Re: [Libusb-devel] udev craze: need /dev/bus/usb rationale

[Kay Sievers <kay.sievers@vrfy.org>]

On Thu, Jan 26, 2006 at 06:08:11PM -0800, Johannes Erdfelt wrote:
> On Wed, Jan 25, 2006, Linus Walleij <triad@df.lth.se> wrote:
> > Having fun making packages for Fedora we have run into a coordination 
> > issue between udev and libusb.
> > 
> > As you know libusb programs (like SANE custom USB backends or my own pet 
> > libnjb) use raw device nodes with no kernel drivers attached to them. A 
> > suitable udev rule found in this mail:
> > 
> > http://sourceforge.net/mailarchive/message.php?msg_id\x12695893
> > 
> > looks like this:
> > 
> > SUBSYSTEM="usb_device", PROGRAM="/bin/sh -c 'X=%k X=$${X#usbdev} 
> > B=$${X%%%%.*}
> > D=$${X#*.}; echo bus/usb/$$B/$$D'", SYMLINK="%c"
> > 
> > has been applied to most distributions set of default rules. (Well Debian 
> > and Fedora atleast, I guess SuSE too.)
> > 
> > This rule creates a dynamic device hierarchy belowe /dev/bus/usb similar 
> > to the good 'ol /proc/bus/usb and everyone is happy. Libusb was patched 
> > to support this device hierarchy. But wait. We found an inconsistency: 
> > that script will create hex-named nodes:
> > 
> >   /dev/bus/usb/01AB1/01BCF
> > 
> > However libusb has this code in linux.c row 336 or so:
> > 
> >   if (!strchr("0123456789", entry->d_name[strlen(entry->d_name) - 1]))
> > 
> > Of course it could be fixed with a patch:
> > 
> >   if (!strchr("0123456789ABCDEF", entry->d_name[strlen(entry->d_name) - 1]))
> > 
> > But we need a clear rationale:
> > 
> > * How shall /dev/bus/usb subdirs and device files be named?
> > 
> > * If they shall be all decimal figures, change all rules accordingly and
> >   libusb will work fine as it is.
> > 
> > * If they shall be hex, make the above suggested change to libusb.
> 
> Coming in late to this thread and I see that the node has been changed
> from hexadecimal to decimal.

Well, they never have been hex. I expect its a "bug" in some shell, I've
never seen this or can reproduce it, even with the old version of the rule
that seems to produce this for some people.

> I want to understand the reason for these device nodes since they seem
> to duplicate the nodes found in /proc/bus/usb. Is this just part of the
> long term plans to remove usbdevfs?

Yes, we want to leave /proc with all stuff that isn't process related.
And as all the major distros depend on udev and have /dev on tmpfs, having
usb devices in /proc makes no sense anymore.

We also want the option to apply access control lists to device nodes,
to allow a set of users (fast-user-switching) access to a device and
this would not work with /proc files, which can only have primary
ownership.

Kay


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