From: Andrey Borzenkov <arvidjaar@mail.ru>

Current 2.5 does not register any device node in devfs for empty media
(capacity == 0) case.  This makes removables unusable with devfs.
Partition rescan is done only on bdev open, but without any device node for
device it is impossible to open it.

In 2.4 it was finally solved by always registering .../disc node as
representation for "whole" disk and using devfsd action to force partition
rescan on access to (non-existing) partition name.  For primary names it
was handled internally by devfs - it kept track of removable devices in
directory and initiated partition rescan when name was not found.

Both are obviously broken now.  You can't do partition rescan because no
node is registered at all and internal handling was removed.  Very nice.

The attached patch makes resgister_disk always register at least disc node.
 This now works for old and new compat names as per devfsd configuration;
canonical names are still broken:

  {pts/3}% ll /dev/scsi/host1/bus0/target4/lun0/part4
  ls: /dev/scsi/host1/bus0/target4/lun0/part4: No such file or directory

but it can be fixed using the same technique as above so I won't push it.