In the old days, Linux pioneers were expected to install new hardware manually, but Linux now offers automatic hardware detection at system boot time, as well as on-the-fly configuration of pluggable devices – whether the device is a USB stick, a digital camera, or a Bluetooth phone.

The Udev subsystem takes the pain out of accessing new devices, and a pair of components known as the Hardware Abstraction Layer (HAL) and D-Bus provide an interface from the hardware to desktop applications.

Under the Hood

Udev [1], which runs in the background as the udevd daemon, creates dynamic device files under /dev whenever it identifies a new device. The Udev daemon finds the devices at system boot time, and it also sets up new devices that are plugged in while the system is running – a feature that is commonly called hot plugging. The program might also create symbolic links for mass storage media such as disks or USB sticks under /dev/disk to point to the physical device files under /dev.

The Udev system makes sure device files are only created for existing devices. The kernel monitors the USB bus and detects the device you just plugged in. It generates a Uevent to report its findings to the Udev daemon. The Udev system processes the information by retrieving the data for the device from the kernel device database, Sysfs. The Udev rules specify what Udev does with the new device. The rules, which are located under /etc/udev/rules.d/, comprise a number of individual files that define actions for various events.

As Listing 1 shows, rule file names start with numbers. The idea behind the numerical approach is to specify the processing order because some dependencies might need to be resolved to complete the configuration.

# ls /etc/udev/rules.d/
05-udev-early.rules          64-md-raid.rules
40-alsa.rules                70-kpartx.rules
40-bluetooth.rules           70-persistent-cd.rules
50-udev-default.rules        70-persistent-net.rules
55-hpmud.rules               75-cd-aliases-generator.rules
55-libsane.rules             75-persistent-net-generator.rules
56-idedma.rules              77-network.rules
60-cdrom_id.rules            79-yast2-drivers.rules
60-persistent-input.rules    80-drivers.rules
60-persistent-storage.rules  90-hal.rules
64-device-mapper.rules       95-udev-late.rules

If you are interested in more detail on creating Udev rules, check out the article at kernel.org [1].

If you are interested, you can watch Udev working. The Udevmonitor tool gives you this option. Udevmonitor shows you the kernel Uevents that occur when you plug in a device (say, a USB stick) along with the matching Udev responses (Listing 2).

# udevmonitor
udevmonitor will print the received events for:
UDEV the event which udev sends out after rule processing
UEVENT the kernel uevent
UDEV  [1212504262.814732] add      /devices/pci0000:00/0000:00:1d.7/usb3/3-1 (usb)
UDEV  [1212504262.814934] add      /devices/pci0000:00/0000:00:1d.7/usb3/3-1/usb_endpoint/usbdev3.7_ep00 (usb_endpoint)
UDEV  [1212504262.815017] add      /devices/pci0000:00/0000:00:1d.7/usb3/3-1/3-1:1.0 (usb)
UDEV  [1212504262.815086] add      /class/scsi_host/host5 (scsi_host)
UDEV  [1212504262.815146] add      /devices/pci0000:00/0000:00:1d.7/usb3/3-1/3-1:1.0/usb_endpoint/usbdev3.7_ep01 (usb_endpoint)
UDEV  [1212504262.815193] add      /devices/pci0000:00/0000:00:1d.7/usb3/3-1/3-1:1.0/usb_endpoint/usbdev3.7_ep81 (usb_endpoint)
UEVENT[1212504263.666057] add      /devices/pci0000:00/0000:00:1d.7/usb3/3-1/3-1:1.0/host5/target5:0:0/5:0:0:0 (scsi)
UEVENT[1212504263.666112] add      /class/scsi_disk/5:0:0:0 (scsi_disk)
UEVENT[1212504263.673463] add      /block/sdb (block)
UEVENT[1212504263.673511] add      /block/sdb/sdb1 (block)
UEVENT[1212504263.673538] add      /class/scsi_device/5:0:0:0 (scsi_device)
UEVENT[1212504263.673565] add      /class/scsi_generic/sg2 (scsi_generic)
UDEV  [1212504263.716085] add      /devices/pci0000:00/0000:00:1d.7/usb3/3-1/3-1:1.0/host5/target5:0:0/5:0:0:0 (scsi)
UDEV  [1212504263.733094] add      /class/scsi_disk/5:0:0:0 (scsi_disk)
UDEV  [1212504263.793736] add      /class/scsi_device/5:0:0:0 (scsi_device)
UDEV  [1212504263.823559] add      /class/scsi_generic/sg2 (scsi_generic)
UDEV  [1212504263.831015] add      /block/sdb (block)
UDEV  [1212504263.900340] add      /block/sdb/sdb1 (block)

All the lines starting with UEVENT relate to kernel messages, whereas the lines that start with UDEV identify Udev actions. The new device is accessible via the /dev/sdb1 device file, as you can see from the last line in Listing 2. A quick glance at /dev/disk/by-label reveals a link with the name the USB stick used to register with the system. The matching symbolic link for the stick in this example is DISK_IMG, which points to /dev/sdb1, matching the Udevmonitor output.

