doc/en/devfs-guide.xml

<?xml version='1.0' encoding="UTF-8"?>
<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/devfs-guide.xml,v 1.10 2005/06/20 09:41:16 neysx Exp $ -->

<!DOCTYPE guide SYSTEM "/dtd/guide.dtd">

<guide link="/doc/en/devfs-guide.xml">
<title>Device File System Guide</title>
<author title="Author">
  <mail link="swift@gentoo.org">Sven Vermeulen</mail>
</author>
<author title="Reviewer">
  <mail link="seemant@gentoo.org">Seemant Kulleen</mail>
</author>

<abstract>
In this document you'll find information on what devfs is really about
and how to work with it.
</abstract>

<!-- The content of this document is licensed under the CC-BY-SA license -->
<!-- See http://creativecommons.org/licenses/by-sa/2.5 -->
<license/>

<version>0.5</version>
<date>2005-07-18</date>

<chapter>
<title>What is devfs?</title>
<section>
<title>The (good?) old days</title>
<body>

<warn>
devfs is <e>obsolete</e> and will eventually be removed from the stable 2.6
tree. Users on 2.6 kernels are hereby advised to switch to udev. For further
information on udev, please refer to the <uri
link="/doc/en/udev-guide.xml">Gentoo udev Guide</uri>.
</warn>

<p>
Traditional Linux implementations provide their users with an
abstract device path, called <path>/dev</path>. Inside this path the
user finds <e>device nodes</e>, special files that represent devices
inside their system. For instance, <path>/dev/hda</path> represents the
first IDE device in their system. By providing device files to the
users, they can create programs that interact with hardware as if the
hardware was a regular file instead of using special APIs.
</p>

<p>
The device files are split in two groups, called <e>character</e>
devices and <e>block</e> devices. The first group consists of hardware
of which read/writes are not buffered. The second group naturally
consists of hardware of which read/writes are buffered. Both devices can
be read one character at a time, or in blocks. Therefore, the naming
might sound confusing and in fact is wrong.
</p>

<p>
If you take a look at a certain device file, you might find something
like this:
</p>

<pre caption="Checking the information of a device file">
# <i>ls -l /dev/hda</i>
brw-rw----    1 root     disk       3,   0 Jul  5  2000 /dev/hda
</pre>

<p>
In the previous example we see that <path>/dev/hda</path> is a block
device. However, more importantly, it has two special numbers assigned
to it: <b>3, 0</b>. This pair is called the <e>major-minor</e>
pair. It is used by the kernel to map a device file to a real device.
The major corresponds with a certain device, the minor with a subdevice.
Seems confusing? It isn't.
</p>

<p>
Two examples are <path>/dev/hda4</path> and <path>/dev/tty5</path>. The
first device file corresponds with the fourth partition on the first IDE
device. Its major-minor pair is <b>3, 4</b>. In other words, the
minor corresponds with the partition where the major corresponds with
the device. The second example has <b>4, 5</b> as major-minor
pair. In this case, the major corresponds with the terminal driver,
while the minor corresponds with the terminal number (in this case, the
fifth terminal).
</p>

</body>
</section>
<section>
<title>The problems</title>
<body>

<p>
If you do a quick check in such a <path>/dev</path>, you'll find out
that not only all your devices are listed, but <e>all</e> possible
devices that you can imagine. In other words, you have device files for
devices you don't have. Managing such a device group is cumbersome to
say the least. Imagine having to change the permissions of all device
files that have a corresponding device in your system, and leaving the
rest of the device files as they are.
</p>

<p>
When you add new hardware to your system, and this hardware didn't have
a device file previously, you would have to create one. Advanced users know
that this task can be accomplished with <c>./MAKEDEV</c> inside the
<path>/dev</path> tree, but do you immediately know what device you have
to create?
</p>

