In Installing Basic System Software, we installed the Udev package. 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 use a static device creation
method, whereby a great many device nodes are created under
/dev (sometimes literally thousands of
nodes), regardless of whether the corresponding hardware devices
actually exist. This is 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
tmpfs 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.
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 suffers from race
conditions that are inherent in its design and cannot be fixed
without a substantial revision to the kernel. It has also been
marked as deprecated due to a lack of recent maintenance.
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
seeing a userspace replacement for devfs became much more realistic.
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 sysfs 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 built-in
drivers registered with sysfs are
available to userspace processes and to udevd for device node creation.
The S10udev
initscript takes care of creating device nodes when Linux is
booted. The script unsets the uevent handler from the default of
/sbin/hotplug. This
is done because the kernel no longer needs to call out to an
external binary. Instead udevd will listen on a netlink
socket for uevents that the kernel raises. Next, the bootscript
copies any static device nodes that exist in /lib/udev/devices to /dev. This is necessary because some devices,
directories, and symlinks are needed before the dynamic device
handling processes are available during the early stages of
booting a system. Creating static device nodes in /lib/udev/devices also provides an easy
workaround for devices that are not supported by the dynamic
device handling infrastructure. The bootscript then starts the
Udev daemon, udevd,
which will act on any uevents it receives. Finally, the
bootscript forces the kernel to replay uevents for any devices
that have already been registered and then waits for udevd to handle them.
To obtain the right major and minor number for a device, Udev
relies on the information provided by sysfs in /sys.
For example, /sys/class/tty/vcs/dev
contains the string “7:0”.
This string is used by udevd to create a device node
with major number 7 and
minor 0. The names and
permissions of the nodes created under the /dev directory are determined by rules
specified in the files within the /etc/udev/rules.d/ directory. These are
numbered in a similar fashion to the CLFS-Bootscripts package. If
udevd can't find a
rule for the device it is creating, it will default permissions
to 660 and ownership to
root:root. Documentation
on the syntax of the Udev rules configuration files is available
in /usr/share/doc/udev/writing_udev_rules/index.html
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 by Udev will cause udevd to call out to
/sbin/modprobe with
the contents of the MODALIAS uevent
environment variable (that 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.
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.
There are a few possible problems when it comes to automatically creating device nodes.
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-2.6.39, 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-2.6.39, 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.
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 an
“install” line in
/etc/modprobe.conf. For example:
install snd-pcm /sbin/modprobe -i snd-pcm ; \
/sbin/modprobe snd-pcm-oss ; true
If the module in question is not a wrapper and is useful by
itself, configure the S05modules 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.
Either don't build the module, or blacklist it in /etc/modprobe.conf file as done with the
forte module in the
example below:
blacklist forteBlacklisted modules can still be loaded manually with the explicit modprobe command.
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 udevadm info.
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.
Please notify the CLFS Development list if you do so and it
helps.
Further text assumes that the driver is built statically into the kernel or already loaded as a module, and that you have already checked that Udev doesn't create a misnamed device.
Udev has no information needed to create a device node if a
kernel driver does not export its data to sysfs. This is most common with third party
drivers from outside the kernel tree. Create a static device node
in /lib/udev/devices with the
appropriate major/minor numbers (see the file devices.txt inside the kernel documentation or
the documentation provided by the third party driver vendor). The
static device node will be copied to /dev by the S10udev bootscript.
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 11.7, “Creating custom symlinks to devices” and Networking Configuration for examples.
Additional helpful documentation is available at the following sites:
A Userspace Implementation of devfs
http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf
The sysfs Filesystem
http://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf