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

James Bottomley
2002-06-17T14:55:13Z

oliver@neukum.name said:
> These may be an exception. You usually want to get drivers involved
> even if only for synchronisation. Failing to do so has already let to
> problems with usb storage.

I don't deny that there may be devices that need some type of vendor specific
probe to get the information.

> That is the point. The driver knows best what kind of devices it works
> on. You can forget about the whole identification method business, if
> you go for the driver. In case of usb storage and firewire that data
> is already sitting there ready for taking. I suspect the same for
> fibrechannel.

But, in SCSI, you can't. Just for a simple device showing up as a SCSI disc
(that's a real SCSI disc, attached say by parallel connectors), there are
several potential ways to get a unique ID. No one way works for all such
disks, that is the nub of the problem.

I think, by driver, we may mean different things. A scsi disc attaches like
this:

+----+
| sd |
+----+
|
+-------+
| mid |
| layer |
+-------+
|
+-----+
| lld |
+-----+
|
+--------+
| real |
| device |
+--------+

but the only piece that has disc specific logic is sd. The lld is specific to
the host adapter card (not shown), not the real device, so it is not the right
element to probe for an ID.

In USB, things look slightly different:

+----+
| sd |
+----+
|
+-------+
| mid |
| layer |
+-------+
|
+---------+
| usb |
| storage |
+---------+
|
+--------+
| real |
| device |
+--------+

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.

The thing I think is a bad idea is having to code the logic to look up a
unique identifier (plus all the exceptions) in sd. But for the pure SCSI
stack, there's nowhere else to place it. Even if you get usb-storage to
supply an API for providing the id, it will be one of the few llds that can
retrieve this, so it will become just another exception sd has to cope with.

James