<p>
When you have programs interacting with hardware using the device files,
you can't have the root partition mounted read only, while there is no
further need to have it mounted read-write. And you can't have
<path>/dev</path> on a seperate partition, since <c>mount</c> needs
<path>/dev</path> to mount partitions.
</p>

</body>
</section>
<section>
<title>The solutions</title>
<body>

<p>
As you can imagine, the kernel hackers have found quite a number of
solutions to the aforementioned problems. However, many of them had
other flaws as described in
<uri>http://www.atnf.csiro.au/people/rgooch/linux/docs/devfs.html#faq-why</uri>.
We are not going to talk about these implementations, but focus on the
one implementation that did make it to the official kernel sources:
devfs.
</p>

</body>
</section>
<section>
<title>devfs as all-round winner ?</title>
<body>

<p>
devfs tackles all listed problems. It only provides the user with
existing devices, adds new nodes when new devices are found, and makes
it possible to mount the root filesystem read only. And it tackles more
problems we haven't discussed previously because they are less
interesting for users...
</p>

<p>
For instance, with devfs, you don't have to worry about major/minor
pairs. It is still supported (for backwards compatibility), but isn't
needed. This makes it possible for Linux to support even more devices,
since there are no limits anymore (numbers always have boundaries :)
</p>

<p>
Yet devfs does come with it's own problems; for the end users these issues
aren't really visible, but for the kernel maintainers the problems are big
enough to mark devfs <e>obsolete</e> in favor of <uri
link="udev-guide.xml">udev</uri>, which Gentoo supports and uses by default on
most architectures since the 2005.0 release when using a 2.6 kernel. 
</p>

<p>
For more information as to why devfs is marked obsolete, please read the <uri
link="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ">udev
FAQ</uri> and <uri
link="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev_vs_devfs">udev
versus devfs document</uri>.
</p>

</body>
</section>
</chapter>

<chapter>
<title>Navigating through the device tree</title>
<section>
<title>Directories</title>
<body>

<p>
One of the first things you might notice is that devfs uses directories
to group devices together. This improves readability, as now all related
devices are inside a common directory.
</p>

<p>
For instance, all IDE-related devices are inside the
<path>/dev/ide/</path> device directory, and SCSI-related devices are inside
<path>/dev/scsi/</path>. SCSI and IDE disks are seen in the same way,
meaning they both have the same subdirectory structure. 
</p>

<p>
IDE and SCSI disks are controlled by an adapter (on-board or a seperate
card), called the <e>host</e>. Every adapter can have several channels.
A channel is called a <e>bus</e>. On each channel, you can have several
IDs. Such an ID identifies a disk. This ID is called the <e>target</e>.
Some SCSI devices can have multiple luns (<e>Logical Unit Numbers</e>),
for instance devices that handle multiple media simultaneously (hi-end
tapedrives). You mostly have only a single lun, <path>lun0/</path>.
</p>

<p>
So, whereas <path>/dev/hda4</path> was used previously, we now have
<path>/dev/ide/host0/bus0/target0/lun0/part4</path>. This is far more
easy... no, don't argue with me... it <e>is</e> easier... ah whatever!
:)
</p>

<note>
You can also use more Unix-like device file naming for hard disks, such as
<path>c0b0t0u0p2</path>. They can be found in <path>/dev/ide/hd</path>,
<path>/dev/scsi/hd</path> etc.
</note>

<p>
To give you an idea on the directories, this is a listing of the
directories which I have on my laptop:
</p>

<pre caption="Directories in /dev">
cdroms/     cpu/        discs/          floppy/
ide/        input/      loop/           misc/
netlink/    printers/   pts/            pty/
scsi/       sg/         shm/            sound/
sr/         usb/        vc/             vcc/
</pre>

</body>
</section>
<section>
<title>Backwards compatibility using devfsd</title>
<body>

<p>
Using this new scheme sounds fun, but several tools and programs make
use of the previous, old scheme. To make sure no system is broken,
<c>devfsd</c> is created. This daemon creates symlinks with the old
names, pointing to the new device files.
</p>

