From: Rob Landley <rob@landley.net>
To: Olivier Galibert <galibert@pobox.com>
Subject: Re: Device enumeration (was Re: CD writing in future Linux (stirring up a hornets' nest))
Date: Wed, 15 Feb 2006 13:19:27 -0500
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On Wednesday 15 February 2006 11:55 am, Olivier Galibert wrote:
> On Tue, Feb 14, 2006 at 07:24:22PM -0500, Rob Landley wrote:
> > I'm torn between "nuts to alsa", pointing out that "ln -s /dev /dev/snd"
> > should shut it up nicely, and thinking up a way to actually do what it
> > wants.
> >
> > If there's a real need for subdirectories, I'm sure we can come up with a
> > way to shunt stuff into them.  (Of course a shellout could do it, but if
> > it's common enough we could build something into mdev...)
> >
> > The easy one's the symlink, assuming there are no name collisions
> > flinging everything into one directory...
>
> Let's see, on a recent kernel and recent udev I have as directories
> under /dev:
>
> - disk, a pure udev creation, so no conflict there
> - loop, devices names under that are numeric.  mount has /dev/loop%d
>   and /dev/loop/%d hardcoded

I just downloaded 2.6.13-rc3 and built it.  That has:

/sys/block/{loop0,loop1,loop2,loop3,loop4,loop5,loop6,loop7}

And those are the names mdev would use.  Turning /dev/loop# into /dev/loop/# 
has to be some kind of udev filter in the config file.

There are no duplicate dev nodes in /sys right now.  Check for yourself.  Grab 
all the names mdev would use:

find /sys -name dev | sed -e 's@^/sys/.*/\(.*\)/dev@\1@'

And then pipe that to sort | uniq -d

No hits on my ubuntu "flatulent badger" system.  I'm not saying there won't be 
any, I haven't tried plugging in a few usb memory keys and such.  (But it 
shouldn't be an overwhelming problem.  We could keep just the first one, 
stick a #2 on the second one...  I'll worry about it when it crops up.)

> - bus/usb, this one collides with itself if flattened
> - snd/sound, names except for seq and timer are pretty much line
>   noise.  "timer" is scary though.
> - net with tun and (I think) tap.
> - pktcdvd with "control", not sure what tool uses it
> - misc is ok (they're all ex-/dev/xx devices)
> - video1394, dv1394, i2c with '0' as device name
> - dri with card%d
> - cpu which self-collides too
> - input with a potentially dangerous event%d

Don't look at the output of udev.  Look at the contents of /sys.

> So, well, I think you're going to need directories for usb and cpu
> without doubt, and some of the rest is potentially risky, long-term
> wise.

The current way sys handles things like "loop0" is to call it "loop0" rather 
than having a "loop" subdirectory with "0" under it.  If they changed that:

A) I'd whimper on the list and make puppy eyes.
B) I could turn loop/0 back into "loop0" if I had to.  Not too hard to do 
mechanically...

>   OG.

Rob
-- 
Never bet against the cheap plastic solution.
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