scsi subsystem in linux

[nidhi mittal hada]

*Setting of sd_prep_fn()*
sd_probe is called* whenever a* *new scsi device is attached to the
system.  *

sd_probe  calls ->  sd_probe_async->
        inside sd_probe_async, request prep fn is set to *sd_prep_fn*
>>>>like this  blk_queue_prep_rq(sdp->request_queue, sd_prep_fn);


*Calling of sd_prep_fn*
?????

Is this request queue sdp->request_queue, different from the request queue,
for which scsi_prep_fn is used??

Are they on different layers ?

Is there some relation between them ?



Thanks
Nidhi


On Tue, Nov 5, 2013 at 6:13 PM, nidhi mittal hada
<nidhimittal19 at gmail.com>wrote:

> 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
>> >
>> >
>> >
>> >
>> > _______________________________________________
>> > Kernelnewbies mailing list
>> > Kernelnewbies at kernelnewbies.org
>> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>> >
>>
>>
>
>
>
>


-- 
Thanks & Regards
Nidhi Mittal Hada

http://nidhi-searchingmyself.blogspot.com/
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