1.2. The Modular Kernel

1.2. The Modular Kernel
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Many components of the Linux kernel may be compiled as modules which the kernel can dynamically load and remove as required.

The modules for a particular kernel are stored in /lib/modules/kernel-version
The best components to modularise are ones not required at boot time, for example peripheral devices and supplementary file systems.
Kernel modules are controlled by utilities supplied by the modutils package
The following is a list of 'userspace' commands that can be used to control kernel modules. Notice that we will spend more time on the depmod,modprobe and modinfo commands below.

lsmod
list currently loaded modules
rmmod
remove a single module
insmod
insert a single module
depmod
create the database of module dependencies modules.dep
modprobe
insert a module and dependencies listed in modules.dep
modinfo
list information about the author, license type and module parameters

depmod. Many modules are dependent on the presence of other modules. A flat file database of module dependencies /lib/modules/kernel-version/modules.dep is generated by the depmod command. This command is run at boot time (for example by the rc.sysinit script).
modprobe will load any module and dependent modules listed in modules.dep (or conf.modules
Search for example for modules that will be loaded at the same time as tvaudio.
```
grep tvaudio /lib/modules/kernel-version/modules.dep
/lib/modules/k-v/kernel/drivers/media/video/tvaudio.o: \ 
/lib/modules/k-v/kernel/drivers/i2c/i2c-core.o
```
This means that the module i2c-core.o will also be loaded when using modprobe. This dependency is also apparent when listing the module with lsmod:
```
lsmod 
Module            Size    Used by       Not tainted
tvaudio          16796    0  (unused)
i2c-core         19236    0  [tvaudio]
```
/etc/modules.conf is consulted for module parameters (IRQ and IO ports) but most often contains a list of aliases. These aliases allow applications to refer to a device using a common name. For example the first ethernet device is always referred to as eth0 and not by the name of the particular driver.
Figure 1.2. Sample /etc/modules.conf file
```
alias eth0 e100
alias usb-core usb-uhc
alias sound-slot-0 i810_audio
alias char-major-108 ppp_generic
alias ppp-compress-18 ppp_mppe

# 100Mbps full duplex
options eth0 e100_speed_duplex=4 
```

	2.6 kernels
	With the latest 2.6 kernels the `/etc/modules.conf` file has been replaced with `/etc/modprobe.conf`

modinfo will give information about modules.

modinfo tvaudio
filename:
/lib/modules/k-v/kernel/drivers/media/video/tvaudio.o
description: "device driver for i2c TV sound decoder \
audiomux chips"
author: "Eric Sandeen, Steve VanDeBogart, Greg Alexander\
 Gerd Knorr"
license: "GPL"
parm:  debug int
parm:  probe short array (min =1, max =48), description \
"List of adapter,address pairs to scan additionally"
parm:  probe_range short array (min =1, max =48), \
description "List of additional adapter triples to scan"
parm:  ignore short array (min =1, max =48), description\
 "List of adapter,address pairs not to scan"
parm:  ignore_range short array (min = 1, max = 48), \
description "List of adapter  triples not to scan"
parm:  force short array (min =1, max =48), description\
 "List of adapter,address pairs to boldly assume to be present"
parm:  tda9874a_SIF int
parm:  tda9874a_AMSEL int
parm:        tda9874a_STD int
parm:        tda8425 int
parm:        tda9840 int

To get information only about parameter option use modinfo -p, to get information about the license type use modinfo -l, etc.

kmod is a mechanism that allows the kernel to automatically load modules as needed (one seldom needs to insert modules manually). This is in fact a statically compiled (resident) module that needs to be configured before compiling the kernel. The command used by the kernel to load the modules is defined in /proc/sys/kernel/modprobe.