5 For those of us who know how to use mdev, a primer might seem lame. For
6 everyone else, mdev is a weird black box that they hear is awesome, but can't
7 seem to get their head around how it works. Thus, a primer.
13 Mdev has two primary uses: initial population and dynamic updates. Both
14 require sysfs support in the kernel and have it mounted at /sys. For dynamic
15 updates, you also need to have hotplugging enabled in your kernel.
17 Here'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
22 Of course, a more "full" setup would entail executing this before the previous
24 [4] mount -t tmpfs mdev /dev
26 [6] mount -t devpts devpts /dev/pts
28 The simple explanation here is that [1] you need to have /sys mounted before
29 executing mdev. Then you [2] instruct the kernel to execute /bin/mdev whenever
30 a device is added or removed so that the device node can be created or
31 destroyed. Then you [3] seed /dev with all the device nodes that were created
32 while the system was booting.
34 For 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.
39 MDEV Config (/etc/mdev.conf)
42 Mdev has an optional config file for controlling ownership/permissions of
43 device nodes if your system needs something more than the default root/root
46 The file has the format:
47 <device regex> <uid>:<gid> <octal permissions>
51 The config file parsing stops at the first matching line. If no line is
52 matched, then the default of 0:0 660 is used. To set your own default, simply
53 create your own total match like so:
56 If you also enable support for executing your own commands, then the file has
58 <device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]
59 The 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.
64 The command is executed via the system() function (which means you're giving a
65 command to the shell), so make sure you have a shell installed at /bin/sh.
67 For your convenience, the shell env var $MDEV is set to the device name. So if
68 the device 'hdc' was matched, MDEV would be set to "hdc".