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Old 03-07-2009, 09:55 PM
Thomas Bächler
Default udev changes - and the comeback of a long lost feature

tpowa did some work today updating udev to 139, cleaning up the rules
and fixing some small bugs.

With it comes the implementation of a recommendation of the udev team:
We now have a directory /lib/udev/devices/ which contains a number of
static devices nodes which are always present. A udev rule also makes
sure that none of these devices is deleted even if udev normally would
do so (the stock udev rule for that is broken, but I fixed it).

This simplifies our rc.sysinit a bit:
http://projects.archlinux.org/?p=initscripts.git;a=commitdiff;h=623e3fac060b0f4b 32c0ec767d76ef57f5040ed1

In that context, I thought of an old feature: "module autoloading" as
Linux 2.4 understood it: Imagine you have a static /dev, and you try to
open a device:

1) /dev/XYZ is a block/char device with major X and minor Y
2) the kernel looks it up, but doesn't find it
3) the kernel issues "modprobe block-major-X-Y" or "modprobe char-major-X-Y"
4) the kernel looks up the device again, it can now find it
Note that step 1 requires that /dev/XYZ exists! This feature completely
broke with the introduction of udev. This feature is obsolete for most
devices, but there are those devices that are not bound to physical
hardware - best examples here are tun and loop.

I am going to add the loop and tun (any maybe fuse) devices to the
static device list that we now support. That means that you can use loop
mounts again without running "modprobe loop" manually, the module will
be loaded as soon as you access /dev/loopX. Same goes for /dev/net/tun
or /dev/fuse. This basically works with all modules that have a
block-major-X-* or char-major-X-* alias, so if you think of any more
modules that do this, we could add them too.

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