Chapter 31. Kernel Modules
The Linux kernel has a modular design. At boot time, only a minimal
resident kernel is loaded into memory. Thereafter, whenever a user
requests a feature that is not present in the resident kernel, a
kernel module, sometimes referred to as a
driver, is dynamically loaded into memory.
During installation, the hardware on the system is probed. Based on this
probing and the information provided by the user, the installation program
decides which modules need to be loaded at boot time. The installation
program sets up the dynamic loading mechanism to work transparently.
If new hardware is added after installation and the hardware requires a
kernel module, the system must be configured to load the proper kernel
module for the new hardware. When the system is booted with the new
hardware, the Kudzu program runs, detects the
new hardware if it is supported, and configures the module for it. The
module can also be specified manually by editing the module configuration
file, /etc/modules.conf.
 | Note |
|---|
| | Video card modules used to display the X Window System interface are
part of the XFree86 package, not the kernel; thus,
this chapter does not apply to them.
|
For example, if a system included an SMC EtherPower 10 PCI network adapter,
the module configuration file contains the following line:
If a second network card is added to the system and is identical to the
first card, add the following line to
/etc/modules.conf:
See the Red Hat Linux Reference Guide for an alphabetical list of
kernel modules and the hardware supported by the modules.
31.1. Kernel Module Utilities
A group of commands for managing kernel modules is available if the
modutils package is installed. Use these commands
to determine if a module has been loaded successfully or when trying
different modules for a piece of new hardware.
The command /sbin/lsmod displays a list of currently
loaded modules. For example:
Module Size Used by Not tainted
iptable_filter 2412 0 (autoclean) (unused)
ip_tables 15864 1 [iptable_filter]
nfs 84632 1 (autoclean)
lockd 59536 1 (autoclean) [nfs]
sunrpc 87452 1 (autoclean) [nfs lockd]
soundcore 7044 0 (autoclean)
ide-cd 35836 0 (autoclean)
cdrom 34144 0 (autoclean) [ide-cd]
parport_pc 19204 1 (autoclean)
lp 9188 0 (autoclean)
parport 39072 1 (autoclean) [parport_pc lp]
autofs 13692 0 (autoclean) (unused)
e100 62148 1
microcode 5184 0 (autoclean)
keybdev 2976 0 (unused)
mousedev 5656 1
hid 22308 0 (unused)
input 6208 0 [keybdev mousedev hid]
usb-uhci 27468 0 (unused)
usbcore 82752 1 [hid usb-uhci]
ext3 91464 2
jbd 56336 2 [ext3] |
For each line, the first column is the name of the module, the second
column is the size of the module, and the third column is the use count.
The information after the use count varies slightly per module. If
(unused) is listed on the line for the
module, the module is currently not being used. If
(autoclean) is on the line for the
module, the module can be autocleaned by the rmmod -a
command. When this command is executed, any modules that are tagged with
autoclean, that have not been used since the previous autoclean
action, are unloaded. Red Hat Linux does not perform this autoclean action by
default.
If a module name is listed at the end of the line in brackets, the module
in the brackets is dependent on the module listed in the first column
of the line. For example, in the line
usbcore 82752 1 [hid usb-uhci] |
the hid and usb-uhci kernel modules depend on the usbcore module.
The /sbin/lsmod output is the same as the output from
viewing /proc/modules.
To load a kernel module, use the /sbin/modprobe
command followed by the kernel module name. By default,
modprobe attempts to load the module from the
/lib/modules/<kernel-version>/kernel/drivers/
subdirectories. There is a subdirectory for each type of module, such as
the net/ subdirectory for network interface
drivers. Some kernel modules have module dependencies, meaning that
other modules must be loaded first for it to load. The
/sbin/modprobe command checks for these dependencies
and loads the module dependencies before loading the specified module.
For example, the command
loads any module dependencies and then the hid
module.
To print to the screen all commands as /sbin/modprobe
executes them, use the -v option. For example:
Output similar to the following is displayed:
/sbin/insmod /lib/modules/2.4.20-2.47.1/kernel/drivers/usb/hid.o
Using /lib/modules/2.4.20-2.47.1/kernel/drivers/usb/hid.o
Symbol version prefix 'smp_' |
The /sbin/insmod command also exists to load kernel
modules; however, it does not resolve dependencies. Thus, it is
recommended that the /sbin/modprobe command be used.
To unload kernel modules, use the /sbin/rmmod command
followed by the module name. The rmmod utility only
unloads modules that are not in use and that are not a dependency of
other modules in use.
For example, the command
unloads the hid kernel module.
Another useful kernel module utility is modinfo. Use
the command /sbin/modinfo to display information
about a kernel module. The general syntax is:
/sbin/modinfo [options] <module> |
Options include -d which displays a brief description
of the module and -p which lists the parameters the
module supports. For a complete list of options, refer to the
modinfo man page (man modinfo).
