systemd/docs/udev_vs_devfs

		udev and devfs - The final word

			December 30, 2003
			Greg Kroah-Hartman
			<greg@kroah.com>


Executive summary for those too lazy to read this whole thing:
	I don't care about devfs, and I don't want to talk about it at
	all anymore.  If you love devfs, fine, I'm not trying to tell
	anyone what to do.  But you really should be looking into using
	udev instead.  All further email messages sent to me about devfs
	will be gladly ignored.


First off, some background.  For a description of udev, and what its
original design goals were, please see the OLS 2003 paper on udev,
available at:
	<http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf>
and the slides for the talk, available at:
	<http://www.kroah.com/linux/talks/ols_2003_udev_talk/>
The OLS paper can also be found in the docs/ directory of the udev
tarball, available on kernel.org in the /pub/linux/utils/kernel/hotplug/
directory.

In that OLS paper, I described the current situation of a static /dev
and the current problems that a number of people have with it.  I also
detailed how devfs tries to solve a number of these problems.  In
hindsight, I should have never mentioned the word "devfs" when talking
about udev.  I did so only because it seemed like a good place to start
with.  Most people understood what devfs is, and what it does.  To
compare udev against it, showing how udev was more powerful, and a more
complete solution to the problems people were having, seemed like a
natural comparison to me.

But no more.  I hereby never want to compare devfs and udev again.  With
the exception of this message...

The Problems:
 1) A static /dev is unwieldy and big.  It would be nice to only show
    the /dev entries for the devices we actually have running in the
    system.
 2) We are (well, were) running out of major and minor numbers for
    devices.
 3) Users want a way to name devices in a persistent fashion (i.e. "This
    disk here, must _always_ be called "boot_disk" no matter where in
    the scsi tree I put it", or "This USB camera must always be called
    "camera" no matter if I have other USB scsi devices plugged in or
    not.")
 4) Userspace programs want to know when devices are created or removed,
    and what /dev entry is associated with them.

The constraints:
 1) No policy in the kernel!
 2) Follow standards (like the LSB)
 3) must be small so embedded devices will use it.


So, how does devfs stack up to the above problems and constraints:
  Problems:
    1) devfs only shows the dev entries for the devices in the system.
    2) devfs does not handle the need for dynamic major/minor numbers
    3) devfs does not provide a way to name devices in a persistent
       fashion.
    4) devfs does provide a deamon that userspace programs can hook into
       to listen to see what devices are being created or removed.
  Constraints:
    1) devfs forces the devfs naming policy into the kernel.  If you
       don't like this naming scheme, tough.
    2) devfs does not follow the LSB device naming standard.
    3) devfs is small, and embedded devices use it.  However it is
       implemented in non-pagable memory.

Oh yeah, and there are the insolvable race conditions with the devfs
implementation in the kernel, but I'm not going to talk about them right
now, sorry.  See the linux-kernel archives if you care about them (and
if you use devfs, you should care...)

So devfs is 2 for 7, ignoring the kernel races.

And now for udev:
  Problems:
    1) using udev, the /dev tree only is populated for the devices that
       are currently present in the system.
    2) udev does not care about the major/minor number schemes.  If the
       kernel tomorrow switches to randomly assign major and minor numbers
       to different devices, it would work just fine (this is exactly
       what I am proposing to do in 2.7...)
    3) This is the main reason udev is around.  It provides the ability
       to name devices in a persistent manner.  More on that below.
    4) udev emits D-BUS messages so that any other userspace program
       (like HAL) can listen to see what devices are created or removed.
       It also allows userspace programs to query its database to see
       what devices are present and what they are currently named as
       (providing a pointer into the sysfs tree for that specific device
       node.)
  Constraints:
    1) udev moves _all_ naming policies out of the kernel and into
       userspace.
    2) udev defaults to using the LSB device naming standard.  If users
       want to deviate away from this standard (for example when naming
       some devices in a persistent manner), it is easily possible to do
       so.
    3) udev is small (49Kb binary) and is entirely in userspace, which
       is swapable, and doesn't have to be running at all times.

Nice, 7 out of 7 for udev.  Makes you think the problems and constraints
were picked by a udev developer, right?  No, the problems and
constraints are ones I've seen over the years and so udev, along with
the kernel driver model and sysfs, were created to solve these real
problems.  I also have had the luxury to see the problems that the
current devfs implementation has, and have taken the time to work out
something that does not have those same problems.

So by just looking at the above descriptions, everyone should instantly
realize that udev is far better than devfs and start helping out udev
development, right?  Oh, you want more info, ok...

Back in May 2003 I released a very tiny version of udev that implemented
everything that devfs currently does, in about 6Kb of userspace code:
	http://marc.theaimsgroup.com/?l=linux-kernel&m=105003185331553

Yes, that's right, 6Kb.  So, you are asking, why are you still working
on udev if it did everything devfs did back in May 2003?  That's because
just managing static device nodes based on what the kernel calls the
devices is _not_ the primary goal of udev.  It's just a tiny side effect
of its primary goal, the ability to never worry about major/minor
number assignments and provide the ability to achieve persistent device
names if wanted.

All the people wanting to bring up the udev vs. devfs argument go back
and read the previous paragraph.  Yes, all Gentoo users who keep filling
up my inbox with smoking emails, I mean you.

So, how well does udev solve its goals:
  Prevent users from ever worrying about major/minor numbers
    And here you were, not knowing you ever needed to worry about
    major/minor numbers in the first place, right?  Ah, I see you
    haven't plugged in 2 USB printers and tried to figure out which
    printer was which /dev entry?  Or plugged in 4000 SCSI disks and
    tried to figure out how to access that 3642nd disk and what it was
    called in /dev.  Or plugged in a USB camera and a USB flash drive
    and then tried to download the pictures off of the flash drive by
    accident?

    As the above scenarios show, both desktop users and big iron users
    both need to not worry about which device is assigned to what
    major/minor device.
   
    udev doesn't care what major/minor number is assigned to a device.
    It merely takes the numbers that the kernel says it assigned to the
    device and creates a device node based on it, which the user can
    then use (if you don't understand the whole major/minor to device
    node issue, or even what a device node is, trust me, you don't
    really want to, go install udev and don't worry about it...)  As
    stated above, if the kernel decides to start randomly assigning
    major numbers to all devices, then udev will still work just fine.

  Provide a persistent device naming solution:
    Lots of people want to assign a specific name that they can talk to
    a device to, no matter where it is in the system, or what order they
    plugged the device in.  USB printers, SCSI disks, PCI sound cards,
    Firewire disks, USB mice, and lots of other devices all need to be
    assigned a name in a consistent manner (udev doesn't handle network
    devices, naming them is already a solved solution, using nameif).
    udev allows users to create simple rules to describe what device to
    name.  If users want to call a program running a large database
    half-way around the world, asking it what to name this device, it
    can.  We don't put the naming database into the kernel (like other
    Unix variants have), everything is in userspace, and easily
    accessible.  You can even run a perl script to name your device if
    you are that crazy...

    For more information on how to create udev rules to name devices,
    please see the udev man page, and look at the example udev rules
    that ship with the tarball.
 

So, convinced already why you should use udev instead of devfs?  No.
Ok, fine, I'm not forcing you to abandon your bloated, stifling policy,
nonextensible, end of life feature if you don't want to.  But please
don't bother me about it either, I don't care about devfs, only about
udev.

This is my last posting about this topic, all further emails sent to me
about why devfs is wonderful, and why are you making fun of this
wonderful, stable gift from the gods, will be gleefully ignored and
possibly posted in a public place where others can see.

thanks,

greg k-h