<pre caption="Created symlinks">
$ <i>ls -l /dev/hda4</i>
lr-xr-xr-x    1 root     root           33 Aug 25 12:08 /dev/hda4 -> ide/host0/bus0/target0/lun0/part4
</pre>

<p>
With <c>devfsd</c>, you can also set the permissions, create new device
files, define actions etc. All this is described in the next chapter.
</p>

</body>
</section>
</chapter>

<chapter>
<title>Administrating the device tree</title>
<section>
<title>Restarting devfsd</title>
<body>

<p>
When you alter the <path>/etc/devfsd.conf</path> file, and you want the
changes to be forced onto the system, you don't have to reboot.
Depending on what you want, you can use any of the two following
signals:
</p>

<p>
<b>SIGHUP</b> will have <c>devfsd</c> reread the configuration file,
reload the shared objects and generate the REGISTER events for each leaf
node in the device tree.
</p>

<p>
<b>SIGUSR1</b> will do the same, but won't generate REGISTER events.
</p>

<p>
To send a signal, simply use <c>kill</c> or <c>killall</c>:
</p>

<pre caption="Sending the SIGHUP signal to devfsd">
# <i>kill -s SIGHUP `pidof devfsd`</i>
<comment>or</comment>
# <i>killall -s SIGHUP devfsd</i>
</pre>

</body>
</section>
<section>
<title>Removing compatibility symlinks</title>
<body>

<warn>
Currently, Gentoo cannot live without the compatibility symlinks. 
</warn>

<p>
If you want the compatibility symlinks that clutter up <path>/dev</path>
removed from your Gentoo system (Gentoo activates it per default), edit
<path>/etc/devfsd.conf</path> and remove the following two lines:
</p>

<pre caption="/etc/devfsd.conf for backwards compatibility">
<comment># Comment the following two lines out to remove the symlinks</comment>
REGISTER        .*  MKOLDCOMPAT
UNREGISTER      .*  RMOLDCOMPAT
</pre>

<p>
You need to reboot your system for the changes to take affect.
</p>

</body>
</section>
<section>
<title>Removing autoload functionality</title>
<body>

<p>
When you load a module, devfs will automatically create the device
files. If you don't want this behaviour, remove the following line from
<path>/etc/devfsd.conf</path>:
</p>

<pre caption="/etc/devfsd.conf, autoload functionality">
LOOKUP      .*  MODLOAD
</pre>

</body>
</section>
</chapter>

<chapter>
<title>Permission Related Items</title>
<section>
<title>Set/change permissions with devfsd</title>
<body>

<note>
These instructions are valid as long as pam_console is disabled in
<path>/etc/pam.d/system-auth</path>. If you enabled pam_console there,
then PAM has the final word on permissions.
</note>

<p>
If you want to set permissions using <path>/etc/devfsd.conf</path>,
then use the syntax used in the following example:
</p>

<pre caption="Permissions in /etc/devfsd.conf">
REGISTER    ^cdroms/.*  PERMISSIONS root.cdrom 0660
</pre>

<p>
The second field is the device group, starting from <path>/dev</path>.
It is a regular expression, meaning you can select several device files
in one rule.
</p>

<p>
The fourth field is the ownership of the device file, and the fifth
field contains the permissions of the device file.
</p>

</body>
</section>
<section>
<title>Manually set permissions and have devfsd save it</title>
<body>

<p>
This is the default behaviour for Gentoo: if you <c>chown</c> (CHange
OWNer) and <c>chmod</c> (CHange MODe) some device files, <c>devfsd</c>
will save the information so that it will persist across reboots. This
is because the <path>/etc/devfsd.conf</path> file contains the
following lines:
</p>

