sd_prep_fn

[rohan puri]

On Thu, Nov 10, 2011 at 4:50 PM, sumeet gandhare
<sumeetgandhare at gmail.com>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 <http://lxr.linux.no/linux+*/+code=blk_queue_prep_rq>(sdp <http://lxr.linux.no/linux+*/+code=sdp>->request_queue <http://lxr.linux.no/linux+*/+code=request_queue>, sd_prep_fn <http://lxr.linux.no/linux+*/+code=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 <http://lxr.linux.no/#linux+v3.1/block/blk-core.c%23L1879>
>
> and blk_peek_request is getting invoked from
>
> scsi_request_fn
> http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_lib.c#L1493 <http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_lib.c%23L1493>
>
>
> 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 <http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_lib.c%23L1688>
>
> And scsi_alloc_queue is invoked from scsi_alloc_sdev
> http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_scan.c#L238 <http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_scan.c%23L238>
>
> 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%23L1004>
>
> http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_scan.c#L1306 <http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_scan.c%23L1306>
>
> 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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