Re: scsi subsystem in linux

[nidhi mittal hada <nidhimittal19@xxxxxxxxx>]

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@xxxxxxxxx> 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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