Re: [linux-usb-devel] Re: /proc/scsi/map

[David Brownell <david-b@pacbell.net>]

> Any SCSI device can return an ID (i.e. INQUIRY VPD page 0x80 or 0x83),
> so the logic need not be in sd. I don't know how removable media should
> be handled (not a SCSI device being added/removed from the system), for
> tape this is probably not an issue.
> ...
> 
>>Here, the usb-storage driver does know about the real device (and already has 
>>a huge exception table), so it has enough knowledge to probe for an identifier.
>  
> usb-storage could emulate VPD page 0x83 to return the GID, and that could
> then be used by the mid-layer or a user level program to extract an ID.

Except that as I recall, that's not practical for all devices; they may not
have built in IDs.  Hence there need to be topology-based IDs available.

Which is why I had pointed out usb_make_path() ... which returns a stable
controller ID for the "CBTU tuple".  It isn't going to get re-assigned by
changing the bus probe sequence or driver load order.  Even PCI has better
IDs available than those simple numbers (pci_device.slot_name).

If the driverfs names for all devices are supposed to be using stable IDs
(ones that don't change unless at least the hardware gets re-configured)
where possible, then those driverfs names will be better answers for the
"what is this device's topological ID" than those purely numeric CBTU tuples.

(And they won't address the volume/media/... ID level problems, which are
of course a separate issue.)

- Dave