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    Subject[PATCH] bio traversal 2/2 - documentation changes

    # Bio traversal (separate submittion/completion bio pointers).
    # Patch 2/2 - Documentation changes.
    # Originally by Suparna Bhattacharya.
    # Bartlomiej Zolnierkiewicz <>

    diff -uNr linux-2.5.66/Documentation/block/biodoc.txt linux/Documentation/block/biodoc.txt
    --- linux-2.5.66/Documentation/block/biodoc.txt Mon Mar 17 19:49:00 2003
    +++ linux/Documentation/block/biodoc.txt Tue Mar 18 21:42:40 2003
    @@ -5,7 +5,7 @@
    Jens Axboe <>
    Suparna Bhattacharya <>

    -Last Updated May 2, 2002
    +Last Updated March 18, 2003


    @@ -204,8 +204,8 @@
    which case a virtual mapping of the page is required. For SCSI it is also
    done in some scenarios where the low level driver cannot be trusted to
    handle a single sg entry correctly. The driver is expected to perform the
    -kmaps as needed on such occasions using the bio_kmap and bio_kmap_irq
    -routines as appropriate. A driver could also use the blk_queue_bounce()
    +kmaps as needed on such occasions using the rq_map_buffer() routine
    +as appropriate. A driver could also use the blk_queue_bounce()
    routine on its own to bounce highmem i/o to low memory for specific requests
    if so desired.

    @@ -399,7 +399,8 @@
    directly by hand.
    This is because end_that_request_first only iterates over the bio list,
    and always returns 0 if there are none associated with the request.
    - _last works OK in this case, and is not a problem, as I mentioned earlier
    + end_that_request_last works OK in this case, and is not a problem,
    + as mentioned earlier.

    1.3.1 Pre-built Commands
    @@ -609,11 +610,20 @@
    unsigned short nr_hw_segments;

    /* Various sector counts */
    + /*
    + * The various block internal copies represent counts/pointers of
    + * unfinished I/O, while the other counts/pointers refer to
    + * I/O to be submitted.
    + */
    unsigned long nr_sectors; /* no. of sectors left: driver modifiable */
    unsigned long hard_nr_sectors; /* block internal copy of above */
    unsigned int current_nr_sectors; /* no. of sectors left in the
    current segment:driver modifiable */
    unsigned long hard_cur_sectors; /* block internal copy of the above */
    + unsigned short nr_bio_segments; /* no. of segments left in curr bio */
    + unsigned short nr_bio_sectors; /* no. of sectors left in curr bio */
    int tag; /* command tag associated with request */
    @@ -623,6 +633,7 @@
    struct bio *bio, *biotail; /* bio list instead of bh */
    + struct bio *hard_bio; /* block internal copy */
    struct request_list *rl;

    @@ -641,9 +652,11 @@
    transfer and invokes block end*request helpers to mark this. The
    driver should not modify these values. The block layer sets up the
    nr_sectors and current_nr_sectors fields (based on the corresponding
    -hard_xxx values and the number of bytes transferred) and updates it on
    -every transfer that invokes end_that_request_first. It does the same for the
    -buffer, bio, bio->bi_idx fields too.
    +hard_xxx values and the number of bytes transferred) and typically
    +updates it on every transfer that invokes end_that_request_first,
    +unless the driver has advanced these (submission) counters ahead
    +of the sectors being completed. The block layer also advances the
    +buffer, bio, bio->bi_idx fields appropriately as well as i/o completes.

    The buffer field is just a virtual address mapping of the current segment
    of the i/o buffer in cases where the buffer resides in low-memory. For high
    @@ -653,6 +666,60 @@
    a driver needs to be careful about interoperation with the block layer helper
    functions which the driver uses. (Section 1.3)

