Re: [PATCH] scsi: fix scsi_get_lba helper function for pc command

["Martin K. Petersen" <martin.petersen@oracle.com>]

>>>>> "Tomo" == FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> writes:

>> scsi LLDs can be pass-through for disks, as well as lots of other
>> type of devices. They should not assume the role of a disk. The use
>> of blk_rq_pos() at LLDs is probably wrong in the first place. Unless
>> it is some kind of scsi emulation driver like scsi_debug.

Tomo> Not only scsi_debug. Have you seen how scsi_get_lba() is used?
Tomo> LLDs sometime needs to know lba in scsi commands.

scsi_get_lba() is used by DIX/DIF-aware HBA drivers to get the first LBA
for use in reference tag verification/preparation.

This code path is only executed by the drivers if the command protection
mode is > SCSI_PROT_NORMAL which can only be true for a READ/WRITE
command.  I don't see any drivers using it for other purposes.

I have some changes in the pipeline to better support protection
interleaving in DIF Type 2.  I was contemplating renaming scsi_get_lba()
to scsi_get_reference_tag() or something to that effect.

As as for the reason scsi_get_lba() looks at the start sector directly:
I was simply trying to avoid growing struct scsi_cmnd.  The "correct"
thing would have been to add an initial reference tag field to scsi_cmnd
and have that be filled out by sd_prot_op().

-- 
Martin K. Petersen	Oracle Linux Engineering