scsi subsystem in linux

[nidhi mittal hada]

scsi_alloc_sdev(must be called once for each scsi device) -->calls
scsi_alloc_queue (that sets up sdev->request_queue
                                    sets up scsi_request_fn and
                                    sets up req prep function as
scsi_prep_fn)

1738 struct request_queue *scsi_alloc_queue(struct scsi_device *sdev)
1739 {
1740         struct request_queue *q;
1742         q = __scsi_alloc_queue(sdev->host,
scsi_request_fn);>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1743         if (!q)
1744                 return NULL;
1746         blk_queue_prep_rq(q,
scsi_prep_fn);>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
1747         blk_queue_softirq_done(q, scsi_softirq_done);
1748         blk_queue_rq_timed_out(q, scsi_times_out);
1749         blk_queue_lld_busy(q, scsi_lld_busy);
1750         return q;
1751 }
And after this call sdev_request_queue is allocated................

Now, scsi_request_fn should be called while processing each request in this
sdev->request_queue...
Please correct me if i m wrong.
Now, inside scsi_request_fn, we get next queueable request by calling
blk_peek_request - to fetch request from request queue ..

blk_peek_request -> calls __elv_next_request to fetch next request-
and calls prep_rq_fn(q, rq);


*which is scsi_prep_fn not the sd_prep_fn isnt it ??*


Then where is sd_prep_fn is used for ??

Thanks
Nidhi




On Tue, Nov 5, 2013 at 5:39 PM, Jack Wang <xjtuwjp at gmail.com> wrote:

> Hi Nidhi,
>
> About the function call trace you can use ftrace to find out, another
> useful tool is scsi_logging_level to set more verbose logging output for
> scsi core.
>
> Regards
> Jack
> On 11/05/2013 12:48 PM, nidhi mittal hada wrote:
> >
> > Hi All
> >
> > i have got a requirement where I need to encrypt/decrypt data that goes
> > from scsi layer to a particular block device.
> > As per my understanding till now on scsi subsystem in linux, i think i
> > need to
> > use crypto api and call appropriate encrypt/decrypt function from sd
> > driver for block device.
> >
> > I need to locate that specific function where this change needs to be
> > made ...
> > I know basic block device driver writing in linux .. But not able to fit
> > scsi in this picture.
> >
> > I have few basic doubts.. kindly help in resolving ...
> >
> > 1) Now, as example block device driver sbull, as given LDD, works on
> > request queue, fetches req from this queue, using function req =
> > elv_next_request(q)),
> > in request function.
> > what is corresponding function in sd layer ?
> > That is the function where i have req->buffer in hand with me..
> >
> >
> > 2) For a write operation from initiator to disk
> > is the hierarchy like this
> > *sd_prep_fn*
> > generic block device request structure -> converted into scsi specific
> > request structure
> > OR
> > what is scsi_prep_fn for??
> >
> > 3)How is Scpnt pointer that is req-> special is used in sd_prep_func..
> > is processed? i mean which layer picks Scpnt up and processes ??
> >
> > 4)Any document any URL any kind of instruction will be extremely helpful.
> >
> > 5)Whenever a *new scsi device is attached *sd_probe is called
> > sd_async_probe() is the async part of sd_probe() So when this is called
> > the prep_fn is set to sd_prep_fn and hence this will be called.
> >
> > *But i thought sd_prep_fn should be called for each and every request
> > .....??*
> > Kindly help me to clear the confusion ..
> >
> >
> > Thanks
> > Nidhi
> >
> >
> >
> >
> > _______________________________________________
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> >
>
>
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