Chapter 38. 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/modprobe.conf.
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Note |
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Video card modules used to display the X Window System interface
are part of the xorg-X11 packages, not
the kernel; thus, this chapter does not apply to them.
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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:
Refer to the Red Hat Enterprise Linux
Reference Guide for an alphabetical list of kernel modules and
supported hardware for those modules.
A group of commands for managing kernel modules is available if
the module-init-tools 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
nfs 218437 1
lockd 63977 2 nfs
parport_pc 24705 1
lp 12077 0
parport 37129 2 parport_pc,lp
autofs4 23237 2
i2c_dev 11329 0
i2c_core 22081 1 i2c_dev
sunrpc 157093 5 nfs,lockd
button 6481 0
battery 8901 0
ac 4805 0
md5 4033 1
ipv6 232833 16
ohci_hcd 21713 0
e100 39493 0
mii 4673 1 e100
floppy 58481 0
sg 33377 0
dm_snapshot 17029 0
dm_zero 2369 0
dm_mirror 22957 2
ext3 116809 2
jbd 71257 1 ext3
dm_mod 54741 6 dm_snapshot,dm_zero,dm_mirror
ips 46173 2
aic7xxx 148121 0
sd_mod 17217 3
scsi_mod 121421 4 sg,ips,aic7xxx,sd_mod
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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 /sbin/lsmod output is less verbose
and easier to read than 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 e100 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/2.6.9-5.EL/kernel/drivers/net/e100.ko
Using /lib/modules/2.6.9-5.EL/kernel/drivers/net/e100.ko
Symbol version prefix 'smp_'
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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 e100 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>
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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).
