Sunday, April 20, 2008

mdev primer

The following is the doc related to mdev which i get from busybox 1.6.1
distribution :

MDEV Primer

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
seem to get their head around how it works. Thus, a primer.

Basic Use

Mdev has two primary uses: initial population and dynamic updates. Both
require sysfs support in the kernel and have it mounted at /sys. For
updates, you also need to have hotplugging enabled in your kernel.

Here's a typical code snippet from the init script:
[1] mount -t sysfs sysfs /sys
[2] echo /bin/mdev > /proc/sys/kernel/hotplug
[3] mdev -s

Of course, a more "full" setup would entail executing this before the
code snippet:
[4] mount -t tmpfs mdev /dev
[5] mkdir /dev/pts
[6] mount -t devpts devpts /dev/pts

The simple explanation here is that [1] you need to have /sys mounted
executing mdev. Then you [2] instruct the kernel to execute /bin/mdev
a device is added or removed so that the device node can be created or
destroyed. Then you [3] seed /dev with all the device nodes that were
while the system was booting.

For the "full" setup, you want to [4] make sure /dev is a tmpfs
(assuming you're running out of flash). Then you want to [5] create the
/dev/pts mount point and finally [6] mount the devpts filesystem on it.

MDEV Config (/etc/mdev.conf)

Mdev has an optional config file for controlling ownership/permissions
device nodes if your system needs something more than the default
660 permissions.

The file has the format:
<device regex> <uid>:<gid> <octal permissions>
For example:
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,
create your own total match like so:
.* 1:1 777

If you also enable support for executing your own commands, then the
file has
the format:
<device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]
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.

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

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