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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