    +2.3.1 The Separation of Submission and Completion State
    +The basic protocol followed all through is that the bio fields
    +would always reflect the status w.r.t how much i/o remains
    +to be completed. On the other hand submission status would
    +only be maintained in the request structure. In most cases
    +of course, both move in sync (the generic end_that_request_first
    +code tries to handle that transparently by advancing the
    +submission pointers if they are behind the completion pointers
    +as would happen in the case of drivers which don't modify
    +those themselves), but for things like IDE mult-count write,
    +the submission counters/pointers may be ahead of the
    +completion pointers.
    +The following fields have been added to the request structure
    +to help maintain this distinction.
    + the rq->bio field now reflects the next bio which
    +is to be submitted for i/o. Hence, the need for rq->hard_bio
    +which keeps track of the next bio to be completed (this
    +is the one used by end_that_request_first now, instead
    +of rq->bio)
    + this keeps track of how many more vecs remain
    +to be submitted in the current bio (rq->bio). It is
    +used to compute the current index into rq->bio which
    +specifies the segment under submission.
    +(rq_map_buffer for example uses this field to map
    +the right buffer)
    + this keeps track of the number of sectors to
    +be submitted in the current bio (rq->bio). It can be
    +used to compute the remaining sectors in the current
    +segment in the situation when it is the last segment.
    +Now a subtle point about hard_cur_sectors. It reflects
    +the number of sectors left to be completed in the
    +_current_ segment under submission (i.e. the segment
    +in rq->bio, and _not_ rq->hard_bio). This makes it
    +possible to use it in rq_map_buffer to determine the
    +relative offset in the current segment w.r.t what
    +the bio indices might indicate.
    +A new helper, process_that_request_first() has been
    +introduced for updating submission state of the request
    +without completing the corresponding bios. It can be used
    +by code such as mult-count write which need to traverse
    +multiple bio segments for each chunk of i/o submitted,
    +where multiple such chunk transfers are required to cover
    +the entire request.
    3. Using bios

    3.1 Setup/Teardown
    @@ -732,6 +799,16 @@
    which don't make a distinction between segments and completion units would
    need to be reorganized to support multi-segment bios.

    +It is recommended that drivers utilize the block layer routine
    +process_that_request_first() while traversing bios for i/o submission,
    +instead of iterating over the segments directly, and use
    +end_that_request_first() for completion as before. Things like
    +rq_map_buffer() rely on the submission pointers in the request
    +to map the correct buffer.
    +rq_map_buffer() could be used to get a virtual address mapping
    +for the current segment buffer, in drivers which use PIO for example.
    3.2.2 Setting up DMA scatterlists

    The blk_rq_map_sg() helper routine would be used for setting up scatter
    @@ -1147,9 +1224,8 @@
    PIO drivers (or drivers that need to revert to PIO transfer once in a
    while (IDE for example)), where the CPU is doing the actual data
    transfer a virtual mapping is needed. If the driver supports highmem I/O,
    -(Sec 1.1, (ii) ) it needs to use bio_kmap and bio_kmap_irq to temporarily
    -map a bio into the virtual address space. See how IDE handles this with
    +(Sec 1.1, (ii) ) it needs to use rq_map_buffer() to temporarily
    +map a bio into the virtual address space. See how IDE handles this.

    8. Prior/Related/Impacted patches
    diff -uNr linux-2.5.66/Documentation/block/request.txt linux/Documentation/block/request.txt
    --- linux-2.5.66/Documentation/block/request.txt Fri Sep 20 16:19:26 2002
    +++ linux/Documentation/block/request.txt Tue Mar 18 21:54:16 2003
    @@ -52,11 +52,15 @@

    sector_t sector DBI Target location

    -unsigned long hard_nr_sectors B Used to keep sector sane
    +sector_t hard_sector B Used to keep sector sane
    + Tracks the location of unfinished
    + portion

    unsigned long nr_sectors DBI Total number of sectors in request

    unsigned long hard_nr_sectors B Used to keep nr_sectors sane
    + Tracks no. of unfinished sectors in
    + the request

    unsigned short nr_phys_segments DB Number of physical scatter gather
    segments in a request
    @@ -68,6 +72,14 @@
    of request

    unsigned int hard_cur_sectors B Used to keep current_nr_sectors sane
    + Tracks no. unfinished sectors in the
    + same segment.
    +unsigned long nr_bio_sectors DB Number of unfinished sectors in first
    + bio of request
    +unsigned short nr_bio_segments DB Number of unfinished segments in first
    + bio of request

    int tag DB TCQ tag, if assigned

    @@ -79,10 +91,12 @@
    struct completion *waiting D Can be used by driver to get signalled
    on request completion

    -struct bio *bio DBI First bio in request
    +struct bio *bio DBI First unsubmitted bio in request

    struct bio *biotail DBI Last bio in request

    +struct bio *hard_bio B First unfinished bio in request
    request_queue_t *q DB Request queue this request belongs to

    struct request_list *rl B Request list this request came from
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