Re: sd_prep_fn

[rohan puri <rohan.puri15@xxxxxxxxx>]

On Thu, Nov 10, 2011 at 4:50 PM, sumeet gandhare <sumeetgandhare@xxxxxxxxx> wrote:

Hi All,
  I am trying to understand the scsi upper layer and middle layer code and found the function sd_prep_fn which seems to convert a request to a scsi command. I want to know when this function sd_prep_fn gets invoked.

It seems sd_probe_async invokes

   blk_queue_prep_rq(sdp->request_queue, sd_prep_fn);



Further following code is invoking the prep_rq_fn

which is the blk_peek_request 

http://lxr.linux.no/#linux+v3.1/block/blk-core.c#L1879



and blk_peek_request is getting invoked from

scsi_request_fn 
http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_lib.c#L1493



scsi_request_fn is registered with the block layer as a request function at the following code

http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_lib.c#L1688



And scsi_alloc_queue is invoked from scsi_alloc_sdev
http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_scan.c#L238


scsi_alloc_sdev is invoked as a part of scsi lun discovery from lots of places


http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_scan.c#L1004

http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_scan.c#L1306



The confusion I have is that the sd.c upper layer driver has not registered for a request function using blk_init_queue

Any help would be appreciated.

Thanks
Sumeet

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Hello Sumeet,

sd_prep_fn function is set to the request queue's prep_rq_fn member as you have mentioned.

Now this function gets called in 

ret = q->prep_rq_fn(q, rq); in function blk_peek_request() which is called from scsi_request_fn() in file scsi_lib.c

Regards,
Rohan Puri
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