lfs/udev-config/doc/80-drivers.txt

Purpose of rules file:

The rules in this file allow Udev to fully replace the old /sbin/hotplug
script.  They automatically load kernel modules as devices are discovered.


Description of rules:

All rules in this file match ACTION=="add", so they only run when devices are
being added.

ENV{MODALIAS} is the value of the environment variable named MODALIAS.  This
environment variable is sent by the kernel when it sends a uevent for any
device that has a modalias.  Modaliases are strings that can be used to load
the appropriate kernel module driver.

Generally a modalias will contain information like vendor ID, device ID, and
possibly other IDs depending on the bus the device is connected to.  (USB, for
instance, has the concept of a "device class" and a "device interface", which
are basically just ways to standardize the USB protocol for various types of
devices.  This is what allows a single kernel module such as hid.ko to drive
many different vendors' USB input devices: all devices that support the USB
HID interface expose the HID interface number in their modalias, and so the
hid.ko driver can be loaded for each device.  When it loads, hid.ko attaches
to the HID interface and does whatever is needed to work with each device.)

Kernel modules that drive hardware expose a list of modaliases.  These
modaliases are matched against the device modalias by /sbin/modprobe (after
shell-style expansion), with the help of /sbin/depmod's modules.alias file.
The upshot of all this is, you can tell Udev to run "/sbin/modprobe modalias",
and it will load the module that claims it can drive the "modalias" device.

The rule that does this inspects ENV{MODALIAS} to ensure it is not empty.  It
does this by comparing it to "?*" -- inside a match, "*" would match *any*
string, including the empty string, so to ensure MODALIAS is not empty, we need
to match against "?*" instead.  ("?" matches any one character.)

The Udev RUN+="" option adds a program to run when the rule matches.  In this
case, we tell Udev to run "/sbin/modprobe $env{MODALIAS}".  Note that Udev does
not do path searches; if the executable is not specified with a fully-qualified
path, it *must* be located under the /lib/udev directory.  If it is not, you
*must* specify a fully-qualified path, as we do here.  Also, "$env{string}" is
replaced by the value of the environment variable "string" when the command
runs, so this adds the modalias to the modprobe command.  The modprobe program
will do the rest.  Finally, the {ignore_error} option is added to the RUN key;
this prevents Udev from failing the uevent if the modprobe command fails.  (The
modprobe command will fail when run during cold-plugging, if the driver was
configured into the kernel instead of as a module, for instance.)

There is still one feature of the old hotplug shell-script system that Udev
cannot provide: blacklisting modules from being auto-loaded.  To accomplish
this, we must use module-init-tools.  In /etc/modprobe.conf, if you use the
"blacklist <module-name>" syntax, modprobe will not load <module-name> under
any name except its real module name.  Any modaliases exposed by that module
will not be honored.


There are also rules in this file for various other types of driver loading.
PNP-BIOS devices, for instance, expose a list of PNP IDs in their sysfs "id"
attribute, instead of exposing a single MODALIAS, so one rule loops through
each ID and tries to load the appropriate module.  Several other types of
devices require an extra module before they will work properly; one example
of this is IDE tapes, which require the ide-scsi module.  Finally, whenever
any SCSI device is found, the file uses the TEST key to check whether the
/sys/module/sg directory exists.  If not, then the "sg" module -- the SCSI
generic driver -- is loaded.  (That driver creates the module/sg directory,
so the module/sg test is just to see whether the driver has already been
loaded.)