Subject: Re: SCSI CD changers supported?
To: Jason Thorpe <>
From: Justin T. Gibbs <>
List: tech-kern
Date: 08/31/1996 17:08:17
>Hrm... I'm currently working on some changes to fix the problem with our 
>SCSI subsystem not handling targets > 7 (we have some wide controllers at 
>work that will need this to work).  In the case of wide SCSI, I'm under 
>the impression that the max LUN can be 63 (I need to look it up to be 

According to section 5.5 of the SCSI II spec, you can only have 8 regardless
of bus width.  The format of the IDENTIFY message would have to change
before the number could be expanded.

>Now, it's probably the case that a wide device isn't going to need the 
>LUN to be stashed in the cdb (indeed, it wouldn't fit).  So, what I was 
>thinking of doing is having the MI layer stash the LUN in the cdb in the 
>case of the scsibus's "maxlun" property being 7 ... some of our SCSI 
>controller drivers do this already ... it should be done in the MI layer, 
>if done at all... and it sounds like it's necessary ... (Gee, how many 
>devices are in the quirk table unfairly? :-)
>SCSI-2 doesn't require this, but it is allowed for compatibility with 
>legacy devices, IIRC.

Quoting the spec:

7.2.2 Logical unit number

The logical unit number is defined in the IDENTIFY message (see
6.6.7). The target shall ignore the logical unit number specified
within the command descriptor block if an IDENTIFY message was
received. It is recommended that the logical unit number in the
command descriptor block be set to zero.

NOTICE: The logical unit number field is included in the command
descriptor block for compatibility with some SCSI-1 devices. This
field may be reclaimed in SCSI-3. New implementations should use
the outbound IDENTIFY message, which is mandatory in SCSI-2, to
establish the I_T_L nexus.

I think we should only send it if the device IDs itself as SCSI I compliant.
Even the spec suggests leaving it as 0 by default.

> -- save the ancient forests - -- 
>Jason R. Thorpe                             
>NASA Ames Research Center                               Home: 408.866.1912
>NAS: M/S 258-6                                          Work: 415.604.0935
>Moffett Field, CA 94035                                Pager: 415.428.6939

Justin T. Gibbs
  FreeBSD: Turning PCs into workstations