1 udev and devfs - The final word
8 Executive summary for those too lazy to read this whole thing:
9 I don't care about devfs, and I don't want to talk about it at
10 all anymore. If you love devfs, fine, I'm not trying to tell
11 anyone what to do. But you really should be looking into using
12 udev instead. All further email messages sent to me about devfs
13 will be gladly ignored.
16 First off, some background. For a description of udev, and what it's
17 original design goals were, please see the OLS 2003 paper on udev,
19 <http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf>
20 and the slides for the talk, available at:
21 <http://www.kroah.com/linux/talks/ols_2003_udev_talk/>
22 The OLS paper can also be found in the docs/ directory of the udev
23 tarball, available on kernel.org in the /pub/linux/utils/kernel/hotplug/
26 In that OLS paper, I described the current situation of a static /dev
27 and the current problems that a number of people have with it. I also
28 detailed how devfs tries to solve a number of these problems. In
29 hindsight, I should have never mentioned the word, devfs, when talking
30 about udev. I did so only because it seemed like a good place to start
31 with. Most people understood what devfs is, and what it does. To
32 compare udev against it, showing how udev was more powerful, and a more
33 complete solution to the problems people were having, seemed like a
34 natural comparison to me.
36 But no more. I hereby never want to compare devfs and udev again. With
37 the exception of this message...
40 1) A static /dev is unwieldy and big. It would be nice to only show
41 the /dev entries for the devices we actually have running in the
43 2) We are (well, were) running out of major and minor numbers for
45 3) Users want a way to name devices in a persistent fashion (i.e. "This
46 disk here, must _always_ be called "boot_disk" no matter where in
47 the scsi tree I put it", or "This USB camera must always be called
48 "camera" no matter if I have other USB scsi devices plugged in or
50 4) Userspace programs want to know when devices are created or removed,
51 and what /dev entry is associated with them.
54 1) No policy in the kernel!
55 2) Follow standards (like the LSB)
56 3) must be small so embedded devices will use it.
59 So, how does devfs stack up to the above problems and constraints:
61 1) devfs only shows the dev entries for the devices in the system.
62 2) devfs does not handle the need for dynamic major/minor numbers
63 3) devfs does not provide a way to name devices in a persistent
65 4) devfs does provide a deamon that userspace programs can hook into
66 to listen to see what devices are being created or removed.
68 1) devfs forces the devfs naming policy into the kernel. If you
69 don't like this naming scheme, tough.
70 2) devfs does not follow the LSB device naming standard.
71 3) devfs is small, and embedded devices use it. However it is
72 implemented in non-pagable memory.
74 Oh yeah, and there are the insolvable race conditions with the devfs
75 implementation in the kernel, but I'm not going to talk about them right
76 now, sorry. See the linux-kernel archives if you care about them (and
77 if you use devfs, you should care...)
79 So devfs is 2 for 7, ignoring the kernel races.
83 1) using udev, the /dev tree only is populated for the devices that
84 are currently present in the system.
85 2) udev does not care about the major/minor number schemes. If the
86 kernel tomorrow switches to randomly assign major and minor numbers
87 to different devices, it would work just fine (this is exactly
88 what I am proposing to do in 2.7...)
89 3) This is the main reason udev is around. It provides the ability
90 to name devices in a persistent manner. More on that below.
91 4) udev emits D-BUS messages so that any other userspace program
92 (like HAL) can listen to see what devices are created or removed.
93 It also allows userspace programs to query it's database to see
94 what devices are present and what they are currently named as
95 (providing a pointer into the sysfs tree for that specific device
98 1) udev moves _all_ naming policies out of the kernel and into
100 2) udev defaults to using the LSB device naming standard. If users
101 want to deviate away from this standard (for example when naming
102 some devices in a persistent manner), it is easily possible to do
104 3) udev is small (49Kb binary) and is entirely in userspace, which
105 is swapable, and doesn't have to be running at all times.
107 Nice, 7 out of 7 for udev. Makes you think the problems and constraints
108 were picked by a udev developer, right? No, the problems and
109 constraints are ones I've seen over the years and so udev, along with
110 the kernel driver model and sysfs, were created to solve these real
113 So by just looking at the above descriptions, everyone should instantly
114 realize that udev is far better than devfs and start helping out udev
115 development, right? Oh, you want more info, ok...
117 Back in May 2003 I released a very tiny version of udev that implemented
118 everything that devfs currently does, in about 6Kb of userspace code:
119 http://marc.theaimsgroup.com/?l=linux-kernel&m=105003185331553
121 Yes, that's right, 6Kb. So, you are asking, why are you still working
122 on udev if it did everything devfs did back in May 2003? That's because
123 just managing static device nodes based on what the kernel calls the
124 devices is _not_ the primary goal of udev. It's just a tiny side affect
125 of it's primary goal, the ability to never worry about major/minor
126 number assignments and provide the ability to achieve persistent device
129 All the people wanting to bring up the udev vs. devfs argument go back
130 and read the previous paragraph. Yes, all Gentoo users who keep filling
131 up my inbox with smoking emails, I mean you.
133 So, how well does udev solve it's goals:
134 Prevent users from ever worrying about major/minor numbers
135 And here you were, not knowing you ever needed to worry about
136 major/minor numbers in the first place, right? Ah, I see you
137 haven't plugged in 2 USB printers and tried to figure out which
138 printer was which /dev entry? Or plugged in 4000 SCSI disks and
139 tried to figure out how to access that 3642nd disk and what it was
140 called in /dev. Or plugged in a USB camera and a USB flash drive
141 and then tried to download the pictures off of the flash drive by
144 As the above scenarios show, both desktop users and big iron users
145 both need to not worry about which device is assigned to what
148 udev doesn't care what major/minor number is assigned to a device.
149 It merely takes the numbers that the kernel says it assigned to the
150 device and creates a device node based on it, which the user can
151 then use (if you don't understand the whole major/minor to device
152 node issue, or even what a device node is, trust me, you don't
153 really want to, go install udev and don't worry about it...) As
154 stated above, if the kernel decides to start randomly assigning
155 major numbers to all devices, then udev will still work just fine.
157 Provide a persistent device naming solution:
158 Lots of people want to assign a specific name that they can talk to
159 a device to, no matter where it is in the system, or what order they
160 plugged the device in. USB printers, SCSI disks, PCI sound cards,
161 Firewire disks, USB mice, and lots of other devices all need to be
162 assigned a name in a consistent manner (udev doesn't handle network
163 devices, naming them is already a solved solution, using nameif).
164 udev allows users to create simple rules to describe what device to
165 name. If users want to call a program running a large database
166 half-way around the world, asking it what to name this device, it
167 can. We don't put the naming database into the kernel (like other
168 Unix variants have), everything is in userspace, and easily
169 accessible. You can even run a perl script to name your device if
170 you are that crazy...
172 For more information on how to create udev rules to name devices,
173 please see the udev man page, and look at the example udev rules
174 that ship with the tarball.
177 So, convinced already why you should use udev instead of devfs? No.
178 Ok, fine, I'm not forcing you to abandon your bloated, stifling policy,
179 nonextensible, end of life feature if you don't want to. But please
180 don't bother me about it either, I don't care about devfs, only about
183 This is my last posting about this topic, all further emails sent to me
184 about why devfs is wonderful, and why are you making fun of this
185 wonderful, stable gift from the gods, will be gleefully ignored and
186 possibly posted in a public place where others can see.