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