<pre caption="/etc/devfsd.conf for saving permissions">
REGISTER        ^pt[sy]/.*   IGNORE
CHANGE          ^pt[sy]/.*   IGNORE
CREATE          ^pt[sy]/.*   IGNORE
DELETE          ^pt[sy]      IGNORE
REGISTER        ^log         IGNORE
CHANGE          ^log         IGNORE
CREATE          ^log         IGNORE
DELETE          ^log         IGNORE
REGISTER        .*           COPY    /lib/dev-state/$devname $devpath
CHANGE          .*           COPY    $devpath /lib/dev-state/$devname
CREATE          .*           COPY    $devpath /lib/dev-state/$devname
DELETE          .*           CFUNCTION GLOBAL unlink
/lib/dev-state/$devname
RESTORE         /lib/dev-state
</pre>

<p>
In other words, changed device files are copied over to
<path>/lib/dev-state</path> as soon as the change happens, and are
copied over to <path>/dev</path> when booting the system.
</p>

<p>
Another possibility is to mount <path>/lib/dev-state</path> on
<path>/dev</path> at boot-time. To do this, you must make sure that
devfs is not mounted automatically (meaning you'll have to recompile
your kernel) and that <path>/dev/console</path> exists. Then, somewhere
at the beginning of the bootscripts of your system, you place:
</p>

<pre caption="Mounting /lib/dev-state on top of /dev">
mount --bind /dev /lib/dev-state
mount -t devfs none /dev
devfsd /dev
</pre>

</body>
</section>
</chapter>

<chapter>
<title>Resources</title>
<section>
<body>

<p>
For more information on devfs, check out the following resources.
</p>

<p>
The devfsd.conf manpage explains the syntax of the
<path>/etc/devfsd.conf</path> file. To view it, type <c>man
devfsd.conf</c>.
</p>

<p>
The <uri
link="http://www.atnf.csiro.au/people/rgooch/linux/docs/devfs.html">devfs
FAQ</uri> explains everything about devfs. It also contains information
about the internal devfs structure and how drivers can support devfs.
</p>

<p>
On <uri link="http://www.linuxjournal.com">LinuxJournal</uri> there is
an interesting article on <uri
link="http://www.linuxjournal.com/article.php?sid=6035">devfs for
Management and Administration</uri>.
</p>

<p>
Daniel Robbins has written a set of articles for IBM's DeveloperWorks
about Advanced filesystems. Three of them are about devfs:
</p>

<ul>
  <li>
    <uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs4/">
    Introduction to devfs</uri>
  </li>
  <li>
    <uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs5/">
    Setting up devfs</uri>
  </li>
  <li>
    <uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs6/">
    Implementing devfs</uri>
  </li>
</ul>

