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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 can't
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 dynamic
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 previous
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 before
executing mdev. Then you [2] instruct the kernel to execute /bin/mdev whenever
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 created
while the system was booting.
For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem
(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 of
device nodes if your system needs something more than the default root/root
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, simply
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".
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 want to know what to name the file in