Mdev

The aim of this page is to document how to replace udev in Linux with mdev, thus allowing a separate /usr partition, without an initramfs. The auther uses Gentoo Linux with Icewm as the WM. The instructions here should be applicable to other distros, assuming you can make the necessary changes.

Will mdev work on my system?

I run a simple Gentoo install, using ICEWM, and no "desktop environment". An xorg.conf is not required on my system. Note that a "desktop environment" is not required to run Abiword, Firefox, Gimp, Gnumeric, etc. However, "Koffice apps" like Kmail seem to pull in most of KDE as a dependancy. In general, if you use KDE or lvm2, you may have problems, and may not be able to get by with mdev. One beta tester reports getting close with lvm2, but it's not there yet.

There is one more sanity check that Gentoo users can run to check for udev dependancy. I don't think this can be duplicated in other distros.

Sanity check

To /etc/portage/package.mask/mdev, add the line

File/etc/portage/package.mask/mdevMask away udev

sys-fs/udev

Execute the two commands:

root # euse -D udev

root # emerge -pvuDN world

If the only errors you get are for not being able to re-install udev as required by virtual/dev-manager, you can proceed to the next stage. Otherwise, sys-fs/udev might be an hard dependency of some package you requested.

Software requiring extra steps

GNOME 2

GNOME version 2 has been made to work under mdev.

Rebuild xorg-server without udev support
- In Gentoo, build with emerge xorg-server
- If you're running a different distro, use your package manager.
Configure the keyboard and mouse explicitly in /etc/X11/xorg.conf (or wherever else you keep your xorg.conf).
- Edit the two InputDevice sections to look like this. The critical lines are noted.

File/etc/X11/xorg.confExplicit input device configuration

<pre>    Section "InputDevice"
            Identifier  "Keyboard0"
            Driver      "evdev"                       # Critical
            Option      "Device" "/dev/input/event3"  # Also critical
    EndSection

    Section "InputDevice"
            Identifier  "Mouse0"
            Driver      "evdev"                       # Critical
            Option      "Protocol" "auto"
            Option      "Device" "/dev/input/event4"  # Also critical
            Option      "ZAxisMapping" "4 5 6 7"
    EndSection

ATI/AMD binary blobs

Some ATI/AMD Radeon cards require binary blobs. The Gentoo radeon-ucode ebuild pulls down all the binary blobs available at http://people.freedesktop.org/~ag5f/radeon_ucode/ to support many different models.

If I leave the binary blobs in the library directory, the kernel needs udev to figure out which one of the many binary blobs to load. If I remove all the other binary blobs, leaving only the correct one in the library directory, it loads automatically.

Warning / Disclaimer

Proceed only if the above stages don't reveal any udev dependencies. Furthermore:

This is a beta.
Use a spare test machine or a VM.
If you don't follow the instructions correctly, the result may be an unbootable Linux.
Even if you do follow the instructions correctly, the result may be an unbootable Linux.

Replacing udev with mdev

Setting up your kernel for devtmpfs

Set up your kernel to support and automount a devtmpfs filesystem at /dev. If you prefer to edit .config directly, set CONFIG_DEVTMPFS=y and CONFIG_DEVTMPFS_MOUNT=y.

If you prefer make menuconfig, the route is as shown below. Note that the "Automount devtmpfs..." option won't appear until you enable the "Maintain a devtmpfs..." option.

Kernel configurationSetting devtmpfs

  Device Drivers --->
    Generic Driver Options --->
      [*] Maintain a devtmpfs filesystem to mount at /dev
      [*] Automount devtmpfs at /dev, after the kernel mounted the rootfs

Once you've made the changes, rebuild the kernel. Do NOT reboot yet.

Emerging busybox

For our circumstance, you will need the mdev USE flag set for busybox. The static USE flag is probably also a good idea. In the /etc/portage/package.use/mdev file, add the line

File/etc/portage/package.use/mdev

sys-apps/busybox static mdev

Now, you may run emerge sys-apps/busybox. If you're using a distro other than Gentoo, or building Busybox manually, do whatever necessary in your situation to enable the mdev option.

Creating /sbin/linuxrc

You will need /sbin/linuxrc to contain at least

File/sbin/linuxrcCustom busybox based linuxrc

<pre>#!/bin/busybox ash
mount -t proc proc /proc
mount -t sysfs sysfs /sys
exec /sbin/init

This should be enough for most users. If you have an unusual setup, you may need to place additional steps in your /sbin/linuxrc. Remember to run chmod 744 /sbin/linuxrc to make the file executable.

Configure the bootloader

Append init=/sbin/linuxrc to your kernel command line. If you use lilo, remember to re-run lilo to implement the changes. If you use a different bootloader, apply any necessary equivalent steps.

Replace the udev service

Remove udev from the services list, replacing it with mdev:

root # rc-update del udev sysinit

root # rc-update add mdev sysinit

Reboot

Reboot to your new kernel. You should now be running without using udev.

Cleanup

Remove udev from your system by running emerge --unmerge sys-fs/udev. In the /etc/portage/package.mask, append the line sys-fs/udev. Create /etc/portage/package.mask if it doesn't already exist.

You should now have a completely udev-free machine.

Setting up a USB printer running under CUPS

Make 'find' available at early boot-time

root # cd /bin

root # ln -s /bin/busybox find

Rebuild the kernel

For libusb (and hence CUPS) to see the USB ports, set CONFIG_USB_DEVICE_CLASS=y and CONFIG_USB_DEVICEFS=y in your kernel configuration. You will also need to enable CONFIG_USB_PRINTER, despite the injunction in the cups-1.5.2 ebuild to disable it. If you use make menuconfig, these items are found here:

Kernel configurationEnabling generation of USB devices at boot time

  Device Drivers --->
    [*] Support for Host-side USB
      [*] USB device filesystem [DEPRECATED]
      [*] USB device class-devices [DEPRECATED]
    [*] USB Printer support

(Don't worry about the "DEPRECATED"s.)

Configure the new kernel into your bootloader and reboot into it

Verify the presence of the new device nodes

You should see a hierarchy of device nodes under /dev/bus/usb/. If you switch on your printer, you will probably see a node for it at /dev/lp0. You should be able to run lsusb successfully, and this should display your printer's details.

Configure your printer in CUPS

You should now be able to configure CUPS in the usual way. CUPS should find your printer, and will give it a URI something like parallel:/dev/lp0.

Notes

This set of instructions was originally written by Walt Dnes and hosted at http://www.waltdnes.org/mdev/. It was imported to the Gentoo wiki with some editing by Michael Mol per discussion on the gentoo-user mailing list.

Mdev

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