Bare mdev is a kernel hotplug helper. To activate it:

echo /sbin/mdev >/proc/sys/kernel/hotplug

It uses /etc/mdev.conf with lines

[-][ENV=regex;]...DEVNAME UID:GID PERM [>|=PATH]|[!] [@|$|*PROG]

where DEVNAME is device name regex, @major,minor[-minor2], or environment variable regex. A common use of the latter is to load modules for hotplugged devices:

$MODALIAS=.* 0:0 660 @modprobe "$MODALIAS"

If /dev/mdev.seq file exists, mdev will wait for its value to match $SEQNUM variable. This prevents plug/unplug races. To activate this feature, create empty /dev/mdev.seq at boot.

If /dev/mdev.log file exists, debug log will be appended to it.