7.3. Overview of Device and Module Handling

In Chapter 6, we installed the Udev package when eudev was built. Before we go into the details regarding how this works, a brief history of previous methods of handling devices is in order.

Linux systems in general traditionally used a static device creation method, whereby a great many device nodes were created under /dev (sometimes literally thousands of nodes), regardless of whether the corresponding hardware devices actually existed. This was typically done via a MAKEDEV script, which contains a number of calls to the mknod program with the relevant major and minor device numbers for every possible device that might exist in the world.

Using the Udev method, only those devices which are detected by the kernel get device nodes created for them. Because these device nodes will be created each time the system boots, they will be stored on a devtmpfs file system (a virtual file system that resides entirely in system memory). Device nodes do not require much space, so the memory that is used is negligible.

7.3.1. History

In February 2000, a new filesystem called devfs was merged into the 2.3.46 kernel and was made available during the 2.4 series of stable kernels. Although it was present in the kernel source itself, this method of creating devices dynamically never received overwhelming support from the core kernel developers.

The main problem with the approach adopted by devfs was the way it handled device detection, creation, and naming. The latter issue, that of device node naming, was perhaps the most critical. It is generally accepted that if device names are allowed to be configurable, then the device naming policy should be up to a system administrator, not imposed on them by any particular developer(s). The devfs file system also suffered from race conditions that were inherent in its design and could not be fixed without a substantial revision to the kernel. It was marked as deprecated for a long period – due to a lack of maintenance – and was finally removed from the kernel in June, 2006.

With the development of the unstable 2.5 kernel tree, later released as the 2.6 series of stable kernels, a new virtual filesystem called sysfs came to be. The job of sysfs is to export a view of the system's hardware configuration to userspace processes. With this userspace-visible representation, the possibility of developing a userspace replacement for devfs became much more realistic.

7.3.2. Udev Implementation

7.3.2.1. Sysfs

The sysfs filesystem was mentioned briefly above. One may wonder how sysfs knows about the devices present on a system and what device numbers should be used for them. Drivers that have been compiled into the kernel directly register their objects with a sysfs (devtmpfs internally) as they are detected by the kernel. For drivers compiled as modules, this registration will happen when the module is loaded. Once the sysfs filesystem is mounted (on /sys), data which the drivers register with sysfs are available to userspace processes and to udevd for processing (including modifications to device nodes).

7.3.2.2. Device Node Creation

Device files are created by the kernel by the devtmpfs filesystem. Any driver that wishes to register a device node will go through the devtmpfs (via the driver core) to do it. When a devtmpfs instance is mounted on /dev, the device node will initially be created with a fixed name, permissions, and owner.

A short time later, the kernel will send a uevent to udevd. Based on the rules specified in the files within the /etc/udev/rules.d, /lib/udev/rules.d, and /run/udev/rules.d directories, udevd will create additional symlinks to the device node, or change its permissions, owner, or group, or modify the internal udevd database entry (name) for that object.

The rules in these three directories are numbered and all three directories are merged together. If udevd can't find a rule for the device it is creating, it will leave the permissions and ownership at whatever devtmpfs used initially.

7.3.2.3. Module Loading

Device drivers compiled as modules may have aliases built into them. Aliases are visible in the output of the modinfo program and are usually related to the bus-specific identifiers of devices supported by a module. For example, the snd-fm801 driver supports PCI devices with vendor ID 0x1319 and device ID 0x0801, and has an alias of pci:v00001319d00000801sv*sd*bc04sc01i*. For most devices, the bus driver exports the alias of the driver that would handle the device via sysfs. E.g., the /sys/bus/pci/devices/0000:00:0d.0/modalias file might contain the string pci:v00001319d00000801sv00001319sd00001319bc04sc01i00. The default rules provided with Udev will cause udevd to call out to /sbin/modprobe with the contents of the MODALIAS uevent environment variable (which should be the same as the contents of the modalias file in sysfs), thus loading all modules whose aliases match this string after wildcard expansion.

