Subject: RE: USB feedback wanted
To: 'Lennart Augustsson' <>
From: Alan Perry <>
List: tech-kern
Date: 06/30/1998 16:59:53
>This is a very good question and I don't know how to do it
>with USB.  Assuming that your speakers really are identical
>I can see no way to guarantee the mapping to minor numbers.
>Unless you want to use the physical position of where the device is
>plugged into the USB tree.  But that is not very appealing since
>physical positions is what USB is supposed to take you away
>from.  If your speakers are not identical the difference may
>be in some device specific descriptor that it would be very
>difficult to make a general locator for.
>Something that could have been helpful here is the device serial
>number (which is supposedly unique given a vendor&product), but
>few devices actually seem to have implemented a serial number
>(since it is optional).

Do USB devices have a GUID like Firewire?  One idea that we have while I
was working on Firewire at Sun was to use the GUID as the device name
and (since Solaris has /dev and /devices) the logical name (/dev) points
to the right physical name (/devices) even if the devices gets moved around
on the Firewire bus.  This doesn't work if USB devices don't have GUIDs.

Of course you still have a device identification problem since the GUID is
not normally on the outside of the device.

>Don't we have the same problem already?  What if I have two identical
>PnP sound cards?  How do I decide which is /dev/audio0 and /dev/audio1?

But PnP sound cards are not hot-swappable ...