Tagged Queueing and Dque Bit

[Daniel M. Eischen]

> But with one box it didn't work, until I figured out that the drive
> claims to have tagged queueing, but has the Dque bit set to one 
> in the control mode page (Dque inhibits tagged queueing).
>
> The driver did time outs and gave me error messages that there is no
> active SCB for this drive.

[...]

> After I figured out the Dque bit, and reset it to zero, it worked flawlessly
> with tagged queueing enabled.
>
> So I would suggest, that the driver should check also the Control Mode
> Page (if available), and should honor the Dque bit (fall back to not use 
> tagged queueing if Dque is set).

The driver is not in the business of making it's own SCSI commands
and sending them to the target.  The mid-level SCSI code and target
drivers do this.  The mid-level SCSI code detects whether the drive
is capable of doing tagged queueing and sets a bit in the SCSI
request structure that the driver looks at.  If this bit is set,
and tagged queueing is enabled in the driver, then you're going
to get tagged queueing.

If a fix is to be made for this, it belongs in the mid-level SCSI
code.

[ See linux-scsi at vger.rutgers.edu ]

Dan Eischen
deischen at iworks.InterWorks.org