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Linux From Scratch - Version 7.0

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7.4.3. Problems with Loading Modules and Creating Devices

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

7.4.3.1. 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-3.1, 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-3.1, 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.

7.4.3.2. 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 an “install” line in any /etc/modprobe.d/

<filename>.conf file. 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 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.

7.4.3.3. 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.

7.4.3.4. 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.

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MTD 248-645A Manual de usuario Pagina 222