kernel Modules

Modules make it easy to develop drivers without rebooting: load, test, unload, rebuild, load...
Useful to keep the kernel image size to the minimum (essential in GNU/Linux distributions for PCs).
Also useful to reduce boot time: you don't spend time initializing devices and kernel features that you only need later.
Caution: once loaded, have full control and privileges in the system. No particular protection. That's why only the root user can load and unload modules.

Module dependencies

Some kernel modules can depend on other modules, which need to be loaded first.
Example: the usb-storage module depends on the scsi_mod, libusual and usbcore modules.
Dependencies are described both in /lib/modules//modules.dep and in /lib/modules//modules.dep.bin These files are generated when you run make modules_install.

When a new module is loaded, related information is available in the kernel log.

The kernel keeps its messages in a circular buffer (so that it doesn't consume more memory with many messages)
Kernel log messages are available through the dmesg command (diagnostic message)
Kernel log messages are also displayed in the system console (console messages can be filtered by level using the loglevel kernel parameter, or completely disabled with the quiet parameter).
Note that you can write to the kernel log from user space too: echo "Debug info" > /dev/kmsg

modinfo modinfo .ko Gets information about a module: parameters, license, description and dependencies. Very useful before deciding to load a module or not.
sudo insmod .ko Tries to load the given module. The full path to the module object file must be given.

When loading a module fails, insmod often doesn't give you enough details! Details are often available in the kernel log.

sudo modprobe Most common usage of modprobe: tries to load all the modules the given module depends on, and then this module. Lots of other options are available. modprobe automatically looks in /lib/modules// for the object file corresponding to the given module name.
lsmod Displays the list of loaded modules Compare its output with the contents of /proc/modules!
sudo rmmod Tries to remove the given module. Will only be allowed if the module is no longer in use (for example, no more processes opening a device file)
sudo modprobe -r Tries to remove the given module and all dependent modules (which are no longer needed after removing the module)

Find available parameters: modinfo snd-intel8x0m
Through insmod: sudo insmod ./snd-intel8x0m.ko index=-2
Through modprobe: Set parameters in /etc/modprobe.conf or in any file in /etc/modprobe.d/: options snd-intel8x0m index=-2
Through the kernel command line, when the driver is built statically into the kernel: snd-intel8x0m.index=-2
- snd-intel8x0m is the driver name
- index is the driver parameter name
- -2 is the driver parameter value