @features:	Required elevator features OR'ed together
- *
- * Tell the block layer that for the device controlled through @q, only the
- * only elevators that can be used are those that implement at least the set of
- * features specified by @features.
- */
-void blk_queue_required_elevator_features(struct request_queue *q,
-					  unsigned int features)
-{
-	q->required_elevator_features = features;
-}
-EXPORT_SYMBOL_GPL(blk_queue_required_elevator_features);
-
 /**
  * blk_queue_can_use_dma_map_merging - configure queue for merging segments.
  * @q:		the request queue for the device

block/elevator.c

@@ -83,13 +83,6 @@ bool elv_bio_merge_ok(struct request *rq, struct bio *bio)
 }
 EXPORT_SYMBOL(elv_bio_merge_ok);
 
-static inline bool elv_support_features(struct request_queue *q,
-		const struct elevator_type *e)
-{
-	return (q->required_elevator_features & e->elevator_features) ==
-		q->required_elevator_features;
-}
-
 /**
  * elevator_match - Check whether @e's name or alias matches @name
  * @e: Scheduler to test
@@ -120,7 +113,7 @@ static struct elevator_type *elevator_find_get(struct request_queue *q,
 
 	spin_lock(&elv_list_lock);
 	e = __elevator_find(name);
-	if (e && (!elv_support_features(q, e) || !elevator_tryget(e)))
+	if (e && (!elevator_tryget(e)))
 		e = NULL;
 	spin_unlock(&elv_list_lock);
 	return e;
@@ -580,34 +573,8 @@ static struct elevator_type *elevator_get_default(struct request_queue *q)
 }
 
 /*
- * Get the first elevator providing the features required by the request queue.
+ * Use the default elevator settings. If the chosen elevator initialization
- * Default to "none" if no matching elevator is found.
+ * fails, fall back to the "none" elevator (no elevator).
- */
-static struct elevator_type *elevator_get_by_features(struct request_queue *q)
-{
-	struct elevator_type *e, *found = NULL;
-
-	spin_lock(&elv_list_lock);
-
-	list_for_each_entry(e, &elv_list, list) {
-		if (elv_support_features(q, e)) {
-			found = e;
-			break;
-		}
-	}
-
-	if (found && !elevator_tryget(found))
-		found = NULL;
-
-	spin_unlock(&elv_list_lock);
-	return found;
-}
-
-/*
- * 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)
 {
@@ -622,10 +589,7 @@ void elevator_init_mq(struct request_queue *q)
 	if (unlikely(q->elevator))
 		return;
 
-	if (!q->required_elevator_features)
+	e = elevator_get_default(q);
-		e = elevator_get_default(q);
-	else
-		e = elevator_get_by_features(q);
 	if (!e)
 		return;
 
@@ -781,7 +745,7 @@ ssize_t elv_iosched_show(struct request_queue *q, char *name)
 	list_for_each_entry(e, &elv_list, list) {
 		if (e == cur)
 			len += sprintf(name+len, "[%s] ", e->elevator_name);
-		else if (elv_support_features(q, e))
+		else
 			len += sprintf(name+len, "%s ", e->elevator_name);
 	}
 	spin_unlock(&elv_list_lock);

block/elevator.h

@@ -74,7 +74,6 @@ struct elevator_type
 	struct elv_fs_entry *elevator_attrs;
 	const char *elevator_name;
 	const char *elevator_alias;
-	const unsigned int elevator_features;
 	struct module *elevator_owner;
 #ifdef CONFIG_BLK_DEBUG_FS
 	const struct blk_mq_debugfs_attr *queue_debugfs_attrs;

include/linux/blkdev.h

@@ -453,8 +453,6 @@ struct request_queue {
 
 	atomic_t		nr_active_requests_shared_tags;
 
-	unsigned int		required_elevator_features;
-
 	struct blk_mq_tags	*sched_shared_tags;
 
 	struct list_head	icq_list;
@@ -958,14 +956,6 @@ disk_alloc_independent_access_ranges(struct gendisk *disk, int nr_ia_ranges);
 void disk_set_independent_access_ranges(struct gendisk *disk,
 				struct blk_independent_access_ranges *iars);
 
-/*
- * Elevator features for blk_queue_required_elevator_features:
- */
-/* Supports zoned block devices sequential write constraint */
-#define ELEVATOR_F_ZBD_SEQ_WRITE	(1U << 0)
-
-extern void blk_queue_required_elevator_features(struct request_queue *q,
-						 unsigned int features);
 extern bool blk_queue_can_use_dma_map_merging(struct request_queue *q,
 					      struct device *dev);