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struct request documentation

Jens Axboe <jens.axboe@oracle.com> 27/05/02

1.0
Index

2.0 Struct request members classification

        2.1 struct request members explanation

3.0


2.0
Short explanation of request members

Classification flags:

        D       driver member
        B       block layer member
        I       I/O scheduler member

Unless an entry contains a D classification, a device driver must not access
this member. Some members may contain D classifications, but should only be
access through certain macros or functions (eg ->flags).

<linux/blkdev.h>

2.1
Member                          Flag    Comment
------                          ----    -------

struct list_head queuelist      BI      Organization on various internal
                                        queues

void *elevator_private          I       I/O scheduler private data

unsigned char cmd[16]           D       Driver can use this for setting up
                                        a cdb before execution, see
                                        blk_queue_prep_rq

unsigned long flags             DBI     Contains info about data direction,
                                        request type, etc.

int rq_status                   D       Request status bits

kdev_t rq_dev                   DBI     Target device

int errors                      DB      Error counts

sector_t sector                 DBI     Target location

unsigned long hard_nr_sectors   B       Used to keep sector sane

unsigned long nr_sectors        DBI     Total number of sectors in request

unsigned long hard_nr_sectors   B       Used to keep nr_sectors sane

unsigned short nr_phys_segments DB      Number of physical scatter gather
                                        segments in a request

unsigned short nr_hw_segments   DB      Number of hardware scatter gather
                                        segments in a request

unsigned int current_nr_sectors DB      Number of sectors in first segment
                                        of request

unsigned int hard_cur_sectors   B       Used to keep current_nr_sectors sane

int tag                         DB      TCQ tag, if assigned

void *special                   D       Free to be used by driver

char *buffer                    D       Map of first segment, also see
                                        section on bouncing SECTION

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 *biotail             DBI     Last bio in request

struct request_queue *q         DB      Request queue this request belongs to

struct request_list *rl         B       Request list this request came from