Oliver Neukum wrote:
What is specific about USB in that regard? It would seem to me that you'd better export something that stays the same across reboot, but don't call it USB_something.I'm not sure I understand your question or implication. Why shouldn't USB-specific identifiers be USB_something?The interpretation is specific to USB. But having a physical path that survives reboot is a problem all bus systems have.
I'd more call it a "basic characteristic" than a problem. Even with multi-pathed I/O frameworks there are physical paths (more than one!) which are stable until the system gets re-cabled. I certainly wouldn't object to having a more generic solution to that issue, but Greg and Pat haven't been amenable to having driverfs address the issue generically.
There are as I see it, two ways of recognising a device, a unique identifier and physical path. User space may want both or one of them. You cannot know thus should export both.
Certainly I agree with that perspective: support policy options for user space. Physical path being one, since every device has such a path. But "unique" identifiers are only "unique in context", and the kernel may not know all of the ones user space cares about. It can expose $DEVPATH/serial (which isn't always even unique!), but for example SCSI devices may have a few other "unique" IDs available. - Dave
Yet the most basic function of such a 'permanent path' is entering it into a table to tell knew devices apart from devices already known which an old configuration is available for. That is not specific to USB, because you can treat the exported information as an opaque token that you simply compare to.DEVPATH indeed seems to be an inferior choice, because you need extra knowledge to extract a permanent path out of it.Knowledge you can't generally have, too: which bus-specific parts of DEVPATH have that information (if any).Yes, taking it is a layering violation. Regards Oliver
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