Re: [Q] In sg_io_v4 what to use as flag for "queue at_head"

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Douglas Gilbert wrote:
Boaz Harrosh wrote:
Douglas Gilbert <dougg@xxxxxxxxxx> wrote ...
struct sg_io_v4 {
    __s32 guard;        /* [i] 'Q' to differentiate from v3 */
    __u32 protocol;        /* [i] 0 -> SCSI , .... */
    __u32 subprotocol;    /* [i] 0 -> SCSI command, 1 -> SCSI task
                   management function, .... */

    __u32 request_len;    /* [i] in bytes */
    __u64 request;        /* [i], [*i] {SCSI: cdb} */
    __u64 request_tag;    /* [i] {SCSI: task tag (only if flagged)} */
    __u32 request_attr;    /* [i] {SCSI: task attribute} */
    __u32 request_priority;    /* [i] {SCSI: task priority} */
    __u32 request_extra;    /* [i] {spare, for padding} */
    __u32 max_response_len;    /* [i] in bytes */
    __u64 response;        /* [i], [*o] {SCSI: (auto)sense data} */

/* "dout_": data out (to device); "din_": data in (from device) */
    __u32 dout_iovec_count;    /* [i] 0 -> "flat" dout transfer else
                   dout_xfer points to array of iovec */
    __u32 dout_xfer_len;    /* [i] bytes to be transferred to device */
    __u32 din_iovec_count;    /* [i] 0 -> "flat" din transfer */
    __u32 din_xfer_len;    /* [i] bytes to be transferred from device */
    __u64 dout_xferp;    /* [i], [*i] */
    __u64 din_xferp;    /* [i], [*o] */

    __u32 timeout;        /* [i] units: millisecond */
    __u32 flags;        /* [i] bit mask */
    __u64 usr_ptr;        /* [i->o] unused internally */
    __u32 spare_in;        /* [i] */
      ...

Currently because of a twisted accident sg and bsg queue their async
request with the "at_head" flag set when calling blk_execute_rq_nowait.

Since sg_io_v4 is new and as plenty of possible choices, what would be
the best place to put the "at_head" flag?

Douglas ?
What was your intention with the above:
     __u32 request_priority;    /* [i] {SCSI: task priority} */

[And while at it, what was:
     __u32 request_attr;    /* [i] {SCSI: task attribute} */

Yes, task attribute. That is the term used by SAM-4
(and earlier). Unfortunately SAM-4 defines the names
but not values; those values are left up to the
transport. That presents a problem for a transport
agnostic SCSI pass-through interface like sg_io_v4.

For backward compatibility we probably need to make the value
0 mean "head of queue" (or whatever the mid-level default
task attribute is for pass-through SCSI commands).

Also we probably don't have a clean pass-through of task
attributes in the SCSI mid-level currently. I mean "clean"
in the sense of "leave them alone" and do not act on the
command completion status (e.g. TASK SET FULL).

Should I define (bsg.h):
enum {
    BSG_AT_HEAD = 0, /* compatible with old code */
    BSG_AT_TAIL,
}
And put it in request_priority

or (adding "_TA_" for task attribute):
  enum {
        BSG_TA_DEFAULT = 0,  // lk 2.4, 2.6 series: head of queue
        BSG_TA_HEAD_OF_Q = 0x1000,
        BSG_TA_SIMPLE,
        BSG_TA_ORDERED,
        BSG_TA_ACA,
        ....

or define:
#define SG_FLAG_AT_TAIL 0x10 /* See sg.h for more/other flags (sg.h flags are ignored by bsg) */

And OR that into the old flags member that is otherwise
unused by current bsg code

I would prefer using the request_attr field.

Ah, another idea: use a SG_FLAG_REQUEST_ATTR_ACTIVE flag
or-ed into the flag mix. Then the transport native
task attributes could be sent through the request_attr
field (and request_priority field).

Doug Gilbert
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