Re: [Bug 80711] [PATCH]SG_FLAG_LUN_INHIBIT is no longer implemented and there's not way to prevent the kernel from using the 2nd cdb byte for the LUN

Alan Stern
2014-08-25T20:12:46Z

On Mon, 25 Aug 2014, James Bottomley wrote:

> On Mon, 2014-08-25 at 10:44 -0400, Alan Stern wrote:
>
> > James, can you explain how the INQUIRY command in scsi_probe_lun()
> > managed to work back in the days when multi-lun SCSI-2 devices were
> > common? sdev->scsi_level doesn't get set when sdev is allocated, so it
> > initially contains 0, so the LUN bits won't get filled in when the
> > first INQUIRY command is sent. Then how could the target know which
> > logical unit the INQUIRY was meant for?
>
> Best guess, some patches over the course of time altered the way we do
> this and no-one noticed. I think it was probably the introduction of
> the unknown SCSI data level that caused the breakage.

Heh. The change was made by commit 4d7db04a7a69 ([SCSI] add
SCSI_UNKNOWN and LUN transfer limit restrictions) back in 2006 -- the
2.6.17 kernel. If nobody has complained in all this time then it's
probably not worth changing.

> Historically, the LUN in CMD bits is left over from SCSI-1; it was
> incorporated into SCSI-2 for backward compatibility (even though SCSI-2
> moved the LUN specification to the identify message). In SCSI-3 and
> beyond, those bits were obsoleted and transports took sole
> responsibility for LUN handling. I'm fairly certain all the SCSI-1
> devices relying on this behaviour have long ago migrated to the great
> data centre in the sky.
>
> Alan's fix looks reasonable because we probe LUN 0 first (for SCSI-1 and
> 2 which has parallel scanning), which is why it doesn't matter (the bits
> are set to zero) and once we have LUN 0 we propagate the data to the
> target and make it the basis for future checks. I'd like to see a
> comment explaining this in the code, though ...

If you think it would be a good idea, I could put it into a separate
patch with an explanatory comment. At the moment, I'm inclined to
forget about it.

Alan Stern

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