For those of us who know how to use mdev, a primer might seem lame. For
everyone else, mdev is a weird black box that they hear is awesome, but can't
seem to get their head around how it works. Thus, a primer.
Mdev has two primary uses: initial population and dynamic updates. Both
require sysfs support in the kernel and have it mounted at /sys. For dynamic
updates, you also need to have hotplugging enabled in your kernel.
Here's a typical code snippet from the init script:
 mount -t proc proc /proc
 mount -t sysfs sysfs /sys
 echo /sbin/mdev > /proc/sys/kernel/hotplug
 mdev -s
Alternatively, without procfs the above becomes:
 mount -t sysfs sysfs /sys
 sysctl -w kernel.hotplug=/sbin/mdev
 mdev -s
Of course, a more "full" setup would entail executing this before the previous
 mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev
 mkdir /dev/pts
 mount -t devpts devpts /dev/pts
The simple explanation here is that  you need to have /sys mounted before
executing mdev. Then you  instruct the kernel to execute /sbin/mdev whenever
a device is added or removed so that the device node can be created or
destroyed. Then you  seed /dev with all the device nodes that were created
while the system was booting.
For the "full" setup, you want to  make sure /dev is a tmpfs filesystem
(assuming you're running out of flash). Then you want to  create the
/dev/pts mount point and finally  mount the devpts filesystem on it.
MDEV Config (/etc/mdev.conf)
Mdev has an optional config file for controlling ownership/permissions of
device nodes if your system needs something more than the default root/root
The file has the format:
[-][envmatch]<device regex> <uid>:<gid> <permissions>
[envmatch]@<maj[,min1[-min2]]> <uid>:<gid> <permissions>
$envvar=<regex> <uid>:<gid> <permissions>
hd[a-z][0-9]* 0:3 660
The config file parsing stops at the first matching line. If no line is
matched, then the default of 0:0 660 is used. To set your own default, simply
create your own total match like so:
.* 1:1 777
You can rename/move device nodes by using the next optional field.
<device regex> <uid>:<gid> <permissions> [=path]
So if you want to place the device node into a subdirectory, make sure the path
has a trailing /. If you want to rename the device node, just place the name.
hda 0:3 660 =drives/
This will move "hda" into the drives/ subdirectory.
hdb 0:3 660 =cdrom
This will rename "hdb" to "cdrom".
Similarly, ">path" renames/moves the device but it also creates
a direct symlink /dev/DEVNAME to the renamed/moved device.
You can also prevent creation of device nodes with the 4th field as "!":
tty[a-z]. 0:0 660 !
pty[a-z]. 0:0 660 !
If you also enable support for executing your own commands, then the file has
<device regex> <uid>:<gid> <permissions> [=path] [@|$|*<command>]
<device regex> <uid>:<gid> <permissions> [>path] [@|$|*<command>]
<device regex> <uid>:<gid> <permissions> [!] [@|$|*<command>]
# block devices
([hs]d[a-z]) root:disk 660 >disk/%1/0
([hs]d[a-z])([0-9]+) root:disk 660 >disk/%1/%2
mmcblk([0-9]+) root:disk 660 >disk/mmc/%1/0
mmcblk([0-9]+)p([0-9]+) root:disk 660 >disk/mmc/%1/%2
# network devices
(tun|tap) root:network 660 >net/%1
The special characters have the meaning:
@ Run after creating the device.
$ Run before removing the device.
* Run both after creating and before removing the device.
The command is executed via the system() function (which means you're giving a
command to the shell), so make sure you have a shell installed at /bin/sh. You
should also keep in mind that the kernel executes hotplug helpers with stdin,
stdout, and stderr connected to /dev/null.
For your convenience, the shell env var $MDEV is set to the device name. So if
the device "hdc" was matched, MDEV would be set to "hdc".
Some kernel device drivers need to request firmware at runtime in order to
properly initialize a device. Place all such firmware files into the
/lib/firmware/ directory. At runtime, the kernel will invoke mdev with the
filename of the firmware which mdev will load out of /lib/firmware/ and into
the kernel via the sysfs interface. The exact filename is hardcoded in the
kernel, so look there if you need to know how to name the file in userspace.
Kernel does not serialize hotplug events. It increments SEQNUM environmental
variable for each successive hotplug invocation. Normally, mdev doesn't care.
This may reorder hotplug and hot-unplug events, with typical symptoms of
device nodes sometimes not created as expected.
However, if /dev/mdev.seq file is found, mdev will compare its
contents with SEQNUM. It will retry up to two seconds, waiting for them
to match. If they match exactly (not even trailing '\n' is allowed),
or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq
IOW: this will serialize concurrent mdev invocations.
If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to
setting mdev to be the hotplug handler. This writes single '\n' to the file.
NB: mdev recognizes /dev/mdev.seq consisting of single '\n' character
as a special case. IOW: this will not make your first hotplug event
to stall for two seconds.