Difference between revisions of "Kernel modules"

Revision as of 19:37, 26 March 2013

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Kernel modules are pieces of code that can be loaded and unloaded into the kernel upon demand. They extend the functionality of the kernel without the need to reboot the system.

Overview

To create a kernel module, you can read this guide. A module can be configured to be build-in or loadable. To dynamically load or remove a module, it has to be configured as a loadable module in the kernel configuration (the line related to the module will therefore display the letter M).

Modules are stored in /usr/lib/modules/kernel_release. You can use the command uname -r to get your current kernel release version.

Obtaining information

To show what kernel modules are currently loaded:

$ lsmod

To show information about a module:

$ modinfo module_name

To list the options that are set for a loaded module:

$ systool -v -m module_name

To display the comprehensive configuration of all the modules:

$ modprobe -c | less

To display the configuration of a particular module:

$ modprobe -c | grep module_name

List the dependencies of a module (or alias), including the module itself:

$ modprobe --show-depends module_name

Loading

The /sbin/modprobe command handles the addition and removal of modules from the Linux kernel. To manually load (or add) a module, run:

# modprobe module_name

Most modules should be loaded on-demand. Modules to be unconditionally loaded at boot can be specified in /etc/modules-load.d/, for example:

/etc/modules-load.d/virtio-net.conf

#Load 'virtio-net.ko' at boot.

virtio-net

Removal

To remove (or unload) a module, use the following command:

# modprobe -r module_name

Or, alternatively:

# rmmod module_name

Configuration

To pass a parameter to a kernel module you can use a modprobe conf file or use the kernel command line. See also Systemd#Kernel_modules.

Using files in /etc/modprobe.d/

The /etc/modprobe.d/ directory can be used to pass module settings to udev, which will use modprobe to manage the loading of the modules during system boot. You can use configuration files with any name in the directory, given that they end with the .conf extension. The syntax is:

/etc/modprobe.d/myfilename.conf

options modname parametername=parametercontents

For example:

/etc/modprobe.d/thinkfan.conf

#On Thinkpads, this lets the 'thinkfan' daemon control fan speed.

options thinkpad_acpi fan_control=1

Using kernel command line

If the module is built into the kernel you can also pass options to the module using the kernel command line. For all common Bootloaders the following syntax is correct:

modname.parametername=parametercontents

for example:

thinkpad_acpi.fan_control=1

Simply add this to your bootloaders kernel-line as described in Kernel Parameters.

Aliasing

Aliases are alternate names for a module. For example: "alias my-mod really_long_modulename" means you can use "modprobe my-mod" instead of "modprobe really_long_modulename". You can also use shell-style wildcards, so "alias my-mod* really_long_modulename" means that "modprobe my-mod-something" has the same effect. Create an alias:

/etc/modprobe.d/myalias.conf

alias mymod really_long_module_name

Some modules have aliases which are used to autoload them when they are needed by an application. Disabling these aliases can prevent auto-loading, but will still allow the modules to be manually loaded.

/etc/modprobe.d/modprobe.conf

#Prevent Bluetooth autoload.

alias net-pf-31 off

Blacklisting

Blacklisting, in the context of kernel modules, is a mechanism to prevent the kernel module from loading. This could be useful if, for example, the associated hardware is not needed, or if loading that module causes problems: for instance there may be two kernel modules that try to control the same piece of hardware, and loading them together would result in a conflict.

Some modules are loaded as part of the initramfs. mkinitcpio -M will print out all autodetected modules: to prevent the initramfs from loading some of those modules, blacklist them in /etc/modprobe.d/modprobe.conf. Running mkinitcpio -v will list all modules pulled in by the various hooks (e.g. filesystem hook, SCSI hook, etc.). Remember to add that .conf file to the FILES section in /etc/mkinitcpio.conf (if you have not done so already) and rebuild the initramfs once you have blacklisted the modules, and to reboot afterwards.

Using files in /etc/modprobe.d/

Create a .conf file inside /etc/modprobe.d/ and append a line for each module you want to blacklist, using the blacklist keyword. If for example you want to prevent the pcspkr module from loading:

/etc/modprobe.d/nobeep.conf

#Do not load the 'pcspkr' module on boot.

blacklist pcspkr

/etc/modprobe.d/blacklist.conf

...
install module_name /bin/false
...

Using kernel command line

You can also blacklist modules from the bootloader.

Simply add modprobe.blacklist=modname1,modname2,modname3 to your bootloader's kernel line, as described in Kernel Parameters.

Tips and tricks

Bash function to list module parameters

Here is a nice bash function to be run as root that will show a list of all the currently loaded modules and all of their parameters, including the current value of the parameter. It uses /proc/modules to retrieve the current list of loaded modules, then access the module file directly with modinfo to grab a description of the module and descriptions for each param (if available), finally it accesses the sysfs filesystem to grab the actual parameter names and currently loaded values.

function aa_mod_parameters () 
{ 
    N=/dev/null;
    C=`tput op` O=$(echo -en "\n`tput setaf 2`>>> `tput op`");
    for mod in $(cat /proc/modules|cut -d" " -f1);
    do
        md=/sys/module/$mod/parameters;
        [[ ! -d $md ]] && continue;
        m=$mod;
        d=`modinfo -d $m 2>$N | tr "\n" "\t"`;
        echo -en "$O$m$C";
        [[ ${#d} -gt 0 ]] && echo -n " - $d";
        echo;
        for mc in $(cd $md; echo *);
        do
            de=`modinfo -p $mod 2>$N | grep ^$mc 2>$N|sed "s/^$mc=//" 2>$N`;
            echo -en "\t$mc=`cat $md/$mc 2>$N`";
            [[ ${#de} -gt 1 ]] && echo -en " - $de";
            echo;
        done;
    done
}

Here is some sample output:

# aa_mod_parameters

>>> ehci_hcd - USB 2.0 'Enhanced' Host Controller (EHCI) Driver
        hird=0 - hird:host initiated resume duration, +1 for each 75us (int)
        ignore_oc=N - ignore_oc:ignore bogus hardware overcurrent indications (bool)
        log2_irq_thresh=0 - log2_irq_thresh:log2 IRQ latency, 1-64 microframes (int)
        park=0 - park:park setting; 1-3 back-to-back async packets (uint)

>>> processor - ACPI Processor Driver
        ignore_ppc=-1 - ignore_ppc:If the frequency of your machine gets wronglylimited by BIOS, this should help (int)
        ignore_tpc=0 - ignore_tpc:Disable broken BIOS _TPC throttling support (int)
        latency_factor=2 - latency_factor: (uint)

>>> usb_storage - USB Mass Storage driver for Linux
        delay_use=1 - delay_use:seconds to delay before using a new device (uint)
        option_zero_cd=1 - option_zero_cd:ZeroCD mode (1=Force Modem (default), 2=Allow CD-Rom (uint)
        quirks= - quirks:supplemental list of device IDs and their quirks (string)
        swi_tru_install=1 - swi_tru_install:TRU-Install mode (1=Full Logic (def), 2=Force CD-Rom, 3=Force Modem) (uint)

>>> video - ACPI Video Driver
        allow_duplicates=N - allow_duplicates: (bool)
        brightness_switch_enabled=Y - brightness_switch_enabled: (bool)
        use_bios_initial_backlight=Y - use_bios_initial_backlight: (bool)

See also

• Disable PC Speaker Beep