The PQ=1 saga

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

I would like to revert commit 948e922fc446 ("scsi: core: map PQ=1,
PDT=other values to SCSI_SCAN_TARGET_PRESENT").

I have been spending quite a bit of time digging through old SCSI and
controller specs. As far as I can tell the original Linux behavior was
correct. Recent SPC specs are very abstract in this department resulting
in unfortunate ambiguity. But originally PQ=1 meant "LUN supports this
peripheral device type but no physical device is currently connected".

Based on this original definition, PQ=1 has been widely used throughout
the industry as a means to avoid associating an ULD driver with a
device. The LUN is accessible (primary commands, etc.) but no media is
present (no physical device connected).

Our original algorithm, which I would like to reinstate, is essentially
the following (in slightly unrolled form):

        if (PQ == 3)
            /* Don't expose device */
        else if ((PQ == 1 || sdev->pdt_1f_for_no_lun) && PDT == 0x1f)
            /* Don't expose device */
        else if (PQ == 1) {
            /* Expose device, don't bind ULD */
        } else /* PQ == 0 */
            /* Expose device, bind ULD if PDT is supported */

I would like to understand why -- in the case of the IBM 2145 --
exposing the sg device caused problems. Li: Can you shed some light on
the problems caused by 2145 LUNs reporting PQ=1?

Thanks!

-- 
Martin K. Petersen	Oracle Linux Engineering