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MDEV Primer: Understanding and Usage Guide

For those familiar with mdev, this primer may seem redundant. For others, mdev is a mysterious black box that is touted as powerful but not easily understood. Thus, this primer.

Basic Use of Mdev

Mdev serves two primary purposes: initial device population and dynamic updates. Both require sysfs kernel support, mounted at /sys. For dynamic updates, kernel hotplugging must be enabled.

A typical initialization script might include the following steps:

A more comprehensive setup involves:

These steps ensure /sys is mounted before mdev execution, enabling device node creation upon device addition or removal.

MDEV Configuration (/etc/mdev.conf)

Mdev offers optional configuration for device node ownership and permissions, deviating from the default root/root 660 settings. The configuration file format is:

For example:

hd[a-z][0-9]* 0:3 660

Parsing stops at the first match. If none match, defaults (0:0 660) apply. To set a default, use a regex like .* 1:1 777.

For advanced use, the format includes command execution:

Special characters trigger commands at specific lifecycle stages. Commands are executed via /bin/sh, so ensure sh is installed at /bin/sh. The $MDEV variable provides the device name for convenience.

This guide provides a clear understanding of mdev configuration and usage, ensuring effective device management on your system.