Re: [RFC/Patch] Device Registry

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

I've just started to wrap my head around the approach in 
http://www.tjansen.de/devreg and how it might get used.

A one sentence summary:  this registry is a text file in a new
format, presenting a logically flat namespace of devices each
with descriptive {attribute-name,value} pairs (X.500-ish).

I like the notion of modeling through attributes, but the flat
namespace bothers me a lot.  Except maybe as a way to
finesse the namespace structure arguments!  I'd rather see
separate naming contexts (directories) grouping different
types of devices.  Disk storage, network interfaces, printers,
and so on.  Sure, use a common root so apps can use ftw()
to make sure they find all the relevant devices.

Also, it seems to me there's some overlap with the "scsimon"
work (see the linux-hotplug links page), which I understand
has relations to similar IDE-focussed work.

But to pick on the specific device attributes for a moment,
here are a few comments:

    - The "Device IDs", if they're unique enough,
      remove need for a "common/number" attribute.

    - Physical topology is a much better way to derive
      the device ids ... in fact, likely the only way that
      can work in general.  Vendor/product IDs don't
      belong there, or even device type/class codes.

    - For PCI, "device" isn't the right focus, it's the
      "function" -- devices can have many functions,
      not necessarily related.  /proc/bus/pci/* has
      per-function files.

    - Similarly for USB, "device" isn't the right focus
      but "interface" (in a given configuration).  This is
      something that /proc/bus/usb does wrong, IMO
      (but it's "preliminary", no worries yet).

After seeing the preliminary reportage of the hotplug
discussions at the kernel summit, I anticipate many
further fruitful (and likely not-so-fruitful :) debates about
naming and hotplugging.  It's pretty fundamental to
taking a systems approach to this stuff.

I hope those debates will continue to use device
attributes when they model the information that's
provided, since that's a good way to start focussing
on the common notions that underly the various
application and kernel issues ... without arguing
about implementation details like how to expose
that information (tree structure, flat file, ioctl, etc).

- Dave




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