struct request documentation

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

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>

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