=====================================================
Notes Written on Jan 15, 2002:
- Jens Axboe <axboe@suse.de>
+ Jens Axboe <jens.axboe@oracle.com>
Suparna Bhattacharya <suparna@in.ibm.com>
Last Updated May 2, 2002
---------
2.5 bio rewrite:
- Jens Axboe <axboe@suse.de>
+ Jens Axboe <jens.axboe@oracle.com>
Many aspects of the generic block layer redesign were driven by and evolved
over discussions, prior patches and the collective experience of several
the same bi_io_vec array, but with the index and size accordingly modified)
- A linked list of bios is used as before for unrelated merges (*) - this
avoids reallocs and makes independent completions easier to handle.
-- Code that traverses the req list needs to make a distinction between
- segments of a request (bio_for_each_segment) and the distinct completion
- units/bios (rq_for_each_bio).
+- Code that traverses the req list can find all the segments of a bio
+ by using rq_for_each_segment. This handles the fact that a request
+ has multiple bios, each of which can have multiple segments.
- Drivers which can't process a large bio in one shot can use the bi_idx
field to keep track of the next bio_vec entry to process.
(e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
3.2.1 Traversing segments and completion units in a request
-The macros bio_for_each_segment() and rq_for_each_bio() should be used for
-traversing the bios in the request list (drivers should avoid directly
-trying to do it themselves). Using these helpers should also make it easier
-to cope with block changes in the future.
+The macro rq_for_each_segment() should be used for traversing the bios
+in the request list (drivers should avoid directly trying to do it
+themselves). Using these helpers should also make it easier to cope
+with block changes in the future.
- rq_for_each_bio(bio, rq)
- bio_for_each_segment(bio_vec, bio, i)
- /* bio_vec is now current segment */
+ struct req_iterator iter;
+ rq_for_each_segment(bio_vec, rq, iter)
+ /* bio_vec is now current segment */
I/O completion callbacks are per-bio rather than per-segment, so drivers
that traverse bio chains on completion need to keep that in mind. Drivers
queueing (typically known as tagged command queueing), ie manage more than
one outstanding command on a queue at any given time.
- blk_queue_init_tags(request_queue_t *q, int depth)
+ blk_queue_init_tags(struct request_queue *q, int depth)
Initialize internal command tagging structures for a maximum
depth of 'depth'.
- blk_queue_free_tags((request_queue_t *q)
+ blk_queue_free_tags((struct request_queue *q)
Teardown tag info associated with the queue. This will be done
automatically by block if blk_queue_cleanup() is called on a queue
The above are initialization and exit management, the main helpers during
normal operations are:
- blk_queue_start_tag(request_queue_t *q, struct request *rq)
+ blk_queue_start_tag(struct request_queue *q, struct request *rq)
Start tagged operation for this request. A free tag number between
0 and 'depth' is assigned to the request (rq->tag holds this number),
for this queue is already achieved (or if the tag wasn't started for
some other reason), 1 is returned. Otherwise 0 is returned.
- blk_queue_end_tag(request_queue_t *q, struct request *rq)
+ blk_queue_end_tag(struct request_queue *q, struct request *rq)
End tagged operation on this request. 'rq' is removed from the internal
book keeping structures.
the hardware and software block queue and enable the driver to sanely restart
all the outstanding requests. There's a third helper to do that:
- blk_queue_invalidate_tags(request_queue_t *q)
+ blk_queue_invalidate_tags(struct request_queue *q)
Clear the internal block tag queue and re-add all the pending requests
to the request queue. The driver will receive them again on the
scheduler for example, to reposition the request
if its sorting order has changed.
+elevator_allow_merge_fn called whenever the block layer determines
+ that a bio can be merged into an existing
+ request safely. The io scheduler may still
+ want to stop a merge at this point if it
+ results in some sort of conflict internally,
+ this hook allows it to do that.
+
elevator_dispatch_fn fills the dispatch queue with ready requests.
I/O schedulers are free to postpone requests by
not filling the dispatch queue unless @force
projects / linux-2.6-omap-h63xx.git / blobdiff