[PATCH 6/8] block: add code to track actual device queue depth

[Jens Axboe <axboe@fb.com>]

For blk-mq, ->nr_requests does track queue depth, at least at init
time. But for the older queue paths, it's simply a soft setting.
On top of that, it's generally larger than the hardware setting
on purpose, to allow backup of requests for merging.

Fill a hole in struct request with a 'queue_depth' member, that
drivers can call to more closely inform the block layer of the
real queue depth.

Signed-off-by: Jens Axboe <axboe@fb.com>
---
 block/blk-settings.c   | 12 ++++++++++++
 drivers/scsi/scsi.c    |  3 +++
 include/linux/blkdev.h | 11 +++++++++++
 3 files changed, 26 insertions(+)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index f679ae122843..f7e122e717e8 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -832,6 +832,18 @@ void blk_queue_flush_queueable(struct request_queue *q, bool queueable)
 EXPORT_SYMBOL_GPL(blk_queue_flush_queueable);
 
 /**
+ * blk_set_queue_depth - tell the block layer about the device queue depth
+ * @q:		the request queue for the device
+ * @depth:		queue depth
+ *
+ */
+void blk_set_queue_depth(struct request_queue *q, unsigned int depth)
+{
+	q->queue_depth = depth;
+}
+EXPORT_SYMBOL(blk_set_queue_depth);
+
+/**
  * blk_queue_write_cache - configure queue's write cache
  * @q:		the request queue for the device
  * @wc:		write back cache on or off
diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 1deb6adc411f..75455d4dab68 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -621,6 +621,9 @@ int scsi_change_queue_depth(struct scsi_device *sdev, int depth)
 		wmb();
 	}
 
+	if (sdev->request_queue)
+		blk_set_queue_depth(sdev->request_queue, depth);
+
 	return sdev->queue_depth;
 }
 EXPORT_SYMBOL(scsi_change_queue_depth);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index fc1894996b12..eee94bd6de52 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -315,6 +315,8 @@ struct request_queue {
 	struct blk_mq_ctx __percpu	*queue_ctx;
 	unsigned int		nr_queues;
 
+	unsigned int		queue_depth;
+
 	/* hw dispatch queues */
 	struct blk_mq_hw_ctx	**queue_hw_ctx;
 	unsigned int		nr_hw_queues;
@@ -681,6 +683,14 @@ static inline bool blk_write_same_mergeable(struct bio *a, struct bio *b)
 	return false;
 }
 
+static inline unsigned int blk_queue_depth(struct request_queue *q)
+{
+	if (q->queue_depth)
+		return q->queue_depth;
+
+	return q->nr_requests;
+}
+
 /*
  * q->prep_rq_fn return values
  */
@@ -984,6 +994,7 @@ extern void blk_limits_io_min(struct queue_limits *limits, unsigned int min);
 extern void blk_queue_io_min(struct request_queue *q, unsigned int min);
 extern void blk_limits_io_opt(struct queue_limits *limits, unsigned int opt);
 extern void blk_queue_io_opt(struct request_queue *q, unsigned int opt);
+extern void blk_set_queue_depth(struct request_queue *q, unsigned int depth);
 extern void blk_set_default_limits(struct queue_limits *lim);
 extern void blk_set_stacking_limits(struct queue_limits *lim);
 extern int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
-- 
2.8.0.rc4.6.g7e4ba36