In this case, the system just used the name.

Many, but not all, devices register with their model designations, or at least with their manufacturers' names. In this example, DISK_IMG is not very helpful.

In the case of USB devices, you can redefine the label for a device you plug in by issuing the lsusb command, as shown in Listing 3. The example here shows a Hewlett Packard keyboard and a digital camera, both of which are registered with their full device designations.

# lsusb
Bus 001 Device 005: ID 03f0:0024 Hewlett-Packard
Bus 001 Device 001: ID 0000:0000
Bus 005 Device 001: ID 0000:0000
Bus 004 Device 001: ID 0000:0000
Bus 002 Device 002: ID 04a9:3073 Canon, Inc. PowerShot A70 (ptp)
Bus 002 Device 001: ID 0000:0000
Bus 003 Device 001: ID 0000:0000

Abstract Art

After Udev has completed its chores, the Hardware Abstraction Layer (HAL) [2] makes its contribution. The idea behind HAL is that applications should be able to access hardware without the need to know anything about the details. HAL acts as an interface between the hardware and the applications.

To allow this to happen, HAL draws information from several sources. For one thing, it listens directly to system buses, for another, it retrieves information from Udev, the kernel, and certain other global and user-specific configuration files.

More information is stored in XML-formatted FDI files under /usr/share/hal/fdi/information/. The FDI files contain long lists of information on devices by individual vendors, such as CD/DVD writers or digital cameras. HAL can draw on this resource to provide detailed information on the hardware.

If you want to view this information, you can either use lshal at the command line or launch a graphical front end, such as the HAL Device Manager (Figure 1), Gnome Device Manager, or KDE HAL Device Manager.

Figure 1: The HAL Device Manager displaying HAL information.

HAL, which is developed by the Freedesktop.org project [3], runs in the background in the form of the hald daemon. On top of this, it launches some auxiliary services when necessary. The services typically appear as entries starting with hald-addon- in the process list. A service typically monitors the status of a device.

If you find an entry for hald-addon-storage: polling /dev/hda, this means the device that follows /dev/hda is currently being polled; it is a CD/DVD drive in this example.

Taking the Bus to Work

Although HAL provides device information for applications, it does not communicate directly with them. This is the domain of D-Bus [4], which is also maintained by the Freedesktop.org project and interacts very closely with HAL. D-Bus is an IPC framework geared to the needs of desktop applications, and it is part of nearly any Linux distribution today.

D-Bus is used for communication between desktop applications within the same desktop session, as well as communication between the operating system and its components.

The idea behind the IPC framework has been around for some time. Both major desktops – Gnome and KDE – made heavy use of completely different approaches in the past, with Gnome mainly relying on Corba [5] and KDE using DCOP [6]. Windows, which also has IPC services, relies on the proprietary DCOM [7]. Other IPC mechanisms also exist on Linux, however, they are restricted to specific tasks.

D-Bus also runs as a daemon and provides two communications channels or buses. The system bus is launched at boot time and is available whether the user logs on to the GUI system or not; that is, it runs all the time.

If a user logs on to the graphical interface, a second bus is launched: the session bus. The daemon process, dbus-daemon, has two options for this: --system and --session.

Whereas the daemon process for the system bus reads its configuration parameters from the /etc/dbus-1/system.conf file, the session bus parses /etc/dbus-1/session.conf. Incidentally, the dbus-launch program is used to launch the daemon.

Dbus-launch runs permanently as a daemon during a graphical user session, just in case it needs to launch further session bus processes.

After the user logs off, dbus-launch and the corresponding session bus terminate, leaving the system bus running. Figures 2 and 3 show the interrelationships.

Figure 2: Although the D-Bus system bus runs permanently, the D-Bus launcher and the session bus only run during the user session.

Figure 3: HAL organizes information on attached hardware and uses D-Bus to make the information available to applications.

If an application requests a specific device class (such as camera, or storage) HAL uses D-Bus to notify the program as soon as the status of a device in this class changes. For example, the Gnome Volume Manager launches the gthumb-import import tool when you connect a camera (Figure 4).

Figure 4: The Gnome Volume Properties tool helps you launch Gthumb-import when you connect your digital camera.

KDE 4 has an applet in the form of a laptop icon labeled Device Monitor. The applet collaborates with D-Bus to display new hardware you plug in.

At the same time, it lets you launch a specific action depending on the device. For example, you could launch the Dolphin file manager to display the content of any USB sticks you plug in.

KDE's Solid framework is configured via the system preferences. So far, hardware detection has only been implemented for HAL management, the network, and Bluetooth.