</body>
</section>
</chapter>
</guide>
1	<?xml version='1.0' encoding="UTF-8"?>
2	<!-- $Header: /var/cvsroot/gentoo/xml/htdocs/doc/en/devfs-guide.xml,v 1.10 2005/06/20 09:41:16 neysx Exp $ -->
3
4	<!DOCTYPE guide SYSTEM "/dtd/guide.dtd">
5
6	<guide link="/doc/en/devfs-guide.xml">
7	<title>Device File System Guide</title>
8	<author title="Author">
9	<mail link="swift@gentoo.org">Sven Vermeulen</mail>
10	</author>
11	<author title="Reviewer">
12	<mail link="seemant@gentoo.org">Seemant Kulleen</mail>
13	</author>
14
15	<abstract>
16	In this document you'll find information on what devfs is really about
17	and how to work with it.
18	</abstract>
19
20	<!-- The content of this document is licensed under the CC-BY-SA license -->
21	<!-- See http://creativecommons.org/licenses/by-sa/2.5 -->
22	<license/>
23
24	<version>0.5</version>
25	<date>2005-07-18</date>
26
27	<chapter>
28	<title>What is devfs?</title>
29	<section>
30	<title>The (good?) old days</title>
31	<body>
32
33	<warn>
34	devfs is <e>obsolete</e> and will eventually be removed from the stable 2.6
35	tree. Users on 2.6 kernels are hereby advised to switch to udev. For further
36	information on udev, please refer to the <uri
37	link="/doc/en/udev-guide.xml">Gentoo udev Guide</uri>.
38	</warn>
39
40	<p>
41	Traditional Linux implementations provide their users with an
42	abstract device path, called <path>/dev</path>. Inside this path the
43	user finds <e>device nodes</e>, special files that represent devices
44	inside their system. For instance, <path>/dev/hda</path> represents the
45	first IDE device in their system. By providing device files to the
46	users, they can create programs that interact with hardware as if the
47	hardware was a regular file instead of using special APIs.
48	</p>
49
50	<p>
51	The device files are split in two groups, called <e>character</e>
52	devices and <e>block</e> devices. The first group consists of hardware
53	of which read/writes are not buffered. The second group naturally
54	consists of hardware of which read/writes are buffered. Both devices can
55	be read one character at a time, or in blocks. Therefore, the naming
56	might sound confusing and in fact is wrong.
57	</p>
58
59	<p>
60	If you take a look at a certain device file, you might find something
61	like this:
62	</p>
63
64	<pre caption="Checking the information of a device file">
65	# <i>ls -l /dev/hda</i>
66	brw-rw---- 1 root disk 3, 0 Jul 5 2000 /dev/hda
67	</pre>
68
69	<p>
70	In the previous example we see that <path>/dev/hda</path> is a block
71	device. However, more importantly, it has two special numbers assigned
72	to it: <b>3, 0</b>. This pair is called the <e>major-minor</e>
73	pair. It is used by the kernel to map a device file to a real device.
74	The major corresponds with a certain device, the minor with a subdevice.
75	Seems confusing? It isn't.
76	</p>
77
78	<p>
79	Two examples are <path>/dev/hda4</path> and <path>/dev/tty5</path>. The
80	first device file corresponds with the fourth partition on the first IDE
81	device. Its major-minor pair is <b>3, 4</b>. In other words, the
82	minor corresponds with the partition where the major corresponds with
83	the device. The second example has <b>4, 5</b> as major-minor
84	pair. In this case, the major corresponds with the terminal driver,
85	while the minor corresponds with the terminal number (in this case, the
86	fifth terminal).
87	</p>
88
89	</body>
90	</section>
91	<section>
92	<title>The problems</title>
93	<body>
94
95	<p>
96	If you do a quick check in such a <path>/dev</path>, you'll find out
97	that not only all your devices are listed, but <e>all</e> possible
98	devices that you can imagine. In other words, you have device files for
99	devices you don't have. Managing such a device group is cumbersome to
100	say the least. Imagine having to change the permissions of all device
101	files that have a corresponding device in your system, and leaving the
