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SubjectRe: USB device allocation

[Nathan Hand <nathanh@chirp.com.au>]
> Agreed. Getting rid of devfs (the dynamic filesystem) makes devfs
> (the concept of dynamic /dev) more acceptable to more punters, and
> honestly doesn't lose all that much functionality.

Que sera sera. I like devfs (the concept of dynamic /dev) but enough
important people don't that this may be the best that can be hoped for.

> Modules contact devfsd when they need a node. The daemon
> creates/deletes nodes as needed on a real filesystem using the policy
> laid out via /etc/devfsd.conf.

If the daemon is deleting nodes upon cleanup_module(), you lose
persistence. If it isn't, you lose the advantage of an uncluttered
/dev with only current devices in it. With a devfs+devfsd, devfsd can
rule the policy organ with an iron fist, squirrel away persistence in
/var/state/devfsd, and still maintain the minimalist /dev. The only
valid objection left that I see is bloat, and it isn't really that much
bloat.

With a real on-disk /dev, manual meddling with /dev nodes cannot be
proscribed from on high or even detected (without evil dcache hacks) so
devfsd won't know if the user has mucked with /dev directly (i.e. not
through /etc/devfsd.conf). If you rename /dev/usbmouse0 to /dev/mouse,
then unplug your USB mouse, then plug it back in, how will you keep
devfsd from creating another /dev/usbmouse0? Any solutions regarding
doing a `find' on /dev in search of the correct major/minor will run
afoul of trying to make those dynamic to alleviate the dev number
shortage.

Sure, tell the user he has to edit /etc/devfsd.conf rather than
changing /dev directly -- and you have now violated the Principle of
Least Surprise. The VFS layer doesn't complain if he plays with /dev
directly, so why can't you do it? With devfs, you can arrange for the
VFS layer to use a warning label of -ENOSYS or -EPERM if you so desire.

> - Devfs gets into the kernel, everyone else in the world is happy.

As I said, if Brave New (Almost-)All-Userspace Devfsd is the only way
to satisfy hpa, tytso and the other VIPs, oh well. AIX does it this
way and I have to admit that it does work, as long as you like the
names and permissions AIX supplies you with.

--
Peter Samuelson
<sampo.creighton.edu!psamuels>

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