In this example, this means that, in addition to snd-fm801, the obsolete (and unwanted) forte driver will be loaded if it is available. See below for ways in which the loading of unwanted drivers can be prevented.

The kernel itself is also able to load modules for network protocols, filesystems and NLS support on demand.

7.3.2.4. Handling Hotpluggable/Dynamic Devices

When you plug in a device, such as a Universal Serial Bus (USB) MP3 player, the kernel recognizes that the device is now connected and generates a uevent. This uevent is then handled by udevd as described above.

7.3.3. Problems with Loading Modules and Creating Devices

There are a few possible problems when it comes to automatically creating device nodes.

7.3.3.1. A kernel module is not loaded automatically

Udev will only load a module if it has a bus-specific alias and the bus driver properly exports the necessary aliases to sysfs. In other cases, one should arrange module loading by other means. With Linux-3.15.6, Udev is known to load properly-written drivers for INPUT, IDE, PCI, USB, SCSI, SERIO, and FireWire devices.

To determine if the device driver you require has the necessary support for Udev, run modinfo with the module name as the argument. Now try locating the device directory under /sys/bus and check whether there is a modalias file there.

If the modalias file exists in sysfs, the driver supports the device and can talk to it directly, but doesn't have the alias, it is a bug in the driver. Load the driver without the help from Udev and expect the issue to be fixed later.

If there is no modalias file in the relevant directory under /sys/bus, this means that the kernel developers have not yet added modalias support to this bus type. With Linux-3.15.6, this is the case with ISA busses. Expect this issue to be fixed in later kernel versions.

Udev is not intended to load wrapper drivers such as snd-pcm-oss and non-hardware drivers such as loop at all.

7.3.3.2. A kernel module is not loaded automatically, and Udev is not intended to load it

If the wrapper module only enhances the functionality provided by some other module (e.g., snd-pcm-oss enhances the functionality of snd-pcm by making the sound cards available to OSS applications), configure modprobe to load the wrapper after Udev loads the wrapped module. To do this, add a softdep line in any /etc/modprobe.d/<filename>.conf file. For example:

softdep snd-pcm post: snd-pcm-oss

Note that the softdep command also allows pre: dependencies, or a mixture of both pre: and post:. See the modprobe.d(5) manual page for more information on softdep syntax and capabilities.

If the module in question is not a wrapper and is useful by itself, configure the modules bootscript to load this module on system boot. To do this, add the module name to the /etc/sysconfig/modules file on a separate line. This works for wrapper modules too, but is suboptimal in that case.

7.3.3.3. Udev loads some unwanted module

Either don't build the module, or blacklist it in a /etc/modprobe.d/blacklist.conf file as done with the forte module in the example below:

blacklist forte

Blacklisted modules can still be loaded manually with the explicit modprobe command.

7.3.3.4. Udev creates a device incorrectly, or makes a wrong symlink

This usually happens if a rule unexpectedly matches a device. For example, a poorly-writen rule can match both a SCSI disk (as desired) and the corresponding SCSI generic device (incorrectly) by vendor. Find the offending rule and make it more specific, with the help of the udevadm info command.

7.3.3.5. Udev rule works unreliably

This may be another manifestation of the previous problem. If not, and your rule uses sysfs attributes, it may be a kernel timing issue, to be fixed in later kernels. For now, you can work around it by creating a rule that waits for the used sysfs attribute and appending it to the /etc/udev/rules.d/10-wait_for_sysfs.rules file (create this file if it does not exist). Please notify the LFS Development list if you do so and it helps.

7.3.3.6. Device naming order changes randomly after rebooting

This is due to the fact that Udev, by design, handles uevents and loads modules in parallel, and thus in an unpredictable order. This will never be fixed. You should not rely upon the kernel device names being stable. Instead, create your own rules that make symlinks with stable names based on some stable attributes of the device, such as a serial number or the output of various *_id utilities installed by Udev. See Section 7.4, “Managing Devices” and Section 7.5, “General Network Configuration” for examples.

7.3.4. Useful Reading

Additional helpful documentation is available at the following sites: