Johannes Erdfelt writes:
> With more and more Hot Swap device busses becoming common, naming of devices
> is starting to become a problem.
>
> As an example, I'll use 2 USB floppy drives. Since they are floppy drives,
> the media and thusly filesystems can change frequently.
[...]
> The first problem is being able to differentiate between similar
> devices. USB has the notion of a serial number, however not all
> devices provide one. Most storage devices do however, which is the
> reason I'm using floppies as my example.
>
> Now that we have a unique way of differentiating 2 seemingly identical
> devices, our next problem is how to store this information. This is
> where devfsd comes in. I'd like to use devfsd to track this information.
> How it does this is not too important right now.
>
> The last problem is name space conflict.
>
> There may be a real device called /dev/floppy0. A possible solution
> that has been discussed is to mount the devfs filesystem on /devices
> (completely aribtrary name) and create symlinks between the virtual
> names on /dev to the physical names on /devices.
I think any move that promotes mounting devfs elsewhere is going to
come back and hurt us in the future. /dev should be the only place
that applications (and administrators) look for device files. As soon
as we start seeing people (distributions) mounting devfs on /devfs, we
will see applications looking in multiple places for device
nodes. This will inevitably cause confusion, and once done, will be
very hard to fix.
And having a pile of symlinks doesn't really solve the namespace
problem, because you can always come up with some name that will
conflict. And besides, a /dev full of symlinks is going to be
butt-ugly.
I think the only solution to namespace conflicts is to admit that it
is real and has to be dealt with firmly. This means coming up with a
policy and "enforcing" it (i.e. various utilities follow that policy
and if you try and change it and it breaks, you get to keep the
pieces).
However, it probably makes sense to put *all* USB devices under
/dev/usb, just like the SCSI and IDE trees.
> What I'd like to get is some opinions on this. I have a reservation
> towards putting so much into devfsd. I fear it may become too
> complicated.
Also, I'd want to avoid "solutions" that are really only partial
solutions. I think we should just bite the bullet on namespace policy.
> Also, not all devices can be differentiated. However, we can do some
> things. For instance, we can determine which port a device is
> plugged into and create a topology tree. This way, we can
> differentiate 2 mice. However, if the device is moved to another
> port, the mouse would be "new".
Yep. Not much to be done about that.
Regards,
Richard....
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