102	rest of the device files as they are.
103	</p>
104
105	<p>
106	When you add new hardware to your system, and this hardware didn't have
107	a device file previously, you would have to create one. Advanced users know
108	that this task can be accomplished with <c>./MAKEDEV</c> inside the
109	<path>/dev</path> tree, but do you immediately know what device you have
110	to create?
111	</p>
112
113	<p>
114	When you have programs interacting with hardware using the device files,
115	you can't have the root partition mounted read only, while there is no
116	further need to have it mounted read-write. And you can't have
117	<path>/dev</path> on a seperate partition, since <c>mount</c> needs
118	<path>/dev</path> to mount partitions.
119	</p>
120
121	</body>
122	</section>
123	<section>
124	<title>The solutions</title>
125	<body>
126
127	<p>
128	As you can imagine, the kernel hackers have found quite a number of
129	solutions to the aforementioned problems. However, many of them had
130	other flaws as described in
131	<uri>http://www.atnf.csiro.au/people/rgooch/linux/docs/devfs.html#faq-why</uri>.
132	We are not going to talk about these implementations, but focus on the
133	one implementation that did make it to the official kernel sources:
134	devfs.
135	</p>
136
137	</body>
138	</section>
139	<section>
140	<title>devfs as all-round winner ?</title>
141	<body>
142
143	<p>
144	devfs tackles all listed problems. It only provides the user with
145	existing devices, adds new nodes when new devices are found, and makes
146	it possible to mount the root filesystem read only. And it tackles more
147	problems we haven't discussed previously because they are less
148	interesting for users...
149	</p>
150
151	<p>
152	For instance, with devfs, you don't have to worry about major/minor
153	pairs. It is still supported (for backwards compatibility), but isn't
154	needed. This makes it possible for Linux to support even more devices,
155	since there are no limits anymore (numbers always have boundaries :)
156	</p>
157
158	<p>
159	Yet devfs does come with it's own problems; for the end users these issues
160	aren't really visible, but for the kernel maintainers the problems are big
161	enough to mark devfs <e>obsolete</e> in favor of <uri
162	link="udev-guide.xml">udev</uri>, which Gentoo supports and uses by default on
163	most architectures since the 2005.0 release when using a 2.6 kernel.
164	</p>
165
166	<p>
167	For more information as to why devfs is marked obsolete, please read the <uri
168	link="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev-FAQ">udev
169	FAQ</uri> and <uri
170	link="http://www.kernel.org/pub/linux/utils/kernel/hotplug/udev_vs_devfs">udev
171	versus devfs document</uri>.
172	</p>
173
174	</body>
175	</section>
176	</chapter>
177
178	<chapter>
179	<title>Navigating through the device tree</title>
180	<section>
181	<title>Directories</title>
182	<body>
183
184	<p>
185	One of the first things you might notice is that devfs uses directories
186	to group devices together. This improves readability, as now all related
187	devices are inside a common directory.
188	</p>
189
190	<p>
191	For instance, all IDE-related devices are inside the
192	<path>/dev/ide/</path> device directory, and SCSI-related devices are inside
193	<path>/dev/scsi/</path>. SCSI and IDE disks are seen in the same way,
194	meaning they both have the same subdirectory structure.
195	</p>
196
197	<p>
198	IDE and SCSI disks are controlled by an adapter (on-board or a seperate
199	card), called the <e>host</e>. Every adapter can have several channels.
200	A channel is called a <e>bus</e>. On each channel, you can have several
201	IDs. Such an ID identifies a disk. This ID is called the <e>target</e>.
202	Some SCSI devices can have multiple luns (<e>Logical Unit Numbers</e>),
203	for instance devices that handle multiple media simultaneously (hi-end
204	tapedrives). You mostly have only a single lun, <path>lun0/</path>.
