Re: sd_prep_fn

[rohan puri <rohan.puri15@xxxxxxxxx>]

On Thu, Nov 10, 2011 at 6:26 PM, sumeet gandhare <sumeetgandhare@xxxxxxxxx> wrote:

On Thu, Nov 10, 2011 at 6:13 PM, rohan puri <rohan.puri15@xxxxxxxxx> wrote:

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

Hi Rohan,
 Thanks for your response and help.
 It seems there is scsi_prep_fn too. I thought the blk_peek_request() invokes scsi_prep_fn and not sd_prep_fn.

http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_lib.c#L1248

scsi_prep_fn is registered here

http://lxr.linux.no/#linux+v3.1/drivers/scsi/scsi_lib.c#L1684


Please could you confirm.

Thanks
Sumeet

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.

sd_prep_fn is for scsi disk, similarly there is a function sr_prep_fn which gets called for scsi cd-rom.

Regards,
Rohan Puri
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