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
2014-08-25T21:19:06Z

On Mon, 25 Aug 2014, Alan Stern wrote:

> 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.

It turns out the code is already there, and I didn't realize because I
was looking at the wrong source file. scsi_sysfs_device_initialize()
already does:

sdev->scsi_level = starget->scsi_level;

Here's the update to the patch, adding an appropriate comment and
setting the new sdev->lun_in_sdb flag properly:

Alan Stern

Index: usb-3.16/drivers/scsi/scsi_sysfs.c
===================================================================
--- usb-3.16.orig/drivers/scsi/scsi_sysfs.c
+++ usb-3.16/drivers/scsi/scsi_sysfs.c
@@ -1238,7 +1238,19 @@ void scsi_sysfs_device_initialize(struct
sdev->sdev_dev.class = &sdev_class;
dev_set_name(&sdev->sdev_dev, "%d:%d:%d:%d",
sdev->host->host_no, sdev->channel, sdev->id, sdev->lun);
+ /*
+ * Get a default scsi_level from the target (derived from sibling
+ * devices). This is the best we can do for guessing how to set
+ * sdev->lun_in_cdb for the initial INQUIRY command. For LUN 0 the
+ * setting doesn't matter, because all the bits are zero anyway.
+ * But it does matter for higher LUNs.
+ */
sdev->scsi_level = starget->scsi_level;
+ if (sdev->scsi_level <= SCSI_2 &&
+ sdev->scsi_level != SCSI_UNKNOWN &&
+ !shost->no_scsi2_lun_in_cdb)
+ sdev->lun_in_cdb = 1;
+
transport_setup_device(&sdev->sdev_gendev);
spin_lock_irqsave(shost->host_lock, flags);
list_add_tail(&sdev->same_target_siblings, &starget->devices);

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