205	</p>
206
207	<p>
208	So, whereas <path>/dev/hda4</path> was used previously, we now have
209	<path>/dev/ide/host0/bus0/target0/lun0/part4</path>. This is far more
210	easy... no, don't argue with me... it <e>is</e> easier... ah whatever!
211	:)
212	</p>
213
214	<note>
215	You can also use more Unix-like device file naming for hard disks, such as
216	<path>c0b0t0u0p2</path>. They can be found in <path>/dev/ide/hd</path>,
217	<path>/dev/scsi/hd</path> etc.
218	</note>
219
220	<p>
221	To give you an idea on the directories, this is a listing of the
222	directories which I have on my laptop:
223	</p>
224
225	<pre caption="Directories in /dev">
226	cdroms/ cpu/ discs/ floppy/
227	ide/ input/ loop/ misc/
228	netlink/ printers/ pts/ pty/
229	scsi/ sg/ shm/ sound/
230	sr/ usb/ vc/ vcc/
231	</pre>
232
233	</body>
234	</section>
235	<section>
236	<title>Backwards compatibility using devfsd</title>
237	<body>
238
239	<p>
240	Using this new scheme sounds fun, but several tools and programs make
241	use of the previous, old scheme. To make sure no system is broken,
242	<c>devfsd</c> is created. This daemon creates symlinks with the old
243	names, pointing to the new device files.
244	</p>
245
246	<pre caption="Created symlinks">
247	$ <i>ls -l /dev/hda4</i>
248	lr-xr-xr-x 1 root root 33 Aug 25 12:08 /dev/hda4 -> ide/host0/bus0/target0/lun0/part4
249	</pre>
250
251	<p>
252	With <c>devfsd</c>, you can also set the permissions, create new device
253	files, define actions etc. All this is described in the next chapter.
254	</p>
255
256	</body>
257	</section>
258	</chapter>
259
260	<chapter>
261	<title>Administrating the device tree</title>
262	<section>
263	<title>Restarting devfsd</title>
264	<body>
265
266	<p>
267	When you alter the <path>/etc/devfsd.conf</path> file, and you want the
268	changes to be forced onto the system, you don't have to reboot.
269	Depending on what you want, you can use any of the two following
270	signals:
271	</p>
272
273	<p>
274	<b>SIGHUP</b> will have <c>devfsd</c> reread the configuration file,
275	reload the shared objects and generate the REGISTER events for each leaf
276	node in the device tree.
277	</p>
278
279	<p>
280	<b>SIGUSR1</b> will do the same, but won't generate REGISTER events.
281	</p>
282
283	<p>
284	To send a signal, simply use <c>kill</c> or <c>killall</c>:
285	</p>
286
287	<pre caption="Sending the SIGHUP signal to devfsd">
288	# <i>kill -s SIGHUP `pidof devfsd`</i>
289	<comment>or</comment>
290	# <i>killall -s SIGHUP devfsd</i>
291	</pre>
292
293	</body>
294	</section>
295	<section>
296	<title>Removing compatibility symlinks</title>
297	<body>
298
299	<warn>
300	Currently, Gentoo cannot live without the compatibility symlinks.
301	</warn>
302
303	<p>
304	If you want the compatibility symlinks that clutter up <path>/dev</path>
305	removed from your Gentoo system (Gentoo activates it per default), edit
306	<path>/etc/devfsd.conf</path> and remove the following two lines:
307	</p>
308
309	<pre caption="/etc/devfsd.conf for backwards compatibility">
310	<comment># Comment the following two lines out to remove the symlinks</comment>
311	REGISTER .* MKOLDCOMPAT
312	UNREGISTER .* RMOLDCOMPAT
313	</pre>
314
315	<p>
316	You need to reboot your system for the changes to take affect.
317	</p>
318
319	</body>
320	</section>
321	<section>
322	<title>Removing autoload functionality</title>
323	<body>
324
325	<p>
326	When you load a module, devfs will automatically create the device
327	files. If you don't want this behaviour, remove the following line from
328	<path>/etc/devfsd.conf</path>:
329	</p>
330
331	<pre caption="/etc/devfsd.conf, autoload functionality">
332	LOOKUP .* MODLOAD
333	</pre>
334
335	</body>
336	</section>
337	</chapter>
338
339	<chapter>
340	<title>Permission Related Items</title>
341	<section>
342	<title>Set/change permissions with devfsd</title>
343	<body>
344
345	<note>
346	These instructions are valid as long as pam_console is disabled in
