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1-------------
2 MDEV Primer
3-------------
4
5For those of us who know how to use mdev, a primer might seem lame.  For
6everyone else, mdev is a weird black box that they hear is awesome, but can't
7seem to get their head around how it works.  Thus, a primer.
8
9-----------
10 Basic Use
11-----------
12
13Mdev has two primary uses: initial population and dynamic updates.  Both
14require sysfs support in the kernel and have it mounted at /sys.  For dynamic
15updates, you also need to have hotplugging enabled in your kernel.
16
17Here's a typical code snippet from the init script:
18[1] mount -t sysfs sysfs /sys
19[2] echo /bin/mdev > /proc/sys/kernel/hotplug
20[3] mdev -s
21
22Of course, a more "full" setup would entail executing this before the previous
23code snippet:
24[4] mount -t tmpfs mdev /dev
25[5] mkdir /dev/pts
26[6] mount -t devpts devpts /dev/pts
27
28The simple explanation here is that [1] you need to have /sys mounted before
29executing mdev.  Then you [2] instruct the kernel to execute /bin/mdev whenever
30a device is added or removed so that the device node can be created or
31destroyed.  Then you [3] seed /dev with all the device nodes that were created
32while the system was booting.
33
34For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem
35(assuming you're running out of flash).  Then you want to [5] create the
36/dev/pts mount point and finally [6] mount the devpts filesystem on it.
37
38-------------
39 MDEV Config   (/etc/mdev.conf)
40-------------
41
42Mdev has an optional config file for controlling ownership/permissions of
43device nodes if your system needs something more than the default root/root
44660 permissions.
45
46The file has the format:
47    <device regex> <uid>:<gid> <octal permissions>
48For example:
49    hd[a-z][0-9]* 0:3 660
50
51The config file parsing stops at the first matching line.  If no line is
52matched, then the default of 0:0 660 is used.  To set your own default, simply
53create your own total match like so:
54    .* 1:1 777
55
56If you also enable support for executing your own commands, then the file has
57the format:
58    <device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]
59The special characters have the meaning:
60    @ Run after creating the device.
61    $ Run before removing the device.
62    * Run both after creating and before removing the device.
63
64The command is executed via the system() function (which means you're giving a
65command to the shell), so make sure you have a shell installed at /bin/sh.
66
67For your convenience, the shell env var $MDEV is set to the device name.  So if
68the device 'hdc' was matched, MDEV would be set to "hdc".
69
70----------
71 FIRMWARE
72----------
73
74Some kernel device drivers need to request firmware at runtime in order to
75properly initialize a device.  Place all such firmware files into the
76/lib/firmware/ directory.  At runtime, the kernel will invoke mdev with the
77filename of the firmware which mdev will load out of /lib/firmware/ and into
78the kernel via the sysfs interface.  The exact filename is hardcoded in the
79kernel, so look there if you need to want to know what to name the file in
80userspace.
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