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.
- 1 Will mdev work on my system?
- 2 Software requiring extra steps
- 3 Warning / Disclaimer
- 4 Replacing udev with mdev
- 5 Setting up a USB printer running under CUPS
- 6 Notes
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.
To /etc/portage/package.mask/mdev, add the line
/etc/portage/package.mask/mdev - Mask away udev
Execute the two commands as root:
euse -D udev 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, udev might be a hard dependency of some package you requested.
Software requiring extra steps
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.
/etc/X11/xorg.conf - Explicit input device configuration
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: Setting 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.
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
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.
You will need /sbin/linuxrc to contain at least
/sbin/linuxrc - Custom busybox based linuxrc
#!/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:
rc-update del udev sysinit c-update add mdev sysinit
Reboot to your new kernel. You should now be running without using udev.
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
cd /bin 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: Enabling 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.
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 unlike udev does not support auto-modules loading thus you will need to use /etc/conf.d/modules and put there all the modules like you used to load (nvidia, wl etc.). Also, /etc/conf.d/modules have own _args variables as it does not support /etc/modprobe.d. You may need to move your configuration there.
- mdev -s does not create /dev/mapper nodes. You may need to manualy create them or use 'dmsetup mknodes' from lvm2 (good idea is to add it after mdev -s in init script).
- you should use mouse and kbd drivers for xorg inputs. evdev need udev to be built. Mousedrv (mouse) may conflict with synaptics driver, when both are loaded, non of them really work. (bug?)