347	<path>/etc/pam.d/system-auth</path>. If you enabled pam_console there,
348	then PAM has the final word on permissions.
349	</note>
350
351	<p>
352	If you want to set permissions using <path>/etc/devfsd.conf</path>,
353	then use the syntax used in the following example:
354	</p>
355
356	<pre caption="Permissions in /etc/devfsd.conf">
357	REGISTER ^cdroms/.* PERMISSIONS root.cdrom 0660
358	</pre>
359
360	<p>
361	The second field is the device group, starting from <path>/dev</path>.
362	It is a regular expression, meaning you can select several device files
363	in one rule.
364	</p>
365
366	<p>
367	The fourth field is the ownership of the device file, and the fifth
368	field contains the permissions of the device file.
369	</p>
370
371	</body>
372	</section>
373	<section>
374	<title>Manually set permissions and have devfsd save it</title>
375	<body>
376
377	<p>
378	This is the default behaviour for Gentoo: if you <c>chown</c> (CHange
379	OWNer) and <c>chmod</c> (CHange MODe) some device files, <c>devfsd</c>
380	will save the information so that it will persist across reboots. This
381	is because the <path>/etc/devfsd.conf</path> file contains the
382	following lines:
383	</p>
384
385	<pre caption="/etc/devfsd.conf for saving permissions">
386	REGISTER ^pt[sy]/.* IGNORE
387	CHANGE ^pt[sy]/.* IGNORE
388	CREATE ^pt[sy]/.* IGNORE
389	DELETE ^pt[sy] IGNORE
390	REGISTER ^log IGNORE
391	CHANGE ^log IGNORE
392	CREATE ^log IGNORE
393	DELETE ^log IGNORE
394	REGISTER .* COPY /lib/dev-state/$devname $devpath
395	CHANGE .* COPY $devpath /lib/dev-state/$devname
396	CREATE .* COPY $devpath /lib/dev-state/$devname
397	DELETE .* CFUNCTION GLOBAL unlink
398	/lib/dev-state/$devname
399	RESTORE /lib/dev-state
400	</pre>
401
402	<p>
403	In other words, changed device files are copied over to
404	<path>/lib/dev-state</path> as soon as the change happens, and are
405	copied over to <path>/dev</path> when booting the system.
406	</p>
407
408	<p>
409	Another possibility is to mount <path>/lib/dev-state</path> on
410	<path>/dev</path> at boot-time. To do this, you must make sure that
411	devfs is not mounted automatically (meaning you'll have to recompile
412	your kernel) and that <path>/dev/console</path> exists. Then, somewhere
413	at the beginning of the bootscripts of your system, you place:
414	</p>
415
416	<pre caption="Mounting /lib/dev-state on top of /dev">
417	mount --bind /dev /lib/dev-state
418	mount -t devfs none /dev
419	devfsd /dev
420	</pre>
421
422	</body>
423	</section>
424	</chapter>
425
426	<chapter>
427	<title>Resources</title>
428	<section>
429	<body>
430
431	<p>
432	For more information on devfs, check out the following resources.
433	</p>
434
435	<p>
436	The devfsd.conf manpage explains the syntax of the
437	<path>/etc/devfsd.conf</path> file. To view it, type <c>man
438	devfsd.conf</c>.
439	</p>
440
441	<p>
442	The <uri
443	link="http://www.atnf.csiro.au/people/rgooch/linux/docs/devfs.html">devfs
444	FAQ</uri> explains everything about devfs. It also contains information
445	about the internal devfs structure and how drivers can support devfs.
446	</p>
447
448	<p>
449	On <uri link="http://www.linuxjournal.com">LinuxJournal</uri> there is
450	an interesting article on <uri
451	link="http://www.linuxjournal.com/article.php?sid=6035">devfs for
452	Management and Administration</uri>.
453	</p>
454
455	<p>
456	Daniel Robbins has written a set of articles for IBM's DeveloperWorks
457	about Advanced filesystems. Three of them are about devfs:
458	</p>
459
460	<ul>
461	<li>
462	<uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs4/">
463	Introduction to devfs</uri>
464	</li>
465	<li>
466	<uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs5/">
467	Setting up devfs</uri>
468	</li>
469	<li>
470	<uri link="http://www-106.ibm.com/developerworks/linux/library/l-fs6/">
471	Implementing devfs</uri>
472	</li>
473	</ul>
474
475	</body>
476	</section>
477	</chapter>
478	</guide>