blk-mq-sched: add framework for MQ capable IO schedulers

This adds a set of hooks that intercepts the blk-mq path of
allocating/inserting/issuing/completing requests, allowing
us to develop a scheduler within that framework.

We reuse the existing elevator scheduler API on the registration
side, but augment that with the scheduler flagging support for
the blk-mq interfce, and with a separate set of ops hooks for MQ
devices.

We split driver and scheduler tags, so we can run the scheduling
independently of device queue depth.

Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 2e99d659b0f1..25564857f5f8 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -154,6 +154,7 @@ struct request {
 
         /* the following two fields are internal, NEVER access directly */
         unsigned int __data_len;        /* total data len */
+        int tag;
         sector_t __sector;              /* sector cursor */
 
         struct bio *bio;
@@ -220,9 +221,10 @@ struct request {
 
         unsigned short ioprio;
 
+        int internal_tag;
+
         void *special;          /* opaque pointer available for LLD use */
 
-        int tag;
         int errors;
 
         /*