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

[Douglas Gilbert <dgilbert@interlog.com>]

Boaz Harrosh wrote:
> Douglas Gilbert <dougg@torque.net> 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.

Tomo, what do you think?


Doug Gilbert