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 [0] mount -t proc proc /proc
19 [1] mount -t sysfs sysfs /sys
20 [2] echo /bin/mdev > /proc/sys/kernel/hotplug
23 Alternatively, without procfs the above becomes:
24 [1] mount -t sysfs sysfs /sys
25 [2] sysctl -w kernel.hotplug=/bin/mdev
29 Of course, a more "full" setup would entail executing this before the previous
31 [4] mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev
33 [6] mount -t devpts devpts /dev/pts
35 The simple explanation here is that [1] you need to have /sys mounted before
36 executing mdev. Then you [2] instruct the kernel to execute /bin/mdev whenever
37 a device is added or removed so that the device node can be created or
38 destroyed. Then you [3] seed /dev with all the device nodes that were created
39 while the system was booting.
41 For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem
42 (assuming you're running out of flash). Then you want to [5] create the
43 /dev/pts mount point and finally [6] mount the devpts filesystem on it.
46 MDEV Config (/etc/mdev.conf)
49 Mdev has an optional config file for controlling ownership/permissions of
50 device nodes if your system needs something more than the default root/root
53 The file has the format:
54 <device regex> <uid>:<gid> <octal permissions>
58 The config file parsing stops at the first matching line. If no line is
59 matched, then the default of 0:0 660 is used. To set your own default, simply
60 create your own total match like so:
63 You can rename/relocate device nodes by using the next optional field.
64 <device regex> <uid>:<gid> <octal permissions> [>path]
65 So if you want to place the device node into a subdirectory, make sure the path
66 has a trailing /. If you want to rename the device node, just place the name.
68 This will relocate "hda" into the drives/ subdirectory.
70 This will rename "hdb" to "cdrom".
72 If you also enable support for executing your own commands, then the file has
74 <device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]
75 The special characters have the meaning:
76 @ Run after creating the device.
77 $ Run before removing the device.
78 * Run both after creating and before removing the device.
80 The command is executed via the system() function (which means you're giving a
81 command to the shell), so make sure you have a shell installed at /bin/sh. You
82 should also keep in mind that the kernel executes hotplug helpers with stdin,
83 stdout, and stderr connected to /dev/null.
85 For your convenience, the shell env var $MDEV is set to the device name. So if
86 the device "hdc" was matched, MDEV would be set to "hdc".
92 Some kernel device drivers need to request firmware at runtime in order to
93 properly initialize a device. Place all such firmware files into the
94 /lib/firmware/ directory. At runtime, the kernel will invoke mdev with the
95 filename of the firmware which mdev will load out of /lib/firmware/ and into
96 the kernel via the sysfs interface. The exact filename is hardcoded in the
97 kernel, so look there if you need to know how to name the file in userspace.