Kernel Korner - udev—Persistent Device Naming in User Space

Starting with the 2.5 kernel, all physical and virtual devices in a system are visible to user space in a hierarchal fashion through sysfs. /sbin/hotplug provides a notification to user space when any device is added or removed from the system. Using these two features, a user-space implementation of a dynamic /dev now is possible that can provide a flexible device naming policy.

This article discusses udev, a program that replaces the functionality of devfs. It provides /dev entries for devices in the system at any moment in time. It also provides features previously unavailable through devfs alone, such as persistent naming for devices when they move around the device tree, a flexible device naming scheme, notification of external systems of device changes and moving all naming policy out of the kernel.

The /dev directory on a Linux machine is where all of the device files for the system should be located. A device file details how a user program can access a specific hardware device or function. For example, the device file /dev/hda traditionally is used to represent the first IDE drive in the system. The name hda corresponds to both a major and a minor number, which are used by the kernel to determine what hardware device to talk to. Currently, a wide range of names that match up to different major and minor numbers has been defined.

All major and minor numbers are assigned a name that matches up with a type of device. This allocation is done by the Linux assigned names and numbers authority (LANANA), and the current device list can be found on its Web site (see the on-line Resources section).

As Linux begins supporting new kinds of devices, these devices need to be assigned a major and minor number range in order for users to access them through the /dev directory. An alternative is to provide access through a filesystem; my 2002 linux.conf.au paper provides more details on how to do this (see the on-line Resources section). In kernel versions 2.4 and earlier, the valid range of major numbers was 1–255, and the valid range of minor numbers was 1–255. Because of this limited range, a freeze was placed on allocating new major and minor numbers during the 2.3 development cycle. This freeze since has been lifted, and the 2.6 kernel had the valid range of major numbers increased to 4,095. More than a million minor numbers are available per major number.

When the kernel finds a new piece of hardware, it typically assigns the next major/minor pair for that kind of hardware to the device. So, on boot, the first USB printer found would be assigned the major number 180 and minor number 0, which is referenced in /dev as /dev/usb/lp0. The second USB printer would be assigned major number 180 and minor number 1, which is referenced in /dev as /dev/usb/lp1. If the user rearranges the USB topology, perhaps adding a USB hub to support more USB devices in the system, the USB probing order of the printers might change the next time the computer is booted, reversing the assignment of the different minor numbers to the two printers.

This same situation holds true for almost any kind of device that can be removed or added while the computer is powered up. With the advent of PCI hot-plug-enabled systems and hot-pluggable buses, such as IEEE 1394, USB and CardBus, almost all devices have this problem.

With the advent of the sysfs filesystem in the 2.5 kernel, the problem of determining which device minor number is assigned to which physical device is much easier to determine. For a system having two different USB printers plugged in to it, the sysfs /sys/class/usb directory tree would look like this:

/sys/class/usb/
|-- lp0
|   |-- dev
|   |-- device -> ../../../devices/pci0/00:09.0/usb1/1-1/1-1:0
|   `-- driver -> ../../../bus/usb/drivers/usblp
`-- lp1
    |-- dev
    |-- device -> ../../../devices/pci0/00:0d.0/usb3/3-1/3-1:0
    `-- driver -> ../../../bus/usb/drivers/usblp

$ cat /sys/class/usb/lp0/device/serial
HXOLL0012202323480
$ cat /sys/class/usb/lp1/device/serial
W09090207101241330

Within the individual USB device directories pointed to by the lp0/device and lp1/device symbolic links, a lot of USB-specific information can be determined, such as the manufacturer of the device and the (hopefully unique) serial number.

As can be seen by the serial files in the above description, the /dev/usb/lp0 device file is associated with the USB printer with serial number HXOLL0012202323480, and the /dev/usb/lp1 device file is associated with the USB printer with serial number W09090207101241330. If these printers are moved around, say by placing them both behind a USB hub, they might be renamed, as they are probed in a different order on startup:

$ tree /sys/class/usb/
/sys/class/usb/
|-- lp0
|   |-- dev
|   |-- device -> ../../../devices/pci0/00:09.0/usb1/1-1/1-1.1/1-1.1:0
|   `-- driver -> ../../../bus/usb/drivers/usblp
`-- lp1
    |-- dev
    |-- device -> ../../../devices/pci0/00:09.0/usb1/1-1/1-1.4/1-1.4:0
    `-- driver -> ../../../bus/usb/drivers/usblp

$ cat /sys/class/usb/lp0/device/serial
W09090207101241330
$ cat /sys/class/usb/lp1/device/serial
HXOLL0012202323480